TMR4225 Marine Operations.

Project work

Load-out, towing to site and installation of Spool-piece transported on barge

Group members: Mads Dalane

Petter Søyland Henrik A. Larsen Stefan Schlömilch

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Preface This is the result of the project work in the subject TMR 4225 Marine Operations, at the university of science and technology (NTNU) in Trondheim. The goal of this project was to enable a group of students to learn about the planning process of a marine operation. The work was to be carried out by groups consisting of 3-4 members. Each group had the choice between 7 different operations and this group chose to carry out task number 3: Load-out, towing to site and installation of a spool piece transported on a barge. The work was carried out as a group at “marinteknisk senter”. All the members shared an office during most of the work time. Each member was given specific task but many problems were discussed amongst all the group members. A lot of different sources were used as references; those sources were: The internet, books lend by professors or the library at Tyholt and most of all: communication with people working in the industry as well as the teaching assistant and professors. We would like to thank Ken Robert Jakobsen, Rasmus Haneferd of Acergy, Finn Gunnar Nilsen, Tor Einar Berg and Robert Indegård of Taubåtkompaniet for their kind help. Trondheim, April 17, 2007 Mads Dalane Petter Søyland Henrik A. Larsen Stefan Schlömilch

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Table of contents Preface.................................................................................................................................. II Table of contents.................................................................................................................. III Table of figures.................................................................................................................... IV Table of tables ..................................................................................................................... IV Summary .............................................................................................................................. V 1. Introduction ....................................................................................................................... 1 2. Scope of work .................................................................................................................... 2

2.1 Location ....................................................................................................................... 2 2.2 Weather........................................................................................................................ 3 2.3 Waves .......................................................................................................................... 4 2.4 Current ......................................................................................................................... 6 2.5 Weight of spool piece................................................................................................... 6

3. Operational phases ............................................................................................................. 7 3.1 Load-out and preparations ............................................................................................ 7 3.2 Towing to site .............................................................................................................. 7

3.2.1 Directional stability ............................................................................................... 7 3.2.2 Static stability........................................................................................................ 8

3.3 Installation ................................................................................................................... 9 4. Vessels............................................................................................................................. 11

4.1 Barge.......................................................................................................................... 11 4.2 Towing tug................................................................................................................. 11

4.2.1 Towing force calculations.................................................................................... 11 4.2.2 Tug efficiency factor............................................................................................ 13 4.2.3 Strength of towline and towline connections....................................................... 13

4.6 Stabilizing tug ............................................................................................................ 14 4.7 Crane vessel ............................................................................................................... 16 5.1 About ......................................................................................................................... 17 5.2 Gantt chart for our project. ......................................................................................... 17

6. Offshore Standard DNV OS H101.................................................................................... 19 7. Forces acting on spool while deploying............................................................................ 21

7.1 Water entry forces, calm water ................................................................................... 21 7.2 Vortex induced oscillations ........................................................................................ 23

8. Use of Remotely operated vehicle ROV ........................................................................... 25 8.1 Observation ROV....................................................................................................... 25 8.2 Work ROV................................................................................................................. 25

9. Feasibility study of the operation...................................................................................... 27 10. Conclusion ..................................................................................................................... 29 References ........................................................................................................................... 30

Pictures ............................................................................................................................ 30 Appendix ............................................................................................................................. 31

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Table of figures Figure 1:Map of Haltenbanken............................................................................................... 2 Figure 2:Voyage Kr.sund-Haltenbanken ................................................................................ 2 Figure 3:Kristiansund............................................................................................................. 2 Figure 4: Wind velocities in July and December. ................................................................... 3 Figure 5: Monthly mean values of Hs for the Haltenbanken site ............................................ 4 Figure 6: Current direction at 3 and 100 meter water depth ................................................... 6 Figure 7:Lifting arrangement ................................................................................................. 7 Figure 8:Tow out ................................................................................................................... 9 Figure 9: Landing................................................................................................................. 10 Figure 10: Viking barge 1 .................................................................................................... 11 Figure 11: Tug Pawlina........................................................................................................ 14 Figure 12: stabilizing tug...................................................................................................... 14 Figure 13: Tug Lieni ............................................................................................................ 16 Figure 14: Normand Mermaid.............................................................................................. 16 Figure 15: Gantt chart example ............................................................................................ 17 Figure 16: Gantt chart .......................................................................................................... 18 Figure 17: α-factors.............................................................................................................. 20 Figure 18: Strouhals number vs. Reynolds number............................................................... 23 Figure 19: Observation ROV, Seaeye Lynx.......................................................................... 25 Figure 20: Connection of spool piece with hydraulically powered tool and Work class ROV ............................................................................................................................................ 26 Figure 21: Hercules, Work class ROV.................................................................................. 26

Table of tables Table 1: Wind data from Haltenbanken. ................................................................................. 3 Table 2: Barge data .............................................................................................................. 12 Table 3: Barge calculations .................................................................................................. 12 Table 4: Towing force on barge............................................................................................ 12 Table 5: Towline pull required related to bollard pull ........................................................... 13 Table 6: Towline pull required and continuous bollard pull .................................................. 13 Table 7:Minimal break loads of towline ............................................................................... 13 Table 8:Towline and towline connection strength................................................................. 14

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Summary Our task in this project was to investigate different aspects of the load-out, tow-out and installation of a spool piece with the help of a barge. Initially we decided that the tow out should take place from Kristiansund to the Njord field situated at Haltenbanken. Metrologic data about Haltenbanken was gathered and we found out that it would be preferable to conduct the operation during the summertime, due to wave- wind- and lighting conditions. We need four vessels for the operation. The barge has to be able to carry the 107 meters long spool, and the selected towing tug requires a towline pull of 65,9 tons. We also need a second tug for maneuvering through narrow areas, increased directional stability of the barge, as well as operational aid under lift off. The crane vessel needs to be able to operate the 31 tons spool with applied heave compensating. This is to make the operation easier and to minimize the risk. We were able to find 4 suitable vessels and the are presented in the full report. The main operational phases were defined and those were: the load out and preparations, the tow out and the installation of the spool. Different aspects of those phases were investigated such as directional stability of the barge, and forces acting on the spool during descent. To get a nice overview of the operation a Gantt chart was established and discussed. We also checked which DNV rules we need to comply with and commented on the most important rules such as wave height and weather forecast uncertainties. We also discussed the use of ROVs to assist the operation and a short introduction into different kinds of feasible ROVs is given. At the end we once more commented on the feasibility.

