dp operators handbook

DP Operator's Handbook by

Captain David Bray FNI

ii

DP Operator's Handbook

By Captain David Bray FNI David Bray has asserted his right under the Copyright, Designs and Patents Act of 1988 to be

identified as the author of this work.

Published by The Nautical Institute 202 Lambeth Road, London SE1 7LQ, England Tel: +44 (0)20 7928 1351 Fax: +44 (0)20 7401 2817 Web: www.nautinst.org First edition published 2008 Reissued 2010 Copyright ©The Nautical Institute 2008 ©

David Bray 2008

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form, by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, except for the quotation of brief passages in review. Although great care has been taken with the writing of the book and production of the volume, neither The Nautical Institute nor the author can accept any responsibility for errors and omissions or their consequences.

The book has been prepared to address the subject of dynamic positioning. This should not, however, be taken to mean that this document deals comprehensively with all of the concerns that will need to be addressed, or even, when a particular need is addressed, that this document sets out the only definitive view for all situations. The opinions expressed are those of the author only and are not necessarily to be taken as the polices or view of any organisation with which he has any connection.

Design and typesetting by Phil McAllister Design Printed in the UK byThanet Press Ltd. ISBN 978 1 906915 15 5

THE NAUTICAL INSTITUTE

http://www.nautinst.org/�

iii

Foreword

D ynamic Positioning as a working system was introduced in the 1970s. As its potential was realised, DP systems were applied to ever more critical operations involving

divers and drilling.

The UK Department of Energy and the Norwegian Maritime Directorate specified the need for training;The Nautical Institute assessed training needs and established a training programme. Captain David Bray from Lowestoft with Jim Simpson from Aberdeen in the UK and Rune Mellum in Kongsberg, Norway, played a leading role in converting the training specification into simulator courses that became the international standard.

As the range of DP applications becomes ever more specialised, so the training must meet these specialist needs. Captain Bray recognised the need for a multi-disciplinary committee to examine DP specialisation and it is pleasing to see the progress being made as DP matures into a new phase.

Currently there are some 55 DP training centres worldwide; The Nautical Institute has issued over 1,200 DP certificates a year since 2005, and numbers continue to increase. Captain Bray has devoted countless hours to the development of effective training methods which he has generously shared with colleagues. He has accredited numerous DP centres and contributed to the DP Training Forum. I would like to acknowledge his dedication and outstanding professional commitment.

In our discussions over the years we concluded that, with so many applications, it might be useful to provide a handbook for DP operators (DPOs) to aid easy transfer from one ship to another which could serve as a handy reference book to be used as a refresher, a source of good management and operational practices and a guide for organising onboard training particularly for the benefit of understudies.

For example, communications has prominence in a number of sections and it is a maxim of all cooperative operational systems to keep lines of communication open at all times, always seek to explain intentions to those who need to know. By keeping lines of communication open and being open to the comments of people engaged in the operation, most problems will be overcome.

Captain Bray has now retired. On behalf of everybody he has helped and trained over the past 20 years and for the wisdom of this handbook, he deserves our special appreciation and thanks.

Julian Parker The Nautical Institute Secretary 1972-2003, Publisher 2003-2008

DP OPERATOR'S HANDBOOK

iv

Acknowledgements

M any individuals and organisations have either directly or indirectly contributed to this book. Inevitably, a work of this type is a digest of existing knowledge,

experience and previously published material.

Thanks are particularly due to my long-time colleague at Lowestoft College in the UK, Richard Lodge, whose help and support have been invaluable, both in the compilation of this book and in the running of training within the DP unit.

For information supplied over the years, I am indebted to the various manufacturers of DP and peripheral systems, in particular Kongsberg Maritime, Converteam, Nautronix, Sonardyne, Guidance Limited and MDL.

Thanks to the following for supplying illustrations: Heerema Marine Contractors, R Clarke, Teekay Shipping and Gary Ritchie FNI.

I must also thank Julian Parker at The Nautical Institute, not only for the opportunity of compiling this volume, but for all the help and encouragement over more than 20 years in the business of DP training, accreditation and competence.

David Bray 2008


V

Contents Chapter 1 Introduction to dynamic positioning .................................................................. 1

Chapter 2 Dynamic positioning principles and systems ...................................................... 3

Chapter 3 Redundancy and equipment class ................................................................... 21

Chapter 4 DP vessel operations ...................................................................................... 31

Chapter 5 Operational planning and watchkeeping .......................................................... 47

Chapter 6 Position referencing ....................................................................................... 57

Chapter 7 Propulsion and thrusters ................................................................................ 87

Chapter 8 Power plant ................................................................................................... 93

Chapter 9 Operator training and human factors ................................................................... 98

Appendix 1 Guidance to Masters of DP vessels in assessing the competency

of DPO candidates ...................................................................................... 107

Appendix 2 References page............................................................................................ 109

Appendix 3 Glossary of Terms and List of Abbreviations..................................................... 111

Index .............................................................................................................117


Chapter 1 1

Introduction to dynamic positioning

Chapter 1 Introduction to dynamic positioning

D ynamic positioning (DP) is now an established technique in the world of offshore- related and specialised shipping. The earliest, very rudimentary DP systems were in

use in the 1960s in deep water drillships, but the technology did not come of age until the late 1970s and early 1980s when DP systems were being fitted to new vessels in large numbers. From these beginnings, there are now in the order of 3,000 DP-capable vessels extant, with more being built.

It is more than 45 years since the first, fully automatic dynamically positioned vessel entered service. After all, the first DP-capable vessel entered service only 34 years after the launch of the last ocean-going cargo-carrying square-riggers. It may be thought that DP is now a fully mature technology, but a visit to any of the conferences regularly held on the subject of DP will soon dispel that myth! The technology is a very rapidly changing one, in every area (propulsion, control systems, position-reference, operational, etc) and it is sometimes quite a challenge to stay current. For the vessel operators and DPOs this is even more of a challenge, as, perforce dynamic positioning operators (DPOs) may find themselves working on a limited range of vessels without the most up-to-date equipment.

The technology related to DP has changed and developed very quickly, and the learning curve has been very steep. The number of personnel involved in DP operations has continually increased and there is now a huge base of expertise and experience. Nevertheless, a large number of personnel are new to DP techniques, and training is a major requirement within the DP vessel industry.

The Nautical Institute has been involved with DPO training and qualification from the beginning: its scheme for the training of DPOs is accepted worldwide. Shore-based training centres all over the world provide elements of training approved and accredited to a common high standard.

This handbook is not intended to be a comprehensive treatise on the subject of DP. It is aimed at those perhaps entering the DP scenario from more conventional vessels. It does not attempt to replace the system handbooks supplied by the system manufacturers, nor does it go into great detail on individual manufacturers' systems. It attempts to provide


2 Chapter 1 DP Operator's Handbook

an overview of the topic, leading the reader into further study and discussion. It also highlights some of the pitfalls and problems that may be experienced, while references to further reading are provided to allow the subject to be studied further if needed.



Operator (DPO). Measured values of position and heading are continually fed back into the controllers, while the difference between Set Point and feedback is termed error or deviation. The controller computers continually adjust thrust commands to reduce these errors to (or maintain them at) zero.

All modern complex control systems, such as DP, use mathematical modelling techniques as part of their control functionality. The system contains a mathematical model, or description of the vessel's dynamics. This is used to continually predict future vessel positions, headings and velocities. This data is continually compared with the corresponding measured values, allowing the computation of corrective thrust commands. A DP system is an example of an automatic Closed-loop control function.

The mathematical model contains static data on the vessel parameters, but is also an adaptive feature. The analogy is that of steering a vessel by hand. Although skilled at steering, a helmsman will take five minutes or so to get the feel of the vessel after taking the wheel. The DP system does the same, the mathematical model taking up to 30 minutes to fully adapt to the present environment and vessel configuration. Subsequent to this period, the system will continually adapt to changes in the vessel or environment, just as a helmsman will adapt his steering to changing sea states. This 30-minute model build period or settling period must be allowed for in the setting-up procedure; until the model is fully complete the vessel may oscillate in position and/or heading.

Controlling computers will, in modern vessels, be configured as part of a fully integrated local-area network covering all vessel control and monitoring functions and facilities.The controller facility may be provided by one processor operating alone, or by an array of


Chapter 2 5

Dynamic positioning principles and systems

two, three or maybe more, in order to provide a level of redundancy. If two processors are provided, then one is on-line while the other acts as a back-up. If three are installed, then there exists the possibility of voting or triple-modular redundancy: one unit on- line and two back-ups. All critical computations are thus triplicated and compared; any discrepancy allows automatic indication and rejection of the errant unit.

2.2 DP system components and layout

As mentioned above, DP is best described as an integration of a number of functions. Central are the controlling computers or processors; these communicate with all other parts of the system via the vessel network.

The system is controlled and operated using the DP console, or desk containing operational controls, buttons, screens and manual joystick. This console should be located in a position affording a good view of the surrounding sea area, and is usually on the bridge or pilot-house. Most modern systems function under a version of Microsoft Windows - a familiar environment to the PC-literate DPO. A vitally important part of the DP system is the DPO. All DPOs must be fully competent to conduct DP operations. Discussion on the subject of human factors is included in Chapter 9 of this handbook.

In order to be able to control any variable function, in engineering terms, it is necessary that the variable be accurately measured. It is thus necessary that the controllers be


6 Chapter 2 I DP Operator's Handbook

provided with accurate and reliable data on vessel position and heading. DP systems are thus interfaced with gyro compasses and a variety of position reference systems (PRS).

Gyro compasses are standard pieces of ships'equipment, and in DP vessels are fitted in duplicate or triplicate dependent upon the level of redundancy desired. It is common to find three compasses installed to fulfil the requirements of equipment Classes 2 and 3 (see Chapter 3 of this handbook covering system redundancy). A more recent development is the fibre-optic compass. This device provides a full compass facility from a solid-state (no moving parts) unit.

Position-reference systems (PRS) provide feedback data on vessel position. In the DP scenario, there needs to be a greater level of precision than that needed in conventional navigation. Vessel positioning can never be more accurate than the precision of the PRS, and typical positioning accuracy of a DP vessel is within 1 to 2 metres. Position-reference is therefore required to be in the area of 1 metre or better.

Position-reference systems are independent systems interfaced into the DP controllers. They can be satellite-based (DGPS), optical laser systems (Fanbeam, CyScan), microwave- based (Artemis, RADius and RadaScan), underwater acoustic systems, or mechanical (Taut Wire). DP systems are enabled to receive and pool data from two or more PRS, determining a best-fix position from all monitored data. The greater the number of PRS in simultaneous use, the greater is the precision of this best-fix, and the lesser is the impact of the loss of any one unit. A detailed consideration of the topic of position-reference systems is given in Chapter 6 of this handbook.

The DP system is interfaced to a variety of sensors and other peripheral equipment. This is detailed further in paragraph 2.4 of this chapter.

The vessel is ultimately under the control of her propulsion units - propellers, rudders and thrusters. It is therefore necessary that all these are interfaced into the DP system. Propulsion commands are sent in respect of pitch, rpm, azimuth and rudder angle, while feedback from all units is continually monitored. The DPO must continually monitor the Set Point and feedback values for each propulsion unit, as, although a discrepancy should generate the appropriate warnings and alarms, a failed thruster does not necessarily trigger an alarm. This is because, if conditions are stable, the commands may not vary by much. A thruster might have failed as-set, ie it is outputting a constant fixed thrust value and not reacting to commands. No alarms are generated as the failure thrust value is very close to the command. This failure mode will only be picked up by the diligent DPO! Further information on propulsion systems is contained within Chapter 7 of this handbook.

Every vessel needs power, and as such, the power supply is very much part of the DP system. It must be remembered that a power problem has an immediate knock-on effect to the DP system and vessel capability. Most DP-capable vessels are diesel-electric,


Chapter 2 7


thus the diesels, alternators, switchboards, cabling, propulsion motors and power- management system are all part of the DP system. Furthermore, diesel upstream systems must all be regarded as part of the DP system; cooling, lube, control and fuel systems. A water-contaminated fuel day-tank may cause the stop of one or more diesels, resulting in an immediate power shortage. The vessel may not have sufficient power for all the thrusters and propellers, thus a position loss may occur. Further notes on the topic of power plant are contained in Chapter 8 of this handbook.

It is not the intention here to describe in detail the operation of any particular make or model of DP system, facility or function. It is vitally important that DPOs fully familiarise themselves with all functions, facilities and procedures appropriate to their system, and that all operational handbooks and manuals are studied in depth. What follows is an outline of the most common functions available.

2.3 DP system functions

The main function of DP is to enable the vessel to maintain a fixed position and heading, and, of course, to make changes to that position and/or heading in a controlled manner. To that end, facilities are provided to allow DPOs to select a new position. This may be defined as a range/bearing from the present position, or in global co-ordinates such as



UTM Northings and Eastings, or Lat/Long. The vessel may then be moved to that new position at a specific speed. Likewise, the heading of the vessel can be adjusted to a new value at the desired rate-of-turn. A further heading-related facility is the optimum heading, or weathervane function, in which the vessel's minimum power heading is continually calculated in relation to the combined wind and current vectors, and the vessel 'hunts' this continuaily changing value. This facility may be of value in some situations, but not in others.

2.3.1 System gain

Gain is the relationship between the vessel's positioning situation and the power used. Most DP systems have three gain settings; low, medium and high. In general, low gain is used in calmer weather conditions, or where positional precision is not a prime concern. Low gain settings may reduce fuel consumption. In a high gain setting, proportionally more power is used in order to maintain a tighter position. This is more useful in position- critical operations or where the weather is more severe. Another function is known as relaxed gain in which the vessel's set-point position is expanded into a disc of specific radius (selected by the operator). Within this area the vessel is allowed greater freedom of movement, power only being increased when the vessel approaches or passes the limit of the area.

In modern DP systems, further adjustment may be made to the positioning ability to facilitate conservation of power and fuel. The latest generation of system from Kongsberg exhibits a 'green DP' function. In this function, complex predictive calculations allow the damping of positioning with the minimum of fuel expenditure. With any of the facilities mentioned above, it is vital that DPOs thoroughly familiarise themselves with


Chapter 2 9


the function in order to avoid unwanted surprises! A gain setting is also applied to the joystick control, with a variety of alternative control selections.

2.3.2 The follow-target facility

Many systems are configured with a variety of this facility. The vessels making greatest use of this might be ROV support vessels and vessels operating Trencher units. These vessels have a need to maintain position relative to a moving target rather than to a geographical location. A common method of conducting such operations is a method often called 'dog-on-a-lead'. With this method, the DP system is configured to use one single PRS, which is an acoustic beacon located on the ROV or trencher. The DP system 'thinks' the vessel is stationary as the beacon is a fixed entity, whereas the vessel is actually trying to maintain position relative to a moving target. This procedure cannot be recommended as there are a number of problems. Often, the agility of the vehicle is greater than that of the vessel, which struggles to keep up. The DP system generates



a false current within the model. Also, the ROV pilot has direct hands-on control of the vessel's movements rather than the DPO.

To avert these problems, the system manufacturers provide a specific follow target or follow-sub mode of operation. In follow target mode the ROV acoustic beacon is designated mobile, thus the DP system does not include it in the PRS pool. Instead, that beacon is designated the follow beacon. The DP system must be configured with other PRS (eg DGPS) in the normal manner. A geographically-fixed circle (radius defined by the DPO) is placed around the ROV, becoming the ROVs operational area. ROV (with beacon) can move freely with the vessel on a fixed location. If the ROV breaks out of the circle, the DP reacts by adjusting the vessel position by an amount equal to the radius in the appropriate direction, and generating a new circle. This continues as many times as necessary. The DP system avoids generating a false current value, while the DPO retains full positional control of the vessel.

Another way in which the follow-target facility may be used is where the vessel must be positioned relative to a slowly moving target such as an FPSO. As the FPSO is anchored and weathervaning, the offtake tanker loading in tandem must match this movement. In this case the operational area consists of a target 'box' located on the FPSOs stern reference point. The tanker carries a mythical 'bowsprit', the end of which must be maintained within the target box. Provided the bowsprit end point remains within the box, the tanker maintains a fixed position. This position is adjusted as and when the bowsprit end breaks out of the box as the FPSO moves. A similar facility may be used by a DP supply vessel working the FPSO.

2.3.3 The autotrack function

The autotrack facility allows a vessel to track slowly along a predefined line, itself defined by waypoints. This facility may be useful in vessels conducting cable- or pipelay operations, dredging, rockdumping or survey operations. Whatever the type of operation, a comprehensive plan of the track will be compiled, with full details of vessel speed and heading on each leg of the track, and points where the tracking may need to be temporarily suspended. A numbered listing of the track waypoints and their co- ordinates is compiled, with due attention to such details as chart datum and co-ordinate frame in use. The track file may be compiled by the surveyor, away from the bridge, and imported into the DP system. If this is the case, then all parameters of the imported file must be carefully checked.

The DP system autotrack function is accessed via a complex menu system. The DPO can enter data manually from his track table or plan using a facility called 'track table' or 'track editor'; this facility will allow entry or deletion of waypoints, insertion of new waypoints at any point, entry of values of vessel heading and speed on each leg. Once a track has been compiled (often considerably in advance of the operation), it can be saved and


Chapter 2 11


recalled at a future time.

A great variety of autotrack settings and configurations is available; DPOs must familiarise themselves with all the permutations available. They must also know the exact reaction of the vessel to each of the menu choices. The DPO specifies how the vessel will handle a waypoint passing (does the vessel stop on the waypoint, or does she proceed around a radiused turn, and does she slow down on the turn?) and any track offset. Offset facilities may be useful for making small adjustments to the track placement while the tracking is underway, and a variety of alternative offset strategies is available.

A further variation on the autotrack theme, is the 'move-up' function. If operating a pipelay vessel, it may be necessary to move the vessel forward a distance equal to one or two pipe-joints (12m or 24m) at frequent intervals. Having compiled the autotrack, the vessel can be operated in the move-up mode, with a single move of the required distance initiated by a button-push.

The commonest mode of autotrack operation is the low speed option. Here, vessel heading is under the full control of the operator - the vessel does not necessarily sail along the track in a conventional bow-first manner. The vessel heading may be adjusted to give a lee for the operational elements, or the vessel may maintain a weathervane or minimum-power heading during the tracking. Vessel speed is limited to 3 or 4 knots in this configuration. An alternative option is'high speed' autotrack, or 'auto-sail', in which the full cruising speed of the vessel is available. In this configuration vessel heading is dictated by the system, the vessel navigating in a more conventional bow-first manner.

It is important that DPOs and all concerned are familiar and practised with the operation of the autotrack function. The commencement of an operation using autotrack is not



the time to investigate and practice the facility. A problem that has arisen in the past concerns a long East/West track which spans more than one UTM zone, and what happens when the zone border is crossed. Points such as this must be resolved ahead of time.

2.3.4 Riser angle mode

This function is vitally important in deep-water drilling operations, in which the critical factor is the management of riser angle. The riser is the pipe containing the drillstring, supported from the drillship and connected to the sea floor wellhead or 'stack' assembly. This connection is known as the lower flex-joint (LFJ) and conducts the rotating drillstring through to the well. It is vital that the riser be perpendicular to the stack, otherwise damage will occur to the drillstring and wellhead components. Riser angle is monitored via sensors located on the LFJ, and fed back to the pilothouse and the DP system. Typical angular criteria are 3° (yellow) and 7° (red). Up to 3° drilling proceeds normally. If riser angle reaches 3°, drilling stops and preparations are made for a riser


Chapter 2 13


disconnect operation, which must be initiated if the riser angle reaches 7°. Riser angle is affected by tidal flow, the riser 'bows' down-tide. The DPO compensates for this by moving the vessel. In deep water there may be a complex tidal shear pattern, resulting in an irregular riser profile. In riser angle mode, the DP system receives feedback from the riser management system, and displays the 3° and 7° watch circles on-screen. These circles will drift on-screen as tidal conditions change, and the DPO will adjust the vessel position and heading in order to maintain angles within limits. Thus, in this type of operation, the DPO is less concerned with the vessel's actual position but more responsive to the riser feedback.

2.3.5 Shuttle tanker functions

Some general information on shuttle taker operations is given in Chapter 4 of this handbook. Obviously, this is a specialist DP configuration, not found or required in other types of vessel, and many of the DP functions are also specialised. A shuttle tanker will always work in a weathervane mode; the power-to-weight ratio of these vessels precludes adopting any other heading. The tanker will be configured to load from one or more specific Offshore Loading Terminals (OLTs) which may be fixed tower structures, floating towers, submerged turrets or FPSOs.The position and other characteristics of the OLT are contained within a file in the DP system, accessed from a 'Select OLT' menu.

The vessel will approach from a down-wind or down-tide direction, keeping the OLT ahead, transferring into DP control at an appropriate point (usually outside the 500m zone). The DP system will have an 'auto-approach' function, allowing the DPO to progressively reduce the distance from the OLT. All the time the vessel is maintaining a weathervane, or minimum power heading. Once on the defined position circle, at the correct distance from the OLT for the loading phase, the DP is selected into the 'Loading' function. With the loading hose connected, the vessel maintains a fixed distance from the OLT while weathervaning around that point. Its position is continually adjusted to keep the OLT ahead and the vessel on the minimum-power heading.

If loading is from a submerged turret, the position of the turret is monitored by underwater acoustic methods, as part of the vessel's hydroacoustic position reference (HPR). Special displays show the position relationship between the turret and the vessel. The vessel's reference point will be the docking cone in the forepart of the vessel's bottom. Once the turret is recovered and locked into the cone, the vessel may simply stop all thrusters and propellers, and weathervane naturally, anchored to the cone. This is only possible if weather and tidal conditions are light to moderate. If more severe conditions are experienced, the DP system may be brought in use in a 'damping' mode.

Many offshore off-take operations are from FPSO installations. Here, the tanker maintains a position in tandem with the anchored FPSO, and must match her movements and heading. The FPSO is not normally DP-capable, but may have thruster control of heading,



within environmental limits. The approaching tanker may request the FPSO adopt a specific heading for the approach phase. The auto-approach facility will be used (see above) until the vessel is at the desired location relative to the FPSO stern reference point. The tanker's' loading' function will then allow a target-follow facility using a position box as described in 3.2 above. Set-point heading for the tanker is the FPSO heading, which must be monitored via telemetry in the tanker.

2.3.6 Auto-area mode

A further variation on the automatic control of positioning is 'Auto-area'. In this function, the DPO is able to specify a geographical area of any desired size, usually circular but may also be elliptical. This becomes a 'loiter'area, in which the vessel drifts with no thruster commands. When the vessel crosses the area boundary, power is gently applied, and the vessel is slowly restored to the centre of the area.

2.3.7 Other DP functions and facilities

It is not the intention here to provide a comprehensive description of all available DP functions, but an overview of common facilities may be useful.


Chapter 2 15


2.3.8 External force compensation

Some vessels under DP control experience a variety of external forces which need to be compensated. An example of this is the pipe construction vessel, laying pipe using a Stinger assembly at the stern. The stinger is a rigid support ramp taking the pipe from the pipe deck into the water. From the end of the stinger to the pipe touch-down point on the sea floor the pipe is unsupported and must be kept under tension to maintain the correct catenary profile. To enable pipe tension to be maintained, the pipelay engine is programmed to pipe feed tension values back to the DP system. Thus the DP system and DPO are able to manage the thrust requirements enabling compensation. Other similar external force compensation facilities may be provided for cable plough operations, or on fire monitors.

