Berkeley Catamaran

7/28/2019 Berkeley Catamaran

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i Acknowledgements

This work has been carried out on the department of Ocean Engineering at The University ofCalifornia, Berkeley, under the supervision of Professor R. G. Bea and his valuable guidance.

The project work started January 2002 and was submitted for review and grading as a part of thecourse OE 180: Design, Construction & Maintenance of Engineered Systems, May 16th 2002.

The group worked together as experts in team, where each member contributed with their ownspecial area of expertise:

Alexandra Koefoed Managed the hydrodynamics, and stabilityGeir Olav Kaase Managed the structural designMathieu Beunier Managed the flow rates and system designShaik Mushtaq Managed the fender and sea fastening designAmir Conrad Osmanagic Coordinated the report and managed the procedures, transport and

risk analysis.All members worked on the design where we followed a design spiral, i.e. the vessel wasredesigned several times.

We wish to thank Chevron Texaco represented by James Stear and Owen Oakley, John HalkyardCSO Aker Engineering, James Wiseman Intec, and Professor Mansour at UC Berkeley for theirhelp, encouragement and participation as coaches during the whole project period.

Berkeley, June 20th 2002


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ii Executive Summary

This report describes an offshore deck installation system capable of installing platformsweighing up to 30 000 tons in open seas. The system is designed to be a multi purposeinstallation system. The desire is to meet the needs of the future deep water offshore fields, in

particular spar structures (where an onsite installation is required).

The system utilizes the float over deck concept, were the offshore platform deck is lowered ontoa substructure by ballasting the carrying vessel and or raising the substructure. The projectfocuses on meeting the challenge of having an ultra heavy capability installation system thatrequires far smaller instillation periods than conventional methods. These goals are met bycreating a catamaran system with unique rapid ballasting capabilities.

The catamaran is a purpose built vessel that consists of two rectangular hulls connected by a boxgirder. Each hull is divided into conventional buoyancy sections and sections that have largeflood valves. The tanks with large flood valves allow for rapid flooding. This action also

compress the existing air in the cells. This air is later vented to lower the system even further.The compressed air is piped to the substructure to increase its buoyancy by evacuating waterfrom its ballast tanks. The ballasting of the catamaran system is studied in detail and rates forlowering are found to be reduced to as little as 40 minutes.

A other time reducing measure that has been introduced is a unique fender system. The fender isprimarily designed to absorb the contact shock, however it will also work as a guide when a sub-structure enters the catamaran. The idea is to reduce hook up time during the Spar installationsequence.

The catamaran is not self-propelled and is transported by way of heavy lift carrier vessel or towed

with tugs.

Important analysis that has been performed is:

Stability calculations for the HLV transport, in field waiting and installation. A linear frequency based hydrodynamic analyze on both the transportation scenario and

the installation scenario.

An overview of a preliminary risk analysis, considering every stage from deck load-out todeck installation. The risk analysis of such a novel system is important due to its inherentunproven characteristics.

A rough cost estimate is done of the system versus a crane lifted deck installation. Structural analysis of the hull


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The system was analyzed to prove its feasibility with respect to construction, transportation,offshore operation and cost. All limiting codes and regulations are taken into consideration. Theanalyses of the actual design of the vessel is reasonably well detailed, due to the fact that thebuilding of the vessel would represent the greatest cost and accurate construction quotes would

be needed to justify the project.

Figure a1. Concept drawing and general layout of Catamaran

100m

88m

20m


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i ACKNOWLEDGEMENTS ..2

ii EXECUTIVE SUMMARY ...3

1 INTRODUCTION............................................................................................................. 8

2 GENERAL ARRANGEMENT AND SPECIALIZED VESSEL DESIGN................... 10

2.1 GENERAL ARRANGEMENT ........................................................................................... 102.2 WEIGHT...................................................................................................................... 132.3 SPECIALIZED VESSEL DESIGN....................................................................................... 152.4 LOWERING PROCESS.................................................................................................... 16

3 OPERATION SEQUENCE AND PROCEDURES........................................................ 23

3.1 STEP-BY-STEP SEQUENCE: ............................................................................................ 233.2 TIME MANAGEMENT OF ACTIVITIES ............................................................................. 30

4 TRANSPORT .................................................................................................................. 32

4.1 TRANSPORT

OPTIONS

.................................................................................................. 324.2 SEA STATE ................................................................................................................. 344.3 STABILITY .................................................................................................................. 35

5 HYDRODYNAMIC MOTIONS AND LOADS.............................................................. 37

5.1 ASSUMPTIONS MADE IN ANALYSES .............................................................................. 375.2 RESULTS..................................................................................................................... 38

6 INSTALLATION PROCEDURE ................................................................................... 41

6.1 STABILITY DURING IN FIELD WAITING .......................................................................... 416.2 FENDERS .................................................................................................................... 42

7 SEA FASTENING........................................................................................................... 458 STRUCTURAL DESIGN AND ANALYSIS.................................................................. 48

8.1 INTRODUCTION ........................................................................................................... 488.2 STRUCTURAL DESIGN ................................................................................................. 498.3 CLASSIFICATION REQUIREMENTS ................................................................................ 538.4 LOAD CALCULATIONS................................................................................................. 548.5 STRUCTURAL ANALYSIS ............................................................................................. 57

9 RISKS FOR SEVERE OR CATASTROPHIC DAMAGE OF DECK, VESSEL ORSUBSTRUCTURE .................................................................................................................. 60

9.1 DECKLOAD FROM QUAY............................................................................................ 60

9.2 DECKINSTALLATION ON SUBSTRUCTURE .................................................................... 619.3 WAITING IN FIELD HAZARDS ...................................................................................... 619.4 TRANSPORT................................................................................................................ 619.5 INSTALLATION RISK TO PERSONNEL AND EQUIPMENT. .................................................. 63

10 COST ANALYSIS ....................................................................................................... 66

10.1 CONSTRUCTION COST ................................................................................................. 6610.2 TRANSPORTATION AND INSTALLATION COST COMPARATIVE STUDY .............................. 66


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11 DELIVERABLES. ....................................................................................................... 69

12 REFERENCES ............................................................................................................ 70

1. APPENDIX 1 ................................................................................................................... 71

2. APPENDIX 2 ................................................................................................................... 81

3. APPENDIX 3 ................................................................................................................... 85

4. APPENDIX 4 ................................................................................................................... 88

FIGURESFigure 2.1 Main Dimensions side view. .................................................................................. 11Figure 2.2 Main Dimensions front view.................................................................................. 11Figure 2.3.Main Dimensions top view..................................................................................... 11Figure 2.4: Pipes and valves arrangement ............................................................................. 16Figure 2.5................................................................................................................................. 17Figure 2.6................................................................................................................................. 20

Figure 2.8................................................................................................................................. 22Figure 3.1................................................................................................................................. 23Figure 3.2. Quay slide.............................................................................................................. 24Figure 3.3, Catamaran on HLV, side view............................................................................. 25Figure 3.4, Catamaran on HLV, top view. ............................................................................. 25Figure 3.5, towing to site ......................................................................................................... 27Figure 3.6, positioning of catamaran...................................................................................... 27Figure 3.7, first stage of lowering, contact is made. ............................................................... 28Figure 3.8, second stage of lowering, 100%weight transfer................................................... 29Figure 4.1, Towing................................................................................................................... 32Figure 4.2, Transport on HLV................................................................................................ 33

