0031_nd rev_2 14-dec-15 guidelines for float-over installations and removals

8/18/2019 0031_ND Rev_2 14-Dec-15 Guidelines for Float-over Installations and Removals

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TECHNICAL STANDARDS COMMITTEE

http://www.dnvgl.com/

GUIDELINES FOR FLOAT-OVER INSTALLATIONS / REMOVALS

0031/ND

14 Dec 15 2 RJP Technical Standards Committee

22 Jun 13 1 RJP Technical Policy Board6 Dec 10 0 RJP Technical Policy Board

Date Revision Prepared by Authorised by

These Guidelines have been updated as part of the first stage of the harmonisation between theGL Noble Denton and DNV heritage marine services requirements.

Refer also to DNVGL-SE-0080 Noble Denton marine services – Marine Warranty Surveyfor further details.

All references to GL Noble Denton apply to the legal entity trading under the DNV GL or GL Noble Dentonname which is contracted to carry out the scope of work and issues a Certificate of Approval, or provides a

marine related advisory or assurance service.

Once downloaded this document becomes UNCONTROLLED.

Please check the website below for the current version.

http://www.dnvgl.com/http://www.dnvgl.com/http://www.dnvgl.com/


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0031/ND REV 2 Page 2 of 37

PREFACE

This document has been drawn with care to address what are considered to be the primary issues in relation to thecontents based on the experience of the GL Noble Denton Group of Companies (“the Group”). This should not,however, be taken to mean that this document deals comprehensively with all of the issues which will need to beaddressed or even, where a particular matter is addressed, that this document sets out a definitive view for allsituations. In using this document, it should be treated as giving guidelines for sound and prudent practice, butguidelines must be reviewed in each particular case by the responsible organisation in each project to ensure thatthe particular circumstances of that project are addressed in a way which is adequate and appropriate to ensure thatthe overall guidance given is sound and comprehensive.

Reasonable precaution has been taken in the preparation of this document to seek to ensure that the content iscorrect and error free. However, no company in the Group

shall be liable for any loss or damage incurred resulting from the use of the information contained herein or

shall voluntarily assume a responsibility in tort to any party or shall owe a duty of care to any party other than to its contracting customer entity (subject always to the terms

of contract between such Group company and subcontracting customer entity).

This document must be read in its entirety and is subject to any assumptions and qualifications expressed therein aswell as in any other relevant communications by the Group in connection with it. Elements of this document containdetailed technical data which is intended for analysis only by persons possessing requisite expertise in its subjectmatter.

© 2015 Noble Denton Group Limited. The content of this document is the copyright of Noble Denton Group Limited. All rights reserved. Any reproduction in other material must have written permission. Extracts may be reproduced

provided that their origin is clearly referenced.


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CONTENTSSECTION PAGE NO.

1 SUMMARY 5

2 INTRODUCTION 6

2.1

General 6

2.2 Start and Completion of Operations 6 2.3

Other GL Noble Denton Guideline Documents & Feedback 6

2.5 Codes and Legislation 7 2.6

Revisions 7

3

DEFINITIONS AND ABBREVIATIONS 8

4 THE APPROVAL PROCESS & SAFETY 11 4.1 General 11 4.2 Scope of Work Leading to an Approval 11 4.3 Limitation of Approval 11 4.4 Health, Safety and Environment 11

5

LOAD-OUT, TRANSPORTATION & MOORING 12

5.1

Structure Load-out 12

5.2 Structure Transportation 12 5.3 In-field Mooring 12

6

DESIGN ENVIRONMENTAL CONDITIONS 13

6.1 Principles 13 6.2 Operational Reference Period 13 6.3

Weather Restricted Operations 13

6.4 Weather Unrestricted Operations 13 6.5

Operational Feasibility 14

7 CONSIDERATIONS INFLUENCING BARGE SELECTION CRITERIA FOR LOAD-OUT,

TRANSPORTATION & FLOAT-OVER 15

8

FLOAT-OVER ENGINEERING 16

8.1 General 16 8.2 Weight & Dimensional Control 16 8.3 Barge Load Conditions 16 8.4 Stability During Installation / removal 16 8.5 Fenders and/or Tethers 17 8.6 Motions and Mating Stages 17 8.7

Draught and Freeboard 18

8.8 Clearances 19 8.9

Barge Mooring and Positioning Overview 19

8.10 Barge Mooring and Stand-off Moorings 19 8.11

Clearances around Mooring Lines and Anchors 20

8.12 Position Keeping during Mating /De-mating with Tethers 20 8.13

Barge Positioning Using Tug(s) with Mooring Lines 20

8.14 Barge Positioning Using a DP System 20 8.15 Host Structure – Structural Considerations 20 8.16 Floating Host Structure – Additional Structural Considerations 20 8.17 Floating Host Structure – Freeboard, Stability and Reserve Buoyancy 21 8.18

Deck Removal 21

9 STRUCTURAL STRENGTH 22 9.1 Codes 22 9.2

Seafastenings 22

9.3

Fenders and Guides 22

10 PUMPING AND BALLASTING 23 10.1 Principles 23 10.2 Host Structure Ballasting Equipment 24


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10.3

Tidal Limitations 24

10.4 Pumping Capacity 24 10.5

Ballasting and Gauging Systems 27

11 MARINE EQUIPMENT 28 11.1 General 28

11.2

Installation / removal Barge Systems 28

11.3 Leg mating units & Deck Support Units 28 11.4

Dynamically Positioned Vessels 28

12

ASSIST TUGS AND SUPPORT VESSELS 29

13 FLOAT-OVER MONITORING EQUIPMENT 30 13.1 Leg Markings / Leg Access 30 13.2

Motions / Clearance Monitoring 30

13.3 Environmental Monitoring 30

14

MARINE OPERATIONS PROCEDURES 31

14.1 Principles 31 14.2 Weather Forecasting 31

14.3

Installation / Removal Manual 31

15 PROJECT SAFETY AND CONTROL 33 15.1

Safety 33

15.2 Management and Organisation 33

REFERENCES 34

APPENDIX A - CHECK LIST OF INFORMATION REQUIRED FOR APPROVAL 35

TABLESTable 10-1

Installation / Removal Class 24

Table 10-2 Sub-Notation for Weather Limitations 24 Table 10-2

Required Pumping Capacity 26

Table 10-3

Example of required pumping capacity calculation 26


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1 SUMMARY1.1

This document has been developed to provide guidelines for the float-over installation or removal ofstructures onto or from a host structure which may be:

A pre-installed structure, e.g. jacket and concrete unit, or A floating structure, e.g. TLP, concrete unit and semi-submersible.

This document is intended to primarily address offshore operations and may be found to be tooconservative for inshore or sheltered water operations. In such cases reference may be made to DNV-OS-H201, Ref. [7], which provides specific guidance for mating operations in sheltered waters.

1.2 These guidelines are intended to lead to an approval by GL Noble Denton, which may be soughtwhere an operation is the subject of an insurance warranty, or where an independent third party reviewis required.

1.3

A description of the Approval Process is included, for those projects which are the subject of aninsurance warranty.

1.4

The document includes the requirements for consideration, intended to represent good practice, for thefollowing phases:

Engineering

Barge selection

Positioning and manoeuvring

Set down

Barge removal

Operational considerations.

1.5 Check lists are appended, to act as a guide to information required.

2


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2 INTRODUCTION

2.1 GENERAL

2.1.1

This guideline refers to the offshore float-over of structures onto or off both fixed and floating

structures. It contains general recommendations and checklists of information required to allowapproval of such operations by GL Noble Denton.

2.1.2

Due to the range of float-over methods, this document does not cover all aspects of every scheme. Alternative proposals and methods will be considered on their own merits, and can be approved if theyare shown to be in accordance with safe and good engineering and operational practices.

2.1.3 This document is intended to primarily address offshore operations and may be found to be tooconservative for inshore or sheltered water operations. In such cases reference may be made to DNV-OS-H201, Ref. [7], which provides specific guidance for mating operations in sheltered waters.

2.2 START AND COMPLETION OF OPERATIONS

2.2.1

Transportation is generally defined as being completed when:

The towage or transportation has arrived at the installation / removal location and authorisationto start installation / removal operation has been received from relevant bodies, or

Installation / removal has started at the installation / removal site (typically within the 500mexclusion zone).