If this compensation facility were not enabled, the external forces would be treated as false current.

2.3.9 Quick-current or fast learn

This facility provides a partial solution to the problem arising during rapid turn-of-tide situations. The tide or current value is deduced by the system, not measured directly. The accuracy of the current value is dependent upon the quality of the mathematical model. If the tide turns quickly the model will lag behind, as time is required for new model data to build. This results in a deterioration of positioning ability. Modern DP systems incorporate a facility allowing a temporary acceleration of the model-build process, which may improve the positioning ability during this time. It must be emphasised that this facility is not perfect or foolproof, and there may still be some positional instability during such slack-water periods.

2.3.10 DP capability plot

DP operational handbooks provide capability diagrams which give an approximate indication of the capability of the vessel in a variety of weather conditions. The value of these diagrams is sometimes questionable, since the vessel and/or environmental conditions rarely match those tabulated in the diagrams. A better assessment of the capability is given by the online capability plot. This is a similar diagram calculated 'live' by the system on demand. DPOs must specify sea state and current values. They must also input a variety of degraded-status failure modes relating to power plant and propulsion. Once set up, the facility calculates and displays a capability plot for that set of conditions. The operator must be aware of the limitations of this display, as it is still only a theoretical, computer simulation of the vessel's capability, and not obtained from real observation.



2.3.11 Drift-off calculation

This facility, when set up correctly, will show the drift-off profile related to an operator- specified failure mode, if, for example, a total blackout is selected, the display will show the predicted positions and headings of the vessel at specific time intervals (eg every minute) subsequent to the blackout.

2.3.12 Autopilot and transit mode

Frequently, the DP system incorporates autopilot functions. When the vessel is in transit, there will be a dedicated thruster configuration. As an example, a vessel with three azimuth thrusters aft may, in autopilot mode, have the two wing thrusters locked for forward thrust, while the centre thruster is enabled to steer with azimuth angle limited to 35 .


Chapter 2 17


2.4 Sensors and peripheral equipment

Feedback is required in respect to the environment in which the vessel is operating. The DP controllers need continuous and accurate data on vessel Roll and Pitch angular values. This

data is provided by a motion reference unit, or MRU. An inertial motion sensor, this device outputs not

only roll and pitch data, but also heave values, together with rate (velocity) data on each of the three

measured values. In earlier installations this function was provided by a device called a vertical

reference sensor or unit (VRS or VRU). Instantaneous roll and pitch data is required by the processors

in order to correct data input from a variety of PRS which measure vessel position as an analogue of a

vertical angle (taut wire systems and underwater acoustic systems). In these PRS, vertical angle is

measured by a sensor that rolls and pitches with the ship; if no correction were applied, the processor

would interpret this as a moving vessel. Roll and pitch data input allows these angles to be corrected

or reduced to the true vertical.

The other element of the environment requiring measurement is the wind, and a number of

transmitting anemometers are installed. Usually fitted in (at least) duplicate, windsensors may be of

the traditional rotating-cup type, or may be of the more modern ultrasonic variety. These read data

into the mathematical model allowing wind loads on vessel hull and superstructure to be computed

and compensated for. However, there is a problem. Wind conditions can change rapidly and radically;

gusting conditions will adversely affect the DP performance of the vessel. It is necessary that the

system reacts rapidly to large changes in wind speed and direction. The mathematical mode! does not

serve to best effect here, as there is a significant delay between changes and system reaction. This is

because, as with most elements that change within the vessel model environment, the changes take

place slowly (eg vessel displacement, tide etc). The model takes 5 to 30 minutes to update, and with

most elements this is appropriate. However, gusting wind conditions can play havoc.

The DP system compensates for this problem with a function known as 'wind feed- forward', in which

radical changes in wind speed and/or direction by-pass the mathematical model and generate

compensating thrust directly. In some DP systems this is known as 'gust-thruster compensation'. It is

important to realise that for this compensation to give satisfactory results, the windsensors must be

reading representative values for the wind. The DPO must ensure that the windsensor selected, of the

array available, is reading clear wind, unobstructed by any windshadow from structure.

The other environmental value needed is current, but it is not possible to obtain representative values

for the current from any vessel-mounted sensor. Instead, current values are deduced or estimated. In

effect, the current value shown on-screen is what forces remain when all known forces are accounted

for. A continuous discrepancy between 'predicted position' within the mathematical model, and

'measured position' derived from the PRS indicates a current. The DPO must be aware that this

'current' value is only a deduced value and not a real measurement, and thus is subject to error. In

fact,



any error within the DP system will be incorporated into the displayed current value. One DP system

manufacturer does not call this vector 'current' at all - it is referred to as 'sea force' and displayed in

tonnes. Errors within the system which may affect the accuracy of current display include erroneous

thruster pitch feedback, or erroneous windsensor data, perhaps from a jammed wind-vane unit.

In specialist-function vessels, other external sensors providing the processors with feedback data

include measurement of external forces and measurement of riser- angle (in a drillship). External

forces may take a number of forms. In a pipelay vessel, pipe tension must be maintained at pre-set values as the lay progresses. Pipe tension is monitored at the pipe tensioners, and fed directly into the

DP system, allowing direct and immediate thrust compensation for tension and tension changes. In a

cable-lay vessel towing a cable plough, hawser tension is monitored and fed back in a similar manner to pipe tension. A shuttle tanker tandem-loading from an FPSO unit ahead will monitor hawser tension

and make similar compensation.

2.5 Some DP problem areas As with any system, DP has its limitations. It is vitally important that the DPO is familiar with these limitations and problem areas. Some of them will be described here:

2.5.1 Operations in shallow water and strong tidal conditions For a variety of reasons, DP vessels may not perform well in shallow water or strong tidal conditions.

One of those reasons is the lack of efficiency of some position-reference systems in shallow waters.

The PRS affected are taut wire systems, and hydroacoustic systems.

The taut-wire system becomes of limited value in shallow water as a result of its limited horizontal scope, or range. The maximum allowable wire angle is typically 28° to the vertical, so if there are only a

few metres under the keel, horizontal range is also only a few metres. Hydroacoustic systems will also suffer limitations in range capability due to considerations of ray-path angle; the flatter the path the

greater the distortion and losses due to refraction. In shallow waters, acoustic systems will also suffer

proportionally greater levels of thruster noise interference. In general, it is always wise to avoid the use of underwater PRS in shallow water.

If there are strong currents or tides (often the case in shallow water) the immediate effect is that the

vessel will be using larger amounts of power and thrust in order to maintain position and heading.

Consequently there will be smaller amounts of both in reserve, affecting the redundancy

considerations, if the vessel is working harder to maintain position, she will undoubtedly be creating

greater levels of noise and water


Chapter 2 19


turbulence, with further detrimental effects on the performance of hydroacoustic position reference. As mentioned in 2.3.9 above, DP systems perform badly during periods of rapid turn of tide, and this is often a feature of areas where depths are limited and tidal conditions are severe.

It must also be mentioned that many underwater operations are adversely affected by strong tidal conditions and shallow water, particularly diving and ROV operations. Any such operation must be the subject of a full risk-assessment process, with all hazards identified and assessed.

2.5.2 Operations in close proximity to fixed structure

Offshore support vessels of all types routinely work in close proximity to fixed platform structures. The main and immediate hazard is that of contact with the structure. DPOs must ensure that they have an immediate and direct view of the nearby structure. It is an axiom of DP work that a positioning problem will usually first be apparent to the Mk 1 eyeball before instruments and alarms start to react. This is, of course, always assuming that DPOs make use of their Mk 1 s!

The nearby presence of a large fixed structure will have detrimental effects on the DGPS position reference. Satellite signals may be blocked by structure causing a reduction in satellite availability. There will be greater levels of multi-path reception caused by reflection of satellite signals from structure. Both of these mentioned effects result in degradation of DGPS performance. Further, the differential correction signal may be lost due to shadow from the platform structure. All of this adds up to a simple truth: DGPS cannot be regarded as a reliable PRS in close proximity to platform structures.

A further concern, when working in such close proximity, is the interference caused to windsensor readings. If a vessel is working close-in on the downwind side of a platform, the windsensor may be badly windshadowed, and not therefore delivering representative wind data. The vessel herself may be feeling the full effects of the wind blowing through the legs of the platform, thus the wind forces are not being properly monitored. The result may be a deterioration in positioning efficiency, and there is no real cure for this problem.

2.5.3 Operations in close proximity to other vessels

DP vessels are by their nature restricted in their ability to manoeuvre, and must always display the appropriate signals. Sometimes in the offshore theatre there will be a number of vessels working in close proximity to each other. A number of hazards exist, chief among which is the risk of collision. DPOs must always bear in mind that any vessel in the area, including his own, may suffer total power loss or blackout. Do they have a



contingency plan for this? That supply boat working the platform crane - if she suffers blackout, will she drift down on us? And is there any action we can take? If own vessel is position-constrained in a safety-critical task, then it might be necessary to suspend operations until a safer situation arises.

Two DP vessels working in close proximity might generate problems also. Thruster wash from one vessel may disturb the positioning of the other, and vice-versa. This could lead to thruster-thruster interference and positional loss by one or both vessels. A nearby vessel may cause line-of-sight loss on laser or microwave based PRS. Other position- reference related problems might occur; if one vessel is using underwater acoustics, the thruster noise or aeration of the other vessel might block acoustic returns. All of these problems should be considered in the risk assessment for the task.

Chapter 3 21 Redundancy and equipment class

Chapter 3 Redundancy and equipment class

3.1 The need for redundancy

DP vessels undertake many tasks and operations, some of which are more safety- critical than others. In any operation the consequences of a DP failure must be taken into account. A DP failure may result in a simple positional excursion (a 'drift-off') or a powered drive-off. Both are regarded as catastrophic failures. The consequences of such a catastrophic failure can be categorised three ways; risk of death or injury to personnel, risk of damage to property or risk of pollution.

Redundancy arrangements within a system ensure that system function remains subsequent to the loss of any single element or subsystem. The function of redundancy is to prevent a catastrophic failure, ie a loss of position and/or heading. By the term 'redundancy' we mean the ability of the vessel to withstand the loss of any component within the DP system without losing position and/or heading. A more practical view of redundancy provision in a DP-capable vessel might be that redundancy arrangements allow the operation or task to be safely suspended, and the vessel to safely exit the worksite, still under the control of the DP system.

Often, especially in safety-critical operations, the task cannot safely be suspended without continued provision of DP ability. Most operations cannot be continued after the loss of DP ability, and in many operations, the DP ability is essential for the safe suspension of the task and vessel worksite exit. In effect, redundancy arrangements within the DP system provide a time-frame in which the operation can be safely suspended and the vessel to move to a position of safety after the failure of a critical component.

A number of methods for the provision of redundancy are available. The most common is to provide back-up or stand-by facilities, a good example here being the DP system control computers or processors. A redundant provision might be two processors, one 'on-line'and the other on 'hot stand-by'. This is a common arrangement in many vessels, a further requirement is a 'bumpless transfer' between units, ie if the on-line processor fails, the stand-by unit takes command automatically without any change in the vessel status, including position and heading. Subsequent to such an event, the vessel is still under DP



control and can safely move to a position of safety, albeit no longer redundant in the area of

computers.

3.2 The I MO guidelines

The need for redundancy will be indicated by a variety of means. The Guidelines for Vessels with

Dynamic Positioning Systems is published by the IMO and specifies three levels of redundancy known as equipment classes. Three equipment classes exist; the higher the class number, the greater the level

of redundancy. Vessels may be described as compliant with equipment Class 1, 2 or 3. These guidelines, issued In 1994 (MSC Circ 645) have become accepted as a world industry standard. DP

vessels will be designed for compliance with a particular equipment class, and will be issued with an

appropriate notation by the classification society. Each classification society has class notations equivalent to the IMO classes.

The IMO guidelines introduce the concept of the single point failure. Redundant operation allows for

the continuance of position and heading ability after any single point failure. The equipment classes are described in terms of the effects of worst-case single point failure. The definitions are as follows:

For equipment Class 1, loss of position may occur in the event of a single fault

For equipment Class 2, a loss of position is not to occur in the event of a single fault in any active component or system. Normally static components will not be considered to fail where adequate

protection from damage is demonstrated, and reliability is to the satisfaction of the administration.

For equipment Class 3, a single failure includes: items as listed above for Class 2, and any normally static component is assumed to fail; all components in any one watertight compartment, from fire or

flooding; all components in any one fire subdivision, from fire or flooding.

Thus, vessels of equipment Class 1 will not be fully redundant in every area. Vessels of equipment

Class 2 have full redundancy in equipment and systems, while Class 3-designated vessels are capable

of maintaining position and heading after the loss of all components in any watertight compartment or

fire subdivision. In Class 3 a single-point failure might be the loss of one complete compartment.

The choice of equipment class determines which vessel is chartered for any particular operation or

contract. The greater the level of risk associated with the operation, the higher the equipment Class of

the vessel tasked for that operation. The IMO guidelines state:

The equipment class of the vessel required for a particular operation should be agreed THE NAUTICAL

INSTITUTE

Chapter 3 23

Redundancy and equipment class

between the owner of the vessel and the customer based upon a risk analysis of the consequence of a loss of position. Else the Administration or coastal state may decide the equipment class for the

particular operation.

Thus the vessel owner and client must design the operation incorporating a full risk assessment process and safety case. The results of the risk analysis will indicate the consequences of a loss of

position and/or heading, allowing a decision to be made as to the equipment class of vessel required

for that operation. The more severe the consequences within the three categories (death/injury, damage, pollution) the higher the equipment class vessel deemed necessary.

Equipment class certification for the vessel may consist of a class certificate issued by the vessel's

classification society, with the appropriate class notation, eg if a DP vessel is classed at Lloyd's Register of Shipping, the appropriate notations for Classes 1, 2 and 3 are respectively DP(AM), DP(AA),

and DP(AAA). In addition to this the vessel may carry an FSVAD (flag state verification and acceptance document) issued by the IMO, again stating clearly the equipment class. Such FSVAD documents are

generally issued by the classification society on behalf of the IMO.

Further guidance on the provision of redundancy is given in Guidelines for the Design and Operation

of Dynamically Positioned Vessels, published in February 1999, by the International Marine

Contractors Association (IMCA) as M 103. Other relevant documents from IMCA are M 161, a

supplement to the above guidelines covering two-vessel operations, published in February 2001, and

M 182, the International Guidelines for the safe operation of DP Offshore Supply Vessels, published in

March 2006.

3.3 The provision of Redundancy

Hereafter follow a few comments upon the provision of redundancy at the various equipment class

levels in respect of the major elements of the DP system, ie controllers, power systems, thrusters and

propellers, position, heading and environment reference systems.

3.3.1 Redundancy in controllers

Computers are the heart and brain of any DP system. For Class 1 a single controlling computer is

adequate, but for Class 2, two parallel identical computers are installed. Each is running

independently in parallel, receiving the same feedback data and performing the same computations.

One is 'on-line' while the other is the back-up. Each continually monitors or 'watch-dogs' the other,

such that, if the two units are not running identically, then an 'A-B difference' warning can be

initiated. This is important, because if such an A-B difference exists then, effectively, redundancy has

been lost. A requirement of this type


24 Chapter 3


of installation is that of 'bumpless transfer' between computers, ie if on-line fails and the back-up takes over, the vessel's position, heading and status are unaffected.

A more extensive installation may incorporate triple modular redundancy (TMR), providing greater security, but not necessarily sufficient redundancy to qualify for Class 3. In such a triple system, three computers are provided, all running in parallel; one on- line and two as stand-by. The arrangement allows the application of voting logic, in which every calculation is compared with the corresponding outcomes from the other two processors. If a malfunction occurs, then it can be automatically detected with the errant unit isolated. Such a system is logically allied to triplicated sensors and position references, such that the voting ability applies to all systems. For equipment Class 2, there must be available three independent position references and three gyro compasses. In DP system terminology, a single-computer system is referred to as 'simplex', a Duplex system refers to those with dual computers, while a system as described above with three computers is referred to as 'triplex'.

The triplex system described above does not necessarily comply with the requirements for equipment Class 3. In a Class 3 vessel, there must be a separate control computer located in a separate compartment. The essential point of Class 3 operations is that of subdivision, or compartmentalisation. The IMO requirement for Class 3 is that a single point failure includes all components contained within any single compartment.


Chapter 3 25


FULLY REDUNDANT (CLASS 3) SYSTEM LAYOUT

Thus, for compliance with the requirements of Class 3, a separate, independent control computer must be located remotely from the main, fully redundant computer system. In practice this means a

fully redundant, two-computer system installed on the main bridge, with an independent back-up DP

system including computer, located at a remote location. A more extensive system may comprise a main Triplex system, backed up by a Simplex system remotely located.

3.3.2 Redundancy in Position and Heading Reference For equipment Class 1, a minimum of two position references must be installed. This seems to be in

excess of the generally minimum requirements for the (otherwise non-



Table 12 - Provision of redundancy


SUBSYSTEM OR COMPONENT

MINIMUM REQUIREMEN TS FOR GROUP DESIGNATION

Class notations: IMO EQUIPMENT CLASS

1 2 3

DNV AUT AUTR AUTRO

LR DP (AM) DP (AA) DP (AAA)

ABS DPS-1 DPS-2 DPS-3

Power System Generators and prime movers

non-redundant

redundant Redundant separate compartments

Main switchboard

1 1 with bus tie

2 with normally open bus ties in separate compartments

Bus tie breaker 0 1 2

Distribution system non-redundant


Power management no yes yes

Thrusters Arrangement of thrusters

non-redundant


Control Auto control: number of control computers

1 2 2+1 in alternative control station

Manual control: joystick with auto heading

yes yes yes

Single levers for each thruster

yes yes yes

Sensors Pos. reference systems

2 3 3, including 1 in alternative control station.

External sensors wind 2 2

VRS 2 2

gyro 1 3 3

other 1 2 2

UPS 1 1 1 +1 in separate compartment

Alternative control

station for back-up unit

no no yes

Chapter 3 27


redundant) Class 1. Operation with a single position-reference has a specific danger; that of PRS freeze. If the input data from a position reference freezes, ie the data simply continues incoming at fixed values, the common resut is that the vessel will drive-off location. Invariably, the vessel will be some small distance from her set-point position when the freeze occurs. The DP system will issue thrust commands to make up the position deviation. The position-reference will continue to give constant data indicating the vessel stationary; in reality she is driving steadily away from set-point. For heading reference, Class 1 is satisfied by a single gyro compass.

For Class 2 and 3, the equipment class requirement is for three independent position reference systems and three gyro compasses. This gives the voting capability referred to above, and allows automatic identification and rejection of an errant position reference or compass. Three is better than two in one major respect; if a function is duplicated, there is always the question 'which one has failed' when the two disagree. That question can only be answered by operator inspection and intervention. Triplication of gyro compasses and position-references provides a partial solution to this problem. It is still possible to defeat the voting.

The DPO must be vigilant to avoid the 'common-mode failure' syndrome. For example, a particular vessel may be using two independent DGPS and a fanbeam laser system for position reference. This in theory complies with the requirements for Class 2 or 3, but there is a major shortcoming to this arrangement. If both DGPS suffer a drift problem, because of the same root cause, then the voting is likely to cause automatic rejection of the Fanbeam - the only system at that moment providing correct position data.

It is always good advice to use three position references, all of which operate on different principles, thus reducing the exposure to common-mode failure conditions. Even so, it is still possible to defeat the system. A vessel might be using three position-references: taut-wire, hydroacoustic position reference, and DGPS. The operator has decided it would be most convenient if the underwater beacon which is the basis of the hydroacoustic system were secured to the taut-wire depressor weight. However, if the weight drags, the beacon goes with it, and the position-reference which is outvoted is the DGPS - again, the only good one at that moment.

Further good advice is to avoid the use of purely underwater position references, ie two taut wires and a hydroacoustic position reference is not a good solution. In rough sea conditions all three can become unreliable.

If working to Class 2 or 3, it is always advisable to consider deployment of four position references instead of three. If three references are in use and one fails, the vessel is immediately out-of-class, and may have to suspend operations until another reference is deployed and on-line. If, however, four references were in use, and one was lost, then the vessel is still operational.



For heading reference, gyro compasses are used, triplicated for Class 2 and 3. In the Class 3 vessel, one gyro must be remotely located to comply with the subdivision requirements.

3.3.3 Propulsion redundancy

For a diesel-electric vessel of Class 2 or 3, the worst-case single-point failure will be the loss of one complete busbar, one complete section of switchboard. This results in the loss of all propeller and thruster units that were powered from that bus. It is thus necessary to distribute power such that subsequent to worst-case failure, sufficient thrust capability remains to manoeuvre the vessel. At its simplest, if a vessel has three propellers at the bow, and three at the stern, powered from a split switchboard, then the loss of one bus leaves three thrusters intact, at least one at the bow and one at the stern. The intact thrust capability must be capable of effective manoeuvrability.

The DPO must be continually aware of the worst-case single point failure mode relative to propulsion. Modern DP systems are configured to give a consequence analysis against this eventuality.


Chapter 3 29


3.3.4 Consequence analysis

In respect of the power and propulsion configuration, DP systems are able to provide an on-line consequence analysis of worst-case single point failure mode. The IMO guidelines state that such a software programme must be running when operating in Class 2 or 3 states. Typically, the consequence analysis programme will repeat its calculation once per minute. Initially the programme scans the switchboard and propulsion configuration, and determines what the 'worst-case' mode is. It then simulates this failure mode, with an indication of the outcome. If the outcome indicates that the vessel will not hold position and/or heading, an alarm will be generated, such as Consequence analysis drift-off alarm. If no such alarms or warnings are activated, the DPO can assume that power/propulsion redundancy is intact, although it is always wise to keep a watching brief on the power and thrust loads.

3.3.5 Power plant

This is not the place for a detailed discussion on the topic of machinery space redundancy, but a few brief comments may be appropriate.

The overall layout of DP vessels' power plants will be designed with redundancy in mind. The diesel-electric configuration is particularly suited to the provision of redundancy. In this type of installation, a number of diesel generators deliver power to the switchboards, so the loss or non-availability of any one diesel is not catastrophic. For Class 3 operations, the machinery space and switchboard rooms will be subdivided, such that a fire will not result in total incapacity. Further, with a number of diesels, flexibility exists in the day- to-day configuration. The number of generators on the board can be varied to suit the loads, redundancy and contingency power existing.

Other systems in the machinery space requiring redundancy include the fuel and cooling system. Arrangements will be in place to prevent water contamination of the ready-use fuel tanks from causing all engines to stop. The cooling system may have an increased redundancy ability compared with that in a non-DP vessel. It must be said, however, that significant redundancy exists in conventional ships, as required by the classification society rules; it is not just DP vessels that have redundancy built-in!

The power distribution layout is designed for redundant operation. The configuration of switchboards, bus tie switches, thrusters and generators will comply with the equipment class requirements. For compliance with Class 3 the bus-tie switches must be open, such that each switchboard section operates in isolation, and a fault cannot transfer across the boards resulting in total blackout. For Class 2 operation, but-tie switches may be open or closed. With bus-tie switches closed there is greater flexibility in terms of power- sharing across the switchboard resulting in more economical operation. If a fault occurs on one bus, the appropriate bus-tie switches should trip, isolating the faulty section.