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1. Introduction In this project work we are going to investigate the load out, tow out and installation of a spool piece with help of a barge. This operation has to be planned and several key parameters such as locations, distances, water depth and environmental factors as well as vessels and main phases need to be defined. A Gantt chart should be established, as well as the compliance with DNV rules. Hydrodynamic and stability analysis should be conducted. At the same time the role of ROV as an aid should be discussed and last but not least the feasibility of the project should be checked. Many of the parameters and options are loosely defined in the exercise so the groups are encouraged to find solutions and locations etc themselves.

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2. Scope of work In this chapter we will look into the scope of work and different limitations we have, due to location, weather, waves, current and weight of structure.

2.1 Location Our operation will take place at the Njord field in the northern North Sea, Norway. According to offshore-technology.com: the Njord field is situated on Haltenbanken, in blocks 6407/7 and 6407/10, 130 km north of Kristiansund and 30 km west of the Draugen field. To be more specific the landing of the spool piece will happen nearby the Njord A platform, and the spool piece shall connect the Draugen PLEM (Pipe Line End Manifold) and the Njord PLEM.. The water depth in this area is around 330 meter. The oil is produced from the Tilje Formation on the East Flank and in the Central Area. The oil-bearing Tilje formation is 100-150m deep, covering an area of around 40km². Reservoir pressure is calculated to be at 390bar, 2,850m below sea level. Temperatures at this depth are roughly 114°C. The topography of the sea bed at Haltenbanken is relatively flat .

Figure 1:Map of Haltenbanken

Figure 2:Voyage Kr.sund-Haltenbanken

Figure 3:Kristiansund

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2.2 Weather All marine operations are weather dependent to some extent. In our operation it is important to have as little wind and waves as possible during the tow out and most important during the lift off, water entry and landing of the spool piece. The northern North Sea is well known for its inhospitable weather during most of the autumn and the winter. Below some wind data from July and December are listed up. Based on these data the operation will take place during summertime, most likely in June or July. Stnr Month FFM [m/s]

71850 04.2006 6,1

71850 05.2006 6,6

71850 06.2006 4,4

71850 07.2006 5,2

71850 08.2006 5,4

71850 09.2006 6,3

71850 10.2006 8,3

71850 11.2006 10,7

71850 12.2006 11,3

71850 01.2007 10,1

71850 02.2007 9,2

71850 03.2007 9,0

Lowest 4,4

Date 06.2006

Highest 11,3

Date 12.2006

Table 1: Wind data from Haltenbanken.1 Figure 4: Wind velocities in July and December. 2

1 Data from e-mail from the Norwegian meteorological institute 2 http://planverk.nofo.no/web_AP3_3.asp?NAVN=Njord+A&Month=Juli

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2.3 Waves Below follow some wave data from the area. Not surprisingly, also the wave conditions are much more suitable in the summertime. The monthly mean value for Hs in July is approximately 1.5 meter.

Figure 5: Monthly mean values of Hs for the Haltenbanken site 3 In Figure 6 the cumulative probability for significant wave height is shown. The dotted line represents data for an average year. For instance the probability of Hs < 2 meter in summertime is 70%. The solid-drawn lines correspond to Hs level of given return periods. Starting with Tp=3 years up to Tp=25 years. From Error! Reference source not found. the wave statistics for Haltenbanken between June and August for all directions are shown. The numbers in the diagram are probability per thousand to get a wave condition with a certain significant wave height Hs and zero crossing periods T0. For instance the probability is 1/1000 to get a wave condition with Hs=6-7 meter and T0=8-9 seconds.

Figure 6: Cumulative probability of Hs, P(Hs) in summer season.4 And wave statistics for Haltenbanken between June and August. 5 3 http://icoads.noaa.gov/jcomm_tr13.pdf 4 Vik and Kleiven 1985. 5 (Global Wave Statistics. N. Hogben 1986)

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2.4 Current In general, current is not a big problem for this area, compared with for instance south of English sector, where the tidal range and currents are large. The maximum current is about 1 knot, and diminishes linearly to zero at sea bed. As shown in Figure 6 the current direction changes 90 degrees from water depth of 3 meter to 100 meter. This phenomenon is very important to take into account during the water entry and landing of the spool piece. The reason for this current twisting is caused by the Coriolis Effect, and is called Ekmans spiral.

Figure 6: Current direction at 3 and 100 meter water depth 6

2.5 Weight of spool piece According to the “Tioga Pipe dimensions and weight”7 the typical weight of a 30’ with 0.625’ wall thickness is:

0.453lbs196.08 291.22

foot 0.305

kgkglbs

m mfoot

⋅ =

The total weight of the spool piece: 107 291.2 31161.3 31kg

m kg tonm

⋅ = ≈

6 (http://www.npd.no/NR/rdonlyres/FAF926A2-5137-4717-9778-79971872DCB4/0/Rapport43.pdf ). 7 (http://www.tiogapipe.com/TiogaChart.pdf)

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3. Operational phases

3.1 Load-out and preparations The spool piece has been assembled on the pier in Kristiansund and will be loaded on to a barge here. We will use the same method to load the spool onto and off the barge – several wires are attached along the spool and assembled together above the center of gravity of the spool. One crane is used to lift the spool and tugger lines are applied at the ends to control the yaw motion of the spool. A crane on the pier is used for on-loading and the main crane on the offshore vessel “Normand Mermaid” is used for off-loading. The spool will be fastened to the barge by welding. If there is calm water and the calculations are correct, there should not be any critical issues in this phase of the operation.