2.2.2

Installation or removal is generally defined as starting when:

all preparations for float-over /float-off have been completed, and

the prevailing weather and weather forecast are acceptable, and

the Certificate of Approval has been issued, and

cutting of seafastenings has started (for installations).

In specific cases, the start of the installation / removal may be defined as the point of handover ortransfer of control of the transportation barge to the installation / removal spread.

2.2.3 Installation / removal is generally defined as being completed when the structure is lowered on the hoststructure or removal barge according to agreed installation procedures and the transportation bargehas left the installation / removal site (e.g. the 500m exclusion zone).

2.3 OTHER GL NOBLE DENTON GUIDELINE DOCUMENTS & FEEDBACK

2.3.1 This document refers to, and should be read in conjunction with other GL Noble Denton Guidelinedocuments, particularly:

0001/ND – General Guidelines for Marine Projects, Ref. [1]

0013/ND - Guidelines for Load-Outs, Ref. [2]

0015/ND - Concrete Offshore Gravity Structures – Guidelines for Approval of Construction,Towage and Installation, Ref. [3]

0027/ND - Guidelines for Marine Lifting & Lowering Operations, Ref. [4]

0030/ND - Guidelines for Marine Transportations, Ref. [5]

0032/ND – Guidelines for Moorings, Ref. [6]

2.3.2

Care should be taken when referring to any GL Noble Denton guideline document that the latestrevision is being referenced.

2.4 Electronic versions of GL Noble Denton Guidelines are available on:

https://www.dnvgl.com/rules-standards/noble-denton-maa-rules-and-guidelines.html

2.4.1 Please contact the Technical Standards Committee Secretary at [email protected] with any queries orfeedback.

2

2

https://www.dnvgl.com/rules-standards/noble-denton-maa-rules-and-guidelines.htmlmailto:[email protected]:[email protected]:[email protected]:[email protected]://www.dnvgl.com/rules-standards/noble-denton-maa-rules-and-guidelines.html


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2.5 CODES AND LEGISLATION

2.5.1 These guidelines are intended to lead to an approval of a specific operation by GL Noble Denton.Such approval does not imply that approval by designers, regulatory bodies and/or any other partywould be given.

2.5.2

Care should be taken that the design and planning of an installation / removal operation comply withrelevant standards and national and international legislation, e.g. established offshore design codes(API, NORSOK, ISO, DNV, etc.) suitably augmented as necessary for local conditions by projectspecific design briefs.

2.6 REVISIONS

2.6.1

Revision 1 superseded Revision 0 of 6 December 2010. Main changes, marked with a vertical line inthe right hand margin, were:

Inclusion of removal of decks or similar structures in many sections.

Much of the Approval Process has been moved from Section 4 to 0001/ND, Ref. [1].

Extension of the use of Metocean Reduction Factors in Section 6.3 which now refers to Ref. [1].

Removal of the 10% risk level in the (previous) Section 6.5.

Weight control has been moved from Section 7 to Section 8.2.1 and refers to Ref. [1].

Dimensional control has been added to Section 8.2.4.

Additional infortmation on fendering and DP for deck removal in Sections 8.5 and 8.14.

Clarification of the use of the Monte Carlo simulations in Section 8.6.4.

Additional information on Deck Support Units (DSU) in Sections 8.6.6 and 11.3.4.

Changes to the minimum barge freeboard in Section 8.7.

Updates to Section 9.1 on Structural Strength which now refers to Ref. [1].

Changes for Barge Systems in Section 11.2, LMUs in Section 11.3.1 and DP vessels in Section

11.4. Safety and Project Control have been moved from Section 15 to 0001/ND, Ref. [1].

2.6.2 This Revision 2 supersedes Revision 1 of 22 June 2013. Main changes, marked with a vertical line inthe right hand margin, are:

Application of document to inshore float-over operations has been removed and reference isnow made to DNV-OS-H201, Ref. [7], in Sections 1.1, 2.1.3 and 8.8.9.

Updates to LMU definitions in Section 3.1.

Changes to the minimum barge freeboard requirements in Section 8.7.

Changes to the minimum vertical entry clearance in Section 8.8.2.

Changes to the minimum horizontal / vertical motions in Section 8.8.3. Additional requirement to confirm that actual vessel draughts correspond to design draughts in

Section 8.8.8

Changes to the operational class definitions in Section 10.3

Changes to the ballast pumping requirements and the addition of an example in Section 10.4

2


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3 DEFINITIONS AND ABBREVIATIONS3.1

Referenced definitions are underlined.

Term or Acronym Definition

50/50 weightestimate

The value representing the median value in the probability distribution ofweight.

AHV Anchor Handling Vessel

Approval The act, by the designated GL Noble Denton representative, of issuing aCertificate of Approval

ASD Allowable Stress Design (effectively the same as WSD)

Barge A non-propelled vessel commonly used to carry cargo or equipment. (for thepurposes of this document, the term Barge can be considered to includePontoon, Ship or Vessel where appropriate)

Certificate of Approval

A formal document issued by GL Noble Denton stating that, in its judgementand opinion, all reasonable checks, preparations and precautions have beentaken to keep risks within acceptable limits, and an operation may proceed

DP Dynamic Positioning or Dynamically Positioned

DSU /Deck Support Unit

Unit installed on the barge grillage to support the structure prior to and duringthe floatover. It can be designed to either provide a rigid vertical support andallow horizontal movement or utilise elastomers to absorb vertical andhorizontal installation motions and forces.

Float-Over The operation of installation / removal of a structure onto or from a hoststructure by manoeuvring and ballasting the transport barge to effect load

transfer

FMEA orFMECA

Failure Modes and Effects Analysis orFailure Modes, Effects and Criticality Analysis

Freeboard Freeboard is defined as the distance from the waterline to the watertightdeck level. In commercial vessels, it is measured relative to the ship's loadline.

GBS Gravity Base Structure

GL Noble Denton The legal entity trading under the DNV GL or GL Noble Denton name whichis contracted to carry out the scope of work and issues a Certificate of Approval, or provides a marine related advisory or assurance service.

HAT Highest Astronomical Tide

HAZID Hazard Identification review

HAZOP Hazard Operability review

Heave Barge motion in a vertical direction

Host Structure The host structure (e.g. jacket, GBS, TLP) onto which the structure orstructure deck will be floated and supported, or from which it will be removed.

Insurance Warranty A clause in the insurance policy for a particular venture, requiring theapproval of a marine operation by a specified independent survey house

LAT Lowest Astronomical Tide

2

2


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LMU /Leg Mating Unit

Unit that is designed and installed between the structure and the hoststructure in order to absorb vertical and horizontal installation motions andforces. The units are normally either installed on the host structure legs to

receive the structure, or on the structure leg stubs in order to interface withthe host structure legs. LMU’s can be also installed on the removal barge.

LRFD Load and Resistance Factor Design

LSF /Load-out SupportFrame

Frame that spans between the underside of the structure and the barge /vessel load spreading grillage

MRU Motion Reference Unit

MSL Mean Sea Level

NDT / Non

Destructive Testing

Ultrasonic scanning, magnetic particle inspection, eddy current inspection or

radiographic imaging or similar. May include visual inspection.

NTE weight / Not ToExceed weight

Sometimes used in projects to define the maximum allowable installation/ removal weight of a structure, excluding rigging.

Operation Duration The planned duration of the operation from the forecast prior to the Point ofNo Return to a condition when the operations /structures can safelywithstand a seasonal design storm (also termed “safe to safe” duration); thisexcludes the contingency period.

Operationalreference period

The Operation Duration, plus the contingency period

PNR /

Point of No Return

The last point in time, or a geographical point along a route, at which an

operation could be aborted and returned to a safe condition

RCS / RecognizedClassificationSociety

Member IACS with recognized and relevant competence and experience inspecialised vessels or structures, and with established rules and proceduresfor classification / certification of such vessels /structures underconsideration.

Sand Jacks A compartment filled with sand that is incorporated into the LMU to allow thefinal controlled lowering of the structure onto the host structure

Seafastenings The means of restraining movement of the loaded structure or cargo on orwithin the barge or vessel

Skidshoe A bearing pad attached to the structure which engages in the skidway andcarries a share of the vertical load

Skidway The lower continuous rails, either on the quay or on the barge, on which thestructure is loaded out, via the skidshoes

Structure The object to be floated onto a host structure

Surge Barge motion in the longitudinal direction

Survey Attendance and inspection by a GL Noble Denton representative.