Nevertheless, the possibility exists of total blackout if the bus-tie switches themselves fail.

All power systems should be under the control and monitoring of an efficient power management system. Low-voltage electrical systems such as computers, bridge consoles and DP peripherals should be power protected via UPS systems. Another design feature contributing to redundant ability is the separate routeing of cabling and pipelines.

3.3.6 Environment Reference

Equipment such as motion reference units (MRU) and windsensors is provided in duplicate to satisfy the requirements of Class 2 and 3. These devices provide data feedback to the DP system, and the loss of this data is usually of lesser impact than the loss of more critical elements. Simple duplication is considered adequate, although triplication is possible.

In general, DPOs must be very familiar with the redundancy arrangements of their vessels, in every area. They must be familiar with any weak points or shortcomings in the particular vessel's redundancy arrangements. They must question redundancy arrangements for modifications or newly installed equipment, for example. If new position-reference systems are installed, are the power supplies properly protected and redundant? The vessel needs to operate with one thruster stopped for operational reasons, has this affected the overall redundancy of the vessel manoeuvrability? What switchboard protection systems are in place and how regularly are they tested? How often are the UPS systems tested? This is the type of question the DPO should ask and obtain answers to.

Chapter 4 31

DP vessel operations

Chapter 4 DP vessel operations

4.1 The range of applications for DP

When DP was in its infancy, the variety of vessels using the positioning ability was limited to deep-water drillships, but the technique rapidly became more widespread. DP soon became a standard feature in a number of vessel types, and the number of applications to which DP- capable vessels were being used increased rapidly. In the early days, DP capability was limited almost exclusively to those vessels engaged in the offshore oil and gas industries, in recent years however, many other areas of commercial and military shipping are using DP techniques.

One reason for this is the tendency for modern vessels to feature a fully integrated vessel control system, combining all vessel monitoring and control functions. DP itself is best described as an integration of a variety of vessel functions (position and heading reference, propulsion, power, environment); so it is comparatively simple (and cheap) to include a DP ability at the design stage. All modern design platform supply vessels and anchor-handling tugs incorporate a DP capability in their design. A few years ago this would be an 'optional extra'. What is prohibitively expensive is the conversion of an existing non-DP vessel into a DP-capable vessel.

What follows is a description of a number of operations in which DP-capable vessels are regularly engaged, with some detail as to the practical application of DP.

4.2 FPSO and Tanker Offtake operations

Shuttle tanker operations may be divided into four groups; systems with hawser moorings, hawserless systems, and submerged turret loading (STL) systems. The fourth grouping consists of those vessels configured to load directly from FPSO installations (floating production, storage and offtake units).

An increasing number of offshore oilfields must conduct export via tanker. In many cases the distance to the beach is too great to warrant the construction of a pipeline, otherwise


32 Chapter 4


the reservoir reserves may only support a limited production period. In such cases the tanker may moor to an offshore loading terminal (OLT) and conduct loading by means of a bow manifold. In many areas, the exposed location of the OLT means that mooring is not possible due to the environmental loads that may be imposed on the OLT structure. It is these areas that need DP-capable tankers.

DP shuttle tankers operate on a position-circle/weathervaning principle. The vessel will position with her bow touching an imaginary circle, centred upon the OLT. The vessel is continuously weathervaning, or actively seeking a minimum-power heading, and adjusting her position to keep the OLT ahead. This allows the vessel's bow manifold to remain within specific maximum and minimum distances of the OLT reference point, ensuring that there is no risk of damage to the loading hose. The OLT avoids having major environmental loads imposed upon it, and the DP system ensures that the vessel's position and heading are maintained in all but the most severe weather.

Tankers built with this functionality are fitted with a conventional DP system configured to handle this weathervane ability. Typically, two or three tunnel thrusters are fitted at the bow, two aft, and single or twin screw main propellers. The DP system may be installed on the main bridge, aft, or in the bow house. Modern shuttle tankers normally feature redundancy levels to Class 2, but many earlier vessels are to Class 1 only.

Position references used may include DGPS, HPR, a Laser system and Artemis. An important consideration here is the distinction between absolute position references, and relative ones. A tanker loading from an offshore terminal needs to maintain position relative to the terminal. If the terminal is mobile in any way, the tanker must match that movement. Many OLTs are floating anchored spar buoys which have motion


Chapter 4 33


characteristics. Other terminals are FPSOs (see below) which are also anchored in a weathervane mode. In both cases, the offtake tanker needs position reference relative to the moving OLT. Laser-based systems and the Artemis system are both relative position references used in these circumstances.

The shuttle tanker will transfer into DP mode early in the approach, allowing a controlled hawser pickup. Once onto the position circle at the designated distance from the OLT, the hawser is latched and the hose connected. A diagram, specific to the OLT, shows the maximum environmental criteria for approach and hook-up, together with the position/ heading criteria for ESD (emergency shutdown and disconnection).


Chapter 4 35 DP

vessel operations

A variation upon this theme is the STL (submerged turret loading) system, in which the loading is carried out from a circular conical subsea turret. This turret is anchored at a depth below keel level, and carries the loading hose. The tanker has a docking cone built into the bottom structure, forward. The vessel manoeuvres over the turret, picking up a messenger line. The turret is located by means of acoustic beacons. The turret is hauled up into the docking cone and locked; once this is complete, the vessel weathervanes around the turret location maintaining position and heading using DP. A number of fields are configured for STL operations.

A further variation is the FPSO tandem loading arrangement. A floating production, storage and offtake unit is usually a ship-shaped vessel moored to a turret arrangement. The FPSO produces into her own tank storage, and at frequent intervals must off-load cargo into a shuttle tanker. The tanker will position astern of the FPSO and load through a bow manifold. Positioning strategy is as for OLT or STL arrangements, with the added complication that the reference point for positioning may be slowly moving; the FPSO will herself be weathervaning. Position reference for the offtake tanker will usually be a combination of Artemis, a Laser system and relative GPS (such as the DARPS system). Both these PRS are relative in nature, as the offtake tanker is positioning in relation to a mobile point.


Chapter 4 35


A variation upon this theme is the STL (submerged turret loading) system, in which the loading is carried out from a circular conical subsea turret. This turret is anchored at a depth below keel level, and carries the loading hose. The tanker has a docking cone built into the bottom structure, forward. The vessel manoeuvres over the turret, picking up a messenger line. The turret is located by means of acoustic beacons. The turret is hauled up into the docking cone and locked; once this is complete, the vessel weathervanes around the turret location maintaining position and heading using DP. A number of fields are configured for STL operations.

A further variation is the FPSO tandem loading arrangement. A floating production, storage and offtake unit is usually a ship-shaped vessel moored to a turret arrangement. The FPSO produces into her own tank storage, and at frequent intervals must off-load cargo into a shuttle tanker. The tanker will position astern of the FPSO and load through a bow manifold. Positioning strategy is as for OLT or STL arrangements, with the added complication that the reference point for positioning may be slowly moving; the FPSO will herself be weathervaning. Position reference for the offtake tanker will usually be a combination of Artemis, a Laser system and relative GPS (such as the DARPS system). Both these PRS are relative in nature, as the offtake tanker is positioning in relation to a mobile point.



The DP system configured for loading from an FPSO facility will feature a 'position box', which is an imaginary area located astern of the FPSO. This box should contain the bow of the tanker. Only if the bow of the tanker breaks out of the box is the DP system triggered to adjust the tanker's position. This arrangement eliminates the continuous manoeuvring needed by the (relatively low-powered) tanker to maintain station on the FPSO. The set-point heading of the tanker is the FPSO heading; this value must be included in the telemetry between FPSO and tanker (and must be redundant). ESD criteria include heading misalignment between FPSO and tanker.

4.3 Diving and ROV support operations

A massive variety of underwater tasks are conducted using a DP-capable vessel as the working platform. These operations will range from routine tasks using ROV (remotely operated vehicle, or unmanned submersible) through to complex tasks involving seabed crawler vehicles or sophisticated ROVs. Human divers may also be deployed by one method or another. Despite the increasing complexity of modern ROV technology, human divers are still often the only real choice for some tasks.

Divers may be deployed in a number of ways. Up to 50m depth the technique is referred to as 'air diving', as the breathing mixture is compressed air. The divers are deployed in a steel basket, or by means of a wet-bell or mini-bell. Upon recovery, the basket or bell is recovered direct to the surface, and the divers will enter a decompression chamber for a controlled return to atmospheric pressure. This is necessary to avoid onset of decompression illness, or 'the bends', which is a major hazard for divers.

In the air-diving range (0-50m) divers (and ROVs) are exposed to a number of hazards. Chief amongst these is the proximity of running propellers and thrusters; the divers and ROV risk the danger of death or injury from being drawn in to propellers. In addition to this problem, there are further problems relating to water turbulence caused by propellers, reduction in visibility and increased noise levels. All of these problems must be carefully considered by the planning team, and must be the subject of a risk-assessment prior to the commencement of the operation. The foregoing paragraph summarises very briefly the problems of operating DP vessels in areas of shallow water and strong tides. Many further sources of hazard exist in these areas, and the DPOs must be familiar with them. Consideration of problems associated with DP operations in shallow water and strong tides forms a significant part of any shore-based DP course.

The most significant hazard relating to dive support operations, is the danger of having a diver ingested into a running thruster. This danger is addressed by ensuring that, at all times, divers' umbilical tethers are restricted in length preventing the diver from reaching any thrusters or propeller. Guideline statements require a minimum 5m margin of safety. Umbilical extensions must be agreed between bridge and Dive Control prior to any diving operation, and will form part of a pre-dive checklist.


Chapter 4 37


Beyond 50m depth, divers will be deployed using a diving bell, forming part of a saturation diving life-support complex. Divers remain at the pressure of the working depth for up to 28 days, shuttling back and forth in the bell between the worksite and the saturation chambers in the vessel. Decompression times at depths of hundreds of metres are measured in days, not hours. The breathing mixture is a helium-oxygen mix (heliox) resulting in the divers' characteristic Donald Duck voices.

A diving bell may well be used instead of a basket in depths of less than 50m, as this represents a greater level of safety. Also, of course, the problems relating to shallow water will also apply to operations where diving is taking place at a shallow depth in deep water, eg inspection of structure on a jacket at the -28m level, on a platform standing in 175m of water.

Obviously, divers in the water are particularly vulnerable to vessel problems, particularly



positioning problems. The diver's only way back to the surface is via the bell or basket. If the vessel has a run-off, then the diver will be dragged on the end of his umbilical, and this may well cause death or injury. The working location of the diver itself has a great bearing on his level of safety. If the diver is working in open water, close to the bell, then his return to the bell only takes a few minutes. If, however, the diver in working inside an enclosed sea bed structure, or habitat, his return-time may be twenty minutes or more. During this period, the vessel must maintain position, irrespective of anything else.

In waters greater than about 450m in depth, the only way divers may be deployed is by means of atmospheric diving suits (ADS) such as the 'newt-suit'. These are pressurised diving suits containing the diver in a 1-atmosphere environment. A DP support vessel may operate two ADS instead of saturation diving. The other alternative for operations in deep water is the deep-water ROV. A more recent development is the AUV, or autonomous underwater vehicle. This is a free-swimming ROV operating without an umbilical on a pre-programmed task, deployed and recovered by its support vessel. One big advantage of using ROVs instead of divers, is the lower levels of redundancy required for non-man-rated operations underwater. Nevertheless, many modern vessels so equipped feature full Class 2 redundancy in consideration of the financial penalties associated with a system failure, also the hazards associated with the close-in locations in which these vessels often operate

4.4 Anchor-handling tugs and platform supply vessels

All new-build vessels in this category are being built with a Class 2 DP capability specified into the design. A few years ago vessels of this type would routinely have been equipped to DP Class 1, but in view of the close-in locations in which these vessels work, and the associated hazards, many clients are demanding DP Class 2 as standard. In the anchor- handling mode, DP may be used to advantage in the manoeuvring to transfer the anchors to the tug from the barge or rig, and in the exact positioning for the laying of the anchors. In the supply vessel mode, the DP allows more reliable positioning of the vessel close to a platform for long periods of cargo-working time.

4.5 Construction vessels and crane barges

A number of heavy-lift vessels routinely use DP to good advantage. The number of facilities capable of lifting in excess of 4,000 tonnes is increasing, and there also exist a number of vessels of lesser crane capacity but greater utility. The ability to do away with the necessity of laying an eight-point anchor spread considerably reduces the time required to complete a particular lifting operation. Invariably these vessels are of Equipment Class 3, meaning that there is no need for back-up anchors to be laid, or for towing capacity to be provided. The largest vessels in this class are the Saipem 7000, the Thialf and the Balder. These vessels are mainly used for installation of large platform elements, and the subsequent pile-driving


Chapter 4 39


operations. They will invariably be utilised in the future for platform removal operations as more fields reach the end of their working lives.

4.6 Drilling Rigs In recent years, drilling operations have been extended into waters of ever greater depth. This has necessitated an increase in the number of DP-capable drilling facilities. The latest drillships are rated to work in water up to 12,000 feet (3,500m) depth. Many DP rigs are of the semi-submersible configuration, or may be very large monohulls.

In deep water, it is not sufficient to simply position the rig directly over the wellhead. Compensation must be made for tidal flow, in that the all-important measurement is that of Riser/stack angle. This is the angle between the riser (containing the drillstring) and the wellhead or LMRP (lower marine riser package). It is vital that this angle remains close to zero. With a tidal stream, the riser will 'bow', necessitating the vessel moving in an uptide direction to accommodate this riser angle. An added complication in some of the latest monohull drillships, is the ability to drill two wells simultaneously, with the need to monitor two riser angle values.

A drilling rig using DP for these functions will usually use dual DGPS and long baseline acoustic position references, as other references are often not available in deep water.



4.7 Dredging and Rock dumping

Many dredging operations are utilising the advantages of DP. Whether the dredging operation is for the purposes of harbour or channel maintenance, or for the recovery of aggregate materials, the precision available from the use of DP makes it an attractive method of operation. A trailing suction dredger may follow a predetermined track with the reference point being located upon the draghead, ie the draghead is the element being positioned rather than the vessel. The DP system is configured to receive and compensate for measured draghead forces, determined from suitably located sensors.

A small number of vessels are configured for rock dumping, to provide protection for underwater elements. This may be an alternative to trenching for a pipeline, or the rock dump may be for the purposes of erosion rectification for platform foundations and the like. Rock dumping vessels are usually mini-bulk carriers, specially fitted for automatic discharge into a hopper adjacent to the fallpipe tower. The fallpipe system is deployed over the side of the vessel from the handling tower. At the lower end of the fallpipe is an ROV which is able to direct the delivered rock accurately on the target corridor. The vessel uses autotrack facilities to follow accurately the required line at a precise velocity. Position reference may utilise conventional PRS, and may be enhanced by a smartwire system, the lower end of which is secured onto the ROV.


Chapter 4 41


4.8 Pipelay and pipe trenching operations

A ever-increasing number of DP pipelay vessels are currently in operation worldwide. Dispensing completely with anchors and moorings, these vessels are able to conduct pipelay more quickly and efficiently than pipelay barges.

Three methods of pipelay are in use: s-lay, reel-lay and s-lay. In s-lay operations the pipe is constructed in a long narrow factory called the 'firing-line' at deck level. Pipe is fabricated, welded, coated and inspected at a number of stations spaced at 12m intervals along the firing-line (standard pipe spools are 12m in length). The pipe is controlled by caterpillar-track pipe tensioners that feed it down the 'stinger'. The stinger is a hefty ramp at the stern supporting the pipe in the overbend area. The pipe is supported by its own tension only in the span between the end of the stinger and the sea bed touchdown point, or the 'sagbend' zone. The DP system must allow the vessel precision positioning on a fixed heading, maintaining pipe tension, and moving the vessel ahead an exact 12m on demand. These moves may occur every 4 minutes. Faster working may be achieved if double-joints are worked, with the vessel moving 24m each time. Pipe tension is fed back into the DP from sensors on the tensioners, and must be maintained within specification tonnages.

In reel-lay operations, the pipe is pre-fabricated and loaded onto a vertical carousel on the vessel. The pipe is laid by passing it from the carousel onto the lay-ramp, thence down the stinger. In very deep water, the only suitable method is J-lay. Here, the stinger is mounted close to vertical. The pipe is fabricated into triple-joint lengths, which are turned to the vertical at the stinger. Large forces are induced at the stinger due to the heavy weights of pipe involved, and these forces must be countered by the vessels DP capability.



Pipelay vessels routinely conduct complicated evolutions using DP. The operations to commence and complete pipelay, conduct an in-water tie-in or to lay down the end of the pipe if necessary, all involve precision positioning and manoeuvring.

Associated with the business of laying pipelines is the need to protect them from damage. DP-capable vessels are used here also. A pipeline may be trench buried by use of a specialist seabed crawler vehicle. This vehicle will be deployed by A-frame over the stern of the trenching vessel, and will follow the pipeline, excavating a trench of the required depth. This will be done using ploughshare elements, and water-jetting. Once a trench has been established, the vehicle will be recovered and re-configured for a back- fill or cover operation. The DP vessel will use a specialist track-follow or vehicle-follow function to maintain station on the trencher. She may also use a specialist position- reference system such as trimcube or smartwire, allowing position relative to the vehicle to be monitored.

4.9 Cable lay and repair operations

The advent of fibre-optics in international communication cables has led to the requirement for greater precision in vessel positioning. Fibre-optic cables have very specific minimum-bend-radius (MBR) and loading limitations; if these are exceeded, then


Chapter 4 43


the cable may be damaged. Most modem and new cable ships are being fitted with a DP capability as standard. The facility is particularly useful when conducting cable repair operations or shore-end connections in shallow waters.

Cable lay operations may involve a seabed crawler vehicle, or a towed plough system. In both these cases the configuration of the DP system may be matched to the requirements of the operation, especially where the cable must be laid and buried to a specific depth in one operation. With a towed plough, the DP system will be enabled to monitor and compensate for the external forces of the plough hawser.

4.10 Accommodation barge and service vessels

During periods of construction, reconstruction or repair of offshore installations, there is a need for a variety of facilities. Sometimes these facilities simply consist of accommodation for extra workmen. In other cases, the facilities required are more complex. A simple flotel barge may be positioned close to a platform providing accommodation facilities; this barge may be connected to the platform by means of a gangway, or in adverse weather conditions, passage must be by helicopter. These barges are often DP semisubmersibles. If a gangway is connected, the gangway itself may perform as a position-reference; a form of horizontal taut-wire. Such vessels may also carry more extensive facilities than just accommodation, and be able to conduct Diving and ROV work, carry out fabrication, assembly or repair work in workshops, conduct crane operations, and have a major role in emergency intervention (firefighting, evacuation, medical, etc). Such a vessel may well be a semi-permanent support facility for one field or a group of adjacent fields.



4.11 Passenger vessels

Large cruise ships are beginning to utilise DP capability. These vessels are increasing in size generally, and have ever-higher freeboards on ever-decreasing draughts. Coupled with the tight schedules vital to these vessels, and the precise manoeuvring associated with some of the out-of-the-way places visited, the business of ship handling becomes very demanding. This is an area where DP is beginning to be used to advantage. A further consideration is that of the environment. In many places it is prohibited to anchor, due to the effects on vulnerable coral reef environments. A further advantage of a DP capability is the ability to give a 'lee' to one side of the vessel for the working of tenders. If the vessel were lying to anchor, head to sea, the sea state may render tendering impossible, while if the DP is used to cant the vessel 20° or so to the wind, then boat embarkation may be a much safer prospect.

4.12 Research and survey vessels

A variety of research and survey vessels make good use of DP capabilities. These include hydrographic research and survey vessels, oceanographic and fisheries research vessels, and logistic vessels such as those operated by the British Antarctic Survey. The great variety of work conducted by these types of vessel will often necessitate the use of DP capabilities. Another category of vessel using DP is the buoy tender.


Chapter 4 45


4.13 Military vessels

A number of naval vessels may use DP to advantage, particularly mine countermeasures vessels. Several classes of MCM vessels, both Royal Navy and overseas, have a DP capability. This allows them a 'hover' capability while an underwater contact is investigated. Another class of military vessel utilising DP is the underwater operations vessel. Increasingly, a form of DP is being utilised in amphibious assault vessels and the like.

4.14 Other applications

Further tasks to which DP capabilities are being put include sea-floor mining, windfarm construction vessels, heavy-lift vessels and the luxury yacht market.

One unusual application of DP has been in the sea-launch facility. Two unique vessels, both featuring DP capability, are the semisubmersible rocket launch platform Sea Launch Odyssey and its supporting assembly and command vessel. Both vessels are dynamically positioned. During fuelling and launch operations, the Sea Launch Odyssey platform will be unmanned, thus her DP is remote controlled from the command vessel. At present, this is the only remote-controlled DP system afloat.

From the above, it can be seen that the practical application of DP can vary from having the vessel maintain one position and heading for long periods, to those vessels which are continually adjusting position and heading for operational reasons. Many vessels' DP



systems will also be specifically configured for the operations to be conducted: while a drillship will have riser angle mode facilities incorporated, a shuttle tanker will have auto- approach and weathervane facilities.

Whatever functions and facilities are provided, it is essential that the DPOs understand fully the operation of their system. A thorough study of the operational and system handbooks is essential. DP systems are, to a certain extent, tailor-made to suit the vessel concerned, and a newly-joined DPO may not be familiar with the exact configuration of the system functions.


Chapter 5 47

Operational planning and watchkeeping

Chapter 5 Operational planning and watchkeeping

5.1 Operational planning

The success of any DP- related operation is totally dependent upon the quality of the planning. The plans must be discussed with the client, who must be in agreement with every aspect of the plan. The planning must include the intended sequence of events, with effective contingency plans covering every aspect of the operation. Every stage of the operation should allow at least one escape route possible with the vessel in a degraded status. Planning must cover the worksite approach and set-up, together with any subsequent manoeuvres, and the eventual exit from the worksite.

5.2 Preparation of operational plans

Frequently, the vessel is simply providing a working platform from which the client can conduct his operation. The vessel may be relatively static on location, or may be engaged in simple or complex manoeuvres. The actual planning may be done on paper charts or worksite diagrams of the area provided by the client, or may be done using computerised NavScreen facilities. Whatever medium is in use, it is essential to check that the data is up-to-date and comprehensive.

When preparing operational plans, the bridge and DPO team must keep in mind a variety of factors, all of which may affect the viability of the plans. Some of these factors are detailed, such as environmental and weather conditions expected in the area. Are any phases of the operation weather-limited? The quality of weather forecast data available. Currents and tidal stream: will the vessel be current-limited? Will any aspect of the operation be current-limited? Is the vessel able to react to changes in weather or current status? Are there any water depth and draught constraints?

What fixed and mobile hazards may be expected in and around the worksite? Are there any restrictions to manoeuvre or placement of sea-floor hardware imposed by the field operator? Are there any factors which might restrict vessel manoeuvrability or escape- routes? Will the vessel be heading-constrained during the operation?



Does the vessel have any limitations in respect of power availability or thrust provision? Will there be any external or other forces which might degrade the vessel's positioning ability?