Figure 4.3, Separate transportation........................................................................................ 34Figure 4.4. GZ-curve for transport......................................................................................... 36Figure 5.1 Response Amplitude Operator for the Catamaran in Head Seas ........................ 39Figure 5.2 Response Amplitude Operator for HLCV in Head Seas...................................... 40Figure:6.1. GZ-curve for in field waiting. .............................................................................. 41Figure 6.2: Fender design........................................................................................................ 43Figure 6.4: Tugs keeping the whole system together ............................................................. 44Figure 7.1: Forces acting on the Sea Fastening and the Launch Rail ................................... 46Figure 8.1: Pontoon cross-section: Web-frame and bulkhead............................................... 50Figure 8.2: Boxgirder cross-section ........................................................................................ 51Figure 8.3: Horizontal Froude-Krilov forces ......................................................................... 54

Figure 8.4: Top view of catamaran with load case 1.............................................................. 54Figure 8.5: Vertical bending of the box-girder....................................................................... 55Figure 8.6 Catamaran on HLV............................................................................................... 55Figure 8.7: Beam model of pontoon outside HLV deck......................................................... 56Figure 8.7: Global forces on pontoon beam ........................................................................... 57Figures appendix 1

Figure 1: Time management................................................................................................... 74Figure 2: Global shear force on a pontoon............................................................................. 77


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Figure 3: Global bending moment on a pontoon.................................................................... 78Figure 4: FEA mesh of bottom plate field .............................................................................. 80Figures appendix 2

Figure 1, flow chart ................................................................................................................. 81

TablesTable 2.1.Main Dimensions..................................................................................................... 10Table 2.2 Heavy Lift Vessel Data............................................................................................ 13Table 2.3. Mass and Inertia Data............................................................................................ 14Table 2.4 .................................................................................................................................. 17Table 3.10, Task time measures.............................................................................................. 30Table 4.4 Stability in transportation....................................................................................... 36Table 5.1 Resonant Frequencies for the Catamaran.............................................................. 38Table 6.1. Stability in field waiting. ........................................................................................ 41Table 6.2 Stability in Installation............................................................................................ 42Table 8.1: Dimensions of structural members in the catamaran........................................... 52Table 8.2: ABS class-rules....................................................................................................... 53Table 8.3: ABS scantling requirements.................................................................................. 53Table 8.6: Global forces .......................................................................................................... 58Table 8.6: Resulting forces and moments on box-girder ....................................................... 59Table 8.7: Box-girder stresses................................................................................................. 59Table 8.8: Plate analysis results.............................................................................................. 59Table 9.1 .................................................................................................................................. 64Table 9.2 .................................................................................................................................. 64Table 9.3 .................................................................................................................................. 64Tables appendix 1Table 1: Weight Data .............................................................................................................. 71Table 2 Time consumption...................................................................................................... 73Table 3: Stress analysis parameters........................................................................................ 75Table 4: Froude-Krilov forces due to load case 1, load case 2 and load case 3:.................... 75Table 5: FEA Properties ......................................................................................................... 79Table 6: Material properties................................................................................................... 79Tables appendix 2

Table 2, hazard analysis.......................................................................................................... 82


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1 Introduction

Offshore structures consist of 3 primary sections foundation, substructure and topside. As the oiland gas industry has ventured into deeper waters the substructures have become floating units.

Such deep-water fields are often far from any land based processing facilities and thereforerequire heavy topside structures. The problem we present a solution to be how an ultra heavydeck can be installed at sea on a floating substructure. The solution has applications forinstallations of decks on a variety substructures but lends its self particularly well to the floatingSPAR substructures.

Our concept is to construct a catamaran vessel that can engulf a substructure, while supporting a30 000 ton deck. The Catamaran can then de-ballast and transfer the deck onto the substructure.

The concept of float-over decks is not new, it extends back to an original patent for trussinstallation for bridges were bridge spans were floated in between pillars on barges and then

lowered by ballasting. Since the 1894 patent there have been approximately 17 float-over decksinstalled on permanent structures in open waters, and 20 decks installed in sheltered waters onCONDEEPS or TLPS.

The largest ever float-over deck in open waters was 10 000 tons, while decks in excess of 50 000tons have been floated onto structures in sheltered waters. Our idea is to create a system thatcould install the ultra heavy decks achieved in sheltered waters in the open seas.

The system is based on installing on the relatively new concept of a Spar substructure. A spar isbasically a long heavily ballasted cylinder floating vertically, which supports a deck system andis moored into position. There are currently multiple Spars in use with their decks being installed

by way of heavy lift crane vessel. Heavy lift crane vessels are limited to maximum lifts of theorder of 10,000 tons and have a steep daily rate. In addition huge costs associated withconnecting the modules make this solution a very expensive one. This is the direct andestablished alternative to the float-over method.

All installation systems have an allowable sea state; in general this sea state is around 1 msignificant wave height and can have direction and wave period limitations. The likelihood ofthis weather occurring and for how long is directly relates to the cost of the system. This is dueto the high cost of heavy lift installation cranes as well as the cost of all of the support personnel,carrying vessel, support boats and equipment on an hourly rate.

There are two ways the weather window can be affected given a certain field. One is to use asystem that has a high allowable weather installation sea state and thereby increase the chance ofthe weather window occurring. The other is to do the operation in a short amount of time andtherefore requiring a smaller span of good weather and hence increase the chance of itsoccurrence, or more correctly its prediction.


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Our idea focused on meeting the challenge of having an ultra heavy capability installation systemthat required far smaller installation periods than conventional methods. We tried to incorporatemeasures that would allow the system to dampen motions and therefore have a higher allowableinstallation sea state. It was felt that these measures could not be sufficiently proven in a

feasibility study and are only commented on in the report.


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2 General arrangement and specialized vessel design

2.1 General arrangement

2.1.1 Objective and functionalities

The objective of the system is to carry an ultra heavy deck and quickly lower into place on asupporting structure. (For this study we have used a spar as target structure.) The following arespecific key functionalities:

stabilizing the pontoons around the substructure, lowering the deck on top of it, controlling the lowering rate.

We adapted the general arrangement of our system to meet the requirements associated withthese special operations.

2.1.2 Main Dimensions

The catamaran that we have designed for installation of the deck on the substructure consists oftwo rectangular pontoons. They are connected aft with a box girder to form one rigid body. Thebox girder has openings in top and bottom to let water and air flow in and out, hence it does notcontribute to buoyancy. The hulls of the pontoons are divided into three sections. The two endsections are conventional buoyancy tanks and the middle section is what we call cells. Thesewere originally thought to have an open bottom. Buoyancy contribution from this section wouldthus be pressurized air. In the final design the middle cells have a closed bottom with large valvesto let seawater in. Reasoning for this decision can be found under the paragraph Constraints onMain Dimensions. The final Main Dimensions for the catamaran are presented in table 2.1.

Table 2.1.Main Dimensions.

Length of Pontoons l 100.0mLength of Cells lc 89.2mLength of Buoyancy Tanks lb 5.4mLength of Box Girder lbg 24.0m

Width of Pontoons w 19.0mWidth of Box Girder wbg 50.0mHeight of Pontoons h 20.0mHeight of Box Girder hbg 6.0m

The abbreviations for main dimensions are shown in figure 2.1. to 2.3. below.


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Figure 2.1 Main Dimensions side view.

Figure 2.2 Main Dimensions front view.

Figure 2.3.Main Dimensions top view.


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2.1.3 Analysis for main dimensions.