Other surveys which may be required for a marine operation, includingsuitability, dimensional, structural, navigational and Class surveys.

Surveyor The GL Noble Denton representative carrying out a survey. An employee of a contractor or Classification Society performing, forinstance, a suitability, dimensional, structural, navigational or Class survey.

2


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Sway Barge motion in the transverse direction

Tether A tether is a mooring line used for pulling and mooring the installation

/ removal barge into the required positionTidal range Where practicable, the tidal range referred to in this document is the

predicted tidal range corrected by location-specific tide readings obtained fora period of not less than one lunar cycle before the operation.

TLP Tension Leg Platform

TMPS /Tug ManagementPositioning System

A system installed on the AHV and the Installation / removal barge to allowthe accurate placing of the tug and anchors.

Vessel A marine craft designed for the purpose of transportation by sea orconstruction activities offshore. See Barge

Weather restrictedoperation

A marine operation which can be completed within the limits of anoperational reference period with a weather forecast not exceeding theoperational criteria. The operational reference period (which includescontingencies) is generally less than 72 hours.

The design environmental condition need not reflect the statistical extremesfor the area and season.

An alpha factor shall be accounted for in defining the design environmentalcondition. See Section 7.4.8 of 0001/ND, Ref. [1].

Weatherunrestrictedoperation

An operation with an operational reference period greater than the reliablelimits of a weather forecast. The operational reference period (whichincludes contingencies) is generally more than 72 hours. The designweather conditions must reflect the statistical extremes for the area andseason.

The design weather is defined in Section 7.3 of 0001/ND, Ref. [1].

WSD Working Stress Design (effectively the same as ASD)


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4 THE APPROVAL PROCESS & SAFETY

4.1 GENERAL

4.1.1 Further information on the approval process appears in DNVGL-SE-0080 - Noble Denton marineservices – Marine Warranty Survey, Ref. [8].

4.2 SCOPE OF WORK LEADING TO AN APPROVAL

4.2.1 In order to issue a Certificate of Approval for a float-over operation, GL Noble Denton will typicallyconsider the topics and information listed in Appendix A.

4.2.2

A Certificate of Approval for a float-over operation covers the marine operations involved in the float-over procedure. Float-Over is normally deemed to start at the time when all preparations for float-overare complete (moorings installed, DP system checks completed, host structure preparations complete,structure prepared) and a suitable weather forecast is received showing that environmental conditionsare expected to be less than the design allowables for the maximum duration of the operation,including contingencies.

4.2.3

Surveys required typically include preliminary surveys of the barge, structure and site; attendance atweighing and load-out operations; surveys of readiness to start float-over and witnessing of float-overoperations.

4.3 LIMITATION OF APPROVAL

4.3.1 See Section 4.7 of DNVGL-SE-0080 - Noble Denton marine services – Marine Warranty Survey,Ref. [8].

4.4 HEALTH, SAFETY AND ENVIRONMENT

4.4.1 See Section 5 of 0001/ND “General Guidelines for Marine Projects”, Ref. [1] for more details.

4.4.2 During the float-over operation there will be a number of simultaneous hazardous activities carried outin a relatively short period of time. The Surveyor, and all others involved in float-over operations,

should be aware of these hazards and participate in the operational safety briefings. The hazardsinclude:

Wires / ropes under tension

Working at height

Trip hazards, grease on deck and hydraulic oil leaks

Openings in the barge deck

High pressure hoses / equipment

Temporary access bridges / scaffolding / wire hand railing

Hot works

Overside working Green water on the deck of the barge

Moving equipment and systems

Relative motions between the deck, host structure and barge.


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5 LOAD-OUT, TRANSPORTATION & MOORING

5.1 STRUCTURE LOAD-OUT

5.1.1

Structure load-out shall be generally carried out in accordance with GL Noble Denton Load-Out

guidelines 0013/ND Ref. [2].

5.2 STRUCTURE TRANSPORTATION

5.2.1 Structure transportation shall be generally carried out in accordance with GL Noble Denton MarineTransportation guidelines 0030/ND Ref. [5].

5.3 IN-FIELD MOORING

5.3.1

In-field mooring of the transportation barge shall generally be carried out in accordance with Section8.9 to 8.13 of these guidelines and GL Noble Denton Moorings Guidelines 0032/ND, Ref. [6].


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6 DESIGN ENVIRONMENTAL CONDITIONS

6.1 PRINCIPLES

6.1.1

For each phase of a marine operation / float-over installation / removal, the limiting design criteria

should be defined, consisting of the design wind speed, wave height and period range and currentspeed. Directional criteria can be specified. Co-linearity should be assumed unless location dataindicates otherwise.

6.1.2 Final selection of the limiting design environmental conditions should be based on the following:

Weather windows analysis

Motions / clearances / interface forces during the installation / removal

Structural strength of the barge, structures and the installation / removal components

Mooring capability assessment (if applicable)

DP capability assessment (if applicable)

Cost benefit analysis considering the items listed above.

6.2 OPERATIONAL REFERENCE PERIOD

6.2.1

Planning and design of the float-over installation / removal shall be based on an operational referenceperiod equal to the planned duration of the operation plus a contingency period.

6.2.2 The planned duration for the installation / removal shall include, typically:

The time anticipated, after the decision to proceed, preparing for installation / removal or waitingfor the correct tidal conditions

The time anticipated for the installation / removal itself.

6.2.3 The contingency period shall include allowances for:

Slower than predicted installation / removal operation, and

Possible mechanical breakdown of key items of equipment.

6.3 WEATHER RESTRICTED OPERATIONS

6.3.1 Section 7.3 of 0001/ND, “General Guidelines for Marine Projects,” Ref. [1], applies for all weather-restricted operations.

6.3.2 Unless agreed otherwise with GL Noble Denton, for marine operations the maximum forecastenvironmental conditions shall not exceed the design wave heights, wind and current speeds multipliedby the Alpha Factor from Table 7-3 of 0001/ND, Ref. [1].

6.4 WEATHER UNRESTRICTED OPERATIONS

6.4.1

For operations that could last more than 1 month, please see Table 7-1 of 0001/ND, Ref. [1].

6.4.2 For other operations, except as allowed in Section 7.3 of 0001/ND, Ref. [1], installations / removalswith an operational reference period greater than 72 hours shall be defined as un-restricted operationsfor the complete operation (this may include connection to and disconnection from a mooring systemas applicable). In such cases the design shall be to the 10-year return period monthly extremes, orany lesser conditions that cause more onerous responses.


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6.5 OPERATIONAL FEASIBILITY

6.5.1 It is recommended that a weather windows analysis be undertaken to demonstrate that weatherwindows with less than the operational design environmental conditions will have a good probability ofoccurrence within the proposed installation / removal period. The weather window analysis results for

a selected installation / removal period should clarify the following: An average duration of the weather windows

The number of windows in a given season

Sensitivity to critical operational design conditions and durations.


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7 CONSIDERATIONS INFLUENCING BARGE SELECTION CRITERIA FORLOAD-OUT, TRANSPORTATION & FLOAT-OVER

7.1.1 The selected barge (in the event that the proposed barge is being converted from its normal usage)

must satisfy certain criteria to ensure suitability for all phases of the operation, including load-out,transportation and the float-over operation.

7.1.2 The barge selection process involves several considerations with regard to geometry, other than thefactors relating to the float-over operation. Relative heights of grillage and LSF have to be optimisedso that grillage height suits load-out by fixing the barge elevation at the quayside to match the yardskidway height, and the combined grillage plus LSF height suits float-over clearances.

7.1.3 Important parameters are:

1. The barge beam and depth must be appropriate for the host structure slot

2.

Barge depth considering minimum and maximum draught, quayside height and tidal range forload-out

3. Grillage minimum height requirement for load spreading considering quayside skidway height.This fixes barge elevation at quayside during load-out

4.

Combined grillage plus LSF height to achieve required clearances at float-over, alsoconsidering effect on barge stability and minimum draught

5. Adequate depth and ballast capacity and ballasting rate to achieve minimum and maximumdrafts for the float-over considering tidal variations

6. Global and local strength to ensure structural adequacy of the barge and attachments for allstages during the float-over, including any contacts /collisions

7.