What equipment class is required for the operation? What is the probability of the vessel becoming unable to comply with the equipment class? Are there any problem areas in the vessels redundancy?

Is there an adequate provision of position-reference? Are there any factors which might result in position-reference systems to become unavailable? Are there any additional position-reference systems on hand to cover unexpected failures?

The planning will include the provision of work-permits, and compliance with the requirements of the client's safety-case. Before any operation commences, the client will require the completion of vessel DP trials. The machinery configuration will also be decided upon in view of the agreed equipment class, and MCR manning agreed. Availability of additional generators or power units must be clarified together with the notice periods of available units.

5.3 Contingency planning and escape routes

A basic principle of DP operations is that the DPO never takes a vessel into a situation from which she cannot be extricated under degraded status. Degraded status means subsequent to worst-case switchboard failure, often the loss of half the power and thrust capability. More practically, a DPO should never take a vessel into a location until he has planned his exit!

Contingency planning means answering questions beginning with the words 'what if?' During the approach stage of any operation, the contingency plan will simply be the least-power escape route to a drift-clear location. The DPO must always be aware of the areas within the worksite that are 'drift-on' or 'blow-on' zones in relation to any fixed hazard (platform structure). These are particularly hazardous areas and their location is continually changing as wind and current conditions change.

Once on the working position and set-up on DP, DPOs should re-affirm escape routes. If the vessel is close alongside a platform, the most obvious escape route may not be the best. Moving the vessel ahead or astern is always preferable to a sideways manoeuvre. Moving sideways always uses more power and thrust for lower velocity and acceleration. It may be that the escape is being made under limited power availability conditions. Under these conditions the DPO should always anticipate a total blackout, thus the vessel must be placed into a 'drift-clear' situation as quickly as possible.

The escape route must be maintained clear at all times. If a supply boat parks across the escape route, the situation must be re-assessed. The DPO must have a good view of the


Chapter 5 49

Operational planning and watchkeeping

escape route direction. If the DP system is located on the aft-bridge of the vessel, facing aft (as is the case in many offshore support vessels) the escape route should ideally be an astern movement. In any situation, the escape route should not be directly against the environment. If that was the case, and the vessel suffered full blackout during an escape manoeuvre, she would just blow straight back into the danger area!

The bridge team must be prepared to make changes to the contingency plans as necessary. Of necessity, DP situations are occasionally fluid, with changing environment



often causing the best-laid planning to become less viable. The DPOs must continually re-assess contingency plans and escape routes, make alternative provision if possible, or to suspend operations if all contingency has vanished. A typical situation might have the vessel close alongside a platform structure, with the vessel bows-on to the forecast (and actual) wind. Wind forecast to remain steady in direction and gradually fall light. The wind very slowly backs round and increases in strength, contrary to forecast. Very soon, the vessel has reached her weather-working limits and contingency plans have been eroded to almost nothing. The DPOs were not expecting such changes, but must react in a timely manner to secure the operation and place the vessel in a position of safety.

5.4 The initial DP set-up

It is assumed that a vessel is proceeding out to an oilfield location in order to commence work close by fixed platform installations. These notes are a general guide to procedure, not intended to be a detailed set of instructions for any particular operation. The first steps generally consist of the transfer of vessel control from conventional navigation to DP control, usually outside of the field 500m exclusion zone. Prior to this point, contact will have been made with the installation and ETA and other relevant information exchanged. Work permit and notice of readiness will be dealt with.

With the vessel outside the 500m zone, control functions will be transferred to the DP system. This may entail a change of control location (eg main bridge to aft bridge); a checklist will ensure that all systems are correctly set up and enabled at the new location. It is essential that the propeller and thruster controls are tested and proved functioning at the new location. The DP system may need to be re-booted or re-loaded prior to operation. The MCR will be informed of the status of the operation. All communication systems must be checked and tested at the DP location. Position-reference systems must be confirmed available, including any back-up references allowed for in the planning. Frequently, the first position-reference to be used will be DGPS, so this will be checked and confirmed operational. When all is ready and checks complete, control may be transferred to the DP system. This usually involves turning a transfer switch on the thruster panel, selecting control input to DP instead of from the manual thruster controls. On the DP system thrusters can be enabled, and the system selected into joystick or manual mode. The DPO must now re-affirm that all propellers and thrusters are responding correctly to the joystick movements - once again proving the thrusters. With the first position-reference enabled and accepted into the DP, the DPO can transfer control of the vessel to full DP. This would be the time to complete the pre-DP checklist.

If the vessel is at the beginning of a charter, the client will require a programme of DP trials to be completed, in which ail DP functions, facilities and peripheral equipment is tried and tested. On satisfactory completion of such trials, assuming no system deficiencies reported, the vessel is ready to commence the operation. Permission to enter the 500m zone will be obtained and logged. The vessel can now commence the


Chapter 5 51 Operational planning and watchkeeping

approach to the worksite.

5.5 The approach to the worksite

It may be more convenient to conduct the earlier stages of the worksite approach in manual (joystick) or partial manual control. The vessel may be navigated using the joystick control to a location perhaps 200m away from her final working position. At this point she can be placed back into full automatic DP control with the vessel on the heading to be assumed for the first stage of the operation. A second position-reference system may be set up and enabled at this point. This position is a good location from which to make a number of checks. The DPO will monitor the vessel's position and heading keeping ability; is she maintaining position and heading comfortably, or (in marginal weather conditions) is she struggling? Are thruster and power levels within limits, or are they occasionally or frequently red-lining? The Master and DPO will make the decision to continue the operation or abort at this point. If there is a need to abort, the client must be fully appraised as to the decision and the reasons.

Assuming that the decision is to continue the approach, contact can be made with the platform giving a revised ETA for the commencement of the operation. The vessel can now be moved towards the intended working position in steps of, initially 20m, reducing to 10 then 5m as the final position approaches. The speed selected for each move will be progressively reduced as the vessel gets closer in. With the vessel moving in from 200m distance, a speed of 0.5 to 1.0 knots may be appropriate, but when she is within 50m of the final position speed should be reduced to 0.3 or 0.2 knots. This is especially relevant if the working position is close in to a fixed structure. When within 50m to 100m of the working position, the third position-reference (Class 2 and 3 operation requires three independent) will be deployed and enabled; this may be an acoustic system, a short range microwave or laser system, or a taut wire reference. With three position references enabled and accepted into the DP system, the final moves can be made to the working position.

Once into the final working position, the vessel should be allowed to 'settle' to this position for a minimum of 30 minutes. The main need for this period is to allow the system mathematical model to build to its optimum state, but the time is also useful for a variety of practical items. The Master and DPO should ensure that the vessel is maintaining position and heading comfortably, with no undue excursions. Power and thruster outputs should remain continuously at acceptable levels with due regard to the redundancy requirements. Escape routes should be re-affirmed. All onboard and external communications should be checked. Appropriate warnings will be passed onboard regarding the vessel status, with revised indications of the time of commencement of operations. Similar information will be communicated to the platform staff. Pre- operational checklists will be completed and signed by the appropriate staff. If the operation is to be conducted under 'traffic-lights' (green, amber and red warning and alarm lamps) these must be tested at all operational locations. All personnel must be



familiar with the correct and required response to amber and red lights.

5.6 Watchkeeping and communications

These notes are written to cover general DP operations. All operations are different and the vessels themselves differ greatly in type, configuration and capability. Some tasks require the vessel to maintain a static or relatively static position for days or even months on end (drill ships, flotels). Other vessels will be continually manoeuvring in order to support their tasks. The nature of the tasks differs widely and the operation must be planned around the requirements of the client. A number of different types of operation are described in various modules of this handbook, but some general watch keeping procedures are included here.

The bridge team must be aware of the significant change in status once the go-ahead or green light is given for the operation to commence, irrespective of the type of operation, prior to this moment the emergency contingency plan is one of safe escape from the location and its hazards. Once, however, the green light is given, the contingency plan must allow for the vessel to maintain position and heading under all circumstances, until the task is aborted.

Some non-redundant DP-capable vessels may have a single-manned bridge when on DP, but the majority of DP operations are carried out with two operators manning the bridge. It is necessary that the DPO mans the DP desk exclusively, while the other watchkeeper carries out all other bridge functions, and that these two individuals ideally swap roles every hour. The watch relief arrangement should allow staggered watch changeover so that there are never two fresh DPOs taking over at the same time. When taking over the watch, the DPOs must familiarise themselves with many aspects of the management of the vessel. The list of information that the bridge team must acquire at this time include (but is not limited to) the following:

• Position and heading of the vessel • Status and recent performance of the DP system and its peripherals • Details of Position Reference Systems in use and their performance • Availability of further PRS on failure of the above • Level of redundancy • Status of the operation in hand. Planned changes and progress for the coming watch. • Details and status of any operational elements (eg if the vessel is a Dive Support Vessel and diving operations are underway, then the status, position, depth of the diving bell or basket, the number of divers in the water, their umbilical lengths and expected return times, also detail of their operational task) • Weather conditions and forecasts • Communications, onboard and external


54 Chapter 5


• Traffic in the area. Any planned traffic movements that may affect the vessel and her operation or positioning

• Any planned helicopter operations

The above is just an outline, of course. The specifics of any watch handover wil! include far more.

One of the items mentioned above is that of communications, which is a vitally important topic to consider. In a typical offshore support vessel, there may be a small number of marine crew providing a working platform for a much larger number of clients and their contractors. It is essential that the bridge team is totally apprised of the progress of the operation, and any changes to the agreed plan are discussed and promulgated. This is the task of the client's representative who liaises with ship's staff.

Physical communication lines must all be working and regularly tested, whether they are simple telephone lines, open talk-back systems, or onboard UHF or VHF radio. With onboard radio systems, procedures must be followed to ensure no ambiguous reception of orders, or misinterpretation of report-back.

With in-field and external communications, all VHF stations in the area must be listed with name, calling and working channels, and call-signs. This listing must be posted on the bridge for quick reference, and must be amended as vessels enter and leave the area.

5.7 Checklists

Many checklists are in use in relation to DP operations. Checklists are intended to be an aid to memory, and a safety feature, but there are dangers inherent in the use of checklists, especially if they are poorly designed. It may be useful at this point to highlight some identified problem areas relating to checklists:

• Checklists may be completed in a hurry, without too much thought for each item. This is sometimes as a result of'checklist overload'where a large number of checklists are completed, taking up valuable time

• The checklist may replace thinking; it must be remembered that the checklist is only and aid to memory

• Vital items may not be included in the checklist. This is particularly the case where 'standard'checklists are in use in different vessels with different facilities. If an item is not included in the checklist, the likelihood is it will not be checked

• Checklists should be regularly reviewed as part of the vessel's safety management system. This is particularly necessary after the vessel has undergone modification or new equipment installed. Proposed amendments should be agreed by operational personnel

• If a checklist needs amendment to include identified vital items, then the correct procedure must be followed. Checklists are usually company-controlled documents; any



unauthorised change may constitute a non-conformance with the company QA system • Occasionally there is ambiguity regarding exactly what check is to be made. A tick box labelled

'Gyro 1'does not mean much • Checklist completion should be conducted by two people. One person will invariably tick items

they are sure they have just looked at. In fact, the item may last have been checked a number of hours ago

• Frivolous or non-essential items must not be included in the checklist. If there are such items, the checklist loses credibility

5.8 Standing orders and logbook records

In any vessel, there must be clear and effective guidance to watchkeepers on the bridge, whether the vessel is operating in DP mode or not. The Master's standing orders will be compiled in relation to all the vessel's operational modes. Standing orders must make reference to company navigational policies, including conduct of the bridge during DP operations. There must be a clear understanding of the immediate chain of command. This is particularly of concern when there is more than one watchkeeper on the bridge. It is often the case that the watch is shared by two watch officers, one of whom is the senior. When in DP mode, one watchkeeper must man the DP console or desk continually, and not engage in any other task. The standing orders will clarify any conflict within the vessel's safety management system. Clear guidance should be included regarding the circumstances under which the Master must be called. In some vessels the Master is one of the DP watchkeeping team, and is backed up by a night Master on the opposing watch. The line of command and responsibility must be clearly stated and understood.

The vessel will have a comprehensive DP operations manual, which is an operational company document. This must be interpreted in conjunction with the operational manuals supplied by the client detailing their tasks. Specific orders or procedures may be written covering very specific operational requirements.

Within the standing orders and operational manuals must be clear and fully-understood criteria for the suspension of the work due to stress of weather or other reasons, and these criteria must be understood by the client. One function of such manuals and standing orders is to protect the bridge team from over-optimistic demands by the client.

Logbook records must be kept, including all times and positions relevant to any DP location or setup. A particularly useful document is a separate record of DP system operation. This logbook will include details of all DP work, with positions, position references used, and any malfunctions attended to. Separate sections within this DP log will detail the operation of each individual position-reference system. This document becomes particularly useful when handing over to newly-joined watchkeepers, and also if the vessel is working for a long period in a specific field or area.


Chapter 6 57

Position referencing

Chapter 6 Position referencing

6.1 The need for position reference The provision of a stable, accurate and reliable input of vessel position is central to the DP function. In control engineering terms, it is not possible to control any variable until that variable can be measured. It is therefore necessary to provide position reference, and this factor may be provided by one or more position reference systems (PRS). In some manufacturers' systems, these are known as position monitoring equipment, or PMEs.

The provision of PRS is a continual concern of the DPO. There is no such thing as a system that is totally accurate, stable and reliable. All PRS have shortcomings, limitations and pitfalls, and it is the DPO's job to correctly evaluate the suitability of individual references for use in any particular situation.

6.2 Characteristics of a Position Reference System Any PRS employed in conjunction with a DP system needs to be accurate, reliable and stable. It is also necessary to provide continuity of positional update. Current PRS vary widely in characteristics, and the DPO must use those which are most appropriate.

Accuracy is often regarded as the most desirable characteristic of a PRS, indeed, vessel positioning can never be more accurate than that of the position reference. Here we must make the distinction between position reference which provides a vessel position to the DP system, and position reference systems which are the individual pieces of equipment contributing to this position reference.

In general, we seek a vessel positioning ability of 1-3m, implying a level of accuracy of individual PRS at 1 metre or better. This is a tall order, and specialist navigational systems are in common use in the DP world.

Accuracy, of course, is a variable quality. Accuracy may vary day-to-day, hour-to-hour


58 Chapter 6


and also with physical location. Accuracy may be dependent upon environmental and human factors.

Reliability is all-important. There is no point in having an ultra-accurate PRS if it is liable to fail without warning, or suddenly become inaccurate. All PRS have drawbacks and limitations. To counter these problems, all modern DP systems have the ability to pool data from two or more PRS into a 'best-fit' position. For operations to Equipment Class 2 or 3, three PRS are required as a minimum. This allows automatic detection and rejection of a failing PRS using a 'voting' system. Lack of reliability may well be down to human error, eg the DPO deploying a PRS which is unsuitable for the situation. One example would be to rely on Differential GPS when in a location on the North face of a large platform in a high Northern latitude. Loss of satellite signal (or differential corrections) due to platform shadow is predictable under these circumstances.

Stability of PRS input is also important. The word 'stability' in this context means freedom from false data jumps. This type of problem can occur when a Laser ranging system jumps onto an unwanted target, or when there is a change of constellation within the GPS.

Continuity of position input is vital. In general, DP systems require a positional update once per second, thus the PRS data should update at this rate or faster. Some references may not provide this continuity, eg an underwater acoustic system uses acoustic interrogation and reply of a sea-floor transponder. Sound travels at approximately 1500m/sec underwater, thus if the water is in excess of 1,000m depth, the reply delay will exceed one second. The update rate under these circumstances is reduced, with a consequent reduction in the value of that PRS.

The reception of differential corrections for update of the GPS data is another area where continuity is an issue. Corrections are sent in 'packets' which take time to reach the receiver, and then need to be decrypted and processed. This all adds to the 'Age of Data' (AOD) displayed on the DGPS screen.

6.3 Pooling of multiple PRS data

As mentioned above, all modern DP systems are able to pool, or combine data from two or more individual PRS, and to obtain a best-fit or 'best-fix' position. Invariably this is done by means of a system of Variance-based weighting. Variance is a measure of the 'spread' of successive positions provided from a PRS; the larger the variance, the less accurate is the overall positional performance. Variance is otherwise known as Standard Deviation (SD), and is a measure of the 68% variation from the norm of any position data. In simple terms, the Variance or SD is a measure of the accuracy of a particular PRS at that moment. SD will change with time and environmental conditions. If a number of PRS are deployed simultaneously, then a comparison of their SDs will provide individual system weightings; the smaller the Variance, the 'tighter' the


Chapter 6 59


positioning from that PRS, and the higher the weighting. The greater the weighting of an individual PRS, the greater the influence of that PRS on the final positional calculation. Weightings are expressed as percentages, adding to unity, thus the larger the number of PRS deployed, the smaller will be their relative weightings. In other words, the greater the number of references deployed, the less significant will be the loss of any individual PRS.

With the above system, weightings are automatically calculated, reflecting the system variances. In some DP systems however, it is possible for the DPO to assign weightings. This must be done with care, as it is possible to add weighting thus reliance onto a PRS that actually does not deserve it. In general, assignment of PRS weighting is best left to the DP system algorithms.

One scenario in which it might be advisable to add weighting to a particular PRS occurs in deep water, where position reference is provided from Long Baseline acoustics, and from DGPS. The update rate of the acoustics may be low due to the water depth, thus the DGPS takes the majority weighting. The DGPS may be suffering occasional noise due to ionospheric activity. It is advisable to reduce the weighting on the DGPS under these circumstances, as, although the acoustics have a low update rate, they are otherwise more reliable.

DPOs monitor the weighting values of PRS by reference to historical graphs. By this means changes to PRS quality can be monitored and predicted. It will be apparent, for instance, that a particular PRS is deteriorating in quality, and may provide problems at a later time.

6.4 Validation of PRS data All DP systems continually apply a testing procedure to incoming PRS data. A simple test is the Freeze Test, which highlights any PRS whose data does not change with time. This is an important test, as although a frozen PRS has actually failed, the DP system could well assign increasing weighting in line with the apparent reduced variance. A frozen PRS should be disabled by the DPO and the cause investigated.

A Prediction Test is able to detect sudden jumps in the measured position, rejecting those that lie outside predetermined limits. When three or more PRS are enabled and accepted, a median test is applied which allows the rejection of a slowly drifting PRS.

Facilities will exist to allow criteria for the rejection of PRS input. For example, the DPO may be able to enter the circumstances under which a DGPS input will trigger a warning, alarm and rejection. The criteria will relate to such factors as loss of differential corrections, minimum number of satellites, maximum HDOP value, and minimum DQI (differential quality index - see paragraph 6.8.6) factor.



6.5 Reference origin

The reference point of the first PRS enabled and accepted into the DP is known as the reference origin. This is in effect a datum zero for the measurement of positions and the calibration of subsequent PRS. If the first PRS is a relative one (eg Fanbeam, Taut Wire) then the reference origin will be local (ie the position of the Taut Wire weight, or the Fanbeam reflector). If however, the first PRS is globally referenced (eg DGPS) then the reference origin will also be globally referenced. The applicable datum must be known (eg WGS 84). In the case of a global reference origin, it is then possible to configure the system to display position information in terms of lat/long or UTM co-ordinates. The DPO must ensure that the datum that the system is using agrees with the one shown on his charts and worksite diagrams. It is possible to convert position co-ordinates from one datum to another internally.

In a DP vessel, there is designated a common reference point within the ship to which is reduced all position reference data. This datum point is often referred to as the 'CG' or centre of gravity. This term is somewhat confusing because it is not the actual vessel CG. Normally this will be the light-ship CG obtained from the shipyard drawings. Within the DP system the x,y and z (alongships, athwartship and vertical) offset co-ordinates of all PRS sensors are configured as a look-up table or database. Thus all and any PRS data is referenced to this common reference datum or 'CG' point.

6.6 The UTM co-ordinate system

In the offshore world, positions are often expressed in Universal Transverse Mercator (UTM) co-ordinates. This is in contrast to the conventional (to the navigator) latitude/ longitude notation. UTM is extensively used by surveyors and others offshore. In the UTM system, latitude and longitude are replaced by Northings and Eastings. UTM is a flat- paper projection covering a local area, or 'zone' of 6° width in longitude. Thus the world is covered by 60 zones. Each zone is based on a Central Meridian, running down the middle of the zone. Zones are numbered from the 180° longitude meridian, thus, Zone 1 spans 180° to 1 74°W, with the central meridian at 177° W. The UK sector of the North Sea is mostly located in Zone 31, spanning Greenwich to 6°E, with central meridian at 3°E. Within the zone, position is defined by Northing and Easting values, both measured in metres. Northing is the distance North of the equator, in metres, usually a seven-digit number. In the southern hemisphere, these Northings would have negative values, so a constant False Northing of 10,000,000m is added to them. This results in Northings always being positive values, and always increasing in a northerly direction. Easting is the distance eastward of the central meridian, with a constant 500,000m false easting added to all values. This means that all Easting values are positive, six-digit values, increasing in an easterly direction. No negative values, and no values increasing in a westerly or southerly direction. Since Northings and Eastings are measured in the same units (metres) plane trigonometry can be used. In the UTM system, there is no polar


Chapter 6 61 Position referencing

convergence, thus no distortion away from the equator. One drawback, however, is that directions on a UTM drawing are 'Grid' rather than 'True'.

6.7 Position Dropout

If the vessel positioning is under automatic control, and all position-reference is lost, the system continues to position the vessel to the best of its ability using the mathematical model. This is known as the 'dead reckoning' mode. This position will initially be stable, but after a few minutes the vessel will begin to pick up speed and move away from her set-point position. The amount of time that the vessel will remain under dead reckoning control will depend upon the quality of the mathematical model, and the environmental conditions. If the model is well established and the weather and sea state is settled, the vessel may remain on position for ten minutes or so. The worst case is where weather conditions are deteriorating, with a consequent deterioration in the quality of incoming PRS data. As a result, the model will be poor at the point that the PRS ultimately drop out, and the vessel will very quickly start to lose position.

6.8 Differential GPS

The global positioning system (GPS) is a well-established navigational medium. Becoming fully operational in 1995, it has since become a standard facility in almost every commercial vessel. The system is operated by the US Dept of Defense as primarily a military system, but is available for use by civilian operators. It must always be remembered that the system is not primarily for the benefit of commercial civilian users, and that there is no technical or political input from any other agency or nation. Also to be remembered is the fact that GPS signals may be jammed or 'spoofed' by third parties.

Despite the above, GPS has become an established mainstream navigational system, with continuous worldwide coverage. The temptation is to use GPS as the ultimate navigational provision, and to assume that it is always correct and reliable. The reality is sometimes very different from this. The DPO must always keep in mind those factors which affect the quality of the positional data from any PRS, including GPS.

Stand-alone GPS is generally of insufficient accuracy to act as a PRS for DP purposes. The stated repeatability figures of 15-20m require that differential enhancement of position data is necessary.

6.8.1 The GPS system

GPS consists of three operational 'segments'; the space segment, the control segment, and the operator segment.