Figure 7:Lifting arrangement

3.2 Towing to site During towing it is important to keep both static and dynamic stability of the barge. After an analysis of the directional stability of the towing system with one tug (see below), we found the need to use two tugs to tow the barge, one in front and one behind. This also helps to maintain good maneuvering of the barge in sheltered waters and to better keep the barge steady when lifting the spool off the barge at the site.

3.2.1 Directional stability We first looked at the possibility to tow the barge with one tug. If the barge is not static and dynamical stable during tow out, it might get unwanted motions in sway and yaw. This is not preferred when maneuvering in narrow waters, and it could cause snatch loads on the towline and in worst case the towline could break.

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3.2.2 Static stability When exposed to a small external force in either sway or yaw, the restoring forces will oppose the external force if the barge is static stable. We assume sway and yaw motions to be uncoupled and the static stability criterion is given by:

0yFMa

δδδψ δψ

+ <

Where ( )yF ψ : Force acting in sway direction due to current on barge when

( )M ψ : Moment acting in yaw direction it is exposed to a yaw angle ψ . and

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yT

FU mψ

δδψ = = − 2

0 22( )T T

MU m x mψ

δδψ = = +

22m : mass of barge and added mass in sway

Tm : 2-D added mass coefficient for the trailing edge

Tx : distance from center of gravity to trailing edge (always negative) U: speed of current on barge (speed of barge)

a: the distance from the center of gravity of the barge to the mooring point for the towing line

Normally 0yFδδψ

< and 0Mδ

δψ> . This indicates that the sway force will act stabilizing on the

vessel while the yaw moment will act destabilizing. To make the barge as hydrodynamically efficient as possible is has a stern which allows the water to “slip” easily away. In other words; it has no transom stern and the area perpendicular to the flow of water is very small. Because the added mass in the stern is directly proportional to the area at the stern we may assume that the barge does not have any contribution to added mass in the stern and set 0Tm = . We get that:

0 0yFψ

δδψ = = and 2

0 22

MU mψ

δδψ = =

Unfortunately, we see that the static stability criterion becomes:

222 0U m <

This expression is false, because both 22m and 2U are always positive. There are a lot of assumptions involved in the preceding paragraph. One of them is the fact that

the formulas for yFδδψ

and Mδ

δψare based on slender body theory, which is more suitable for a

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ship than a barge, because of the L/B ratio of the barge. In addition to this, the slender body theory implicates non-viscous flow, but in reality there will be viscous flow.

Positively, the effect of viscous flow will ensure a negative value of yFδδψ

, which will give a

contribution to the restoring forces, but as the criterion for static stability is not fulfilled, we should do a closer analysis. The problem is to determine the magnitude of the restoring forces

on the barge due to viscous effects (which will contribute to the term yFa

δδψ

in the static

stability criterion), when the barge is given an angleψ in yaw. Since the towing system is not statically stable it will neither be dynamically stable, since it doesn’t have enough restoring forces to oppose the external forces it is exposed to. To increase the static stability one would normally increase the distance between the center of gravity of the vessel and the mooring point for the towing line. To see the effect of this, we need to do a further analysis on the effect of viscous forces on the barge. Instead of doing this we plan to use two tugs, one in front of the barge and one behind. This is frequently done in similar operations, and in addition to assist in towing out, two tugs will better keep the barge in position at the site. A brief calculation of the needed bollard pull for the second tug to maintain static stability is done in chapter 4.6 “Stabilizing tug”.

Figure 8:Tow out

3.3 Installation We will use the main crane on “Normand Mermaid” to lift the spool off the barge and lower it down to the seabed. This operation is divided into five phases, each phase containing critical issues that need attention. The whole operation must be done when the weather and sea state allows for it.

1. The spool is lifted off the barge in the same manner as we did at the pier in Kristiansund. After the spool piece has been lifted off, “Norman Mermaid” will move sideways to get clear of the barge. When considering this operation, we have a coupled system with 18 degrees of freedom – barge, spool and vessel, each with 6 degrees of freedom. Limiting parameters would be possible snatch loads in the crane wire and damage to the spool if impact with barge should occur. This is directly related to the wave conditions. Also, the operation cannot be executed if there is too much wind, because this could result in unwanted drift of barge/vessel during lift off. This could result in horizontal sliding of spool on the deck and cause damages to spool, equipment or even personnel.

2. Tugger lines are applied to better control the motion of the spool when it is in the air. An

important issue will be to avoid collision with the ship side. This can occur if the waves are too big resulting in large motions of the spool, if there is too much wind accelerating

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the spool, or if an instance of Mathieu instability emerges and we get large pendulum motion.

3. The crane on “Normand Mermaid” is heave compensated and even though there are

wave forces acting on the vessel, the spool can be lowered at even speed. Given this, we must estimate the vertical hydrodynamic and static forces on the spool as it enters the water. Again, possible snatch loads on the wire must be considered. We must also avoid forces that can unbalance the spool. If the spool piece has any kind of bended shapes, it is important to think of rotational stability as the pipe enters the water. It has happened before that curved spool pieces have twisted when entering the water due to change in buoyancy.

4. As the spool descends towards the seabed, we must keep an eye on possible current

forces acting on the spool. These forces can push the spool away from its landing site on the seabed, and it might be necessary to move the vessel to counteract this. In general one must be aware of vertical resonance as the spool is lowered down, but the heave compensated crane should help us here. To avoid any snatch loads, it is important that the crane cable is tight at any time.

5. Finally, the spool will arrive at the landing site on the seabed. Before the spool piece can

be lowered the last part, it has to be rotated exactly over the second PLEM. This is simply done by moving the crane vessel in the right position. In advance the ROV connection tools are already lowered down, and prepared at the PLEMs, and the pipe can be connected once it has touched the sea bed. “Normand Mermaid” is equipped with two ROVs - one fairly large work class ROV of about 5 tonnes and one observing ROV. The work class ROV will then connect with the tools, and couple the spool piece to the PLEMs. We must avoid large impacts between the spool and the seabed and the existing subsea installations.