The main challenge associated with selecting the main dimensions was the new concept of anopen cell bottom. This configuration implies that conventional buoyancy calculations dont

apply. Three basic physical principles were applied to calculate the contribution from air underpressure in the cells to total buoyancy in the system. Ideal gas law governs the compression of airinside the open cells. Assuming an isothermal process, temperature is constant duringcompression, and that no air escapes so that the mass is constant the equation for our systembecomes: (for definition of symbols see figures above or abbreviation list.)

ttanconsp

a

=

gives,int

atmintdh

hpp

= (2.1)

Where dint is draft inside the open cells and pint is internal pressure in ballast tanks.Bernoullis law along a streamline assuming no velocity in the fluid provide the relation between

water level inside the open cells and draft of the pontoons.

ttanconsgzp w =+ gives, ( )outintintatm ddgpp += (2.2)

Where dout is draft of the pontoons measured on the outside. Finally the system has to be inequilibrium. Forces from pressurized and conventional buoyancy tanks have to equal weight ofdeck and catamaran.

( )int2 ( )out b w atm cM g d w l g p p w l = + (2.3)

The three relations above constitute a set of three equations that can be solved for the threeunknowns: dint , pint and dout.

2.1.4 Constraints on Main Dimensions.

All modes of operation for the catamaran represent different constraints on the main dimensions.In transport, dimensions have to fit the capacity of heavy lift vessel, for our design we have usedthe dimensions of Mighty Servant 1. From a selection of heavy lift vessels that are in operationtoday it had the more optimal dimensions. For data see table 2.2. Constraints from the heavy liftvessel include total length and width and area of pontoons to comply with size of cargo deck and

load per square meters respectively. Furthermore the heavy lift vessel sets a limitation to draft ofpontoons when they float on and off the cargo deck.


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Table 2.2 Heavy Lift Vessel Data.

Name of vessel Deadweight Deck Load Deck Width Deck Length Launch Draft

[metric tons] [t/sq.m] [m] [m] [m]

Black Marlin 57021 27.5 42 178.2 10.03Blue Marlin 57021 27.5 42 178.2 10.03

Mighty Servant 1 40190 19-40 50 150 14

Mighty Servant 3 27720 19-25 40 140 10

Transshelf 34.03 19-25 40 132 9

However the most important constraints come from the installation procedure, in view of the factthat this is the catamarans purpose. The catamaran has to fit around the target installationstructure, in our study a spar of 48 m diameter. Accordingly the catamaran gets a considerablewidth, which has a very positive effect on initial stability as a result of increased waterline

moment of inertia. The catamaran needs a large range of drafts. This was one of the reasons forclosing off the bottom. Conventional buoyancy is more effective than air under pressure and thusit gives us larger range of drafts. Closed bottoms create the possibility to have three differentmodes of operation: Closed bottom and top valves, open bottom and closed top valves and openbottom and top valves. The conventional buoyancy tanks are dimensional zed to keep thecatamaran floating just above water level when the deck is off loaded and all valves are open.This minimizes the risk of the pontoon floating up and getting jammed under the deck afterinstallation. The three modes of operation acquired by closing the bottom also provides flexibilitytowards what speed the catamaran is lowered with. Next with closed bottom the stability is betterand easier to calculate. We have good stability while waiting for a sufficient weather window tostart the installation.

2.2 Weight.

This section covers the assumptions, calculations and results for mass and mass-moments ofinertia (also known as radius of gyration.) that are used in computations throughout the design.More details can be found in Appendix 1. Weight is assumed uniformly distributed in all bodies.Two systems are calculated, System 1:deck on catamaran and System 2:deck and catamaran onthe HLV. The results are presented in Table 2.3. For system 1, the origin of the coordinate systemis located aft at the keel and center plane is plane of symmetry. For system 2 the origin is aft atthe keel of the HLV and center plane in the HLV is plane of symmetry.


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Table 2.3. Mass and Inertia Data

System 1 System 2

Total Mass 3.84E7 kg 7.73E7 kg

Center of Gravity x-coordinate 50.0 m 91.3 m

Center of Gravity y-coordinate 0.0 m 0.0 mCenter of Gravity z-coordinate 26.8 m 22.3 m

Global Moment of Inertia Ixx 5.53E10 m4 8.54E10 m4

Global Moment of Inertia Iyy 1.48E11 m4 8.57E11 m4

Global Moment of Inertia Izz 1.39E11 m4 8.14E11 m4

Global Moment of Inertia Ixy 0 m4 0 m4

Global Moment of Inertia Ixz 0 m4 1.53E11 m4x-coordinate is longitudinal, y coordinate is transversal and z coordinate is vertically.


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2.3 Specialized vessel design

2.3.1 Fenders

In order to reduce the time required to hook up the catamaran to the spar hull, it is decided to usea method that would require less mating lines than usual. A fender system is added to thepontoons and the box girder to give the entire catamaran a U shape. A complementary malefender system is added to the substructure hull that fits neatly the catamarans fenders. Tugstension the catamaran against the substructure to maintain contact.

2.3.2 Pipes and valves

The lowering of the catamaran carrying the deck is performed by filling the pontoons with water.Each pontoon is divided into three sections. One of them is a ballast tank. For structural reasons,it is actually made of six independent and identical cells. The total ballasting volume is

4 33.41*10 m . The other two, located at the ends, are buoyancy tanks. They are sealed and provideenough buoyancy to carry the pontoons (without deck load). Hydraulic valves on the bottom ofthe cells are used to control the incoming water flow rate. There is one valve per cell. 30 inchesdiameter valves were chosen to maximize the flow rate. For the given range of draft and thus of

water pressure at the bottom of the catamaran, the flow rate is estimated to 3max 1.5 /Q m s= . Thisgives a total maximum ballasting rate capacity of

3 3

max, max2 6* 18 / 6.48 /totQ Q m s m hr = = =

At the top of each cell there are air valves. The air can be either released to the atmosphere, or, inorder to reduce the lowering time, be injected into the ballast tanks of the substructure, therebyforcing it to rise as the catamaran sinks. This latter option is made possible by connecting airvalves on top of the pontoons to the substructure by flexible pipes. To ensure equilibrium ofpressure between the different cells, they are interconnected with pipes. The design pressureinside the cells is two times the atmospheric pressure.


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Figure 2.4: Pipes and valves arrangement

As shown in Figure 2.4, both air and water valves can be operated from a cat way located abovethe top of each pontoon. At this elevated position, operators can work the equipment withoutgetting wet.

2.3.3 De ballasting pumps

At the end of the lowering process, the cells of the catamaran are filled with water. Thecatamaran is towed away from the substructure, and, in order to put it back on the HLV, waterhas to be pumped out (according to the results, the final draft of the catamaran would have to bereduced by 2 meters). We will be using standard ballasting pumps for this operation, which willbe installed on top of the catamaran.

2.4 Lowering process

The entire lowering process can be divided into two successive phases. First, the deck, supportedby the catamaran, is lowered over the substructure by means of letting water into the pontoons. Incase of a fixed substructure, the catamaran then sinks a little bit more and is towed away, whilethe deck remains on top of the substructure. For a floating substructure, once the deck comes intocontact, a second phase starts: the deck weight is progressively transferred from the catamaran tothe substructure. This can be achieved by pumping water out of the substructure ballast tank or byinjecting compressed air from the pontoon tanks (by closing the air valves during the first phase,the water entering the tank is compressing the air). These two phases will be described in thefollowing sections.