Adequate stability for the transportation (see 0030/ND, Ref. [5]) and to meet all Flag Statestability requirements

8. Adequate freeboard (to avoid green water) and stability at all stages of the operations

9.

Adequate motion characteristics at all stages of the operations.

7.1.4 Submersible barges. Barges that can be totally immersed in the intact condition should be classedas submersible barges. Submersible barges are normally classed as such by the RCS (RecognizedClassification Society).


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8.5 FENDERS AND/OR TETHERS

8.5.1 Devices to assist or control the safe entry of the installation / removal barge into the host structure slotshall be provided. These devices can be on the installation / removal barge, the host structure, orboth. The engineering properties (strength, stiffness, damping, hysteresis, elastomeric creep) of all the

components and systems for mating shall have been verified by tests which cover the full range ofconditions (e.g. forces, displacements) anticipated for the installation / removal operation.

8.5.2 Fenders. In most cases a suitable fendering system will be required to reduce stresses in the hoststructure and barges in the event of contact by spreading the load and limiting the relative motions.Fenders on the host structure should be of sufficient depth to ensure that they engage the side of thebarge at all stages of the floatover operation.

8.5.3

Tethers. During the mating operation, the barge may be held in position by tethers connected to thehost structure reacting against the surge fenders (if fitted). Alternatively tethers may operate in bothdirections, replacing surge fenders.

8.5.4 No fenders or tethers. In exceptional cases when DP is proposed as the primary method of barge

station keeping, fenders or tethers may not be used but any possible impact between the barge and

the structure shall be demonstrated to have a low probability of occurrence and minimal consequenceby rigorous risk assessments and analysis respectively, and shall be agreed by GL Noble Denton. Insuch situations:

a.

The requirements in Section 8.14 shall be used to show that, in the proposed operationalenvironmental conditions, accidental contact between the host structure legs and the bargewill not cause significant damage.

b.

Contact velocities and forces shall be determined from a comprehensive range of realisticscenarios. The damage to the barge, structure and host structure shall be quantified andassessed against the probability of the incident occurring in order to provide a suitable lowlevel of risk.

c. Minimum static vertical and horizontal clearances between the host structure and float-off

vessel shall be established. As a guide, minimum static horizontal clearances of 5m betweenthe extremities of the host structure and the float-off vessel shall be provided if using vesselswith a Class 3 notation and with DP reference systems that meet the Class 3 notation.

8.6 MOTIONS AND MATING STAGES

8.6.1 The motions of the transportation barge and associated docking, mooring line and fender loads shallbe analysed in the time domain for docking, load transfer and undocking positions, including non-lineareffects of the stiffness of the host structure / deck / barge, mooring configuration, shock absorbers,fendering system, etc.

8.6.2 The motions of the barge and associated docking, mooring line and fender loads shall be analysed inthe time domain for several docking and undocking positions such as:

Pre-docking, free floating motions with the barge aligned with the first row of the host structure; Docking, intermediate stage with the whole barge engaged with the host structure;

Docked, with the barge offset from the pre-mating position, prior to finally tensioning the matingmoorings;

Undocking, with the barge in an offset position, after full transfer of the deck weight to the hoststructure and release of the mating moorings.

8.6.3

Mating stages shall be selected and analysed in the time domain to identify associated loadings to allthe interfaces associated in the weight transfer process. As a minimum this shall include bargedocking, load transfer and barge removal:

Pre-mating, with the barge and structure positioned in the host structure, and aligned with but

prior to engaging the stabbing cones or positioning system(s) on the LMU’s / host structure;

First contact between the structure and host structure;


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Intermediate load transfer, with the structure weight partially transferred from the barge to thehost structure, without any separation or lift off at the support points;

Last contact between the structure and the support point on the barge;

Post-load transfer, with the vessel positioned in the host structure after complete separation

from the host structure.

8.6.4 A Monte Carlo simulation or multiple seed simulation shall be performed to define maximum values.The simulation period for each stationary stage shall reflect the actual operational period multiplied bya factor of two to capture a contingency period. The time step to be used shall be selected so as toachieve results that differ by no more than a few percent when the time step is halved and besufficiently small to ensure that the maximum peak motion is identified.

When a Monte Carlo simulation is used the design value shall have a probability of exceedance of notmore than 63% and the number of simulations shall be such that the design values change by no morethan 10% when the number of simulations is doubled.

When a multiple seed simulation is performed, the number of seeds shall be no fewer than 10 and the

average of the maxima shall be used as the design value.Note: Design values determined from the above are applicable only when the operational metoceanlimits are reduced below the design values with the applicable Alpha Factor(s) from 0001/ND, Ref. [1].

8.6.5

The Leg Mating Unit (LMU) design shall be carried out taking into account the loads, stroke and motionresponse expected to be applied during load transfer operations. The LMU performancecharacteristics shall be included in the mating analysis.

8.6.6

Where a Deck Support Unit (DSU) is included as part of the load transfer system, the design shall becarried out taking into account the loads, stroke and motion response expected to be applied duringload transfer operations. The DSU performance characteristics shall be included in the matinganalysis. If the DSU is configured without shock absorbers but has a low friction mating surface toallow the vessel to move freely during the floatover operation, the mass of the topside can be

considered independent from the barge so that the vessel’s inertia does not add to any significanthorizontal loading to the LMU /top of jacket.

8.6.7

If any other passive or active heave-compensation systems are used to compensate for relativemotions, the specification, capacity and design of these systems shall be stated in the operationalprocedures.

8.6.8

An assessment shall be made to consider the speed at which the structure and barge can separateduring installation. As the barge starts to separate from the structure there will be a tendency for re-contact at the LSF / structure interface due to the barge motions. Mitigations shall be considered toavoid damage to structure and LSF / barge. During removal operations the vertical speed beforemating shall be demonstrated.

8.6.9

Where a rapid load transfer is required, a jacking system may be utilised. The jacking system shall bedesigned to ensure the stability and restraint of the structure as it is raised above / lowered to itstransportation position. Redundancy shall be provided in the jacking system so that there is no singlepoint of failure in the system. Detailed HAZIDs will be required of the jacking system if used.

8.6.10 Where float-over operations are conducted in the shelter of a breakwater (e.g. for tanker loadingfacilities at coastal locations), the adverse effects of the breakwater on the waves and current shouldbe considered when determining the environmental loading on the installation / removal barge.

8.7 DRAUGHT AND FREEBOARD

8.7.1 The maximum draught of the installation / removal barge during float-over shall not exceed themaximum loadline draught, without a class exemption (but this is not normally needed for semi-submersible heavy lift barges or vessels). For operations involving semi-submersible barges or heavylift ships with watertight main decks, wave crests may be allowed to over-top the vessel deck provided

that all hatches and downflooding points are suitably protected and that raised walkways are added toall areas affected by water on deck where personnel movement is required.

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8.7.2

The minimum freeboard is defined as the minimum distance from the waterline to the watertight decklevel after accounting for static trim and heel. The minimum freeboard shall be sufficient to maintainthe vessel’s water-plane area and to ensure sufficient stability range to meet the requirements ofSection 8.4.1 at all stages of the operation, the minimum freeboard should usually be at least 1.0m. Alower minimum freeboard may be acceptable if adequate precautions and procedures are in place toensure that the stability of the vessel is maintained. The minimum freeboard used during the operationshall be confirmed with the barge owner.

8.8 CLEARANCES

8.8.1

Exclusion zones should be defined in the early phase of the project in order to minimise and avoidclashes during the installation / removal operation.

8.8.2 During approach of the structure for installation, the minimum vertical clearance between the structurestabbing cone and host structure receptacles / jacket leg / piles shall be 0.5m after accounting formaximum dynamic vertical motions. The same minimum vertical clearance applies during removaloperations for relevant structures and stabbing aids.

8.8.3 The maximum vertical / horizontal movement of the stabbing cone should not normally exceed +/-

0.5m during entry and weight transfer unless suitable systems and/or engineering are provided tocompensate for movements in excess of +/- 0.5m.

8.8.4 The as-built clearances between the stabbing cones and the host structure receptacles / jacket legsshall be checked after the load-out operation for installation and the procedures modified, if necessary.Similarly for removal operations the relevant measurements shall be checked before operations start.