Figure 9: Landing

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4. Vessels As presented before, we need four vessels. Two tugs, one crane vessel and one barge. The choice of tugs depends on the specifications of the barge. The towing tug must be able to tow the barge at requested speed. The other tug may be smaller and will operate behind the barge. This barge will apply directional stability as well as navigation in narrow fjords. The crane vessel should be dimensioned to lift the spool from the barge. All calculations and numbers are presented in (Calculations, Appendix A).

4.1 Barge An appropriate barge for transporting the spool is the (Viking barge 1) from Viking supply ships AS. The most critical criteria for transporting the spool is its length, 107 meters. Viking barge 1 is 91 meters, and will therefore fit almost all the length of the spool, except 8 meters on both sides. Because of this we had to take some precautions avoiding destructive slamming on the spool. Some wave limitations are applied to this, but because of the barges design, the waves are to be extreme if they could harm the spool.

Figure 10: Viking barge 1

4.2 Towing tug To find an appropriate tug, we had to find the total towing force of the barge. According to (Taubåtkompaniet), the pulling force of the tug should be 60 tons with the selected draught and velocity. We will do some calculations to see if the estimate is right.

4.2.1 Towing force calculations The data we have collected from the barge are taken from the web page (Viking barge 1), as well as from (Taubåtkompaniet). The most relevant values are presented as follows. Input data Lenght over all (LOA): 91,44[m] Breadth 27,4[m] Depht: 6,1[m] Light weight: 1850000[kg] Towing velocity 3,5[knots] Towing velocity 1,8[m/s]

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Load 31000,0[kg] Cb at 1,8 meter waterline 0,8735[-] minimal draught bow 1,8[m] minimal draught stern 2,3[m] Table 2: Barge data Because the spool is light compared to the barge, we have to add ballast to fulfill the minimum requirements for towing of the barge. The minimal required draught respectively in the bow and the stern are 1,8m and 2,3m. The barge will then have a slight aft trim. This is to avoid slamming forces under the barge in heavy weather. The block coefficient presented is correct at that specific draught. Calculations are shown below. Calculations Displacement: 4486,457[m3] Total weight 4621,1,[tons] Needed ballast: 2740,1[tons] Table 3: Barge calculations According to (Noble Denton) minimum towline pull required (TPR) shall be computed for zero forward speed against a 20m/s wind, 5,0m significant sea state and 0,5m/s current, acting simultaneously. The wave resistance is calculated by the formula (Faltinsen 90):

2 2

2W

BF Rρ ξ= ⋅ ⋅

Where R is the shape factor, and ξ is the wave amplitude. The effects needed because of the propeller race are also included in respect to the extra velocity the propeller induces on the barge. The towing cable length is assumed to be 50m. (Haneferd 07). Calculated values and coefficients are presented in the Calculations in appendix B. The formulas and values are taken from tug specifications (Tug Pawlina) and (Nielsen 03). The extra induced velocity according to these assumptions is 1,2m/s. This extra velocity is added to the towing velocity used to calculate the drag force. The drag force of the barge is calculated by Morrison’s drag equation:

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2D D SF v C Aρ= ⋅

The drag force coefficient DC is given by (Taubåtkompaniet), and consists of drag forces and viscous forces on the hull. The velocity induced by the propeller race as well as the current according to (Noble Denton) are also added to the towing velocity. The wind resistance is calculated with Morrisons formula, where Cd is estimated to be 1. This gives the following values: Forces on barge Wave drift force, Fwa 17,9[kN] Drag force, Fd 318,1[kN] Wind resistance Fw 31,6[kN] Towline pull required (TPR) 367,5[kN] Table 4: Towing force on barge

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4.2.2 Tug efficiency factor The tug efficiency factor has to be applied to the TPR according to (Noble Denton). This is due to decreased performance on tug under harsh weather conditions as well as necessary safety factors. For Hs = 5m, BP can be related to TPR by:

Table 5: Towline pull required related to bollard pull According to the table above, the results are given in tons: Towline pull required and continuous bollard pull Towline pull required (TPR) 37,5[tons] Continuous bollard pull (BP) 65,9[tons] Table 6: Towline pull required and continuous bollard pull The calculations of the continuous bollard pull, 65,9 tons is close to the suggested BP from the (Taubåtkompaniet) and must be regarded as reliable. This bollard pull is calculated regarding the worst possible excepted weather conditions. More calm weather as well as longer towline will decrease the pollard pull considerably. However, if these towing terms should occur, the tug will need this bollard pull to reach the operation area in time.

4.2.3 Strength of towline and towline connections The minimum breaking loads (MBL) of the towline and the bridle legs shall be related to the bollard pull (Noble Denton). Following rules are applied in our calculations:

Table 7:Minimal break loads of towline Because of our offshore operation location at Haltenbanken, we have to use the “other areas” rules. The ultimate load capacity (ULC) of towline connections including bridle legs have following restrictions:

1,25

40

ULC MBL

ULC MBL

= ⋅= +

Whichever is less to be utilized. Towline and towline connection strength Minimum breaking loads (MBL) 163,5[tons]

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Ultimate load capacity (ULC) 203,5[tons] Table 8:Towline and towline connection strength Now we have calculated the total bollard pull of the tug. The criteria of the tug is to be functionally offshore, and to have a bollard bull of at least 65,9 tons. (Tug Pawlina) with its bollard pull of 67,1 tons should be appropriate to tow the barge.

Figure 11: Tug Pawlina

4.6 Stabilizing tug The second tug is used to improve maneuvering, directional stability and to keep the barge in position on the site. It will be moored at the stern of the barge. During tow out we want to take a closer look at what the bollard pull of this tug must be to improve the directional stability of the barge. This will be useful in deciding the size of the tug. To estimate the second tug’s bollard pull we must look back at the static stability criterion and

look at the derivative of the destabilizing moment Mδ

δψ and compare it to the restoring moment

provided by the second tug.