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2.4.1 First phase: lowering of the deck over the substructure

The motion of the catamaran and of the deck is governed by a set of four equations: Newtonssecond law for the entire system, conservation of water flow rate, adiabatic compressibility of theair and equation of motion for the incoming water.

The position of the structure is described using a verticalaxis (Oz) pointing up, the origin being on the sea level.We use the following notations:

Table 2.4

Depth at the bottom of the pontoon 2z

Depth of the water level inside the tank 1z

Total height h Beam w

Cumulated length of buoyancy tanks bl

Length of ballast tank cl Incoming flow rate (per valve) Q

Number of water valves per pontoon N

Surface area of a valve S

Figure 2.5

Considering a mechanical system that consists of a deck (mass dM ), rigidly attached to the

catamaran (mass pM ), whose ballast tanks contain a mass wM of water. It is subjected to the

following forces:

its weight: ( )p d wW M M M g = + + where wM increases with time, a damping force: 2dampF z= & where is the heave damping coefficient of the

catamaran,

a buoyancy force: 22 ( )b w c bF gw l l z= + if assuming the box girder is flooded and doesnot contribute to buoyancy.

Newtons second law of motion for the system will be:

2 2 2( ) ( ) 2 ( )p d w p d w w c bM M M z M M M g z gw l l z + + = + + +&& & (3.1)

The mass of water in the tanks at time t is:0

( )t

w wM Q d = The conservation of water flow rate between the valve and the tank is

1cNQ wl z= & (3.2)

The adiabatic compressibility of the air governs the pressure inside the tank. If the air is released,it is simply


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in atmp p= (3.3)

If the air is compressed, the pressure has to be related to the volume occupied by the air. It iscommonly done assuming an adiabatic transformation, and therefore the conservation of the

quantity pV , where Vis the volume occupied and is a constant ( 1.4 = for the air):

1 2( ( ( )) ( )in c atm cp wl h z z p wl h = (3.4)

Finally, to relate the incoming flow rate to the internal pressure, the equation of motion is writtenfor a system made of:

At time t: the water inside the tank (its linear momentum is 0P) plus the water that will

enter the tank in a period dt (its linear momentum is2

w

Qdt

S ).

At time t dt+ : the same volume of water as at t (unchanged linear momentum) plus theincrease of water level due to incoming water (add linear momentum is 21( )w cwl z dt & ).

This system is experiencing its weight, equal and opposed to the support reaction from thepontoon, and two pressure forces:

At the valve:2

2 22

watm w

Qp S p gz S

S

= using Bernoulli theorem to relate the

pressure 2p at the valve to the atmospheric pressure and the flow velocity at the valveQ

S.

At the interface with the air: 1 cp wl (negative sign indicates downward direction).The equation of motion is:

222

1 2 1( )2

ww c w atm w c

Q Qwl z p gz S p wl

S S

=

& (3.5)

The unknowns are 1z , 2z , Q and 1p (internal pressure). Equations (3.1), (3.2), (3.4) and (3.5)

can be combined to get a system of nonlinear differential equations governing the motion of thestructure (if the air is released, use equation (3.3) instead of (3.4)). The main difficulties to solvethe equations are:

The quadratic term 2Q , which cannot be neglected because of the high values of theincoming flow rate, especially just after the opening of the valves.

The discontinuity of pressure at the valve when it is opened.

The inclusion of the operators action on the valve, modifying the flow rate.

After some simplifications, this system is solved using Matlab, which yields the time evolution of

the internal draft ( 1z ) and the external draft ( 2z ), shown in Figure 2.6. For this simulation, the

parameters are set to the following numerical values:


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h 20m pM

38440*10 kg

w 19m dM

330000*10 kg

bl 10.6m atmp 5 21.01*10 /N m

cl 89.6m

1.4 N 6m 62*10 /kg s

S 20.45m

The initial conditions are:

1,0 2,0 9.9z z m= = (draft of the pontoons carrying the deck with empty ballast tanks),

zero velocity, internal pressure equal to atmospheric pressure 30 1.5 /Q m s=

In Figure 2.6 one can see that the water level inside the tank (1

z ) increases rapidly at the

beginning, but then it quickly reaches a maximum value of roughly 7 meters, and oscillates

slightly around this value. The external draft ( 2z ) increases in a faster way at the beginning. The

horizontal line ( 12.8outd m= ) corresponds to the point where the top of the deck reaches thespar, for an initial gap of 3 meters. For the considered flow rate, this occurs after 25 minutes.


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Figure 2.6

Equation (3.1) has to be modified when the deck reaches the substructure (from now onconsidered to be a floating structure). In order to predict the motion of the new system, consistingof the deck, the catamaran and the substructure. It now includes the mass of the substructure andits added buoyancy:

2 2 2 2, 2( ) ( ) 2 ( ) ( )

p d s w p d w w c b w s rM M M M z M M M g z gw l l z gA z z + + + = + + + + && & (3.5)

where 2,rz is the external draft of the catamaran when the deck reaches the substructure, and sA

is the surface area of the substructure ballasting tank. After 25 minutes, Figure 2.6 shows that theexternal draft oscillates over a mean value, which corresponds to the stabilization of the deck and

the pontoons supported by the substructure.

2.4.2 Second phase: deck weight transfer

During this stabilization, the last of the sea fastenings (about 25%) are cut off. The deck is stillalmost entirely carried by the catamaran, but is now attached to the substructure. The timerequired to perform the release is estimated to 40 minutes. During this time, both air and water


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Figure 2.8


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3 Operation sequence and procedures.

The installation planning and procedure will be a key issue when organizing an operation of this

size. Several hundreds of millions of dollars are at stake and every step must be carefullyconsidered and thought through.

Our target installation deck is 30 000 tons and can be built at many construction yards whichhave both the capacity for building decks this size and a quay slide option. The quay slide willoperate on a greased rail system, where stran jacks and the low friction surface allows the deck toslide from the quay onto the pontoons.

The weather-window surveillance is one of the most critical elements during the installationsequence. A constant surveillance of the field and transport route is necessary through out allsteps because a 24-hour weather window is a requirement for installing the deck. Time is

measured from arrival at target installation site. If this requirement is not fore-filled the operationneeds to abort and wait for the next possibility in a sheltered area.The limiting sea state for transport and installation is addressed and discussed in chapters 4 and 6.

3.1 Step-by-step sequence:

1 Fit-up to quay, pontoons slides on rail:

Figure 3.1

Figure 3.1 shows how the rails between the quay and each of the pontoons are connected. Thejoint between them is flexible. The connection between the rails in the bow and the box-girderin the stern will provide sufficient stiffness so that the pontoons do not spread out when

exposed to the load of the deck.

Quay Pontoon

Rails


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2 Slide-on, de-ballast compensation.

The deck will slide on greased rails onto the catamaran, illustrated in figure 2.2. The slide ratewill be dictated by the de-ballasting capacity of the submerged ballast pumps.

Figure 3.2. Quay slide

All the equipment onboard the catamaran is manually operated, thus it will require operatingpersonnel on deck for any adjustments. Each pontoon has six separate watertight compartments

with individual pumps compensate the changing loads.

Uneven leverage onto the catamaran could cause severe structural stresses which exceeds thestructural capacity. The consequences of any failure in this order is catastrophic and the risk hasto be eliminated in the design process. Several large structures have in a similar manner beenslide onto barges, thus regarded feasible.