The space segment consists of the satellites or space vehicles (SVs) in orbit. A baseline number of 24 SVs plus three in-orbit spares make up the constellation. These satellites are located in six orbital planes inclined at 55° to the equator, with four SVs in each plane. The DPO must remember that if he is located at high latitude, SV distribution in the sky will be biased away from the direction of the pole, ie if one's latitude is 61 °N, then all satellite passes will be to the southward of the observer. This must be realised if the working location is North-face of a large platform, as a significant number of SVs will be blocked by the structure.

SVs are placed in 12-hour orbits, so from the observer's viewpoint the constellation is continually changing. New SVs will be periodically acquired while others drop below the horizon and are lost.

The control segment consists of the ground control centre at Colorado Springs in the USA, together with a worldwide array of monitoring and uplink stations. The third segment is the user, comprising of the equipment located on board.

The principles of operation of GPS are well known so will only be reiterated here in outline. All SVs transmit time-referenced codes containing the SV spatial coordinates. These codes are received at the vessel, the time reference allowing the calculation of a 'pseudo range' to the SV. Errors in the ranging down to receiver clock error can


Chapter 6 63


be eliminated if four or more SVs are observed simultaneously. The vessel position is determined from a knowledge of SV co-ordinates and ranges.

The GPS system has been declared operational by the US Dept of Defense, thus available to foreign civilian users as the Standard Positioning Service (SPS). This service utilises the C/A code on a single frequency, L1 (1575 MHz) and gives a limited level of accuracy of around 15 - 20m. Greater levels of accuracy are available to military users; the precise positioning service (PPS) utilises two frequencies, L1 and L2, with an additional code (the P code) transmitted on both frequencies.

When using raw (non-differential) GPS generally, two quality indicators are available to the navigator; HDOP and number of SVs. Obviously the greater the number of SVs being tracked, the better will be the resolved position. With a full constellation of 24 SVs aloft, the number available should be typically between 7 and 14, dependent upon a clear view of the sky. Another quality factor is SV geometry. This is akin to the 'angle of cut' considerations when plotting terrestrial position-lines. If observed SVs are clustered together in the sky, then the resolved position quality will be poor compared with that resulting from a more even spatial spread of SVs. The factor indicating the geometrical quality of the position is known as HDOP, or horizontal dilution of position. HDOP is a unitless number ranging from one (the most favourable) to infinity, thus the larger the value of HDOP, the poorer the geometrical quality of the fix.

Not all SVs above the horizon give useful data. If the SV altitude is high, then the horizontal component of the positioning calculation becomes less significant. Equally, if SV altitude is low, then unacceptable signal delays occur caused by refraction of signal within the atmosphere. For this reason, GPS imposes an elevation mask at 10° or 15°; SVs below this altitude are tracked but not included in the positional calculation.

A problem area with GPS is the reception of multipath signal. Multipath is the reception of SV signals by non-direct route, ie reflected from nearby structure. Multipath is likely to be a problem in any situation where nearby structure rises above antenna location. Reflected signals have travelled a greater distance than direct ones, and it is, of course, the length of the path travelled that is the measured navigational value. A further effect of nearby high obstructions is the blocking of the line-of-sight to SVs, thus causing loss of signal from that SV. DP vessels routinely work close to platform structures, and in some vessels antenna location is far from ideal, thus multipath and signal loss can have a major detrimental effect on the position accuracy and stability. It cannot be emphasised too strongly that, under the situations described above (close proximity of large structures), that GPS and DGPS cannot be implicitly relied upon.

6.8.2 Differential GPS

Raw GPS is of insufficient accuracy for use as a PRS for DP, even if the P-code were



available, further enhancement of accuracy is needed. This enhancement is the provision of differential corrections to the raw GPS data. Differential services are commercially available on a contract basis. A number of differential service providers operate in this field, offering a variety of differential correction formats tailored for different types of user.

The principle is simple. A reference station is established at an accurately known location ashore. This reference station observes SV data, enabling calculation of SV ranges. These calculated ranges are independent of the observed or measured ranges, obtained in the conventional manner (signal travel time). The differences between the calculated and measured ranges are effectively the range errors at that position and at that moment. These differences are then transmitted to the user vessel, becoming corrections to be applied to the user's SV ranges.

In reality, a differential service is more complex than this illustration.The service provider will maintain and operate a network of reference stations covering much of the globe, and the vessel system is able to receive and process differential corrections from typically three of the nearest reference stations. This is often referred to as network DGPS. Service providers offer a variety of services based on the user requirements. A vessel working primarily in the North Sea might subscribe to a service giving availability from reference stations located around the North Sea area. Likewise, another vessel may avail itself of a Gulf of Mexico service, or an offshore Brazil service. For vessels working worldwide, a wide-area service is available with a much larger choice of reference station. Reference stations tend to be located in areas where offshore oil and gas activity is common, as the services are purely commercial.



Differential corrections are transmitted to the vessel from a central control station known as the hub. Individual reference station data is centralised at the hub, then after a comprehensive QA analysis, the data is transmitted to subscribing vessels in the field. The transmission medium is often Inmarsat or alternatively the Spotbeam communications satellites. DPOs must be able to contact the hub administrator at any time; this is their helpline in the case of a malfunctioning system, loss of differential signal, etc.

Differential services are a contracted service from a service provider, and must be paid for on a subscription basis. An occasional problem arises when the subscription inadvertently lapses, resulting in the immediate loss of all differential corrections from that time!

A vessel using network differential corrections will thus have a resolved position from the GPS system contributed to by the corrections from the nearest three reference stations. A least-squares analysis of the data will yield the most probable position from the incoming data.

6.8.3 Accuracy and quality of DGPS data

Accuracy of DGPS is dependent upon a variety of factors. Already mentioned are the number of SVs available, and the HDOP value. When using differential corrections, a number of other factors arise. The time taken for corrections to be observed, processed, relayed to the hub, further processed, thence transmitted to the vessel via Inmarsat may be considerable. A factor known as age of data (AoD), or latency, comes into play. AoD values affect the accuracy of the positioning significantly. If AoD values are in the 3-6 second bracket, then the data is relatively recent and thus valid, but if the AoD is showing as 15-20 seconds, then the data becomes suspect. AoD is a displayed guality factor.

Part of the AoD argument is the distance from reference stations. The more distant the reference stations, the lower the value of the received corrections. This is not just an AoD consideration; the errors in the system vary rapidly with time and also with geographical location. Received corrections refer to the reference station location, not the vessel location. Data is of greatest value if reference stations are within about 400 km. If the vessel is thousands of kilometres away from the reference stations, the data becomes suspect. One reason for this is that, at those distances, there are fewer common SVs (satellites visible from both the vessel and the reference station) available.

In general, if reference stations are within around 400 km, and differential corrections are available, then DGPS should yield accuracies of 1 -3m.

A significant vulnerability of DGPS as a PRS is the likelihood of loss of differential corrections due to line-of-sight blocking, caused by the presence of large nearby structure. If the differential signal source is Inmarsat, then a continuous line of sight to



the Inmarsat satellite is vital. If the observer is in high Northern latitude, then the Inmarsat satellite is located low down toward the observer's southern horizon. If the observer's vessel is working North-face of a large platform, then there is a predictable loss of corrections. This factor, in combination with the multipath problems referred to earlier may well render the use of DGPS non-viable. This should have been allowed for in the planning of the operation.

6.8.4 Selective availability

Selective availability (SA) is a deliberate downgrading of the accuracy of the commercial C/A code GPS data for civilian users. SA was applied on a continuous basis by the US Dept of Defense, the result being that the stated accuracy of GPS for commercial users was no better than approximately 100m. In May 2000 it was announced that the SA was to be 'set to zero' for the foreseeable future. The US Dept of Defense has reserved the right to re-apply SA at any time, but that warnings to that effect will be promulgated.

6.8.5 Ionospheric interference and Dual-frequency DGPS

GPS data can be severely degraded in equatorial regions by the effects of solar activity. The effects are more severe during times of 'Solar Max' - the periods of greatest solar radiation witnessed by the greatest number of sunspots. Solar radiation repeats on an 11-year cycle, the most recent maximum being 2002/03. Effects on GPS reception were quite severe. Total loss of GPS data for up to 12 hours daily was not uncommon. Other effects were reduced availability of SV signal and larger errors within the system due to heavily distorted and delayed signals.


Chapter 6 67


To counter these effects, dual-frequency differential services were initiated. As mentioned earlier, GPS uses two channels; L1 and L2. L2 decryption is not available to commercial users, but user systems are able to monitor the L2 carrier as well as the L1 code. A dual- frequency service provides differential corrections based upon both channels. Dual frequency receiving equipment is necessary on board, and the receiver must access corrections from dual-frequency reference stations. This is a more expensive service, and only provides a partial solution to the 'sunspot activity' problem. Currently (2007) we are approaching Solar Minimum and the problems are negligible, but the next Solar Max is due in 2013.

6.8.6 Quality monitoring of DGPS

The International Marine Contractors' Association (IMCA) has published the results of a study into quality monitoring of DGPS data. This study was initiated when it was apparent that DPOs were not necessarily able to interpret the quality of the data provided. IMCA proposed a DQI, or differential quality index. This index provides a simple, single-digit quality number indicating quality; the larger the number the better the quality of the data. A DQI of zero is a failed solution (no GPS), while DQI = 1 is a raw GPS solution with no differential corrections. DQI = 2 to 9 are increasing quality, redundancy and accuracy of the differentially corrected solution. The DQI can be monitored by the DPO as a direct indicator, also automatically by the DP system as a warning, alarm and rejection criteria for the DGPS position reference input.

Other quality indicators are: • Number of SVs • HDOP • Presence or absence of differential corrections • Number of reference stations in use • Distance to reference stations • Age of data • Selective availability on or off • Single or dual-frequency differential • DQI (see above)

6.8.7 Specialist DGPS applications

Some DP-capable vessels are more concerned with a relative mode of positioning. One such group are the shuttle tankers which tandem-load from a floating FPSO (floating production storage and offtake unit). Since the FPSO is turret-moored and weathervaning, the reference position for the tanker is a moving point. A conventional DGPS installation would be of limited value; the tanker needs to match the FPSO movements. Thus relative PRS are needed, and one such is Relative GPS. In this system,



the FPSO is equipped with GPS and gyro, and the data is telemetered across to the tanker via UHF link. The tanker system also monitors position. Any positional errors will be identical to those from the FPSO, and thus cancel out. The information yielded on the bridge of the tanker is range and bearing between the stern reference point in the FPSO, and the bow coupler position of the tanker. This data is used as a direct PRS into the tanker's DP system. Commercially available Relative GPS systems include the Seatex DARPS system, and the G-Vec system from MDL.

In some specialist vessels, there are great difficulties in obtaining good antenna locations for the GPS and differential corrections receivers. Examples include the large construction

vessels and crane barges. In these vessels, moving crane booms are liable to produce major shadow and multipath problems. A system has been developed (Multifix) which integrates the input from multiple GPS receiver/antennae, and multiple differential correction receiver/antennae. The data is correlated in a processor which is also fed with antenna locations relative to the vessel CG. Antenna locations may be mobile, as


Chapter 6 69


if mounted upon crane booms or 'A' frames, thus offset data is needed relating to vessel heading, roll and pitch, as well as crane boom attitude. The result is a very stable and redundant position input.

6.8.8 Merits and Limitations of DGPS as a PRS

The advantages and disadvantages of DGPS as a position reference for DP can be summed up as follows:

Advantages: • GPS is a globally referenced system (Datum WGS 84) • GPS is available world wide 24/7 • The equipment is (relatively) low-cost and simple to operate • GPS is free of charge • DGPS is of sufficient accuracy for DP PRS

Disadvantages and limitations of DGPS • Differential services must be contracted and paid for • Major multi-path problems exist close to platform or other structures • Loss of SV data and/or differential corrections close to nearby structure • Poor SV geometry, especially when SV availability is poor; high HDOP • Poor quality of positioning during periods of high solar activity

In general, DGPS has become established as the most popular and heavily used PRS for DP purposes. While accurate and convenient, however, it must always be used with care and vigilance.

6.9 Laser-based position references

A number of laser-ranging systems are available for use as PRS for DP. Among these are the Fanbeam system from MDL, and the CyScan system from Guidance Ltd. Laser systems are very popular and widely fitted in a variety of DP vessels.

In a laser system, a laser beam of white light is projected horizontally, spread out to a vertical 'fan'of approximately 22°. This beam of light is pulsed at a very high frequency. The projector, or scan unit, is placed in a suitable masthead location with a good view of the horizon. When in operation the Scan Unit continuously scans horizontally through approx 20° left and right. A reflective target is located aboard a nearby platform or other fixed structure. In operation, reflections obtained from the process are detected and processed. The time-lapse determined is a direct analogue of the range to the reflector. The azimuth of the reflector is determined from the bearing of the scan unit. This is a



This system is thus best described as a white-light laser radar. Reflectors placed upon platforms may be reflective prism stacks, cylindrical reflective units or vertical reflective panels. These reflectors may be permanently mounted units, or placed on location for the duration of a specific job. Other than the need to keep them clean, there is no maintenance, power supply or other attention necessary. Laser-based systems may be configured to operate in multi-target mode, giving independent range/bearing data from two or more targets.

In operation, once the operator has initiated the system and correctly identified and acquired the designated target, the system runs automatically giving a very accurate range and bearing (typically 0.5m and 0.5°). The requirement to maintain an 'eye-safe' rating means that laser power must be limited, and this effectively limits the maximum range of operation. Laser based systems are generally limited to a range of one to two kilometres.

A laser-based system as described is of necessity a relative reference, giving range and bearing from the reflector. It does not directly give global co-ordinates, simply range/ bearing data from a fixed point. It can also be used as a relative reference against a slowly moving target, eg a shuttle tanker engaged in offtake operations from an FPSO. In this scenario a laser system may well be used in conjunction with other relative PRS such as DARPS (see above).

Because a laser system determines bearing relative to ship's head, it is necessary to integrate a vessel gyro heading into the position calculation. Thus, the accuracy of such a system is dependent upon gyro accuracy and stability.

Laser systems are very simple to set up and operate, and provide accurate positional data, THE

NAUTICAL INSTITUTE

Chapter 6 71


but they have a number of vulnerabilities. It is necessary to maintain an unobstructed line-of-sight to the reflector. The 'signal' will degrade if the lenses or reflectors become dirty, salt encrusted or misted. Similar degradation will occur if the atmospheric visibility is reduced, or if funnel smoke drifts into the line-of-sight. It is necessary for the DPO to monitor the signal strength values, and thus be forewarned of a slow degradation in performance. A further problem is stray reflections from nearby reflective targets. Platform personnel walking past behind the reflector wearing reflective vests are likely to seduce the laser! The system can become 'blinded' by very bright sunlight illuminating a white-painted bulkhead immediately behind the reflector.

The advantages and disadvantages of laser systems as position reference for DP can be summed up as follows:

Advantages: • High accuracy • Quick and simple to set up and operate • Low-cost system

Disadvantages: • Limited range capability • Line-of-sight requirement • Degradation of data when lenses and/or reflectors dirty or misted • Signal link may be seduced by platform personnel wearing reflective vests

6.10 Hydroacoustic position reference

The medium of underwater acoustics is used for a great variety of functions, including echo sounding, survey sonar, fish-finding and military application sonar. Hydroacoustic position reference (HPR) is also a major position reference for DP purposes. A large number of DP vessels are equipped with HPR.

In addition to position-reference, acoustic techniques may be used for monitoring and control of underwater functions, thus a drillship might be using a long baseline acoustic system for positioning, and the system will incorporate acoustic control and monitoring of wellhead functions as a backup to hard-wired control. Similar acoustic telemetry and monitoring may be used in offshore loading terminals and submerged turret installations.

When used as a PRS for DP, two main principles are current, with some variations. The two main principles are ultra-short baseline (USBL) HPR and long baseline (LBL) HPR. Both of these methods involve the transmission of interrogation acoustic pulses from a transducer mounted on the vessel's bottom, to be received and retransmitted from sea-floor transponders. Reception of the transponder replies allows positional data to be determined.



6.10.1 Ultra-short baseline HPR

Ultra-short baseline (USBL) principles are used in the majority of acoustic PRS applications. This principle is sometimes known as SSBL, or Super-short Baseline; the terms are synonymous. In the USBL system, a single transponder is laid upon the sea floor. This transponder is battery-powered, and is programmed to respond to interrogation 'pings' at particular coded frequencies. The ship is equipped with a transducer, typically located at the bottom of a probe or 'pole' projecting 4m to 5m below the vessel's keel level. Acoustic pings are generated within a transceiver unit, and transmitted from the transducer, travelling downward through the water. These are detected by the transponder, which replies after a known fixed time interval called the turn-around delay (TAD). The reply is received at the transducer and passed to the transceiver for processing. The time-lapse measured determines the slant range to the transponder. The other piece of data required is directional; the transducer is directionally sensitive, able to determine an accurate, three-dimensional direction of the reply path.

This principle enables position-reference to be obtained from acoustic communication between the vessel transceiver/transducer and a single transponder placed upon the sea floor. The accuracy of position measurement is a variable factor, but in general USBL systems typically yield accuracies of around 1% to 2% of water depth. The acoustics themselves may well be of greater accuracy than this, but other factors intervene. Since the measurement frame is vessel-referenced, the measured data must be corrected for vessel attitude, ie heading, roll and pitch. Any errors in gyro or MRU (motion reference



unit - roll and pitch data) will directly transfer into the error budget of the HPR system. Nevertheless, in water depths of up to about 250m, USBL techniques constitute a useful PRS for DP. The horizontal range of such a USBL configuration is typically between 50% and 100% of the water depth. At greater horizontal ranges than this, significant errors begin to appear.

Although communication with only one transponder will give position-reference, greater reliability is obtained from the deployment of multiple transponders. Each of these will be interrogated in sequence, giving separate sets of position-reference data. The DP system will treat each transponder return as a separate PRS, but it is important that the DPO treats this array as a single PRS. He cannot count two transponders being interrogated by the same HPR system as two independent PRS; this configuration can only be considered to be one PRS as interrogation and reply processing is via a single transceiver.

6.10.2 The long baseline HPR system

In deeper water the accuracy of USBL systems may be insufficient for use as a PRS for DP. There are many deep-water developments worldwide, and alternative solutions are needed. One such is the long baseline principle.

The accuracy of USBL is limited by the resolution of the angular measurements at the transducer head. In the long baseline system we do away with the need to measure such angles.

In a long baseline system, acoustic communication is obtained between a transducer on the bottom of the ship, and a calibrated array of transponders located on the sea floor. These transponders are all located to yield an acoustic ray-path between 20° and 40° to the vertical. All transponders are interrogated by a common transmission from the vessel transducer. Each transponder replies after known, fixed, turn-around delays. Each reply arrives at the transducer at a different time, allowing a slant range to be determined for each transponder. Since the transponder locations are known, triangulation of the slant ranges allows determination of the vessel position.

Note that, with the LBL configuration, an array of transponders (eg four) yields a single position. The array of transponders must be laid, tested and calibrated before use. This operation may be contracted out to a survey vessel, the resultant data communicated to the surveyor in the DP vessel prior to arrival on the worksite. The array configuration may then be entered into the vessel's HPR system complete with geographical co-ordinates of the transponders. If this is the case then the LBL system is effectively globally referenced.

Since the LBL system does not involve the measurement of vertically-referenced angles, the accuracy of position measurement is not affected by vessel roll and pitch. The accuracies obtainable from LBL systems are in the order of 0.2% to 0.4% of water depth.



In a typical LBL configuration for a deep-water drillship, redundancy may be provided by operating three separate and fully independent HPR systems, each one using a separate transceiver and transducer, and each interrogating four or five sea-floor transponders. The total array on the sea floor may be between 15 and 20 transponders forming a 25° cone around the spud-in location for the vessel.

In any HPR system, the reference point being navigated is the transducer on the bottom of the vessel. The DP system must therefore be configured with offset values of the transducer from the CG allowing all position reference to relate to this CG datum.

6.10.3 Transponders and transponder deployment

The transponders deployed may belong to the ship and be deployed on a downline, or they might well be units belonging to a third party, eg a transponder laid by a pipelay barge marking the end of the pipeline, or a transponder marking a wellhead. If using such a third-party transponder, then care must be taken as the condition of the transponder may be unknown, particularly battery condition.


Chapter 6 75


Various types of transponder are available from the HPR system manufacturers. A standard transponder simply gives positional data, but many transponders are fitted with a variety of sensors giving further data. Transponders may be equipped with sensors such as temperature, density, depth, compass and inclinometers. Information from these devices may be displayed or incorporated in various ways. An inclinometer transponder may be used to monitor Riser angle in a drilling operation. Riser angle is a major factor in determining required position for the drillship.

Transponders may be deployed in suitable locations using a downline from the ship. A common method is to attach the transponder (complete with buoyancy float) to a mudweight, and to lower it to the sea floor on a wire. Once located on the sea floor, the wire can be slacked 50 or 100m to give freedom of movement to the vessel. If the vessel movements result in the transponder becoming out of range, then it is a simple matter to 're-spot' the transponder by picking up the wire and redeploying the transponder. The transponder must be de-selected from the HPR system before doing this, of course. Another method of transponder deployment is to buoy-off the downline. This may be preferable, but gives difficulties in transponder recovery. Another method is to lower the transponder by downline fitted with an off-load hook, allowing recovery of the downline. The transponder will be of the type equipped for acoustic-command release of the mudweight. Sending the release command should result in the transponder coming to the surface for recovery by boat.

Specialist transponders are used in some applications. The inclinometer transponder has been mentioned above. Another acoustic function is the tracking of ROVs and sea- floor tracked vehicles. It is possible to equip such vehicles with standard transponders, but a problem arises with the noise level at the vehicle. This may preclude the efficient reception of the interrogation commands. The alternative is to interrogate the transponder via hard-wire, ie the ROV umbilical. This is known as 'electrical triggering' as opposed to 'acoustic triggering'. The device is known as a responder, and it solves the problem of noise interference at the ROV location. It must be noted that, in most circumstances a responder located on the ROV is not actually used as a position- reference, as it is a moving point. In this case the responder is designated mobile (instead of fixed) within the HPR system, the DP system thus simply displays it on-screen and does not include it in the PRS pool.

6.10.4 Underwater acoustics

Communication with underwater transponders is by its very nature a slightly hit-or-miss process. The operator (or HPR system manufacturer) has no control of the quality of the water column. There are many factors which may have detrimental effects upon acoustic communication. The biggest enemies of underwater acoustics are noise and aeration, and by far the greatest noise source in the DP theatre is propellers and thrusters. It may also be stated that controllable-pitch propellers and thrusters are orders of magnitude noisier than



fixed-pitch units, mainly because controllable-pitch units are usually running at full rpm all the time, while fixed-pitch propellers are often running at relatively low revs. All DP vessels are equipped with an array of propellers and thrusters, thus noise is ever-present. The DPO must take steps to avert noise interference by locating transponders away from thrusters and thruster wash. In this respect it is necessary that the DPO predicts the pattern of noise and aeration in the light of changing weather conditions and tidal conditions. It may be useful to deploy two transponders in different locations; one for use on the flood tide, the other for the ebb, switching between the two at the appropriate times.