8.8.5 To allow safe removal of the installation / removal barge the minimum clearance between the keel ofthe installation / removal barge and any part of the submerged host structure shall be 1.0m afteraccounting for vessel maximum motions at maximum draught.

8.8.6 The minimum vertical clearance between the LSF and the underside of the structure followingcompletion of load transfer shall be 0.5m after accounting for vessel maximum motions to allow saferemoval of the installation / removal barge.

8.8.7

The as-built levels of the host structure should be verified as they can differ from the nominal valuesdue to seabed tolerances, especially where dredging operations are carried out.

8.8.8 The minimum clearances shall be calculated based upon the design draught of the vessel. The actualvessel draughts shall be verified prior to commencing the operation to confirm that they are inaccordance with requirements and that acceptable clearances are achievable. Note that the datumused for vessel draughts may need to be corrected to account for any parts of the vessel that extendbeneath the datum.

8.8.9 For inshore clearance requirements, reference may be made to DNV-OS-H201, Ref. [7], whichprovides specific guidance for operations in sheltered waters.

8.9 BARGE MOORING AND POSITIONING OVERVIEW

8.9.1

Mooring and positioning of the barge into the host structure can be with a barge with a DP system (seeSection 8.14) or by one or more of:

a. pre-laid mooring lines/anchors (see Sections 8.10 and 8.11),

b. tethers (see Section 8.12).

c.

anchored tugs or free-running tugs (see Section 8.13).

d.

using the barge’s propulsion systems with constant thrust against surge fenders.

8.10 BARGE MOORING AND STAND-OFF MOORINGS

8.10.1 A stand-off mooring system shall be provided except for vessels with suitable DP systems, where a DPCapability analysis shall be provided.

8.10.2

When required, the installation / removal barge mooring system shall be designed to resist theenvironmental loads, allowing the barge to maintain position prior to docking. Line integrity and anchoruplift shall also be verified for operational AND extreme environment standby cases.

2

2

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8.10.3

A mooring analysis shall be carried out for the installation / removal barge at the stand-off location andat the incremental stages that comply with the installation / removal procedural steps. The mooringanalyses shall satisfy the requirements of GL Noble Denton Mooring Guidelines, 0032/ND, Ref. [6].

8.10.4 All installation / removal barge mooring lines and tethers shall be capable of being tensioned by the

use of winches or capstans.8.11 CLEARANCES AROUND MOORING LINES AND ANCHORS

8.11.1

Clearances around mooring lines and anchors should comply with GL Noble Denton MooringGuidelines, 0032/ND, Ref. [6]. Exemptions may need to be made for the final float-over stages whenclose to the host structure; these shall be subject to risk assessment and the results agreed with GLNoble Denton.

8.11.2 All anchor lines shall be pre-installed and pre-tensioned to maximum operating loads with a safetyfactor and holding period to be agreed.

8.12 POSITION KEEPING DURING MATING /DE-MATING WITH TETHERS

8.12.1 The tethers shall be designed to hold the barge in the mating /de-mating position to ensure that the

barge motions do not exceed the capture radius of the LMU’s during the mating /de-mating operation.The characteristics of the tethers shall be accurately modelled in the analysis.

8.12.2 Temporary mooring tethers shall be designed for the maximum analysed dynamic tensions and sizedbased on a factor of safety of 1.67 against the certified break load.

8.12.3

The barge may be also held in position using the barge ’s propulsion systems with constant thrustagainst surge fenders.

8.13 BARGE POSITIONING USING TUG(S) WITH MOORING LINES

8.13.1 A combination of mooring lines and tug(s) can be used for barge positioning. Tugs connected toprelaid moorings may be used to provide extra control in variable currents, using their tow winches toadjust the barge position.

8.14

BARGE POSITIONING USING A DP SYSTEM8.14.1 If the barge has dynamic positioning capability (minimum DP Class 2), consideration can be given to

the use of DP in place of barge moorings, subject to review of station keeping analyses and DPoperating procedures. The requirements in Section 13 of 0001/ND, Ref. [1], will apply.

8.15 HOST STRUCTURE – STRUCTURAL CONSIDERATIONS

8.15.1 The host structure will be subject to a variety of loading conditions during the docking and matingoperations. The host structure shall be documented to have adequate global and local strength towithstand such loading. Accidental loadings shall also be considered. For decommissioning, it maybe possible to accept local host structure damage, provided that nothing falls off the structure, and theglobal strength of the host structure shall be demonstrated to show no loss of strength to support thetopside.

8.15.2

Horizontal restraint shall be provided between the barge and the host structure to absorb shock loadsduring mating and to prevent lateral movement of the deck after initial engagement of the two parts.

8.15.3

The effects of un-even load distribution during the mating operation shall be calculated anddocumented.

8.15.4 In the case whereby a float-over operation is planned to be executed without the use of fenders, theglobal and local capacity of the host structure shall be documented for an accidental impact scenario(see Section 8.5.4).

8.16 FLOATING HOST STRUCTURE – ADDITIONAL STRUCTURAL CONSIDERATIONS

8.16.1 Hydrostatic loads on the host structure at the deepest draught can be the governing loadcase. It shallbe demonstrated that a thorough independent check of the calculations covering this loadcase has

been carried out, and that the design and reinforcement details assumed in the calculations concurwith the as-built condition.


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8.16.2

Any limitations on the maximum allowable duration of deep immersion, in relation to the structuralstability of the host structure, should be established and the procedures planned accordingly.

8.16.3 The horizontal restraint between deck and floating host structure after mating / before de-mating shallbe capable of taking the loads resulting from the inclination due to the design storm for the deck mating

/de-mating window, or an inclination of 5 degrees, whichever is the greater, at any applicable draught.Friction may be taken into consideration, but a safety factor against sliding of at least 3 shall bedocumented.

8.17 FLOATING HOST STRUCTURE – FREEBOARD, STABILITY AND RESERVE BUOYANCY

8.17.1 The stability and reserve buoyancy for a floating host structure shall be agreed on a case by casebasis for intact and damaged conditions. In general it will not be possible to have one compartmentdamaged stability at mating / de-mating draught so alternative arrangements must be made to keepthe Risk As Low As Reasonably Practicable (ALARP).

8.17.2 Freeboard to the lowest downflooding point on a concrete gravity base host structure shall never beless than 6 metres, and the stability requirements of 0015/ND, Ref. [3] shall also apply.

8.17.3

The time at the maximum draught, when reserve buoyancy is at a minimum and structural loadings

could be at a maximum, should be as short as possible.

8.17.4 Additional freeboard will be required to allow for the response of the host structure, unless pitch, rolland heave can be shown to be negligible under the conditions expected.

8.17.5 The substructure without the deck shall be capable of being deballasted to a freeboard at which thehost structure has damage stability within 24 hours. An initial deballasting capability of not less than 2metres per hour is recommended.

8.17.6 It should be possible to deballast the deck off the barge(s) / ballast the deck onto the barge(s) withinthe planned weather window. If this is not practicable then the substructure / deck / barge combinationshall be able to survive the 10 year return period seasonal storm at an intermediate condition.

8.17.7 Provision shall be made for the detection of any likely movements of fresh water (freshets) that couldcause significant draught changes. The host structure shall always be able to deballast to maintainfreeboard.

8.17.8 Before the immersion for deck-mating, a comprehensive commissioning and testing programme shallbe agreed, undertaken and documented to prove the integrity of the host structure (and systems).

8.17.9

Risk management techniques, as outlined in Section 5.4 of 0001/ND, Ref. [1], shall be applied asappropriate.

8.18 DECK REMOVAL

8.18.1 Where mechanical devices are used to effect load transfer to the removal vessel, the design capacity,functionality and certification of these systems shall be provided for review. The method of use ofthese systems, and testing to ensure that they will function correctly, shall be detailed in theoperational procedures. This should include any:

Passive or active heave compensation systems

Lifting or load transfer devices

Motion compensation systems (vertical and horizontal – as applicable)


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9 STRUCTURAL STRENGTH

9.1 CODES

9.1.1

The primary structure shall be of high quality structural steelwork with full material certification and

NDT inspection certificates showing appropriate levels of inspection. It shall be assessed using themethodology of a recognised and applicable offshore code including the associated load andresistance factors for LRFD codes or safety factors for ASD/WSD codes. Further details appear inSection 9 of 0001/ND, Ref. [1].