Figure 12: stabilizing tug

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Here we assume that the barge will rotate around the point P, to simplify the calculations. The calculations we did for the one tug system to check if the system was static stable or not, told us that the destabilizing moment, ( )M ψ needs to be balanced to obtain stability. We can

compare 20 22

MU mψ

δδψ = = with the derivate of the moment from the second tug acting on the

barge tugMδδψ

. The contribution from the tug should at least equal this. Viscous effects will also

provide restoring forces (we get a negative contribution from the term yFa

δδψ

in the static

stability criteria), so if

222

TUGM MU m

δ δδψ δψ

= ≤ it should be ok.

If we assume the yaw angle ψ to be small, we can writeTUG BM F L ψ= ⋅ ⋅ .

This gives us tugB

MF L

δδψ

= ⋅ , which again gives us the force on the barge from the second tug

2

22B

U mF

L≥

Calculations give these results U: 3 knots = 1,54 m/s L: length of barge = 90m

22m : mass and added mass of barge, M + 22A M: mass of barge = 4600 e3 kg

22A : added mass of barge:

222

22,7

DA

aρπ= (Faltinsen 90)

2 2

22 22 2,7 782492DA A L a Lρ π= ⋅ = ⋅ ⋅ ⋅ ⋅ = kg

And finally

2 222 1,54 (4600 3 780 3)

14176990B

U m e eF N

L

⋅ +≥ = =

This corresponds to a pull of about 15 tons. The tug operating behind the barge is not required to be as powerful as the barge in front. An example could be (Tug Lieni) with its bollard pull of 35 tons. The estimate of the pull is rather rough, and the parameters can be discussed. Among them are the effects of propeller race and

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current on the barge, and how much this affects the directional stability and thereby the need for pull from the second tug. We have not included the pull of the second tug in the calculations of the bollard pull of the towing tug. This is because the calculation of the pull of the second tug only is rough estimates, and questions of uncertainty are involved.

Figure 13: Tug Lieni

4.7 Crane vessel The crane vessel requires a capacity of at least 31 tons at sufficient distance from the crane foundation. It must also have a heave compensator to make the operation easier and to minimize the risk. The vessel (Normand Mermaid) is recommended by (Haneferd 03). This vessel meets the requirements of crane capacity of 100 tons. Normand Mermaid is also fit to use the heave compensator because the load does not exceed 1-2% of the vessel weight (Nielsen 03).

Figure 14: Normand Mermaid

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5. Gantt chart

5.1 About8 The Gantt chart has in the recent decades become one of the most popular project scheduling aides. It is a relatively simple chart which, with the help of bars and time indicators, gives a wide overview of a projects subtasks and the planned duration of those.

ID

1

2

3

4

5

0 6 12 18 0 6 12 18 0 6 12 18 0 6 12 18 0 6 12 18 0 6 12 18 0 6 12 18 0 6 12 18 0 6 12 18 0 6 12 18Mon 16 Apr Tue 17 Apr Wed 18 Apr Thu 19 Apr Fri 20 Apr Sat 21 Apr Sun 22 Apr Mon 23 Apr Tue 24 Apr Wed 25 Apr

Figure 15: Gantt chart example One is able to define simple constraints such as

• start to start; meaning that two subtasks need to start simultaneously • start to finish; meaning that a task may start when the preceding task is finished • finish to finish; meaning that two or more tasks should end at the same time • work time; how many hours a day work should be executed, weekends etc. •

While the project is being executed one is also able to define things such as progress and completion of tasks and it is relatively simple to assess if one is on schedule or not. One of the drawbacks with a gantt chart is that one is not able to see how labor intensive, weather restricted or expensive etc. the various tasks are. In addition it may be hard to keep it organized and tidy when the number of sub tasks is large. But as a simple tool for scheduling and overview purposes, the gantt chart is perfect.

5.2 Gantt chart for our project. We have decided to divide the project into the following tasks and subtasks

• Planning of marine operation • Preparation of harbour • Harbour Operation

o Preparation of tug o Preparations of spools o Lifting of spools onto barge and sea fastening o Connect tug and barge

• Offshore preparations o Transit of barge o Transit and preparation of crane vessel o Connection of crane vessel and barge o Deployment of ROV

• Lifting operation o lift off barge o pass splash zone o lowering to bottom

8 http://en.wikipedia.org/wiki/Gantt

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o landing of spool and preliminary connection In our case we have decided that the work time is 24 hours a day. This is made possible by two facts:

• The operation will take place during the summertime, relatively far north, meaning that the lighting conditions will be sufficient throughout the night. Still, very critical operations like entering the splash zone should be avoided in the middle of the night

• The crew is most probably working in a 6 hours on 6 hours off schedule, meaning that they already have an artificial diurnal rhythm and they will have the same performance capabilities regardless of day and night.

The times which are indicated in the Gantt chart are approximate values which either are “guessed” qualitatively or estimated based on assumptions which we think are reasonable. Some examples are given below:

• The transit distance is 130 km and average speed is 3 knots [1.54 m/s]; this gives us a time of 23.5 hours, this number is rounded up to 24 hours.

• The same thing can be done about the lowering time. 330 m / 0.5 m/s= 660 s=11 minutes. This would be a very small post on the Gantt diagram such that it is rounded up to one hour.