The accuracy of positioning using acoustics is dependent upon the use of a correct value for sound velocity. An acoustic system may have a default sound velocity of typically 1480m/sec, and this may well be used uncorrected. If a more accurate value is available, it can be entered into the system, while it is also possible to download a sound profile. A sound profile is obtained from a 'T/P dip' in which a bathythermograph is lowered to the sea fioor and recovered. The device has recorded the water column profile of density and temperature. This data yields a sound velocity profile which can be used by the HPR system.

Typically, HPR systems use acoustic channels which incorporate frequencies within the 1 8 - 32 kHz band. Lower frequencies than these will suffer interference from noise, while higher frequencies have the penalty of limited range performance.

6.10.5 HPR System Hardware

A typical HPR system consists of a transceiver, an operator terminal, a transducer hull unit, plus the sea floor transponders. The operator terminal may be integral with the DP operator console, and is where the DPO will control and configure the HPR system from.



A position-plot display shows the vessel and transponder deployment, while Microsoft Windows dialogues and menus allow full operation of the system.

The transducer will be mounted into a hull unit, which allows deployment and retraction of the pole. A sea chest enables the hull watertight integrity to be ensured when the pole is raised. A motorised raise and lower function can be operated locally or from the bridge. It is essential that procedures are followed ensuring that the sea chest valve is open before deploying the pole, retracting the pole after DP operations are concluded before getting underway, and closing the sea chest after retraction. With the pole retracted (and the sea chest closed) an inspection panel can be removed allowing access to the retracted transducer head.

Transducers will differ in characteristics. A typical modern unit is the HiPAP 500 transducer from Kongsberg Maritime, one of the largest suppliers of acoustic systems. This transducer unit is a golf-ball unit fitted within a 500mm sea chest. It incorporates 241 acoustic elements. When initiating acoustic communication with a transponder the unit will search at a 'wide-beam' configuration, the unit thus has a 100° beam-width around 360° of azimuth. This covers the entire underwater hemisphere. When a transponder is detected, the unit switches automatically to narrow beam focusing onto the position of that transponder. If the transponder moves relative to the vessel, the transducer automatically tracks the transponder.



6.10.6 Hydroacoustic-aided inertial navigation

In deeper water, long baseline acoustics suffer a number of problems. The travel-time of the acoustic energy may mean a low update rate for the position, while poor acoustic conditions in the water may further reduce update rate. Under these conditions the HPR system may become less reliable and accurate. A possible remedy for this problem is given by incorporating an inertial measurement unit and processor within the HPR system. Kongsberg market such a system as hydroacoustic-aided inertial navigation (HAIN). A characteristic of inertial navigation systems is very low noise but significant drift over time. Conversely, HPR systems may suffer high noise and occasional data dropout. Combining these two principles allows the inertial system to provide infill positioning during periods of dropout or low update rate of the acoustics. This enables greater reliability of the system in deeper water situations.

6.10.7 Multi-user acoustic systems

A further concern in deep water fields is acoustic saturation. This is brought about by the limited variety of position references available in deeper water, added to which is the fact that nearly all floating installations will be using a form of DP. Everybody will be competing for acoustic channels, and acoustic interference is likely. Both the USBL and LBL principles are only applicable to single-user operation. To provide a solution to this problem, a number of manufacturers of acoustic systems have developed multi-user acoustics.

An example of such a multi-user system is the Nautronix NASNet, or Nautronix acoustic subsea network. This system is showing promising development, and consists of a calibrated local-area array of subsea acoustic stations. Each station transmits time- referenced acoustic signals which are detected by hydrophone in the vessel. A comparison between time of transmission and time of reception is enabled, giving a range to the subsea station. The system is, in effect a subsea GPS. The vessel is passive, ie listening only, so the system may be used by any number of vessels without saturation. Also, since the subsea stations are globally calibrated, this system is suited for globally- referenced positioning (UTM co-ordinates).

6.10.8 Merits and limitations of HPR as a PRS

In general, HPR has proven to be a useful position reference system, and is widely fitted in DP-capable vessels. It does have a number of limitations, however. One of the most significant limitations relates to operation in shallow water. Acoustics perform poorly in shallow water due to the increased noise factors coupled with the difficulties in obtaining a suitable location for the transponder. It must also be mentioned that HPR is a complex system to operate, and that operator training is a necessity.



The advantages and disadvantages of HPR as a position reference for DP are:

Advantages:

• HPR is a versatile system, with many functions additional to position reference • HPR is an accurate position reference system • The system is vessel-centred, ie no dependence on third parties • Ability to track moving targets (eg ROV) • May be configured as a global reference • Various configurations available: USBL; LBL; multi-user systems

Disadvantages and limitations of HPR: • Expensive and complex • Limited range capability • Suffers degradation from interfering noise and aeration (particularly thrusters) • Also degraded by water layering, turbulence and impurity • Accuracy affected by presence of large underwater structures • Ineffective in shallow water conditions • Can suffer interference from other HPR users • Operator training required



6.11 Microwave-based position reference systems

A number of PRS based on microwave transmission are available. One of them, Artemis, has been established as a PRS for DP for many years, but has become less popular in recent years as other positioning systems have become available. Nevertheless, Artemis still finds favour as a relative PRS for tanker offtake operations. Other more recent developments include FMCW microwave systems such as the RADius system from Kongsberg, and the RadaScan system from Guidance Ltd, both used for short-range relative positioning.

6.11.1 FMCW microwave systems

A number of newly-developed microwave systems have arrived on the market in recent years, including those mentioned above. These systems have been developed in response to the increased need for short-range PRS, particularly those operating in a relative mode. Many platform supply vessels, anchor-handling tugs and other support vessels are routinely being equipped with DP capability. Because of the operational proximity of nearby platforms, these vessels are frequently specified as equipment Class 2, with a requirement for three PRS. If the support vessel is working close to an FPSO, for example, the vessel must match the FPSO movements, and there is a requirement for relative PRS.

The RADius system uses an interrogator located high up in the vessel. In the supply vessel mode, the Interrogator will be aft-facing with a good horizon view; the Interrogator has an operational arc of 90° horizontally. The interrogator is a flat-panel phased array FMCW radar unit approximately 1m square, mounted vertically (typically bracketed to monkey-island handrails facing aft). Similar-looking but smaller units called Transponders are mounted at appropriate positions on the platform or FPSO. The operation of the system is via a PC workstation. In operation it is simply necessary to enter the three-digit identification code of the transponder, which, if within the 90° field of view and range, will be automatically acquired. The system can operate simultaneously with two or more transponders.

The field of view can be extended to 180° by the installation of two Interrogator panels configured 90° to each other. Further interrogator installation allows extension in azimuth coverage to the full 360°. The main drawback is the limited range capability; full accuracy in range and bearing (0.5m and 0.5°) is only obtained up to a range of 200m. At greater ranges, bearing resolution is poor or nonexistent.

The RadaScan system is a similar FMCW radar, but utilising more conventional components. The vessel antenna is a conventional parabolic scanner/horn unit, rotating at 180 rpm, mounted in a 1.2m diameter dome. Transponder units are located on the nearby platform or FPSO. These transponders reflect the transmitted signal, introducing a coding, which allows reliable identification of the reflected data using digital signal processing techniques. Its working range is up to 1000m, with accuracies of typically 0.5m.



6.11.2 The Artemis system

As mentioned above, use of Artemis today is much less widespread than in earlier years. During the 1970s and 1980s Artemis was one of the few 'mainstream' PRS for DP-capable vessels. Alternatives were HPR and taut wire systems, both of which are underwater systems. Use of all-underwater PRS is unwise due to the risks of common-mode failure. Artemis was the only surface-based PRS and was thus widely used.

The Artemis system consists of two microwave transceiver/antenna units; one located at a fixed location (platform), the other (the mobile unit) located aboard ship. The antennae are double-slotted waveguide units physically resembling radar antennae. A continuous microwave link is established between fixed and mobile units, with the two antennae automatically tracking so as to continually face each other. Range measurement is obtained by a signal initiated at the mobile unit, received and re-transmitted by the fixed unit, and the reply received by the mobile. Observed time-lapse is thus an analogue of the range, and measured by the mobile (vessel) unit. Bearing is measured by the fixed unit as a value determined by antenna shaft-encoder. Since the fixed unit is, indeed, fixed in position the measured bearing is not dependent upon vessel gyro accuracy, nor is it degraded by vessel movement. The fixed unit antenna is calibrated for true bearing. The bearing thus determined at the fixed unit is transmitted to the mobile as part of the reply.

In practice, Artemis position reference is reliable, accurate and has long range ability. For DP purposes the range is 5km, with accuracies of 0.5-1.0m within this range. Artemis is operational at longer ranges, but the accuracy deteriorates. Artemis has four working channels within the 'X' band, and security is provided with a three-digit 'Address Code'- the same number configured at fixed and mobile stations to prevent interference from other Artemis installations.

The system must maintain a line-of-sight between fixed and mobile antennae. This meant that Artemis-equipped platforms needed two or three permanently-mounted fixed units to cover the whole sea area. During the 1980s, many North Sea platforms were fitted in this way.

A number of drawbacks are evident with the system. In the set-up phase, it is necessary to involve platform staff. Further, no control could be exercised over the fixed unit. If it was moved or shadowed, or if a problem arose with its power supply, then the vessel crew had no control over this. Artemis suffers interference from nearby X-band (3cm) radar, so this must be isolated in own vessel and those within a mile or so.

The vertical beamwidth performance of Artemis is approx 22°, thus there exists the possibility of loss of microwave link at short ranges if the elevations of fixed and mobile antennae are widely different. Fixed and mobile antenna elevations also generate 'dip zones'- specific ranges at which the signal strength drops to zero. This is not a significant problem as generally dip zones occur at ranges greater than the 5km maximum. The



only other drawback of Artemis is interference from radiated heat; if the line of sight is interrupted by flare-stack exhaust, or platform generator exhaust, then the link may become unreliable.

Nowadays, very few mainstream DP vessels are equipped with Artemis. The exception to this is in the offshore offtake application, where shuttle tankers load from offshore loading terminals or FPSOs. Nearly every such installation is equipped with either an Artemis fixed station, or a simpler Artemis beacon. The latter is a microwave transponder giving a reply but no bearing. In beacon mode, the bearing is measured by the mobile unit at the vessel end. This is a particularly good solution, as an Artemis system gives a relative position reference (and the FPSO is a slowly moving target) and the beacon does not require manual intervention to set up.

6.12 The taut wire position reference system

Taut wire systems have been in use since the earliest days of DP, indeed, the first DP vessel Eureka of 1961 had taut wire position reference. Generally taut wire systems are less popular and numerous than hitherto, but are still being fitted on occasion. As with the Artemis system, taut wire technology has been overtaken by alternatives such as



DGPS and Fanbeam.

A taut wire system consists of a davit or A-frame located on deck, usually at the vessel side. A weight of up to 500kg is lowered to the sea floor. Once located, the wire is placed under continuous tension, paying in and out to compensate for vessel movements. The position of the vessel is thus defined by the vertical angle of the wire, and the length of wire deployed. Wire length is read into the DP system at the moment the weight lands and the system switches to 'tension' mode (known as 'mooring' in the Kongsberg/Bandak taut wire). Wire angle is determined from the angular offset of the Gimbal head at the end of the A-frame through which the wire is deployed. These angular measurements are transmitted to the DP system as voltages from gimbal potentiometers.

The taut wire system is robust and simple to operate. Often the vessel will be fitted with two units; one on each side of the ship. The accuracy of the data is dependent upon a number of factors, chiefly water depth. A typical taut wire system has a depth capability of 350m, with an accuracy of typically within 1-2m up to about 200m depth. At greater depths the accuracy starts to deteriorate, and is dependent upon sea state and strength of tide.

The taut wire system requires a bit of planning for successful operation. Maximum scope (horizontal range) is determined by wire angle, limited to 28° or 30°; horizontal range is thus related to water depth. This means that in very shallow water the horizontal range may be only a few metres. Deep-water capability is, of course, limited by the wire length.

DPOs must carefully check the sea floor configuration to ensure that they do not lower the weight onto sea floor hardware, or endanger the operation the vessel is supporting (divers do not like having a taut wire weight lowered onto their heads!). It is normally advisable for the taut wire in use to be that on the side of the vessel away from the operation, eg if a vessel is working port-side to a platform, then the starboard taut wire will be used. With the vessel manoeuvring in the approach stage of the operation, it might be necessary to re-spot or reset the taut wire one or more times. This involves deselecting the taut wire from the DP, lifting the weight a few metres, waiting for it to swing to the vertical, then redeploying it.

When the vessel is engaged in underwater operations (dive or ROV support, for example) it is vital that the divers or ROV pilots are informed of the presence of the taut wire(s) and of their positions. Care must be taken in re-spotting the taut wire if divers or ROV are in the vicinity, and they must be informed of this.

In a conventional monohull vessel, with the taut wire located at the vessel side, there is a deployment limitation on the bilge keel. The 28° or 30° wire angle scope is obviously not available in this direction. On the DP system, a display view will show taut wire status and maximum operational areas.


Strictly a mechanical system, the taut wire requires regular maintenance, and it is necessary to carry spare wires and weights; wires occasionally break and weights are occasionally lost! A recommended move is to crop the wire at intervals. If the vessel is working continually using taut wire, then the wire will become worn where it spools on the drum and gimbal head. Cropping the wire by 10m at approximately two-week intervals refreshes the area of wear. It is necessary to record the crop and the remaining wire length.

Other types of taut wire system may be encountered. A moonpool taut wire is available for vessels which need to work in areas of surface ice. This type of system works directly out of the vessel bottom, the unit being low in which the wire is led directly to the nearby platform and secured. Both these types of unit are nowadays very rarely seen. A further alternative is the deadweight taut wire, in which the tension capability is provided by a suspended weight. This system has the benefit of not requiring mains power for operation, thus will continue to give a position during a blackout.




The advantages and disadvantages of a taut wire system as a position reference for DP can be listed as follows:

Advantages: • Accurate • Reliable • Quickly and easily deployed • Not dependent upon third parties

Disadvantages: • Vulnerable to fouling by ROV or other underwater operations • Limited horizontal range of operation • Limitations in very shallow and very deep water • Mechanical in nature, regular maintenance required • Need for spare weights and wires to be carried



6.13 Positioning reference system redundancy

Although more comprehensive guidance on PRS redundancy is contained in Chapter 3 of this book, a few points may be usefully reiterated here:

For operations at equipment Class 1, two PRS are required. It is never good practice to rely on one PRS only, as the system is vulnerable to PRS freeze. Freeze causes the data to simply repeat, no longer indicating the vessel position. This is sometimes referred to as a 'perfect position reference' (a rather strange term for one that has failed!). If the vessel were displaced from her set-point position at the time of the freeze (by even a small amount), the DP system will try to make up the discrepancy by applying thrust, to no apparent effect, and the vessel drives off!

Equipment Class 2 and 3 operations both require a minimum of three PRS. This is to allow the function of voting. The stated requirements are that the three PRS should be 'independent', but it would be better if the three are also different. If a mix of PRS is deployed such as DGPS and two taut wires, there is the possibility of common-mode failure, in which an identical problem affecting the two taut wires can result in the DGPS being out-voted. Any two-and-one configuration is vulnerable to this scenario.

It is good advice not to rely totally on sub-sea PRS (eg taut wires and HPR only) as common-mode failure possibilities exist.

If an operation is deemed to require a spread of three PRS (Class 2 or 3), then it might be advisable to deploy four. This is not really overkill, as if three are deployed, and one is lost the vessel is immediately degraded, and may have to suspend operations. If, on the other hand, four PRS were deployed and one is lost, then the vessel is still operational. DPOs should always have in mind their response to the loss of a PRS and check what back-up systems are available, and what steps are required to deploy an additional PRS.


Chapter 7

87 Propulsion and thrusters

Chapter 7 Propulsion and thrusters

7.1 Modes of propulsion

When considering the whole-ship system in relation to the DP capability, it is necessary to include all propulsion systems as part of the DP ability. Propulsion systems include the vessel's main propellers and rudders, as well as installed thrusters.

As the majority of DP vessels are of a diesel-electric power configuration, most propellers and thrusters are driven by AC electric motors. In the past this has meant that propellers and thrusters were configured with controllable-pitch (CP) propellers, however, in more modern installations fixed-pitch (FP) propellers are fitted in conjunction with AC variable speed drives (VSDs). This type of installation allows full control of shaft direction and speed, thus obviating the need for CP propellers.

A DP vessel must have adequate thrust capability to provide control of surge, sway and yaw motions. A minimum of three thrusters is required for effective DP, and more are invariably installed to provide the necessary level of redundancy.

In general, three types of propulsion unit are found in DP vessels: main propellers (including associated rudders), tunnel thrusters, and azimuth thrusters. A small number of vessels are equipped with Voith-Schneider propellers or Jet thrusters.

7.2 Main Propellers and Rudders

A vessel may be configured with a conventional propeller/rudder combination, optimised for transit mode; this may be single or twin screw. The DP capability will use these propellers in addition to an array of thrusters providing sway and yaw forces. Main propellers may, of course, be CP or FP. Rudders are usually DP-active, ie fully integrated into the DP control function and used as part of the manoeuvrability of the vessel. With a twin-screw configuration, main engines may operate independently, or in a 'push-pull' mode. This latter configuration has one propeller running continually ahead, the other astern. Surge force is obtained by varying the thrust between them, while stern sway



force is controlled by use of differentia! (or 'split') rudder control.

A disadvantage of conventional propulsion is that in DP the thrust and power demands are much less than that required for transit, so if main propellers are diesel-powered, those diesels are likely to spend long periods running on light loads.

7.3 Azimuth Thrusters

Many DP vessels are fitted with azimuth thrusters as part of their capability. In some cases the vessel's main propulsion consists of twin (or triple) azimuth thrusters at the stern. In some vessels (eg drilling rigs) all thrusters are of the azimuth type. Azimuth thrusters are very widely fitted and popular, but they are complex, and can be troublesome.

A conventional azimuth thruster consists of an inboard horizontal drive shaft powered by electric motor or diesel engine. A bevel gearbox turns the drive vertical, passing out through the hull via a deep sea seal. A further bevel box in the hub turns the drive to the horizontal again. The propeller itself may be FP or CP. Thrust direction is controlled by steering the unit using a hydraulic steering actuator.

Units of this type are very versatile, giving thrust ability in any direction at any force value ranging from zero to maximum. A number of designs and configurations are offered by propeller manufacturers. Some azimuth thrusters are fixed in location, while others are retractable into the hull. Some designs of retractable azimuth thruster retract vertically,


Chapter 7 89 Propulsion and thrusters

while others pivot through 90° to retract. It may be that retraction is necessary in shallow waters, but it must be remembered that, if retracted, then the thrusters cannot be used! Azimuth thrusters are particularly vulnerable to grounding damage.

DPOs must be familiar with the action to reverse thrust. Some azimuth thrusters will only operate in the 'forward' direction, while others (particularly CP) are able to operate in an 'astern' mode up to a limited thrust value. After reaching this limiting value, if increased thrust is commanded, the thruster will rotate 180° in azimuth in order to work in the ahead direction. The DPO must be aware of what is happening under these circumstances.

A variation on the azimuth theme is the azipod thruster. These units are becoming widely fitted as main propulsion in a variety of vessels. In an azipod unit the VSD motor is mounted in the unit's hub, driving a fixed pitch propeller via a very short propeller shaft. The whole unit rotates about a vertical axis to provide steering and DP control. Several modes of operation are provided. With the vessel underway in transit, the pods are in transit mode, ie providing main propulsion with steering control limited to about 35°. In manoeuvring or DP mode, twin pods may be configured such that one unit is locked fore-aft, while the other is locked port-starboard. A further operational mode has both pods under full azimuth control.

7.4 Tunnel Thrusters

Widely fitted in DP vessels, tunnel thrusters are invariably fitted at bow and stern. As with other types, propellers may be CP or FP but are directionally fixed athwartships. Their efficiency is affected by a number of factors. Depth of immersion is sometimes an issue at light draughts, especially if there is a moderate to heavy sea running. Vessel pitching can lead to aeration or 'ventilation' which reduces efficiency drastically. The design of the tunnel is important, the longer the tunnel the less efficient the thruster. Also, as is well known, tunnel thrusters are badly affected by vessel headway or sternway.

An ingenious solution to a particular problem is provided in the retractable azimuth/ tunnel thruster. This is an azimuth thruster which is retractable into the hull, but once housed it lines up with a tunnel, and can still operate in a tunnel mode.

7.5 DP Control functions for thrusters and propellers

The DP system must be able to control all propellers, rudders and thrusters, ie send commands and receive feedback of values of rpm, pitch, azimuth and rudder angle.

In conventional manoeuvring mode, thrusters and propellers are controlled directly from the propulsion panel on the bridge. This may be totally independent of the DP system. On this panel will be a transfer switch with, at its simplest, two positions; 'manual' and 'DP'.


90 Chapter 7


A third choice may be 'joystick'. Typically there will be three status indicators for propellers within the DP, indicated on the panel and/or screen; 'running' 'ready' and 'enable'. The DP system receives the running signal from the thrusters. Thrusters are designated 'ready' if the transfer switch has been set to the 'DP' position. This configures the thrusters and propellers to accept commands from the DP system rather than from the independent thruster panel controls. Once a propulsion unit is showing as 'ready', it can be enabled into the DP system by pressing its individual 'enable' button. As soon as this transfer has been made it is essential that the DPO enters DP joystick mode, and 'proves 'the thrusters. By making small joystick movements in different directions (surge, sway and yaw) a DPO can verify that ALL thrusters and propellers are behaving correctly.

The number and configuration of the thrusters/propeller layout of a ship can vary widely. Modern design tends to smaller numbers of thrusters being used. It is common to find an offshore support vessel compliant with eguipment Class 2 having only four thrusters installed: two tunnels at the bow, and two azimuth thrusters aft. This complies with the redundancy requirements if the electrical arrangement has each thruster powered from a separate section of switchboard; worst-case single-point failure is the loss of one thruster.

DP sometimes performs poorly in flat-calm, in which all environmental vectors are very small and may be variable. With no substantial forces to overcome, the vessel may oscillate around the set-point position. In particular, azimuth thrusters perform poorly, and may spin in azimuth continually. In order to avert this problem, DP systems may


Chapter 7 91

Propulsion and thrusters

be configured with one or more fixed azimuth modes; selecting one of these modes causes the azimuth thrusters to assume a fixed pre-set azimuth configuration, effectively becoming tunnel thrusters. This reduces the wear and tear on the thruster's steering gear, and may improve positioning performance. The DPO must be aware of the impact on the propulsion redundancy, and whether or not the fixed azimuth mode is automatically dropped on the failure of a thruster.

Frequently, azimuth thrusters are configured with prohibited azimuth zones, or 'azimuth barring'. This is a pre-determined azimuth arc in which the thruster is limited in thrust output. There are several reasons for this facility. If two azimuth thrusters are adjacently located, then there is the possibility of one thruster discharging high-velocity water directly into the intake of the other one. This would cause the downstream thruster to overspeed and maybe trip off the board or be damaged. This azimuth prohibition is often a permanent configuration, unable to be changed by the DPO.

Another type of azimuth barring is operationally-related, eg if, in a dive support vessel, the diving bell location is close to an azimuth thruster, there may be a facility to impose an azimuth prohibition during times when diving operations are taking place. This prevents the thruster blasting high-velocity water at the diving bell or basket during operation. The selection of this facility will, of course, be part of the vessel's pre-dive checklist.