9.1.2

The extent of NDT testing shall be submitted for review.

9.2 SEAFASTENINGS

9.2.1

Seafastenings on the installation / removal barge shall be designed to:

Resist seafastening forces during the voyage to /from the float-over location (see 0030/ND,Ref. [5])

Minimise offshore cutting or welding, possibly by the use of mechanical devices

Provide restraint after cutting (for installation) or after landing on the barge (for removal)equivalent to 5% of the structure weight acting horizontally

Permit separation (for installation) or mating (for removal) without fouling.

9.2.2 A design case shall be established for any seafastenings that remain after initial seafastening cuttingwith the installation barge in a stand-off position prior to the barge being manoeuvred into the dockingslot.

9.2.3 All cut lines should be clearly marked. If cutting in 2 stages, the two sets of cut lines should preferablybe marked in different colours.

9.2.4 Where a jacking system is used to achieve clearances during the initial docking and subsequent

operations, the jacking system shall be suitable to provide lateral restraint equivalent to 10% of thestructure weight acting horizontally.

9.2.5 Where mechanical seafastening systems are used their capacity to resist design loads shall bedemonstrated.

9.3 FENDERS AND GUIDES

9.3.1 The design of the system shall be such that it acts to reduce the motions of the installation / removalbarge, provide protection to the host structure and guidance for barge entry.

9.3.2 Barge fender design loading shall be derived from the mating analysis. The design friction coefficientshall account for any facings applied to the host structure fenders.

9.3.3

Surge fenders (if applicable) may be fitted to the barge to control the longitudinal position during the

mating operation by making contact with the host structure fenders.9.3.4

Unless agreed otherwise, sway fenders shall be fitted to the barge sides to reduce the clearancebetween the host structure and the barge during the mating /de-mating operation and so improve thelateral positioning relative to the structure supports. Barge side-shell strength shall be checked for itscapacity to resist the applied fender loads.

9.3.5 To facilitate the docking operation, docking guides (if applicable) shall be provided on either side of thedocking end of the installation / removal barge(s).


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10 PUMPING AND BALLASTING

10.1 PRINCIPLES

The required pumping capacity shall be determined from

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10.1.1

Table 10-3 as a function of the installation / removal class determined from Section 10.3 for the set ofsystem conditions given in section 0.

10.1.2 A detailed ballasting procedure shall be developed for each stage of the installation / removaloperation. The ballast calculations shall include the quantity of water in each ballast tank for each

installation / removal stage. The ballast procedure shall consider float-over clearances, keelclearance, load transfer, tidal range, expected timings and barge freeboard.

10.1.3 Allowances shall be made for the effects of residual ballast water in near-empty tanks and achievablemaximum ballast quantities in full tanks.

10.1.4

The ballast sequence should be developed so that, if possible, separate tanks are used for tidalcompensation and weight compensation.

10.1.5 A risk assessment of the effects of potential errors in ballasting shall be undertaken, documented andthe results accepted by GL Noble Denton.

10.2 HOST STRUCTURE BALLASTING EQUIPMENT

10.2.1 The ballasting equipment used for floating concrete gravity structures shall be designed, constructedand operated in accordance with Section 12 of 0001/ND, Ref. [1].

10.2.2

Before the immersion for deck mating/de-mating, a comprehensive commissioning and testingprogramme shall be agreed, undertaken and documented to prove the integrity of the ballast systems(and structure).

10.3 TIDAL LIMITATIONS

10.3.1

The installation / removal operation will be classed according to the tidal conditions and whether it is aweather restricted operation or a weather unrestricted operation. Requirements for design, reservesand redundancy of mechanical systems will vary according to the class of installation / removal. Subnotations declaring if the installation / removal is to be weather restricted operation or weatherunrestricted operation shall be as per Table 10-2.

Table 10-1 Installation / Removal Class

Class Tidal Limitations

1 The tidal range is such that regardless of the pumping capacity provided, it is not possible tomaintain the mating interfaces with the required level throughout the full tidal cycle, and thefloat-over must be completed within a defined tidal window.

2 The tidal range is such that whilst significant pumping capacity is required, it is possible tomaintain the mating interfaces with the required level during the full tidal cycle, and for atleast 24 hours thereafter.

3

Tidal range is negligible or zero, and there are no practical tidal constraints on the float-overoperation. Pumping is generally required only to compensate for weight changes as the loadtransfer proceeds.

This class will also apply for mating over a floating host structure.

Table 10-2 Sub-Notation for Weather Limitations

Weather Limitations Sub-Notation

Weather Restricted a

Weather Unrestricted Operations b

10.4 PUMPING CAPACITY

The required pumping capacity is given in

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10.4.1

Table 10-3, depending on the Class of installation / removal as defined in Table 10-1 and to satisfyeach condition as defined below:

Condition A: Normal Operating ConditionThe pump capacity required for the normal operating condition. For Condition A,

100% pump capacity represents the nominal pump capacity required for theinstallation / removal operation at the planned speed (to compensate for tidalchanges and weight transfer) with no contingencies.

Condition B: Contingency requirement, load transfer haltedThe pump capacity required as a contingency, assuming the load transfer is halted.For Condition B, 100% pump capacity represents the nominal pump capacity tohold the mating interfaces with the same level, at the maximum rate of rising orfalling tide, assuming the load transfer is halted.

Condition C: Contingency requirement, one pump system failedThe pump capacity required, to continue the operation, at the maximum rate of arising or falling tide, assuming the failure of any one pump, component or pumping

system. Where two or more pumps are supplied from a common power source, thisshall count as a single system. For Condition C, 100% pump capacity is the samenominal pump capacity as determined for Condition A.

Condition D: Contingency requirement, load transfer halted and one pump system failedThe pump capacity required as a contingency, assuming the load transfer is haltedand the failure of any one pump, component or pumping system. Where two ormore pumps are supplied from a common power source, this shall count as asingle system. For Condition D, 100% pump capacity is the same nominal pumpcapacity as determined for Condition B.

10.4.2

Pump capacity shall be based on the published pump performance curves, taking account of themaximum head during the operation and pipeline losses.

10.4.3 If the barge pumping system is used as part of the main or back-up pump capacity, then a bargeengineer familiar with the system shall be attendance throughout the operation. The float-overcommunication system should include the pumproom.

10.4.4 All pumps and systems shall be tested and shown to be operational before transportation to theinstallation / removal site. At the discretion of the GL Noble Denton surveyor, a verification of pumpcapacity may be required.

10.4.5 Pumps which require to be reversed in order to be considered as part of the back-up capacity shall becapable of such reversal within 10 minutes, and adequate resources shall be available to perform thisoperation.

10.4.6 Pumps which require to be moved around the barge in order to be considered as part of the back-up

capacity shall be easily transportable, and may only be so considered if free access is provided at allstages of the float-over between the stations at which they may be required. Adequate resources shallbe available to perform this operation.

10.4.7 Where a compressed air system is used, the time lag needed to pressurise or de-pressurise a tankshould be taken into account, as should any limitations on differential pressure across a bulkhead. Itshould be remembered that compressed air systems cannot always fill a tank beyond the externalwaterline.

10.4.8

Where “drop tanks” are used to change vessel trim and draught, the operational features and control ofthese tanks as part of the ballasting system shall be documented.

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Table 10-3 Required Pumping Capacity

Installation/ removal Class ConditionPump capacity required, as a percentage of

nominal capacity for that condition

1a and 1b

(Tidal window)

A 200% or 150% see note 1

B 150%

C 120%

D 100%

2a

(Constant deck level >24hrs,weather restricted)

A 130%

B 150%

C 100%

D 100%

2b

(Constant deck level >24hrs,weather unrestricted)

A 130%

B 150%

C Contingency procedures – see note 2

D 100%3a

(Little tide, weather restricted)

A 130%

B No requirements

C 100%

D No requirements

3b

(Little tide, weather unrestricted)

A 130%

B No requirements

C Contingency procedures – see note 2

D No requirements

Note 1: Pumping capacity to meet the requirement of the maximum of the following percentages:

150% of the nominal capacity for the condition where the installation / removaloperation at the planned speed and the planned duration including contingenciesis less than the tidal window duration.