3 Secure fastenings

After the deck is safely transported onto the catamaran it has to be secured with proper seafastening (design of sea-fastening is discussed and analyzed in section 7). The sea fastening is

categorized as internal and external. The internal fastenings cover all equipment on the deck,whilst external fastening is the connections between the deck and the catamaran. Internal seafastening will not be analyzed, it is assumed to be standard and provided by the deck owner.

Once at installation site time will be a critical factor, therefore an optimization of the amount ofsea-fastenings will be of importance.

Male/female steel guides, making the installation precision between the spar hull and deck moreaccurate will be attached to the lower parts of the deck while still in quay slide area, where themales are welded onto the deck. Total number of guides/pins will vary with different structures,each with a length of 2 meters.

4 Check in sheltered water

A last and final check in sheltered water of sea-fastenings, and equipment is done beforepreparing for load-up on HLV. All possible work should be carried out in this protectedenvironment, since cost and risk increases further from shore.

Deck

CatamaranQuay


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5 Load-up onto heavy-lift vessel

For transportation of deck and catamaran we have assumed that a HLV such as Mighty Servant 1or a similar vessel is available and used for this purpose.

Figure 3.3, Catamaran on HLV, side view

Figure 3.4, Catamaran on HLV, top view.

Transportation on HLV is proven feasible in chapter 2 and 4, where a selection of commerciallyavailable vessels has been analyzed

6 Sea-fastenings and securing

The loads experienced at the maximum design wave during transportation will be the

dimensioning criteria for this system. The accelerations at this state will also govern the capacityof the fastenings between the catamaran and deck, chapter 7


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7 Check all systems

This is the last possibility to check all systems before the transportation starts.

8 Transportation, route selection.

Transport route selection and traffic clearing will be achieved by working together with localmaritime authorities. Alternative routs and contingency plans needs to be mapped out, togetherwith traffic modeling in critical sections of the routes. High-density traffic fairways will typicallybe in narrow straits and around load-up area. Lessons learned from previous transportationaccidents where offshore platform decks had to be replaced, are that transportation should beconducted in well-known water and fairways.

Weather-window monitoring, and route selection in terms of minimizing the exposure of thevessel to large and high period waves will be a crucial factor for relieving the sea-fastenings ofcyclic loads and the danger of fatigue.

9 Arrival at installation site and load off

All sea fastening between HLV and catamaran is removed and the HLV lowers until thecatamaran is floating.

10 Tug hook up to catamaran.

There will be a maximum significant wave height in which the installation can proceed, whichmeans that any sea-state higher than this will result in abortion of mission. A re-confirmation of a24-hour weather window will be necessary.

Tug hookup starts during the last two hours of ballasting/lowering the HLV.

From this point of time the cost of reversibility increases drastically. Having a fully operatinginstallation organization at a remote installation site in extremely costly and it exposes bothequipment and personnel to higher risk.

Catamaran is temporarily moored.

As load-off is initiated all systems must be ready to go.

11 Hook-up preparation time

Hook-up and preparation is estimated to take 2 hours. The estimation is based on similar projectswhere tugs and personnel time frames are evaluated. The hook-up is defined as getting tugs inposition, and connecting towlines (with redundancy) to standardized connections on thecatamaran and getting personnel on each of the pontoons.


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Figure 3.5, towing to site

All our analysis is based on a three tug tow-out as shown in figure 3.5, where the tug at the sternacts as a rudder.

12 Reduction of sea fastening.

As preparation for installation is in process, a gradual reduction of sea-fastenings are performed.The idea is to minimize the risk of losing the deck if the weather window closes or there is an in-field collision between tug/catamaran, support vessels/catamaran or unwanted collision betweenspar hull and catamaran. The reduction will stop at 75%, the last 25% will be removed during thetwo stages of lowering.

13 Pull into position.

Figure 3.6, positioning of catamaran.

Pulling into position will require a great deal of precision. Tug-tow coordinators will have toguide the catamaran through a 20 cm clearance on each side of the spar hull. This will probablyrequire additional mating lines between hull and catamaran.


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14 Hook-up mooring lines.

When in final position, mating lines between deck and spar hull are attached. The mating lineswill ensure stability and precision when exposed to unwanted roll motions under the loweringsequence.

15 Slow ballasting for line-up.

At the first stage of the lowering sequence, the bottom valves will slowly release water into theballast tanks (There are six watertight compartments on each pontoon with a correspondingvalve). As the lowering starts, the male/female guides mate. The deck will make contact with thespar hull of 12.8 m draft. With fully open valves the system (catamaran/deck/hull) will get to anequilibrium draft at 13.2 m, and there will have been a minor amount of weight transfer from thecatamaran to the spar (figure 3.7).

All the sea fastening will now be removed. The final removal of the 25% is estimated to take 40minutes.

At this point the catamaran buoyancy is a result of pressurized air trapped in air pockets insideeach of the pontoons. All airflow analysis, weight transfers, and drafts at different stages areincluded in section 2, General arrangement, and specialized vessel system design

Figure 3.7, first stage of lowering, contact is made.


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16 Rapid ballasting.

At the second stage of the lowering sequence, there will be flexible pipes connected from the airrelease valve at the catamaran deck to the ballast tanks on the spar hull. The idea is to evacuatewater from the upper (close to the water line) spar ballast tanks using the trapped air in the

pontoons.

Since contact between hull and deck already is obtained, the last lowering stage should be donefast and controlled.

Figure 3.8, second stage of lowering, 100%weight transfer.

As the pressurized air enters the hull, the spar raises and the weight transfer is now complete(figure 3.8). Two watertight compartments on each of the pontoons designed for reservebuoyancy is keeping the catamaran afloat at the waterline.

17 Un-hook, secure

All mating lines are unhooked and the securing of the platform deck to the hull starts.

18 Pull away, tow back

Last stage of the installation sequence is getting the catamaran safely away from the area. The

installation is now complete.


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An overall installation sequence flowchart is summarized in figure 1 in appendix 3 wherecriterias for abortion is visualized.

3.2 Time management of activities

As a part of our time reduction measurers we have developed two significant time savingfeatures.

One of the effects introduced by our air injection lowering system, is the reduction in exposuretime while in a fragile state. By decreasing the lowering time the catamaran is not as vulnerableand the risk of closure of the weather window is reduced, but more importantly we only need asmall window.

The second reduction comes from the fender and self-align guide system. This will aid the tugs toguide the spar hull into position faster than a conventional fender system. The fender/guide

design is described in section 6.3.

Table 3.10, Task time measures.

Task No. Sub-activitiesTime to task

(hours)Task duration

(hours)

1 Removal of HLV-Pontoon sea fastenings 0Load off HLV

2 Ballasting of HLV 1

3 Attaching tow lines 1.5Hook up, after sea launching

4 Getting tugs in position 1 0

5 Preparing people and equipment 1.5Reduce sea fastenings

6Welding operations

2.57 Towing deck 4

8 Coordinating tugs 3

9 Hook up mooring lines 6

Pull into position

10 Positioning 7

11 Hook up fenders 6

12 Hook up flex. Air pipes 8

13 Getting operators in position 7

Hook up, before final lowering

14 Eliminating all sea fastenings 9

15Slow ballasting for line-up, opening of bottomvalves 10

16 Rapid ballasting, opening of top valves 13

Lowering

17

De-ballasting of Spar hull with air pressure frompontoons 15

18 Un-hook all connections 19

19 Secure deck 22

Pull-away

20 Tow back 23


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Figure 1 in appendix 1 gives a graphic illustration of the time spent on each task, and adescription of when in the installation process we are. It takes 24 hours from the point whereour system is loaded off the HLV to the securing of the deck onto the substructure and tow backis performed.