Where azimuth thrusters are fitted in pairs, a further possibility arises: thruster bias. Freguently, DP vessels are operating in light to moderate weather conditions, with low power loads on the generators. Light loadings on diesel engines are detrimental to their performance and efficiency, leading to increased maintenance requirements. Operation with fewer generators is not possible because of redundancy considerations. Under these circumstances it is possible to azimuth bias these thrusters to work against each other. The resultant thrust vector is a function of the difference in output between the two biased thrusters. This produces lower overall thrust output while increasing generator loadings. This may be preferable to simple considerations of fuel economy.

In general, thrusters and propellers are limited to about 80% of maximum rating when in DP mode. This is to prevent propeller blade overload which might occur at high thrust conditions with the vessel almost stationary. The DPO must be familiar with the facility to override this limitation. Usually, there is a 'max power' facility which can be selected for a limited time period. This 80% limitation is also (usually automatically) deselected at times when the vessel is in transit.

In ships in which the autopilot is integral within the DP system, a variety of configurations are available for the transit mode. In a vessel with main propulsion consisting of twin stern azimuth thrusters, then in autopilot mode these thrusters are configured to steer the vessel with a maximum 'helm' angle of 35°. In another example of a vessel having triple azimuth thrusters at the stern, in autopilot mode, the wing thrusters are locked in the forward-thrust position, while the centre thruster steers.



7.6 Thruster failure modes, effects and actions

All propellers are vulnerable to damage and subsequent failure. A common cause of such damage is contact with sea floor or any solid object, or simply fouling by ropes. Such damage may reduce or destroy the efficiency of the seal, allowing sea water to enter. If lubricating oil is water-contaminated, then subseguently damage will occur to bearings and thruster hub gearing. As thrusters and propellers are 'wet' technology, the resulting effect is the need for a dry-docking.

Many thrusters are CP, and these have many failure modes. Failures may occur in the hub actuator, the inboard controller and/or spool valve, the hydraulic power unit or the feedback mechanism. Any of these will result in propeller malfunction, downgrading the DP capability. The DPO must continually monitor the thruster Set Point and feedback values for not only pitch, but also rpm and azimuth. A major discrepancy should generate a system alarm, but a relatively minor error may go otherwise unnoticed.

All CP units should have a fail-safe pitch. In a traditional vessel this may be full-ahead pitch, or full-astern. In a DP vessel the requirement is for the fail-safe mode to be zero pitch, however, not every failure will automatically result in this. It is always possible for any DP thruster to fail without warning to any pitch setting from full ahead to full astern. Perhaps the commonest failure mode is the fail-as-set situation; the pitch simply stops working, leaving the pitch value as it was on failure.

If a propeller or thruster fails in any way, the DPO MUST stop that unit immediately. In this instance, stop means just that, ie stop the propeller rotating. It is not sufficient to simply deselect the thruster from the DP panel. If a thruster is running out-of-control and it is deselected, it is still running and still out of control. The DPO must be familiar with the location of the thruster stop buttons, located on the propulsion control panel (not on the DP panel). If an enabled thruster is stopped using the thruster Stop button, it will automatically be dropped from the DP panel.

In these circumstances, correct and rapid decisions and actions are essential. If a thruster runs away on full power, irrespective of the level of redundancy, the vessel's positioning will be badly affected. The vessel may run away out of control. As mentioned above it is essential that the DPO stops the thruster immediately, and equally importantly, must correctly identify the thruster which has failed. It may well occur that No 1 bow thruster has failed to full starboard thrust, and the DP system has compensated by commanding full port thrust from No 2. The DPO looks at the screen and sees red lines on both bow thrusters. It is obviously essential that the correct (failed) thruster is stopped and not the good one. The DPO must compare Set Point command and feedbacks from each thrusters, and check alarm and warning messages to ensure that the right thruster is tripped.


Chapter 8

93 Power plant

Power Plant

8.1 Power Plant

DP-capable vessels are totally reliant upon power supply, both for propulsion, and for the electronic elements of the system. It is vitally important that DPOs have a working knowledge and familiarity with the power systems on their vessels. The reason for this requirement is simple: DP problems are instant, necessitating rapid, if not instant decision-making. Under crisis circumstances a familiarity with the vessels power plant layout and operation is of great benefit in the decision-making process.

DP vessels divide, power-wise into two groups; those that are diesel-electric, and those that are not. The non-diesel-electric group includes hybrid arrangements, in which direct-drive diesels power main propellers, while thrusters are electrically driven. Other arrangements include all-diesel, in which each propulsion unit is driven by its own diesel engine.

8.2 Diesel Electric Power plant

By far the biggest grouping is diesel-electrically powered vessels. In a vessel of this type, the power is supplied on a 'power station' basis, with electrical power being generated centrally, thence distributed to all vessel consumers at a variety of voltages. All consumers are electrically powered.

Power is generated by a number of 'prime movers', usually medium-speed diesel engines, which turn alternators, which generate electrical power at high voltage (HV). Typical HV values range from 3,000 Volts (3kV) through to 11 kV. Typical voltages for DP vessels are 3.3kV, 6kV and 6.6kV. Higher voltages are preferred where high power consumptions are present, as currents are maintained at low levels. This results in greater efficiency and smaller line losses.

[The HV generated is transmitted to the HV switchboard, which is split into two or more sections for the purposes of redundancy. From the HV switchboards power is conducted



to the thruster and propulsion motors. Almost all modern DP vessels use AC (alternating current), although a few DC vessels are in operation.

In order to understand the vessel power system, it is essential that the DPO is familiar with the power distribution diagram, sometimes known as the 'one-line' diagram. This diagram shows the relationship between generators, HV switchboards and propulsion units. A study of the power distribution diagram will indicate the power and thruster failure possibilities, and the 'worst-case' failure mode.

One feature of a diesel-electric plant is the value 'available power', otherwise known as 'spinning reserve'. Generators are not often running on full load, the power generated being that which is demanded moment-to-moment. Spinning Reserve is the difference between present demand, and the maximum available from running generators. An indication of this level of reserve is commonly provided on the bridge, and this value can be monitored by the DPO.


Chapter 8 95

Power plant

8.3 Bus tie switches

Where, in a diesel-electric layout, the busbars are divided into two or more sections, the argument will arise regarding operation with the bus tie switches open or closed. If closed, power can be shared across the board between all running generators, giving greater flexibility in the number and configuration of running generators, if a power problem such as an earth fault or short circuit occurs on one bus, then the bus tie switch will automatically break to isolate the faulty section, the vessel will continue to operate on the remaining sections of switchboard. However, there is the possibility of failure of the bus-tie, which could result in blackout. For operations at equipment Class 1 or 2, the bus-tie switches may be open or closed.

For operations at equipment Class 3, bus-tie switches must be open, such that each section of busbar is running independently. In theory a fault or failure on one bus cannot transfer to other bus sections. The disadvantage of such a configuration is that, since power cannot be shared across the whole board, that a larger number of generators must be connected to provide the necessary contingency on each switchboard section.

8.4 Lower voltages

In a vessel configured for HV power to propulsion units, power is supplied to lower voltage consumers by using step-down transformers. Thus the main switchboard may power the majority of vessel functions at 660V or 440V. Individual items of heavy equipment (cranes, pipe tensioners, etc) may be powered by dedicated transformers from the HV board. Large items of equipment installed in exposed locations, and liable to water ingress, should be powered by dedicated transformers.

Separate switchboards may be provided for safety-critical areas such as the diving spread, including all life-support services. This switchboard will be split and fully redundant. A further switchboard will be fitted to handle the domestic supplies at 220V or 240V, this power again transformed down from the 440v of the main switchboard.

8.5 Hybrid systems

If a vessel is powered by a combination of direct-drive diesels (eg on main propellers) and electric drive on thrusters, the arrangement is known as 'hybrid'. Many hybrid arrangements feature constant-speed diesel drive to controllable-pitch main propellers, with electrical power provided from shaft alternators. Bow and stern thrusters are electrically driven. Sometimes, a hybrid arrangement is found in vessels which are DP conversions, with additional thrust and power capacity added during the conversion process. A number of modern vessels have been built with a hybrid configuration in order to allow economical operation in a variety of specific operational modes. A recent



hybrid layout features twin-screw main propellers driven via twin-input gearboxes. One input is from a large diesel, the other from a smaller electric motor. When in DP mode, main propellers are lightly loaded and are electrically powered. When in transit, the electric motors are clutched out, and the large diesels are clutched in. The diesels are thus run at full power (their most efficient mode of operation) to give the transit speed of 14 knots. In this arrangement, the large diesels are rarely running on Sight loads, which is always detrimental to efficiency and need for maintenance.

8.6 Power Management

All modern vessels are configured with computerised power management. The main function of this system is to ensure an adequate number of generators 'on the board' for the present demand, including a power contingency. The system monitors power demand and reserve, and is able to auto-start additional generators if spinning reserve is deemed inadequate. The power management system allows input of required equipment class, and the switchboard will be monitored accordingly for redundancy and power reserve level. When power demands reduce (eg when weather conditions improve) the system will either auto-stop excess generator capacity, or issue a generator stop recommendation. The watch engineer can react by stopping the generator if deemed appropriate.


Chapter 8 97 Power plant

Power management systems will act in a biackout-prevention capacity in a number of ways. A variety of switchboard protection devices prevent faults such as under-voltage or overload from causing part or full blackout conditions. Start-blocking will prevent the inadvertent start-up of heavy consumers (eg thrusters) if insufficient power is available. If this were not the case, the current surge from starting a large motor could lead to blackout.

Power management will react to power shortages by preferential trip, where non- essential circuits are dropped off the board at appropriate moments, in inverse order of importance. When in DP and a power shortage becomes critical, the system will invoke 'no further pitch increase' or pitch reduction commands to preserve power supplies. The philosophy is that it is better to suffer a slow position excursion than to suffer total blackout with the resulting large, rapid uncontrolled position loss.

A common problem with diesel-electric plant in fully-redundant DP ships is the running of diesel engines on light loadings. This is necessary to give the required level of redundancy, but results in engine inefficiency. Diesels run for lengthy periods in this manner may require reduced maintenance intervals.

8.7 Uninterruptible Power Supply

UPS units are commonly installed to provide a fully redundant and fully conditioned power supply to DP and related electronics. UPS systems typically output 24V and 48V to the DP computers, bridge consoles and ancillary low-voltage systems such as Position Reference Systems, gyros, vertical reference sensors, windsensors etc. It is important to realise that UPS systems do not supply power to thrusters and propellers, nor to thruster controllers. UPS systems are required to have a battery back-up with a minimum capacity of 30 minutes.

As part of the redundancy consideration, the DPO must be aware of any shortcomings in power supply arrangements. For example, the DP system and peripherals may be fully powered via UPS systems, but what about the differential corrections? If supplied via Inmarsat, is the Inmarsat terminal fully power protected? If position reference includes the deployment of taut wire systems, then sometimes these are not UPS protected. Some other off-vessel functions (eg HPR transponders) cannot be provided with power redundancy.



Chapter 9 Operator training and human factors

9.1 The need for competence

In common with any other area of shipboard management, key DP personnel are required to be fully competent. When considering the functioning of the DP system, it is clear that the DPOs are very much part of the system, as are other personnel involved in DP operations. Competence is assured by a combination of factors: background, experience, training, assessment and qualification.

The personnel most directly involved with the management of DP systems and vessels are the DPOs, usually the watchkeeping navigating officers, and the Master. Until 1985, DP-related training was a very hit-or-miss affair. Training was conducted by the DP system manufacturers, usually on real DP equipment, but with poor or non-existent simulation. Often, training was only available to the officers that started working onboard a new- build vessel. Officers joining later had to learn on-the-job. Clearly, improvements were required.

9.2 The Nautical Institute Scheme

Since 1984 The Nautical Institute has administered a training programme intended to provide a verifiable level of competency for DP operators. This scheme was developed as a result of the deliberations of a working committee, consisting of serving experienced DP Masters, Operations managers, representatives from the UK's Department of Transport, the UK's Department of Environment, UKOOA and the AODC, together with those concerned with training matters.

The progress of a trainee through The Nautical Institute scheme is witnessed by means of records kept in a logbook. Trainees keep their own logbooks and are responsible for maintaining the records up to date. Six phases of training constitute the recommended scheme. The six phases are:


Cnapter9 99

Operator training and human factors

i. Satis factory completion of a shore-based DP induction or basic operator 's course . ii . Sea-going DP familiarisation. A minimum of 30 days aboard a DP-capable vessel. ii i. Satis factory completion of a shore-based DP Simulator or advanced operators ' course. iv. Satis factory completion of six months supervised DP Operations. v. Having completed phases i to iv above, trainee DPOs must obtain verifica tion from the

Masters of their vessels as to the sui tability of trainees to undertake full watchkeeping responsibility on board thei r vessels.

vi. Issue of the certi ficate .

The majori ty of courses conducted at phases i and iii above are organised and run by nautical colleges or other training establishments.

i. The DP induction course (Phase 1)

This course is normally run at a shore-based establishment, either a nautical training establishment or a t the premises of one of the manufacturers of DP systems. The induction course is of four or four and a half day's duration, and provides a balanced introduction to the principles and practice of DP operations and systems. The induction course is intended for ships' offi cers, already qualified as watchkeeping officers, but wi th little or no DP experience.

ii. Sea-going familiarisation (Phase 2)

The DP logbook contains a familiarisa tion log lis ting various activi ties to be completed during this one-month period, it is necessary that the vessel be engaged in DP operations for the majority of this period, and that the candidate be actively involved in the bridge watchkeeping. The successful completion of each activity must be endorsed wi th the signature of the vessel 's Master .

iii. The DP simulator course (Phase 3)

This shore-based course is, again, of four or four and a halfday's duration. It is conducted at a training centre equipped with a sophistica ted and capable DP simulator facility. This course is intended for those who have successfully completed phases 1 and 2 of The Nautical Institute scheme. The DP simulator course provides the trainee wi th the opportunity to plan and conduct DP operations using a shore-based DP system. This system is interfaced with a simulator, and is able to provide trainees with realis tic simulation of DP scenarios, including error and failure mode simulation. This type of course provides realistic training in a sa fe environment, wi thout incurring the risks that would accompany any such training conducted using the real-ship environment.



iv. Supervised DP watchkeeping (Phase 4)

This phase is intended to allow the consolidation of the various training experiences, and to build up experience in the variety of tasks and operations conducted by DP vessels. Space is provided in the DP logbook for this experience to be detailed, suitably endorsed by the Master of the vessel/s.

v. Assessment of suitability (Phase 5)

This assessment is intended to be carried out by the Master of the vessel in which the candidate completed their DP watchkeeping within Phase 4 of the scheme. It constitutes the only actual assessment of the competence of the DPO candidate, and places the responsibility for that assessment with the person best able to comment upon the DPO candidate's competence. This assessment is recorded in the DP logbook. (See Appendix 1, Guidance to Masters of DP vessels in assessing the competency of DPO candidates).

Once this has been completed, the logbook may be forwarded to The Nautical Institute together with any supporting documentation. If all is in order, the candidate will be issued with a DP operator's certificate (Phase 6).

Grades of DP certificate

The Nautical Institute issues two grades of DPO certificate: full and limited. The full certificate is issued to those whose sea-time has been spent working in vessels of equipment Class 2 or 3.

Any officer who has undertaken the complete scheme of training, but whose service has been limited to vessels of equipment Class 1, ie where a loss of position may occur in the event of a single fault - will be issued with a certificate endorsed with the word 'limited'. In order to acquire an unlimited certificate the officer must meet the following requirements:

(a) Six months documented practical experience in the use of DP systems on units/ vessels operating in equipment Class 2 or 3.

(b) If the six-month DP watchkeeping experience has been exclusively aboard DP Class 1 vessels, a DPO certificate endorsed with the word 'limited 'will be issued. To have the limitation removed the applicant needs to accumulate the equivalent of six months DP watchkeeping experience aboard DP Class 2 or 3 vessels, where two months DP Class 1 is equivalent to one month DP Class 2. However, all applicants for such upgrading must have at least two months actual DP watchkeeping time aboard DP Class 2 or 3 vessels.


Chapter 9 101 Operator training and human factors

For details relating to the equipment classes referred to above, see IMO MSC/Circ 645, Guidelines for Vessels with Dynamic Positioning Systems, 1994. Equipment class descriptions are also contained in Chapter 3 of this handbook covering system redundancy.

The scheme described above has been running since 1984 and, as detailed elsewhere, over 50 shore-based establishments are approved for the purpose of conducting formal courses fulfilling the requirements of phases 1 and 3 of the scheme. The scheme is generally accepted as being the worldwide standard for this type of training, and is regarded as working well. The Nautical Institute maintains a central register of all certificates issued, with appropriate details.

9.3 IMO and IMCA -Training and qualification of key personnel

The International Maritime Organization (IMO) published in June 1996 Maritime Safety Committee (MSC) Circular 738, Guidelines for Dynamic Positioning Operator Training, inviting member governments to bring that publication to the attention of the bodies concerned and apply them to the training of key DP personnel.

This guideline referenced the Internationa! Marine Contractors Association (IMCA) document Training and Experience of Key DP Personnel originally published in January 1996 as IMCA M 117, and revised as IMCA M 117 Rev 1, published in February 2006.

The above IMCA guidelines are intended to improve the safety of DP operations by defining minimum industry standards for the formal training and competence of key DP personnel, and for the familiarisation programme for such personnel new to a vessel.

The Nautical Institute scheme is recognised within the IMCA document as being the standard method of acquiring training and experience for bridge watchkeeping DPOs. As such, The Nautical Institute scheme is described in IMCA M 117. Additionally, however, the following remarks are made:

Achievement of an Nl DPO certificate does not guarantee provision of a fully qualified senior DPO capable of handling any DP situation. The certificate provides evidence that a DPO has been trained in the basics of DP and may require additional vessel and equipment specific training.

It is recognised, of course, that 'key DP personnel' include those other than the senior and junior DPOs. These will include the Master (whether or not DP certificated) the chief and watchkeeping engineers, and electrician/electronics maintenance staff. All these personnel must be fully competent, experienced and must have familiarity with own ship's specific systems.



IMCA M 117 details required levels of competence, experience and vessel/project familiarisation. In terms of experience, a detailed specification is laid down within the IMCA guidance. Each personnel rank is listed, with minimum levels of experience, both in the present and in previous vessels. Also detailed is the need for familiarisation in a new vessel, and the nature and duration of the familiarisation programme.

In March 2006, IMCA published International Guidelines for the Safe Operation of Dynamically Positioned Offshore Supply Vessels (IMCA M 182). Covering the whole spectrum of operation of DP OSVs, this document also highlights the training and competence requirements in this particular class of vessel.

9.4 Incident reporting

It is recognised that great benefits can result from a study of reported DP incidents. In view of this, IMCA have for many years operated a DP incident reporting scheme among its members. The results of these reports are published annually. The statistics resulting from this scheme are vitally important in identifying trends, which themselves relate to areas that need addressing. In the analysis for 2004, loss of position incidents showed the following distribution: operator error 16%; references 32%; power 24%; thrusters 12%; while the remainder was split between computer, electrical and environmental factors.

A study of reported incidents over the years highlights a number of common operator- related problem areas. One such consists of the poor use of position-references. If, for example, a spread of three PRS is in use, but this includes two identical systems plus one other (eg 2 x DGPS plus a fanbeam), the system is vulnerable to common-mode failure. If both DGPS fail in the same manner, the system voting causes rejection of the fanbeam, which was at that moment the only reliable PRS. Although the primary cause of this incident will be position references, it must be recognised that operator error is also a factor, as the operator should have been aware of the common-mode vulnerability, and should have taken steps to avoid it.

A number of incidents highlight another syndrome. This is the assumption that, if the vessel is correctly set-up to Class 3 status, then the vessel is 'bullet-proof' and nothing can go wrong. A proportion of incidents come into the category' this cannot happen'. Unfortunately, ships are complex systems, usually built to the lowest tender. Sometimes corners are cut, particularly when an existing vessel is being converted into a DP role. In a vessel with comprehensive switchboard blackout protection, a full blackout can still occur. In a vessel with six thrusters supplied independently from three separate switchboards, five thrusters out of six can trip together. In a vessel with four separate and independent PRS, then position-reference can be lost as a result of losing all PRS simultaneously. These things can all happen, and a study of the IMCA incident reports will verify this.



9.5 Human factors

The most comprehensive study on human factors in DP operations was the Human Factors Pilot Project written by Captain J Hughes in 1994, and published by the DPVOA (now IMCA). Still relevant, some of the findings of this report will be reiterated here.

Analyses of DP-related incidents by IMCA over the period 1987 - 2008 indicated that human error still accounts for around 20% of incidents. That figure, although quite high, is an improvement on the approximately 50% recorded in the late 1980s.

A number of reasons can be proposed as accounting for this reduction. One of these reasons is the more systematic and regulated training undertaken by DPOs and others, administered by The Nautical Institute. Prior to this scheme becoming commonplace, training was usually an extremely piecemeal business, often only available to personnel taking charge of a new vessel. The establishment of the DP Operators' logbook enabled key personnel to record their experience.

In more recent years, equipment has become more reliable and user-friendly. Often, an incident is regarded as being primarily caused by operator error, but the design and layout of the eguipment was a contributing factor. Modern equipment operated by means of a Windows platform is much more intuitive and user-friendly to operate.

The wealth of experience in the operation of DP vessels has led to more relevant and comprehensive operations handbooks and manuals being available. Redundancy levels and provision is much improved, and the techniques of hazard-identification and risk- assessment have all contributed.