200% of the nominal capacity for the condition where the installation / removaloperation at a reduced speed with the combined duration for the reduced speedplus contingencies equal to the tidal window duration.

Note 2: The contingency procedure shall contain details covering pump system failure and theassociated actions required to correct the pump system failure.

10.4.9 The following Table 10-3 gives an example for a Class 2a operation that assumes that the worstsingle system failure reduces the pumping capacity to 80% of the full capacity (with any consistentunits).

Table 10-4 Example of required pumping capacity calculationCondition Nominal capacity Factor Required capacity

A 1,000 130% 1,300

B 1,100 150% 1,650

C 1,000 / 80% = 1,250 100% 1,250

D 1,100 / 80% = 1,375 100% 1,375

Required 1,650 (Condition B)

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10.5 BALLASTING AND GAUGING SYSTEMS

10.5.1 The installation / removal barge shall have a permanent ballast system if used offshore, and shall becapable of ballasting the barge within one tidal cycle (as applicable). The total ballast pump capacityshall be sufficient to maintain the required pumping rates with failure of any one primary pump or

power source. Control of the pumping systems and ballast valves shall be from a centralised ballastcontrol room.

10.5.2 The barge shall have a remote tank gauging system capable of continuously monitoring the level ofliquids in all ballast tanks simultaneously. The readout of the tank gauges shall be in the ballast controlroom. The ballast tanks shall also be fitted with sounding tubes or ullage access to allow manualmeasuring of the tank levels.


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11 MARINE EQUIPMENT

11.1 GENERAL

11.1.1

All the equipment on the installation / removal barge, including ship loose items, shall be properly

fastened to the deck for the tow and installation / removal phases.

11.1.2 The equipment installed on the installation / removal barge (e.g. winches, fairleads, towing andmooring lines, etc.) shall comply with the requirements of the Marine Warranty Surveyor and have validcertificates.

11.2 INSTALLATION / REMOVAL BARGE SYSTEMS

11.2.1 The installation / removal barge shall have electrical, hydraulic and/or pneumatic power plants with anindependent 100% back up to supply all power for installation operations. It shall have sufficientlighting to illuminate the complete barge deck and other operating areas to allow the float-overoperation to proceed safely on a 24-hour basis.

11.2.2 In particular, all critical systems must be shown to have adequate:

Reserve capacity Back-up power

Testing and commissioning before use

Failure mode identification and acceptability

Fail-safe condition (where practicable)

Over-rides and alternative controls for emergencies

Marinisation of key components

11.3 LEG MATING UNITS & DECK SUPPORT UNITS

11.3.1

Shock absorbers and leg mating units (LMUs) may be provided at the mating interface between thestructure and the host structure or removal vessel. The LMUs will dampen vertical and horizontal

motions and help to distribute the loads evenly.11.3.2 A hydraulic jacking system or a rapid ballast system may be used in combination with a mechanism

which allows for rapid transfer of the structure to /from the host structure and establishment ofclearance between the structure and the barge / LSF. The system shall be optimized to reduce boththe risks of weather downtime and the potential for impact damage between the structure, barge andhost structure.

11.3.3 Once the structure weight is fully transferred to/from the host structure, final lowering to achievesteel/steel contact may be required, often after barge removal.

11.3.4 Deck support units (DSUs) may be provided at the mating interface between the structure and thesupport structure on the vessel. The DSUs can be supplied with or without shock absorbers. WhereDSUs are supplied with shock absorbers, this will dampen vertical motions and help to distribute the

loads evenly. Where there are no shock absorbers in the DSU but there is a low friction slidingsurface, the topside can move freely in a horizontal direction thus reducing the horizontal loads into theLMU or top of jacket. See Section 8.6.6.

11.4 DYNAMICALLY POSITIONED VESSELS

11.4.1 Where DP vessels are planned to be used to execute a float-over operation, the requirements ofSection 8.14 will apply.


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12 ASSIST TUGS AND SUPPORT VESSELS12.1.1

A main tug capable of controlling the barge must be provided. This tug should be an AHV orequivalent.

12.1.2

Additional tugs can be required for the mating operations (anchor handling and barge matingassistance). When required these shall be highly manoeuvrable tractor tugs with a specification tomeet the needs of the operations. The tugs shall be classed for offshore work (if appropriate) andcrewed for 24 hour operations.

12.1.3

Any AHV used for anchor handling shall be fitted with a Tug Management Positioning System (TMPS)which is sufficiently accurate to allow anchors to be positioned within 5m of target.

12.1.4 An accommodation / work vessel may be required for offshore personnel and to permit host structurepreparations prior to, during and after the structure float-over. The vessel specification shall bedeveloped to suit the specific requirements of the project.

12.1.5

A work boat for personnel transfer shall be operated by a competent trained coxswain and havesufficient crew members to assist during personnel transfers.


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13 FLOAT-OVER MONITORING EQUIPMENT

13.1 LEG MARKINGS / LEG ACCESS

13.1.1

The identity of each leg shall be clearly marked with row and line reference.

13.1.2

Draught mark elevations shall be painted on the host structure legs. After host structure installation, asurvey shall note corrections to be made to the markings for accurate tide measurement. Levelmarkings shall be floodlit so that they are clearly visible during darkness.

13.1.3

Tide boards can be used in case the painted leg markings in the host structure are not adequate forthe installation / removal purposes.

13.1.4 Design elevations shown on the host structure legs shall relate to the lower edge of the mark, and shallbe clearly visible at a distance of not less than 50m and shall include increments at a maximum of200mm. Corrections by which these marks may be related to MSL, HAT or LAT shall be known.

13.1.5

Host structure leg access platforms shall be incorporated with safe access from the sea for operationand inspection of LMU’s. These platforms can also be used for host structure to deck leg weld out.

13.2

MOTIONS / CLEARANCE MONITORING13.2.1 The following critical factors shall be monitored using an MRU (Motion Reference Unit) for the float-

over installation / removal:

The six degrees of freedom motions of the barge in a free-floating mode. This is to ensure thatthe motions can be compared with those predicted by the motion analysis. These are usuallypresented as motions and accelerations at the system centre of gravity and should be used tocheck that the loads and clearances remain at acceptable levels.

The vertical clearance between the leg mating units (LMUs) on the structure, and the dockingcones on the underside of the host structure during the initial entry of the barge into the hoststructure, or any other critical vertical clearances during installation or removal operations.

13.2.2

The system shall be calibrated and tested prior to sailaway to the mating / removal site

13.3 ENVIRONMENTAL MONITORING

13.3.1 The environmental monitoring system has two primary functions:

To confirm that conditions are suitable for the docking and mating operations to proceed.

To provide input for the vessel or barge’s DP system (if applicable).

The secondary function is to predict weather and environmental trends prior to and during the float-over installation / removal.

13.3.2 The environmental conditions which require monitoring are:

Wind speed and direction Wave and swell heights and periods

Current speed and direction

Tidal height against time.

13.3.3

A tide gauge should be installed in the field, as close to the host structure as is practical, and should bemonitored for at least two tide cycles before installation / removal to allow actual levels and cycle timesto be compared with predictions. During installation / removal corrections derived from thiscomparison shall be used in conjunction with visual readings of the level marks on the host structurelegs. A tide gauge may also be fitted to the host structure for reference purposes.

13.3.4

An infield directional wave rider buoy and associated hardware recording wave height, direction, periodand spectral energy shall be considered for use to enhance operability of operations.


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14 MARINE OPERATIONS PROCEDURES

14.1 PRINCIPLES

14.1.1 If the marine operations (Load-out, Transportation, Float-over, Float-off) are performed by differentcontractors, then the scope split between the contractors must be clearly defined, to ensure that allparties are aware of their responsibilities, handover points and reporting lines.

14.1.2 The installation / removal procedure shall include detailed step by step procedures and contingencyprocedures for each phase of the installation / removal operation including all operational and limitingenvironmental conditions e.g. minimum and maximum tidal heights at all stages of the float-overoperation. Required weather windows for critical float-over operations shall be stated, referenced todetailed hourly installation / removal schedules.

14.1.3 Allowable environmental criteria and barge motions shall be established for each phase of theinstallation / removal by analysis. The decision to proceed from one phase of the installation / removalto the next shall be based on a comparison between the allowable environmental criteria for the nextphase, the data obtained from the environmental monitoring systems, MRU and weather forecasts.