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4 Transport

For the feasibility study no specific site for the deck and catamaran construction was considered.It was thought that the system could be built anywhere and be transported to a field anywhere.

This meant making some general assumption about the sea state during travel. The feasibility ofthe transportation was proven using static stability and a more detailed analysis would have to beperformed to meet the finer points of the certifying authorities codes. Three transportationoptions were considered and a final choice of transporting the entire system together was deemedthe best.

4.1 Transport Options

The three transportation scenarios were considered are described below with their advantages anddisadvantages

4.1.1 Tow Combined System

Figure 4.1, Towing

Issues

This system is most likely cheaper assuming the towing distance is not to great This system will require the vessel to be the most robust and will also mean that a

higher stability requirement will be placed on system

This system will be the slowest and will have the highest risk factors


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4.1.2 Use a HLCV for the Entire Transport

Figure 4.2, Transport on HLVIssues

This is the fastest transport system but for ultra heavy decks it would be limited to ten orless capable vessels based on the static stabilty requerment used in the anaylisis describedin section 4.3.

Dynamics transport and dynamic stabity conditions are not yet understood. It is thoughtthat the roll accelerations may be too high therefore making the sea fastening for the deckand the pontoons to the HLCV excessive.

This system has the advantage of being a temporary operation once offloaded and thepontoons only have to meet the standard of a positive GM.

This is a standard transportation scenario and the risk is well midigated.


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4.1.3 Transport deck and Pontoons separately and connect at site.

This system involves transporting the deck and catamaran separately and then matting them atthe field. They can then perform the installation when the weather window arises.

Figure 4.3, Separate transportation

Issues

The deck transfer at sea requires more engineering in the system as well as a longerweather window.

The sytem requires either two separate transport vessels or 1 vessel and a tow sytem for

the pontoons This system has the advantage of being a temporary operation and only has to meet the ofa positive GM.

Transfering the deck to the catamaran in an open sea poses an additional risk The deck and hlcv will have a lower center of gravity during tranport and therefore have a

smaller roll acceleration. This means less sea fastning.

Option number twowas choosen because it lessoned the constraints on the design of the pontoon.It was also qualititvly deemed the safest tranporation scenario. When considering largetranportation distences option 2 is also the cheapest scenario.

4.2 Sea State

The Sea state plays an important role in determining the motions of the system. The following seastate was considered for transportation.Significant wave height, HS: 5 m.Time Period of the wave, T Z: 10 sec.


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For the installation process, the following sea state was considered.Significant wave height, HS: 2 m.Time Period of the wave, T Z: 7 sec.

A severe sea state would result in increased motions (accelerations).

4.3 Stability

4.3.1 Stability during transportation.

There are three main cases for which stability needs to be checked. GZ-curves for the two firstcases, (transportation and waiting) are generated by calculations in ProSurf3. GZ curves areimportant because they form the foundation for stability requirements. Conditions placed by theauthorities are taken from IMOs General intact stability criteria for all ships., and listed below.

The area under the GZ-curve up to 30o should not be less than 0.055 [m*rad] The area under the GZ-curve up to 40o or flooding angle f1 whichever comes first should

not be less than 0.09 [m*rad]

The difference between the two abovementioned areas should not be less than 0.03[m*rad]

The GZ-arm should be at least 0.20m at an angle of heal equal to or larger than 30degrees.

The maximum righting arm should occur at an angle of heel preferably exceeding 30o butnot less than 25o.

The initial metacentric height should not be less than 0.15m.

In figure 4.4. the GZ- curve for the transportation mode, when the catamaran and deck are on theheavy lift vessel is exhibited and table 4.1 shows how the criteria above are satisfied. The GZ-curve for transportation is calculated taking into account the extra buoyancy and water plane areafrom immersion of the pontoons. This happens at 5.7o as can be clearly seen in the plot. It shouldbe noted that only stability for the static case is considered here.

1 Where f is an angle of heel at which openings in the hull , superstructures or deckhouses which cannot be closedwatertight immerse.


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Figure 4.4. GZ-curve for transport.

Table 4.4 Stability in transportation.

Regulations Transport mode.

Area under GZ up to 30o 0.055 [rad*m] 3.941 [rad*m]


Area under GZ between 30o and 40o 0.03 [rad*m] 1.509 [rad*m]

GZ-arm at 30o 0.2 [m] 11.63 [m]

Max righting arm occur at XXo 25o 35oInitial Metacentric height. 1.5 [m] 11.0 [m]


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5 Hydrodynamic Motions and Loads

The frequency domain hydrodynamic program Wamit was used to model the transportation of the

deck and pontoons on the HLCV and the pontoons and deck floating in the field. The modelingproduced the following results

First order wave excitation forces and moments Hydrodynamic added mass and damping Rigid body motions (see fig 1-12) Sectional forces and moments Steady drift forces (see fig 13-18) Inertia loads Pressure loads for structural shells

For the preliminary design these results were not used to their full potential in the structuraldesign. Given the need for a more precise design the above forces could be directly applied in afinite element program. The response amplitude curves were used to approximate maximumaccelerations that were then used in sea fastening design for both systems.

5.1 Assumptions made in analyses

5.1.1 -Pontoons System

The analysis is a 1st order approximation and therefore assumes a mean waterlevel and that the shape of the body above the water has straight sides. The mass distribution for the system was considered constant for the pontoons and

deck.

The deck was not restrained by a temporary mooring system

5.1.2 HLCV

The analysis is a fist order approximation and therefore assumes a mean waterlevel and that the shape of the body above the water is the same as the water planearea. This means that the submergence of the over hanging pontoons (occurring at5.7 deg) is not modeled.

The mass distribution for the system was considered constant for the HLCV,pontoons and deck

The forward speed of the vessel was not modeled.


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5.2 Results

5.2.1 Pontoons System

The catamaran RAOs, drift forces and force transfer functions all with respect to the center ofmass are shown in Appendix 4.1. The amplitudes are non-dimensionalized to a unit wave input

and are given in meters or degrees. Forces are non-dimensionalized by the density of water timesthe submerged volume times gravity (*V*g) for a given unit wave. The resonant frequency forthe unrestrained catamaran is listed in the table below.

Table 5.1 Resonant Frequencies for the Catamaran

Motion Peak Period Amplitude in m or deg Sea Direction

Heave 11 7.2 Head Sea

Pitch 11 7.9 Head Sea

Roll 10 4.61 Beam Sea

Surge 11 2.5 Head Sea

Sway 11 1.5 Beam Sea

Yaw 10 1.7 Beam Sea

The peak period of around 10 sec can be a problem in some installation sea states. The designengineer may want the installation restraint mechanisms to shift the period up depending on thesea state. A RAO for a head sea state is given below


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Figure 5.1 Response Amplitude Operator for the Catamaran in Head Seas

5.2.2 HLCV System

The HLCV RAOs, and force transfer functions all with respect to the center of mass are shown inAppendix 5.2. The amplitudes are non-dimensionalized to a unit wave input and are given in

meters or degrees. Forces are non-dimensionalized by *V*g for a given unit wave. The HLCVresults are a crude approximation of the system due to the possible large roll motions of thesystem and the forward speed. The numbers were however used to get a feel for accelerationsduring the voyage.