• Most other times are assumptions. This gives us the following Gantt chart (can be seen in a larger version in the appendix) :

ID Task Name

1 planning of marine operation2 preparation of harbour3 Marine Operation4 Harbour Operation5 Preparation of tug6 Preparations of spools7 Lifting of spools onto barge and seafastening8 Connect tug and barge9 Offshore preparations10 Transit of barge11 Transit and preparation of crane vessel12 Connection of crane vessel and barge13 Deployment of ROV14 Lifting operation15 lift off barge16 pass splash zone17 lowering to bottom18 landing of spool and preliminary connection

18 20 22 0 2 4 6 8 10 12 14 16 18 20 22 0 2 4 6 8 10 12 14 16 18 20 22 0 2 4 6 8 10 12 14 16 18Mon 18 Jun Tue 19 Jun Wed 20 Jun

Figure 16: Gantt chart

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6. Offshore Standard DNV OS H101 The offshore standard DNV OS H101 (OS hereafter) is a relatively extensive standard written for industrial and commercial marine operations. The scope of our project is not as large as such projects, so we have assumed that most of the rules are satisfied and are just going to comment one the rules we found to be most important. Seen from a weather point of view there are two kinds of operations considering weather and time restrictions: unrestricted operations (lasting for more than 72 hours) and restricted operation (lasting for less than 72 hours).(see section OS Section 4 B 500) The largest difference between those two is that one is allowed to introduce restrictions on the operations lasting for less than 72 hours, which are stricter than the official OS states. This means that a structure which is used in an operation which lasts for less than 72 hours may be constructed weaker than a construction which is designed for an operation lasting for more than 3 days. The reason for this is that the accuracy of weather forecasts only is within an acceptable limit for 3 days. As can be seen from the Gantt chart (Figure 16: Gantt ) our total operation time is approx. 2.5 days (60 hours). According to the OS Section 4 B 500 our operation may thereby be characterized as a weather restricted operation, as it has duration of less than 72 hours. Moreover we have three sub operations which all have a duration of less than one day. Those sub operations have different weather restrictions and are relatively independent of each other. The harbor operations are of course taking place in the harbor. The harbor of Kristiansund is situated in relatively sheltered waters such that there probably won’t be any waves larger than 1 meter in the area. The transit operation includes two tugs towing a relatively light barge. The barge has a large freeboard and the spool only extends 5 meters on either side of the barge. This makes it possible for the towing operation to take place in relatively high waves. Haneferd 07 referenced that Acergy uses a maximum significant wave height of 5 meters in towing operations. This wave height is only exceeded by 1,4% of the sea states during June- August in that region, according to “global wave statistics” (Hogben 86). Moreover the transit only takes about 24 hours, which allows for quite accurate weather and wave predictions. It is the lifting operation which is of highest importance considering the wave restrictions. The “bottleneck” of the operation is the relative motion of crane vessel and barge. Here Haneferd recommended a maximum significant wave height of 1,5-2 meters. Again referring to Hogben we can see that this wave height is exceeded by 41,7% of the sea states in the region within the period of June-August. From this one sees that the event which is the most dependent on the weather is the event which is furthest away in time, meaning that the weather predictions will have a certain uncertainty. Let’s investigate what weather predictions must be satisfied, to give the order for mobilization: The spools may be loaded on to the barge almost independently of weather. One restriction might be rain or heavy winds, considering that the spools will be connected to the barge by welding. After the spools have been attached and the tug connected to the barges one needs 24 hours of waves below 5 meters, followed by at least a 10 hour period of less than 2 meters wave. Preferably this period should be extended to 14-16 hours to include unforeseen happenings. The operational criterion is defined by C0=α*OPLim where OPLim is the operational limit: 5 meters for the first 24 hours and 2 meters for the following 16 hours. The α symbolizes an uncertainty factor in the forecasts. This uncertainty factor is unique for different areas and is found from tables. We us table A-3 in appendix B of OS and find assume that we have a

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forecast level B and monitoring on board. Monitoring means that all design criteria such as wave height and wind etc are monitored within reasonable accuracy and update rate. Any abnormalities should be reported and investigated and variation borders should be defined. This gives us the following α-values for the initial forecast:

Figure 17: α-factors With time and monitoring those factors will of course increase because the uncertainty is constantly decreasing. The weather forecasts should be updated no fewer than at least once every 12 hours. When the operation is starting at the pier the forecast should thus not have a larger forecasted value for the wave height than C0=α*OPLim=0.62*2=1.24 meters. In this particular operation wind and current velocities are of small importance. The ship is equipped with a good DP system which enables it to counteract the currents. When the spool is drifting off course the crane vessel will change its position to counteract the drift off.

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7. Forces acting on spool while deploying

7.1 Water entry forces, calm water The water entry is a critical phase, and it is necessary to find the vertical hydrodynamic forces. We assume calm water, and use this formula to find the water entry forces:

2333 33 z z

dAF gV A U U

dhρ= + ⋅ + ⋅

i

Assumptions:

• Ideal, non-viscous, non-compressible fluid • No viscous effects • Body dimensions << wave length

The first part in the equation is hydrostatic force, the second added mass force and the last is “slamming” force. The hydrostatic force is a function of the volume V of the submerged spool. The acceleration in the added mass force, is found under the assumption that the lowering velocity before entering the water is U=1m/s, the retardation time is 0.5 seconds and the sinking velocity is Uz=0.5 m/s.

20.25z

zU U m

Ut s

−= =i

The slamming force can according to equation (9.39) in (Faltinsen 90) be written as:

2 233 22z s z

dAU C RU

dh

ρ⋅ = Where Cs is:

5.15

0.2751 8.5

s

hC

h RR

= ++

The next problem is to find the volume of the submerged spool as it is sinking into the water. First we found the area of a circle segment:

2RA= ( sin )

2θ θ−

Below follows an Excel spreadsheet with calculations of the vertical hydrodynamic force. The force is also plotted as a function of submerged spool, h. This calculation is as mentioned only for calm water, and the added mass- and slamming forces would in real life be larger. From the spread sheet underneath, we se that the hydrostatic force is the main contribution to the total force. We also notice that the slamming force is largest the moment it hits the water, witch is