14.1.4

The start of the barge docking procedure may consider a specific period of the tide cycle. However,after the installation / removal barge is docked in the slot, the installation / removal setup should beable to ensure the installation / removal can proceed safely for any specified tide.

14.1.5 Criteria for stopping or aborting each stage of the operation, and a critical “point of no return” for thefloat-over installation / removal shall be identified.

14.1.6

The installation crew shall be fully trained on the details of the installation / removal procedures and theoperation of all related equipment.

14.1.7 Detailed installation / removal barge mooring and anchor running procedures shall be developed takingdue account of the AHV and assist tugs being provided.

14.1.8 A readiness meeting shall be held shortly before the start of float-over, attended by all involved parties.

14.2

WEATHER FORECASTING14.2.1 See Section 7.4 of 0001/ND, Ref. [1], for more details.

14.3 INSTALLATION / REMOVAL MANUAL

14.3.1 A comprehensive Installation / removal Manual shall be written to identify all aspects of the operationsin detail, cover all likely contingencies and specify exactly how the float-over will be conducted.


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14.3.2

The following information (as a minimum) shall be included in the manual(s):

Installation / removal schedule (hourly)

Installation / removal test plan

Installation / removal organisational structure

Roles and responsibilities of key personnel

Communication procedure

Key contacts and installation / removal personnel information

Safety (emergency response procedures)

Environmental limitations for operations

Tidal and current predictions

Weather forecasting procedure

Weather windows studies

Support facilities and vessel information

DP design and operational requirements (as in Section 13 of 0001/ND, Ref. [1])

Barge station keeping procedures

Preparation check lists

Pre-departure activities

Pre-docking activities

Docking operation

Mating /de-mating operation

Un-Docking operation

Installation / removal related drawings

Ballast procedure

Change procedure

Installation / removal sequence drawings, anchor patterns and catenaries

General arrangement drawings of LMU, LSF, seafastenings, fenders

Detailed make up drawings and specifications of all mooring lines and tethers Specifications for all installation / removal equipment and systems


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15 PROJECT SAFETY AND CONTROL

15.1 SAFETY

15.1.1 See Section 5 of 0001/ND, Ref. [1], for information on Health, Safety and Environment.

15.2 MANAGEMENT AND ORGANISATION

15.2.1 See Section 6 of 0001/ND, Ref. [1], for information on Organisation, Planning and Documentation.


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REFERENCES

[1]

GL Noble Denton 0001/ND - General Guidelines for Marine Projects.[2] GL Noble Denton 0013/ND - Guidelines for Load-Outs.

[3] GL Noble Denton 0015/ND - Guidelines for Concrete Gravity Structure Construction & Installation.

[4]

GL Noble Denton 0027/ND - Guidelines for Marine Lifting & Lowering Operations.

[5]

GL Noble Denton 0030/ND - Guidelines for Marine Transportations.

[6] GL Noble Denton 0032/ND - Guidelines for Moorings.

[7] DNV-OS-H201 – Offshore Standards for Load Transfer Operations.

[8]

DNVGL-SE-0080 - Noble Denton marine services – Marine Warranty Survey..

GL Noble Denton Guidelines are available on:

https://www.dnvgl.com/rules-standards/noble-denton-maa-rules-and-guidelines.html

https://www.dnvgl.com/rules-standards/noble-denton-maa-rules-and-guidelines.htmlhttps://www.dnvgl.com/rules-standards/noble-denton-maa-rules-and-guidelines.htmlhttps://www.dnvgl.com/rules-standards/noble-denton-maa-rules-and-guidelines.html


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APPENDIX A - CHECK LIST OF INFORMATION REQUIRED FOR APPROVAL

A.1

STRUCTURE A.1.1 Structural analysis reports, including;

Structural drawings including any additional float-over/load-out steelwork

Description of structural analysis software used

Structural models and description of boundary conditions

Loadcases including derivation of weights and contingencies

Structural strength checks for members, jonits and connections

Justification of any over-stressed members or joints

Detailed design of structure support points, padeyes, winch connection points

Proposals for structure reinforcement if required.

Host structure drawings and limiting design loadcases

Weight report for structure and results of weighing operation

Checks on the effect of any weight changes after weighing or final weight calculations on thefloat-over operation

Dimensional control report for mating points (see Section 8.2.4).

A.2 SITE A.2.1

Site plan, showing host structure position, infield pipelines, flowlines and subsea infrastructuredocument by recent reliable surveys.

A.2.2 Drawings showing heights above datum of host structure legs, LMUs, structure support points, barge,and water levels.

A.2.3

Recent bathymetric survey report of area adjacent to the host structure, related to the same datum asitem A.2.2

A.3 BARGE A.3.1 General arrangement and compartmentation drawings.

A.3.2 Hydrostatic tables and tank tables.

A.3.3

Details of class.

A.3.4

Trim and Stability booklet.

A.3.5 Barge allowable still water bending moment and shear force values.

A.3.6 Allowable deck loadings and skidway loadings if applicable.

A.3.7

Specification and capacity of all mooring bollards.

A.3.8

Details of any additional steelwork such as grillages or winch attachments.

A.3.9

Structural strength checks for grillage, seafastening additional steelwork and load-transfer areas.

A.3.10 Details of barge pumping system.

A.3.11 Barge boarding ladders (4 minimum) for the range of draughts in question and wave height range.

A.3.12 Office/control room container suitability and equipment.

A.3.13

Barge power sources (generators) and redundant equipment.

A.3.14

Method of fendering between barge and host structure showing any sliding or rolling surfaces.

A.3.15

Pumps

A.3.16

Specification and layout of all pumps, including back-up pumps and control systems. A.3.17 Pipe schematic and details of manifolds and valves where applicable.

A.3.18

Pump performance curves.


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A.4 JACKING, WINCHING & LOAD TRANSFER EQUIPMENT A.4.1 Jack/winch specification.

A.4.2

Layout of jacking/winching systems including power-packs.

A.4.3

Layout of contingency systems.

A.4.4 Calculations showing friction coefficient and loads on attachment points and safety factors.

A.4.5 Details of LMUs and any heave-compensation equipment

A.4.6 Details of any other load transfer equipment

A.5 BALLAST CALCULATIONS A.5.1 Planned date, time and duration of float-over, with alternative dates, tidal limitations and operational

windows.

A.5.2 Ballast calculations for each stage showing;

Time,

Tidal level, Structure position,

Load on host structure and barge,

Ballast distribution,

Barge draught, trim and heel,

Pumps in use, and pump rates required,

Moment to change heel and trim,

Freeboard (to ensure no green-water on deck).

A.5.3 Stages to be considered shall include as a minimum;

Pre-mating,

First contact,

Intermediate load transfer, initial range 10-60%,

Last contact,

Barge exit.

A.6 MOORINGS A.6.1 Limiting design and operational weather conditions for float-over.

A.6.2 Mooring arrangements for float-over operation and barge/vessel standby position.

A.6.3

Calculations showing environmental loads, line tensions and attachment point loads for limitingweather condition for each stage of the float-over operation.

A.6.4 Specification and certificates of all wires, ropes, shackles and chains.

A.6.5

Specification for winches, and details of foundation/securing arrangements.

A.7 TUGS A.7.1

Details of any supporting tugs including bollard pull, thrusters and towing equipment.

A.8 DP SYSTEMS(see Section 8.14 herein and Section 13.8 of 0001/ND, Ref. [1], for more details)

A.8.1

Detailed DP system description

A.8.2

DP system FMEA.

A.8.3 DP reference system procedures.


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A.8.4

DP capability analysis

A.8.5 DP trials procedures and checklists

A.9 MANAGEMENT

A.9.1

Organogram showing management structure and roles and responsibilities.

A.9.2 Location of key personnel.

A.9.3 Details of manning levels, showing adequate coverage for all operations and emergency procedures.

A.9.4 Times of shift changes, if applicable.

A.9.5

Weather forecast arrangements and procedures.

A.9.6

Communications procedure.

A.9.7 Operational bar-chart showing times and durations of all critical activities including;

Mobilisation of equipment

Testing of pumps and winches

Barge/vessel movements Initial ballasting

Seafastening removal

Float-over /float-off operation

Seafastening

Barge