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Figure 5.2 Response Amplitude Operator for HLCV in Head Seas


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6 Installation Procedure

6.1 Stability during in field waiting

Rules governing this state are the same as for transport given chapter about transportationStability calculated under this section would also be the static stability of the pontoons duringtow. In this mode the pontoons are off loaded from the heavy lift vessel and are waiting forsufficiently good weather to proceed with installation. GZ curve for this scenario is shown infigure 6.1. and related results are shown in table.6.1. The fact that the maximum righting armoccurs at 15o is due to the fact that one of the hulls starts getting lifted out of the water. Thisshould not represent a major problem since the righting arm is also large at heeling anglesexceeding 25o. The GZ-arm starts decreasing at about 22 deg, when one hull is out of the water.The relatively slow decrease can probably be explained by the large depth of the pontoons.

Figure:6.1. GZ-curve for in field waiting.

Table 6.1. Stability in field waiting.

Regulations Waiting mode.



Area under GZ between 30o and 40o 0.03 [rad*m] 2.16 [rad*m]GZ-arm at 30

o0.2 [m] 20.94 [m]

Max righting arm occur at XXo 25o 15o

Initial Metacentric height. 1.5 [m] 101.7 [m]


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6.1.1 Stability during Installation.

Given the fact that installation only lasts for a relatively short period of time and that the seaconditions will be very moderate, the common stability requirement by the Coastguard for thisoperation mode is that the system has a positive initial metacentric height, GM, which is given

by:

KGBMKBGM += ,where

= IBM (6.1)

KG is the distance from keel to center of gravity, KB is the distance from keel to center ofbuoyancy, I is area moment of inertia of the water plane and is the catamarans volumedisplacement. To take account for the cells with open valves at the bottom correction for theeffect of free fluid surface is used. It reduces the GM by a imaginary elevation of the catamaranscenter of gravity GG:

= ni

'GG (6.2)

where i is the moment of inertia of the free surface around its own area center and n is number oftanks with free surface. It is summed up over the number of compartments in the cells, which are12, six in each pontoon. This method is not accurate, but the best approximation. It is valid forclosed tanks with free fluid surface. Both transversal and longitudinal stability is checked and arepresented in table 4.6. below.

Table 6.2 Stability in Installation

Transversal Longitudinal

Initial Metacentric height GM 101.8m 62.6m

Metacentric height corrected forfree surface GM

99.1m 60.9m

6.2 Fenders

A way to make sure that the installation process goes smoothly is to design a fender system thatguides the Spar hull in position quick and safe. The deck and the spar hull will be joined in amanner so that the deck guides will penetrate into their respectively Spar hull female receiversaccurately. Figure 6.2 illustrates the tight fender design, which will hold the hull in place.


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Figure 6.2: Fender design

The fenders provide better accuracy and help save time as the hull enters the catamaran. A rubbersheet is placed at the joint to take the compressive forces generated by the waves/tugs to keep the

pontoons and the spar hull together. The rubber sheet will also take some of the impact forces ifthere are any.


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Figure 6.4: Tugs keeping the whole system together

The fenders as shown in figure 6.4 are made of pipes of outer diameter of 0.2 m and thickness of0.02 m. The fender attached to the pontoons box girder is 0.6 m wide and the shortest width ofthe fender attached to the spar hull is also 0.6 m.

The tugs pull the catamaran in opposite directions so that the hull and vessel stays together. Theforce would be a little more than the highest surge acceleration expected during the installation.Rubber sheet each of thickness 0.5 m. is attached to the fenders of pontoons and spar hull. Thetugs pull with a minimum force of 200 ton so that the catamaran and deck stay together.

According to calculations, rubber with a thickness of 1 m will be compressed by 0.2 m. Naturalrubber with a suitable hardness to satisfy the above condition is to be chosen.


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7 Sea Fastening

The catamaran and the deck are subjected to six degrees of motions during its transportation. The

HLV carries the catamaran with the deck from the manufacturing site to the installation site. Thetransportation phase is crucial and proper sea fastening has to be designed to prevent the loss ofthe deck. The HLV undertakes the mission when the weather is suitable i.e., the probability ofencountering high waves is very small.

The design case for the sea fastening is the worst expected sea state. For a given sea state, theRAOs can be used to get the Response Spectra for the six modes of motion from which theangular accelerations for each motion are calculated. The Response Amplitude Operators or theTransfer Functions are described in chapter 6, Hydrodynamics.

There are 31 sea fastenings each on port and starboard side of the deck to encounter the roll and

sway motions and 10 sea fasteners each on aft and fore of the deck to encounter the pitch andsurge motions. Totally 41 on each pontoon.

The sea fastenings are placed with a distance of 2.4 meters between them. They are pipes weldedto a plate 1cm thick, which is welded to the deck of the pontoon. This is done to avoid stresslocalization.The pipe dimensions are:

Outer Diameter, D = 0.7 m.Thickness, t = 0.06 m.Inner diameter, d = 0.6 m.

Length, l = 3 m.Angle of the pipe to the deck to the pontoon, = 45 deg.

The following is the force calculation on the launch rail and the sea fasteners.

The assumed sea state from section 4.2:Significant wave height, HS: 5 mTime Period, TZ: 10 sec

System I : HLV, catamaran and deckThe Area under the Response Spectra, A = 7.42*10

-6rad

2from SESAM

Assuming that the transportation phase has N = 100,000 peaks

The expected value of the largest roll is0.5772

2. .ln( )2. .ln( )

A NA N

+ = 0.77 degrees assuming a

Rayleigh distribution.Peak period is 19 sec from SESAM

Therefore, maximum expected roll angular acceleration, = 0.034 rad/sec2

Similarly from the RAOs of other motions,


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maximum expected heave acceleration is 2.7 m/sec2

maximum expected sway acceleration is 0.22 m/sec2

System II : Catamaran and deckThe Area under the Response Spectra, A = 1.86*10-3 rad2 from SESAM

Assuming that the transportation phase has N = 1000 peaksThe expected value of the largest roll is

0.57722. .ln( )

2. .ln( )A N

A N+ = 5.9 degrees assuming a

Rayleigh distribution.Peak period is 9.97 sec from SESAM

Therefore, maximum expected roll acceleration, = 0.052 rad/sec2

Similarly from the RAOs of other motions,Maximum expected heave acceleration is 3.27 m/sec2

Maximum expected sway acceleration is 1.12 m/sec2

Forces acting on the sea fastening are shown in the figure 7.1

Vertical force acting on the launch and the sea fastening= weight of the deck+ force due to heave

Horizontal force acting on the launch and the sea fastening:= Partial weight due to gravity+ force due to roll+ force due to sway Frictional force by launch

Figure 7.1: Forces acting on the Sea Fastening and the Launch Rail

mass of the deck, m = 30,000 metric tonsacceleration due to gravity, g = 9.81 m/sec2


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8 Structural design and analysis

8.1 Introduction

Prior to this structural design and analysis part of the project the only design constraint is themain dimensions of the pontoons in the system. The main dimensions are established fromgeometrical dimensions of the Spar-deck, the Spar itself, the complicated buoyancy and the loadsa floating structure will encounter in a sea environment.

The purpose of the structural design and analysis is to verify that the catamaran main dimensionsare appropriate considering the different loads it is exposed to. And design a connection betweenthe two pontoons that is able to keep the two pontoons together in the sea load the catamarancould encounter during operation, and an overall design of the structural support framing insidethe pontoons and the box-girder.