A study of human factors will focus on five key areas of particular significance:

Man/machine interface DP management issues Training and competence Identification of stressors Aspects of human behaviour

Captain Hughes' report included a number of specific results of the study, some of which will be paraphrased here. For more comprehensive information on human factors, the reader is recommended to study the full document.

i. DP Consoles

An identified problem is physical and/or information overload. DPOs have to operate a large quantity of equipment surrounding the actual DP consoles. Another problem is the



quantity of information on display, much of which is largely irrelevant.

ii. Work overload and under-stimulation

Overload and under-stimulation are features of a DPO's workload. There are many periods when the amount of work is excessive, and the DPO may well be expected to carry out duties additional to simply manning the DP console. This leads to an increase in stress levels with a commensurate increase in the probability of an operator-related incident. In other parts of this handbook, reference has been made to the need for the DPO to concentrate on the DP system and operation only, and not to engage in any other tasks; these being the duty of the other watch officer. The direct opposite problem is under- utilisation; long hours of enforced idleness at the DP console leading to concentration lapses.

iii. Temporary handover arrangements

Although watch handover arrangements are generally thorough and effective, occasionally a short-term handover causes problems. Relevant information is sometimes not passed over, and the incoming DPO may not be made aware of the status of the DP operation. This can happen, for example, when the Master takes over from the DPO, while the latter attends to another task.

iv. Distracting presence of non-DP personnel

In many vessels the DP control area is freely accessible to other personnel, resulting in distraction and noise. It is vitally important that the DP control area is an appropriate working environment, dedicated exclusively to the safe conduct of DP operations, free from all other distractions and influences.

v. Inadequate manuals and documentation

This is less of a problem nowadays than hitherto, but it occasionally happens that equipment and system manuals are either not available or insufficiently comprehensive or user-friendly.

vi. Risk taking

A variety of causal factors result in risks being taken, departing from accepted standards of operation. This is often the case with the management of position-references. While



DPOs may assert that their standards of professionalism are high, the analysis of DP incidents shows risk-taking to be a significant causal factor.

vii. DP incident reporting

While IMCA maintains a database of reported incidents, it is evident that not all incidents are reported, and that some incident reporting is misleading or not fully accurate. The human factor here is that of avoiding blame or responsibility.

viii. DP operator training and competence

An inherent limitation in shore-based training is the lack of vessel-specific reality in DP simulator provision. Nevertheless, training is one of the most powerful tools in ensuring DPO competency and is directly linked with the human-factor element.

ix. Knowledge base - vessel specific

Cases are reported in which DPOs had insufficient knowledge of their vessels and their systems.

x. Geographical differences

Standards vary according to geographic location. There is considerable room for abuse of operational and safety procedures in certain parts of the world. The temptation to relax safety constraints must be countered by the implementation of acceptable company- specific standards, and the professionalism of the DPOs themselves.

xi. Fatigue

Principal causes of DPO fatigue are sleep-related, noise-related and operational stress. Sleep-related fatigue is greater where the DPO is working the night shift, and adequate rest is not obtained during the off-watch periods.The two-man watch system goes some way to addressing this problem.

xii. Irresponsible behaviour patterns

Not of major significance, but DPOs have been known to behave in a dangerous manner, violating rules and procedures.



xiii. Employment conditions

There may be a correlation between conditions of employment, job security, and safety attitudes. Morale is made up of a number of variables, all difficult to measure, and it is recognised that low morale means lower levels of safety.

xiv. Status of DP operators in the industry

It is evident that DPOs operate more positively where efforts have been made to integrate them into the management of the operation and in the planning of the project. Improved performance is directly related to job satisfaction and self-esteem.


Appendix 1 107

Appendix 1 Guidance to Masters of DP vessels

in assessing the competency of DPO candidates.

A

watchkeeping DPO must complete all phases of training, sea-going and shore- based, as defined within The Nautical Institute Scheme (see Section 9 of this

handbook). This period of training will be recorded in the candidate's DP Logbook. Phase 5 of the Scheme is the final assessment of the candidate as a suitable person to keep a DP watch in the vessel. This assessment is to be made by the Master of the vessel at the time the candidate completes all preceding phases. The Nautical Institute recognises that the Master is most advantageously placed to make such an assessment.

In making their assessment, Masters should take into account a candidate's ability to demonstrate competence in the following areas:

Ability to set the vessel up on DP in a satisfactory manner, using the appropriate facilities for the transfer from conventional manual control into DP joystick control, thence into full auto-position mode

Ability to operate the DP system in al! of its modes of operation (eg follow target, autotrack etc).

The setting-up and use of all position reference systems, both individually and in combination. Recognition of the merits and limitations of individual PRS

The function of the various peripheral systems associated with the DP system (eg.windsensors, MRUs, gyro compasses)

The use of facilities for the manoeuvring of the vessel; changing position and heading, selecting suitable values for velocity, rate-of-turn, gain, etc.

A comprehensive knowledge of all the functions, facilities, operational controls and selections available at the system panel or desk. All DP systems are different; DPOs should


108 Appendix 1


be fully conversant with the system in their own vessels

• A practical understanding of the levels of redundancy in the vessel systems, with particular reference to the vessel's FMEA

• An ability to manoeuvre the vessel in both joystick mode, and in conventional manual mode. This is essential. Watchkeeping DPOs must be familiar with the shiphandling characteristics of their own vessel, and must be practised in such techniques. The DPO may be faced with the task of extracting the vessel from location in severe weather conditions, after the DP system has suffered partial or total failure

• Ability to manoeuvre the vessel in manual mode in a degraded status (ie without the full spread of thrusters available, or with severe power restrictions)

• Whilst engaged in DP operations in deteriorating conditions, recognise the appropriate time for the operation to be suspended on safety criteria

• Recognising a variety of failure modes, and subsequently making the correct responses and decisions

• Ability to interpret the variety of messages generated by the systems, eg warning, alarm and information messages

• The functions of the ships power management system

Once the Master is satisfied that the candidate can be trusted to act as senior DPO on watch, then he/she should complete Section F of the candidate's Nautical Institute DP Logbook. Please note that this logbook entry must not be completed at an earlier stage of the training process. The candidate may then forward the Logbook to The Nautical Institute in London for processing and issue of the DPO certificate.


Appendix 2 109

References

Appendix 2 References

The Nautical Institute monograph Dynamic Positioning Operator Training, second edition. 1999. Captain D Bray FNI.

Dynamic Positioning. Captain D Bray FNI. 2003. Oilfield Publications Ltd.

Developing DP Visual Systems. Captain G Fiander. Pub. Seaways January 2006

MSC Ore 738. Guidelines for Dynamic Positioning (DP) Operator training. IMO. June 1996

MSC Circ 645. Guidelines for vessels with Dynamic Positioning Systems. IMO. June 1994

The Training and Experience of Key DP Personnel. IMCA M 117 Rev 1, Feb 2006 International Guidelines for the Safe Operation of Dynamically Positioned Offshore Supply Vessels. IMCA M 182. Rev 1. Aug 2009

The Use and Requirements for Simulators in Training and Operations. IMCA C 009, May 2006 The

Common Marine Inspection Document. IMCA M 149 Issue 7. Mar 2009 Guidance on failure Modes

and Effects Analyses (FMEAs). IMCA M 166. April 2002 FMEA Management Guide. IMCA M 178. April

2005

Guidance on the Investigation and Reporting of Incidents. IMCA SEL 016, January 2007

Analysis of Station Keeping Incident Data 1994-2003. IMCA M 181, January 2006 Considerations about the use of hold-back vessels during DP diving operations. IMCA M 185. Feb 2007

Deep Water Acoustic Positioning. IMCA M 200. Oct 2009


110 Appendix 2 DP Operator's Handbook

Guidance on Simultaneous Operations (SIMOPS). IMCA M 200. Mar 2010 Bridge

Procedures Guide. International Chamber of Shipping, 2007 edition


Appendix 3 111

Glossary

Appendix 3 Glossary of Terms and List of Abbreviations

ADS - Atmospheric Diving Suit, enabling diver access to depths greater than w450m

Centre of Rotation-The reference spot within the vessel

AoD - Age of Data. The latency of the corrections within the DGPS system

c.p. - Controllable pitch (propeller)

Cycloconverter - A variable-speed a.c. drive system

Artemis - A microwave position reference system using one shore and one shipboard transceiver, giving range and bearing information

CyScan - A laser-based position reference system manufactured by GCS

Autotrack - A DP function allowing the vessel to follow a predetermined track

DARPS - Differential Absolute and Relative Positioning System, a DGPS- related PRS used by shuttle tankers and FPSOs

AUV - Autonomous Underwater Vehicle Depthtransponder - An acoustic transpomder giving depth data

BOP - Blow-out preventer. A component of the wellhead "stack"

DGPS - Differential GPS (see GPS)

C/A Code -The Coarse Acquisition code used with the GPS system

C/R - Centre of Rotation

DNV - det Norske Veritas, the Norwegian Classification Society

DoD-The US Department of Defence

DOP - Dilution of precision

Central Meridian - The datum meridian for measurement of Easting in a Transverse Mercator projection

DP - Dynamic Positioning


Appendix 3 111

Glossary

Appendix 3 Glossary of Terms and List of Abbreviations

ADS - Atmospheric Diving Suit, enabling diver access to depths greater than w450m

AoD - Age of Data. The latency of the corrections within the DGPS system

Artemis - A microwave position reference system using one shore and one shipboard transceiver, giving range and bearing information

Autotrack - A DP function allowing the vessel to follow a predetermined track

AUV - Autonomous Underwater Vehicle

BOP - Blow-out preventer. A component of the wellhead "stack"

C/A Code - The Coarse Acquisition code used with the GPS system

C/R - Centre of Rotation

Central Meridian -The datum meridian for measurement of Easting in a Transverse Mercator projection

Centre of Rotation-The reference spot within the vessel

c.p. - Controllable pitch (propeller)

Cycloconverter - A variable-speed a.c. drive system

CyScan - A laser-based position reference system manufactured by GCS

DARPS - Differential Absolute and Relative Positioning System, a DGPS- related PRS used by shuttle tankers and FPSOs

Depthtransponder - An acoustic transpomder giving depth data

DGPS - Differential GPS (see GPS)

DNV - det Norske Veritas, the Norwegian Classification Society

DoD - The US Department of Defence

DOP - Dilution of precision

DP - Dynamic Positioning


DP(AA) - The Lloyd's Classification for DP vessels of Equipment Class 2

DP(AAA) - The Lloyd's Classification for DP vessels of equipment Class 3

DP(AM) - The Lloyd's Classification for DP vessels of Equipment Class 1

DP(CM) - Lloyd's Classification for DP vessels of limited capability

DPO - Dynamic Positioning Operator

DQI - DGPS Quality Indicator

DSV - Dive Support Vessel Duplex - A DP system having two computers to

provide redundancy

ECR - Engineering Control Room

ED - Emergency Disconnection (drilling rig)

ESD - Emergency Shutdown and Disconnection procedure for offshore loading shuttle tankers

ETO - Electrical and Technical Officer

Fanbeam - An optical Laser radar position reference system

FMEA - Failure Modes and Effects Analysis

FOC - Full Operational Capability of the GPS system, declared on 17th July 1995

fp - Fixed pitch (propeller)

FPSO - Floating Production, Storage and Offtake Vessel

FSVAD - Flag State Verification and Acceptance Document

Galileo - The proposed European satellite navigation system

GNSS - Global Navigation Satellite System

GPS - The Global Positioning system of satellite navigation

G-Vec - A Relative GPS position reference system manufactured by MDL

HAIN - Hydroacoustic-aided Inertial Navigation

HDOP - Horizontal Dilution of Precision, a measure of accuracy in the GPS and DGPS systems

HiPAP -The Kongsberg Maritime High Precision acoustic positioning system

HPR - Hydroacoustic Position Reference

HSE OD - The Health and Safety Executive Offshore Division

HV - High Voltage

IMCA - International Marine Contractors Association

IMO -The International Maritime Organization

IMU - Inertial measurement unit


Appendix 3 DP Operator's Handbook

Appendix 3 113

Glossary


INS - Inertial navigation system

LBL - Long Baseline HPR

LFJ - Lower Flex Joint. The connection between the riser and the wellhead structure

LMRP - Lower Marine Riser Package

MBR - Minimum Bend Radius (cable lay &

repair) operations

MCR - Machinery Control Room

MLBE - Mooring line buoyancy element

MMI - Man-machine interface

MOU - Mobile Offshore Unit

MRU - Motion reference unit Network DGPS - DGPS using multiple reference

stations instead of a single one

NMD -The Norwegian Maritime Directorate

NMEA 0183 - A standard format for the transference of data in marine navigational equipment

OIM - Offshore Installation Manager

OLT-Offshore Loading Terminal. A buoy, articulated tower or rigid tower used by shuttle tankers for loading crude oil for export

PLEM - Pipeline end manifold

PME - Position Monitoring Equipment (synonymous with PRS, see below)

PPS -The Precise Positioning Service from the GPS system, only available to approved military users

PRC - Pseudo-Range Corrections provided within the DGPS system

PRS - Position Reference System

Pseudo-range - The basic navigation measurable of GPS. Also referred to asPSR

Relative GPS - A position reference used by shuttle tankers (see DARPS)

Responder - A form of transponder where the interrogating signal is conducted by hard-wire rather than through water

Riser Angle mode - The mode by which DP control is exercised in deep water drilling applications

RMS - Riser Management System

ROV - Remotely Operated Vehicle, usually an unmanned submersible

SA - Selective Availability

SD - Standard Deviation, or Variance in data from a position-reference system

Simplex - A non-redundant DP system, single computer

SPS - The Standard Positioning Service from the GPS system



Stinger - A heavy gantry deployed at the stern of a pipelay vessel, supporting the pipe as it is laid. Used in the S-lay method

STL - Submerged Turret Loading.

Surge - Vessel movement if the fore-and-aft direction

SV - Space vehicle or satellite (GPS etc)

Sway - Vessel movement in the transverse direction

Taut Wire - A position reference system consisting of sensors detecting the length and angles of a tensioned wire leading to a weight on the sea bed

TMR -Triple Modular Redundancy; the Voting concept of redundancy

TMS - ROV tether management system

Tp -Transponder, used as part of an HPR system

Transducer - The acoustic probe on the vessel's bottom used as part of the HPR system

Transponder - A sea bed beacon used with HPR (seeTp)

Trencher - A sea bed crawler vehicle intended to trench and bury a previously laid pipeline

Trimcube - A specialist taut wire sensor used in trenching applications

Triplex - A DP system having three

computers providing redundancy through voting logic

Variance - A measure of the spread of successive positions derived from a PRS

Voting - A system of redundancy using triplicated systems and a "two- out-of-three" vote on all critical values

UPS - Uninterruptible Power Supply

USBL - Ultra Short Baseline, a principle used with HPR

UTM - The Universal Transverse Mercator projection and co-ordinate system

VRS - Vertical Reference Sensor, a device giving accurate values for vessel Roll and Pitch values

VRU-Vertical Reference Unit - see VRS

VSD - Variable Speed Drive

WGS 84 - The World Geodetic Spheroid upon which the GPS system is based

Yaw - Vessel rotation about the vertical axis; Change of heading

Index 115

Index


A

Accommodation barge 43 Air-diving 36 Alternator 5, 7, 90, 93, 95 Anchor-handling tug 38 AoD (age of data) 58, 65,67,111 Artemis 6,14, 32, 33, 35, 55, 80, 81, 82,111 Assessment 15,19,20, 23,36,98, 100, 103, 107 ADS (atmospheric diving suit) 38 Auto-Approach function 13,38,111 Auto-area mode 14 Autopilot 16,91 Autotrack 10,11,40,107,111 Available power 94 Azimuth barring 91 Azimuth thrusters 16, 87,88, 89, 90,91 Azipod 89

B Busbar 28,95 Bus tie switch 29, 30, 55,90, 95

c Cable-lay 18, Cable repair 43 Capability plot 15,16 Central meridian 60,111 Checklists 51, 54 Common-mode failure 27,81,86,102 Communications 3, 51, 53, 54, 65 Consequence analysis 28, 29 Construction vessel 15, 38,45

Contingency planning 48 CP propeller 87,88, 89,111 Crane barge 38,39, 68 Current (sea) 8,12,15,17,18,47,48,49,55 Current (voltage) 93,94, 97

CyScan 6,69,111

D DARPS (differential absolute and relative positioning system) 35, 68, 70,111.113 Datum 10,55,60,69, 74, 111 Dead reckoning mode 61 Degraded status 15,47,48,108 DGPS 6,10,19, 27, 32, 39, 50, 52, 55, 58, 59, 60, 63, 64, 65, 66, 67, 69, 83, 86.102.111.112.113 Diesel electric 6,28, 29, 87, 93,94,95, 97 Differential GPS 58, 61, 63,111 Dive control 36 Dive support 36, 53,91,112 Diving bell 37, 53,91 Diving operations 53, 91,109 Diving umbilical 36, 38, 53, 75, 76 DP Induction Course 99 DP Operator's Certificate 3,99,100, 101,108 DQI (differential quality index) 55, 59, 67,112 Dredging 10,40 Drift-off calculation 16 Drilling rig 12, 39, 74, 79,88,112 Drillship 1,12,18, 31,39, 46,71,74,75 DSV (dive support vessel) 53, 91,112

116 Index DP Operator's Handbook

THE NAUTI CAL I NSTITUTE

Dual DGPS 39

Dual-frequency DGPS 66

Duplex 24,112

E Equipment class 6,21, 22,23,24, 25, 26, 27, 29, 38, 48, 58, 80, 86, 90, 95, 96,

100, 101,112 Escape routes 48,49, 50, 51, 55

ESD (emergency shutdown and disconnection) 33,35, 36,112

F False current 10,15 Fanbeam 6, 27, 52,55,60, 69,83, 102, 112 Fast learn 15, Fixed azimuth 91 FMCW radar 80 Follow-sub function 10,14 Follow-target function 9,10,14,42, 107 FP propeller 87, 88, 89,112 FPSO (floating production, storage and offtake unit) 10, 13,14,18, 31, 33, 35, 36, 67, 68, 70, 80, 82,111,112

FSVAD 23,112

G Generator 26, 28, 29, 48, 55,82,91, 94, 95, 96 GPS 6, 10, 35, 58,61,62, 63, 64, 65, 66, 67, 68, 69, 78,111,112,113,114 Guidance Ltd iv, 69, 80 G-vec 68, 112 Gyro 25, 26, 55, 56, 68, 70, 72, 81, 97 Gyro compass 5,6, 24,25, 27, 28,107

H HAIN (hydroacoustic aided inertia l navigation) 78,112 HDOP (horizontal dilution of precision) 55, 59, 63, 65, 67, 69, 112 Heavy-lift vessel 38,45 High voltage 5,28,90, 93,112 HiPAP 55, 77,112 HPR (hydroacoustic position reference) 13,14, 32, 55, 71, 72, 73, 74, 75, 76, 78,

79,81,86,91,112,113,114 Hull unit 76, 77 Human factors 5, 58, 98,103 Hybrid systems 95

I IMCA (International Marine

Contractors Association) 23, 55, 67,101,102, 103, 105,109,110, 111 Incident reporting 102,105 Inclinometer transponder 75 Inertial navigation 78,112,113 Inmarsat 65,66, 97 IMO (The International Marit ime Organization) 22, 23, 24, 26, 29,101, 109

Interrogator 80

J Jet thruster 87 J-lay 41,42

K Kongsberg iii, iv, 8, 77, 78, 80,

83,112

L LBL (long baseline) 39, 59, 71, 73, 74, 77, 79,113

Index 117


M

Mathematical model 4,15,17, 51,61 MCR (machinery control room) 48, 50, 113 MDL iv, 68, 69, 112 MRU (motion reference unit) 17, 24, 25, 30,55, 72, 84,107,113

Multifix 68 Multipath 63, 66,68 Mobile transponder 75 Multi-user acoustic system 78-79

Moonpool taut wire 84

N NASNet 78, 79 Nautronix iv, 78 Network DGPS 64,113

Notice of readiness 50

0 OLT (offshore loading terminal) 13,14, 32,33,35,71,82,113

Operational planning 47-56 Optimum heading (weathervane) function

7,8

P Passenger vessel 44 Pipelay 10,11,15,18,41-42, 74,114 Pipelay vessel 11,18,41,42,114

Pipe tensioner 18,41,95 Platform supply vessel 31,38,80 Position dropout 61 Power distribution 29, 94,96 Power management 26, 30,96-97, 108

Power systems 23,30,93 PPS (precision positioning service) 63,113 Prediction test 59

Propellers 5,6, 7,13,23, 24, 25, 28, 32, 36, 50, 75, 76, 87, 88, 89, 90, 91, 92, 93, 95, 96, 97

Propt Ision 1, 3, 5, 6, 7, 15, 24, 25, 28, 29,31,87-92, 93, 94, 95

PRS (position reference systems) 6, 9, 10, 17,18, 19, 20, 27, 35, 40, 53, 55, 57-

86,102,107,111,113, 114 Pseudo range 62,113

Q QA (quality assurance) 54,65

R RadaScan 6,52, 80 RADius 6, 55, 80, 113 Redundancy 5,6,18,21-30,32, 38, 48, 51, 53, 67, 74, 86, 87, 90, 91, 93, 96, 97,101,103,108,112,114 Reel-lay 41 Reference origin 55,60 Relative GPS 35,67, 68,112,113 Relaxed gain 8, 55 Research vessel 44,45, 88 Responder 75,76,113 Riser angle mode 12-13,46,113 Riser management system 13,113 Rockdumping 10 ROV (remotely operated vehicle) 9,10, 19, 36-38,40,43, 55, 75, 76, 79, 83, 85,113,114

Rudders 6,87-88,89

S SA (selective availability) 66, 67,113 Saturation diving 37,38 Sea force 18 Seatex 68

Set point 3,4,6,8,14, 27, 33, 36, 61, 86, 90, 92

118 Index DP Operator's Handbook


Shuttle tanker 13-14,18,31-36,46, 67, 68, 70, 82, 111, 112,113 Shuttle tanker offtake 31-36 S-lay 41,114 Simplex 24-25,113 Single point failure 22, 24, 28, 29, 90 Spotbeam 65 SPS (standard positioning service) 63, 113 SSBL (super short baseline) 72-73 Standard deviation 58,113 Standing orders 56 Stinger 15,41,42,114 STL (submerged turret loading) 31, 34, 35,114 Sunspot activity 67 SV (space vehicle) 62, 63,64,65, 66, 67, 69, 114

System gain 8-9 Spinning reserve 94, 96

Surface diving 36-38

T TAD (turn around delay) 72, 73 Taut wire 6,17,18,27,43, 51, 55,60, 81,82-85,86, 97,114 Thruster failure modes 92 Thrusters 23, 24, 25, 26, 87-92, 93, 95 Training 98-106 Transducer 74, 76, 77,114 Transponder 74-75, 80 Trenching 40,41-42,114 Triple modular redundancy 5, 24,114 Triplex 24,25, 114 Tunnel thrusters 32, 87,89, 91

u UPS (uninterruptible power supply) 30, 97, 114 USBL (ultra short baseline) 71, 72-73, 78, 79, 114

UTM (universal transverse mercator) 8, 12, 60-61,78, 114

V Variance 58, 59,113,114 Variance based weighting 58-59 Voith-Schneider propeller 87 Voting 5, 24, 27, 58, 86,102,114 VRS (vertical reference sensor) 17, 23, 97, 114 VSD (variable speed drive) 5, 87,114

w Watchkeeping 3,47-56,98, 99,100, 101,107,108 WGS 84 60, 69, 114 Wind feed-forward 17 Windsensor 17,18,19, 24, 25, 30, 55, 97,107 Work approach 47, 51

About the author 119

About the Author

Captain Bray first went to sea as a cadet with Ellerman Lines in 1965. He stayed with the company until 1972 when he joined The British Antarctic Survey and sailed on the RRS John Biscoe for the next three seasons. Afterwards he sailed on a variety of offshore vessels, gaining his foreign going Master's Certificate in 1976.

He then moved ashore and was appointed as a lecturer in the maritime department at Lowestoft College. In 1986 he founded the internationally-recognised DP training unit at the college, and won the UK government's prestigious National Training Award for Excellence in 1989.

The author was instrumental in helping to develop The Nautical Institute's training scheme for DP Operators and contributed to the advisory board for a number of years, for which he was elected a Fellow.

The purpose of writing a Handbook was to provide re-enforcement of good operational discipline and to cover some of the human element issues that need to be addressed by all operators working in an international industry.

The author has written a number of books concerning dynamic positioning and contributed numerous papers to conferences for which he is recognised internationally.

Captain Bray retired in 2008 but retains a passion for maritime history and is widely sought as a guest lecturer on cruise liners.


120 Also available


dp operators handbook

Documents

dp training centres

dp training forum

captain bray

dp specialisation

dp systems

dp certificates

dp operators handbook

dp operators dpos