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reasonable. In this case we have used a constant negative acceleration for the added mass force. This is not quite correct, as the acceleration will change during the water entry. As mentioned above, this analysis is for calm water only. With waves in addition, we can expect a larger contribution from the slamming- and added mass force. L 107m D 0,762m R 0,381m lowering vel, U 1m/s sinking vel, Uz 0,5m/s time of retardation 0,5s negative accel. 1m/s^2 rho 1025 g 9,81

submerged spool, h [m]

Area of circular segment Cs

Hydrostatic force

Added mass force

Slamming force

Total vertical

force [kN]

0,00 0,00 5,15 0,00 0,00 1005,60 1,010,10 0,04 1,67 38002,21 3873,82 325,33 42,200,20 0,10 1,09 102723,55 10471,31 212,30 113,410,30 0,17 0,89 179423,91 18289,90 173,00 197,890,40 0,24 0,81 260898,23 26595,13 157,71 287,650,50 0,32 0,78 341278,85 34788,87 153,20 376,220,60 0,39 0,79 414427,34 42245,40 154,46 456,830,70 0,44 0,82 471802,09 48093,99 159,17 520,060,76 0,46 0,84 490543,36 50004,42 163,12 540,71

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7.2 Vortex induced oscillations When the spool is lowered towards the bottom, vortex shedding may occur. It is important that the vortex shedding frequency is not in the area of the resonance frequency of the spool. A simple analysis will be conducted here.

The vortex shedding period is according to (Faltinsen 90) found by 1

VC

DT

St U= i where St is the

Strouhals number which is Reynolds number dependent, D is the diameter of the spool and Uc is the sinking velocity.

Figure 18: Strouhals number vs. Reynolds number

First the Reynolds number cU DRn

ν= needs to be found. ν is the cinematic viscosity of

saltwater. Rn in this case is 56 2

0.5 / *0.7623.2*10

1.2*10 /cU D m s m

Rnm sν

= = = . According to Figure 18:

Strouhals number vs. Reynolds number this is exactly in the critical Strouhal region, meaning the Strouhals number is somewhere between 0.2 and 0.5. This gives us a eigenperiod range of 7.62[s] < T < 3.05[s] . It is critical that the eigenperiods of the spool are not in that region. But there are several ways of preventing resonance oscillations. By either increasing or decreasing the sinkage speed one will get a more disambiguously defined Strouhals number. At the same time one can adjust the effective distance between the crane wires which are attached to the spool. By changing the distance between those wires one will change the length of the oscillating element and thereby also change the eigenperiod. The eigenperiod/frequency of a spool may be found by literature such as (Bergan 86). The

formula for the frequency is given by 4n n

EI

mlω ω= where nω can be found by determining

different border conditions. We will assume it is fixed in either end due to symmetry effects.

Then nω =22.37 for the first eigenvalue. 92,1*10E = . I is given by

where r is the inner and R the outer diameter. This gives us the

result 4 4 4 4( ) (0.381 (0.381 0.016) ) 0,00264 4yI R rπ π= − = − − = m is approx 290kg/m and the

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length can be varied. In this exemplified calculation we will use 10 meters. This gives us the

following result: 11

4 4

2.1*10 *0.002622.37* 22.37*4.33 97 0.01

290*10n n

EIHz T s

mlω ω= = = = → = .

Here we can clearly see that resonance oscillations are of no importance when the crane wires are situated so close to each other and the descending velocity is low.

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8. Use of Remotely operated vehicle ROV In almost any marine operation the use of ROV is important. After the structure has entered the water, the ROV becomes eyes and hands of the engineers. For our operation we need both observation ROV and work class ROV.

8.1 Observation ROV After the spool piece has been launched into the water, the Observation ROV is in use until the operation is done. In the lowering–phase it is important to check that the spool piece is stabile and that the crane cable is tight at any time. During the landing phase it is crucial to have good and clear images of the spool and the PLEM (Pipe Line End Manifold) at all time. On the Njord site the water depth is too large to use divers (max depth usually 100 meter), so the only way to get visualization is by ROV. The Observation ROV is also very useful for the pilot of the Work class ROV, to get a kind “bird's-eye” view of what he is doing. Below is typical vehicle data for an Observation ROV:

• Maximum working depth: 1500 m • Length: 1260 mm • Height: 625 mm • Width: 825 mm • Forward thrust: 66 kg • Lateral thrust: 47 kg • Vertical thrust: 43 kg • Launch weight (basic vehicle): 210 kg • Payload: 40 kg

Figure 19: Observation ROV, Seaeye Lynx9

8.2 Work ROV The Work ROV is a much more robust and heavy vehicle than the Observation ROV, that can be equipped with heavy duty manipulators and hydraulic tooling. The Work class ROVs can either be permanently fitted with different tools, or it is set up with tools dependant on the task to be carried out. In our case the connection tools are so heavy that it has to be placed at the sea bed by guiding wires. The tools are located in connection with the PLEM, and the ROV is free to land on and hook up with the tool. The connection tools are hydraulically powered, and flow of hydraulic oil is controlled by the ROV. Below is an illustration of how the connection could take place:

9 (http://www.seabed.pl/firm/seaeye/seaeye.html )

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Figure 20: Connection of spool piece with hydraulically powered tool and Work class ROV 10 The Work ROV we are going to use is called Hercules. Length: 2400 mm Width: 1850 mm Height: 2050 mm Weight (in air): 2750 kg Through frame lift: 3000 kg Payload: 150 kg Maximum working depth: 3000 m Shaft power 120 hp Hydraulic power 103 hp

Figure 21: Hercules, Work class ROV

10 (http://www.verderg.com ).

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9. Feasibility study of the operation The operation we have studied in this project work has been to transport a 107 meter long spool piece on a barge from Kritsiansund to the Njord field, and install it at the sea bed. The first phase of the operation is Load out and preparation of the spool piece. The spool is lifted onto the barge with a large crane located on the pier, and fastened to the barge by welding. This part of the operation is quite straight forward, since the harbour in Kristiansund is well protected from waves and weather. During towing of the barge we will use two tugs, one in front and one behind. This phase is quite weather dependant, and the maximum significant wave height we can handle during the tow out is 5 meters. There are a couple of other ways to handle the tow out, and also installation of the spool piece. One way is to tow the spool piece submerged between two tugs. Another method is to tow the spool underneath the crane vessel (Acergy). In addition to the tugs and the barge, we are going to use a multi-purpose Inspection, Maintenance and Repair (IMR) and construction ship called “Norman Mermaid”. By towing the spool piece through the moon pool of the ship, both tugs and barge can be skipped. Another great benefit with the subsurface towing is that the critical water entry phase can be neglected. An operation like this requires a lot more preparation at the harbour before towing. In our case spool piece is a bit to long to be transported like this. It would be a great challenge to get long pipe stabile as it hangs under the ship. After the towing to the Njord field, the spool piece has to be lifted off the barge. This is the most weather critical phases during the operation, and the maximum significant wave height Hs is put to be 1.5-2 meter due to relative motions between the barge and the crane vessel ( Haneferd 07). If it is possible to carry the spool piece onboard the crane vessel, the whole problem with relative motions between the vessels could be disregarded. After the spool piece has been lifted of the barge, “Norman Mermaid” will move sideways to get clear off the barge. Another option could be to move the barge after the Lift off, but this would probably be more troublesome. When free off the barge, the crane operator can start to lower the spool piece towards the water. The water entry phase is also a very critical, as the pipe goes from one medium to another; air to water. The first thing to consider is the water entry forces. In our report we have carried out some calculations of these forces, but only for calm water. To get a real picture of especially the slamming forces, the calculation should have included waves. If the spool piece got any kind of bended shapes, it is important to think of rotational stability as the pipe enters the water. It has happened before that curved spool pieces have twisted and rolled when entering the water due to change in buoyancy. As mentioned above the whole water entry phase can be disregarded by subsurface towing. The journey towards the sea bed is more comfortable stage. The spool piece will be connected to a guide line, and continuously lowered until it is some meters over the first PLEM. To avoid any snatch loads, it is important that the crane cable is tight at any time. Before the spool piece can be lowered the last part, it has to be rotated exactly over the second PLEM. This is simply done by moving the crane vessel in the right position.

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In advance the ROV connection tools are already lowered down, and prepared at the PLEMs, and the pipe can be connected once it has touched the sea bed. The work class ROV will then connect with the tools, and couple the spool piece to the PLEMs. This method as it is described above is actually being used in the industry. Companies like for example Acergy AS have used this kind of method and are going to use it in future projects. (Haneferd 07) They have proven it to be a feasible and accepted method of installing spools. The feasibility analysis and use of well known procedures is a very important part of a marine operation. DNV have included two topics in their rules about the use of new technology:

And we seem to comply with those two rules. Therefore this operation is feasible.

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10. Conclusion We were able to do a satisfactory analysis of many factors involved in the load-out, tow-out and installation of a spool on a barge. Our insight into the planning of a marine operation was greatly expanded by investigating such things as vessels, stability, directional stability, lifting forces; vortex induced motions, weather windows and forecast accuracies. Several of the group members will have summer internships in companies like Acergy and Aker Marine Contractors, which have such kind of operations as main business. This project has given us a good introduction into the potential work tasks we will be given, and we are looking forward to experience the difference between our planning and how the planning is conducted in the industry. If the work on this project should be continued it would be natural to investigate safety factors and weather windows even further. At the same time economical aspects should be considered. Most of all; a variety of different solutions should be investigated to find the best combination of safety, feasibility and economy.

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References • Nielsen, F.G.; “Lecture notes in Marine Operations”; Tapir trykk. Trondheim 2003 • Faltinsen, O:”Sea Loads on Ships and Offshore Structures”; Cambridge University

Press; Cambridge 1990 • Hogben,N ; N.M.C.Dacunha; G.F. Olliver, “Global wave statistics”, Unwin brothers

limited, 1986 • Haneferd, Rasmus; Acergy AS: E-mail correspondence April 2007 • Bergan, P.G., P.K. Larsen og E. Mollestad (1986): Svingning av konstruksjoner. Tapir,

Trondheim • DNV: Offshore standard DNV OS H101:

http://www.ivt.ntnu.no/imt/courses/tmr4225/exercises/teamwork/2007/DNV-OS-H101.pdf

• http://www.offshore-technology.com/projects/njord/ (accessed on April 17th, 2007) • Information about Viking barge 1 are collected through information from Robert

Indegård, employee at Taubåtkompaniet, whom has commercial management for barges within Viking Barges KS. http://www.boa.no/

• Noble Denton – General guidelines for marine transportations http://www.nobledenton.com/guidelines/0030-2.pdf

• Viking barge 1, barge within Viking Barges KS http://vikingsupply.egroup.no/barges.asp

• Tug Pawlina, Utilized towing tug for operation http://www.tugmalta.com/pawlina.html

• Tug Lieni, Utilized stabilization tug for operation http://www.tugmalta.com/lieni.html

• Normand Mermaid, Crane vessel http://www.acergy-group.com/publicroot/webresources/6Q5ANARDUN/$file/Normand%20Mermaid.pdf

Pictures • Sources found in footnotes for each picture

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Ap

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ID Task Name

1 planning of marine operation2 preparation of harbour3 Marine Operation4 Harbour Operation5 Preparation of tug6 Preparations of spools7 Lifting of spools onto barge and seafastening8 Connect tug and barge9 Offshore preparations

10 Transit of barge11 Transit and preparation of crane vessel12 Connection of crane vessel and barge13 Deployment of ROV14 Lifting operation15 lift off barge16 pass splash zone17 lowering to bottom18 landing of spool and preliminary connection

18 20 22 0 2 4 6 8 10 12 14 16 18 20 22 0 2 4 6 8 10 12 14 16 18 20 22 0 2 4 6 8 10 12 14 16 18Mon 18 Jun Tue 19 Jun Wed 20 Jun