The structural analysis consist of what you could call a first stage analysis, that is establishingscantlings of all elements like shell, frames, beams and stiffeners in the catamaran. Then thescantling against the ABS classification rules are checked to verify that they meet minimumrequirements. Then the global loads are calculated, and stresses from these loads are calculatedfor the pontoon cross-section to see if it has sufficient moment of inertia. When all the scantlingsare set for the pontoons ,a plate analysis of a local plate-field on the bottom shell is done. That is,where the global moment is largest. The stresses in the box-girder are analyzed to see if it is ableto carry the stresses induced by the most extreme load cases during operation.

If the catamaran were to be built and put into service, the structural analysis should be continuedto a second stage of analysis. That would be an analysis of the stresses in the box-girder having

a much more accurate estimate of the hydrodynamic forces acting on the catamaran, which mightlead to less support and decrease support scantlings.This stage would also involve finite element analysis of other parts in the system, like seafastening, valve connections and the box-girder pontoon intersection that certainly would havestress concentrations.


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8.2 Structural Design

8.2.1 Pontoon

When the catamaran main dimensions are set the next stage is to give it an internal supportingstructure that is able to withstand external loads form deck and waves, and internal loads due toballast water.The pontoons are designed in a way very similar to an ordinary crude oil tanker with bulkheads,web-frames, longitudinal beams, transverse beams and stiffeners on all shell plating andbulkheads.

As a first support for the shell and the bulkheads, all plates are provided with stiffeners to preventbuckling and increase the strength of the plates. The stiffeners are placed with a spacing of 1 mhorizontally on the shell and vertically on the bulkheads.

Then the pontoon has bulkheads and web-frames to support the shell against external/internalpressure, load from the Spar-deck and global torque from angled waves.The bulkheads are placed with 15 m spacing except in the ends where the last compartment isonly 5 m long. This means that there is one bulkhead in the middle of the pontoon and threebehind and three in front of the mid-bulkhead.In addition to the bulkheads a web-frame is placed halfway between the bulkheads. Accordinglythe pontoon has transverse support for each 7.5 m, measured from mid-ship, and totally six web-frames.

For global bending in horizontal and vertical direction, and for supporting the plates, the pontoon

has one longitudinal beam under deck, one on bottom and two on each side plating. For supportof bulkheads, the stiffeners on the bulkhead, and the longitudinal beams each bulkhead isprovided with two transverse horizontal beams, in same height as the longitudinal beams on thesides, and a vertical beam in the centerline, supporting the longitudinal beams under deck and onbottom.

Table 8.1 gives all scantlings for the supporting members, and Figure 8.1 shows sketches of thestructural design with dimensions.


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Figure 8.1 show a transverse cross-section of a pontoon on two different places. The left figureshows a web frame, the six longitudinal beams and some stiffeners on bottom and side plating.The right figure shows a bulkhead, two horizontal transverse beams on the bulkhead, the verticalbeam on bulkhead and some stiffeners on bottom plating and bulkhead.

Figure 8.1: Pontoon cross-section: Web-frame and bulkhead


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8.2.2 Box-girder

One of the biggest challenges in the structural design of the catamaran is to come up with adesign of the connection between the two pontoons that are able to take the substantial loads that

could be induced in the connecting structure due to extreme hydrodynamic wave loads and inertiaforces when the entire catamaran system is moving in sea.

Initially two design options were proposed for the connection. We could either make it of somekind of truss, and there could be several ways to design this truss, or make the connection of aclosed structure like the pontoons with shell and supporting structure. Since one main objective inour system is to come up with cheap, cost competitive solution, we chose to connect the pontoonsby a closed structure. This kind of structure is much cheaper to build than a tubular-truss that isexpensive mainly because of the all the joints and that it has to be designed specifically for thisstructure.A closed structure is a standard shipyard design and can easily be made by any shipyard with

customized dimensions.

The box-girder is a 50 m long, 6 m high and 24 m wide box made up of top, bottom and sideshell plating. The internal supporting structure consisting of ten beams in longitudinal direction,three longitudinal bulkheads, mid-deck halfway between bottom and deck, and stiffeners on allplates.The dimensions of the box-girder are constrained by several factors. The length is constrained bythe diameter of the Spar, and the dimensions of the Spar-deck. Since the pontoons are 100m long,the radius of the Spar is 24 m and a 2 m fender is put between Spar end box-girder there is 24 mleft for the box-girder width. The length of the box-girder are set to 50 m so the gap between thepontoon is sufficient to fit a 48 m diameter Spar and a 1m clearance on each side for rubberfenders.The height of the box-girder is constrained by the expected wave height during installation. Theheight of the pontoons is 20 m, with a 10 m draft while waiting on site for installation, and amaximum wave height of 3m, plus 1m clearances, leaves 6 m for the box-girder height.

Dimensions of the box-girder are showed in Figure 8.2 and scantlings of supporting memberslisted in Table 8.1.

24 m

z

6 m

x

Figure 8.2: Boxgirder cross-section


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Table 8.1: Dimensions of structural members in the catamaranShell plate,bulkheads,web-frames

Long.beam

pontoon

Long.beam box-

girder

Verticalbeam

pontoon

Transversebeam

pontoon

Stiffeners

Thickness 0.03 m - - - - -SM - - - - - 4700 cm3

Flangethickness

- 0.02 m 0.02 m 0.02 m 0.02 m 0.015 m

Flangelength

- 1.0 m 1.0 m 1.0 m 1.0 m 0.4 m

Webthickness

- 0.02 m 0.02 m 0.02 m 0.02 m 0.015 m

Weblength

- 3.0 m 2.0 m 2.0 m 2.0 m 0.9 m


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8.3 Classification Requirements

When all the main dimensions are set and the internal supporting structure is established a check

of all scantlings are done to see if they satisfy the minimum requirements set by a classificationagency like the American Bureau of Shipping, ABS. Since the pontoons are both structurallydesigned and loaded much like a crude tanker, the class-rules for a vessel with oil in bulk isapplied as a first minimum demand for structural dimensions. The scantlings that are checked arebottom-, side-, and deck shell plating, stiffeners, longitudinal beams and transverse bulkheads.The rules that are used are ABS Rules for building and classing, Steel Vessels Part 5 Specializedvessels and services, 1998-99, Section 2 Vessel Intended to carry oil in bulk.The scantlings given by the ABS rules are just a minimum requirement and the design scantlingshould exceed the class requirements.

The calculations are shown in Appendix 1, Table 8.2 lists the particular rules that are applied and

Table 8.3 summaries the resulting dimensions.

Table 8.2: ABS class-rules

Section Structural part

ABS 5/2A.4.3.2 Shell platingABS 5/2A.4.3.3 StiffenersABS 5/2A.4.3.4 BeamsABS 5/2A.4.5.2 Bulkheads

Table 8.3: ABS scantling requirementsStructural member ABS requirements Design dimensions

Bottom plate thickness 19.2 mm 30 mmSide plate thickness 19.2 mm 30 mmDeck plate thickness 19.2 mm 30 mm

Longitudinal beam thickness 9 mm 20 mmStiffener section modulus 4600 cm^3 4700 cm^3

Transverse bulkhead thickness 13.6 mm 30 mm

The results prove that the design dimensions for the catamaran structural members are inside therequired dimensions set by ABS.


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8.4 Load Calculations

In the following section the simplified hydrodynamic forces acting on the pontoons are calculatedfor some extreme cases and will be used for calculation of extreme stress values in the box-

girder. The forces are calculated by Froude-Krilov assumption, that is, the wave forces are actingon the structure as if the structure was not there. By applying Froude-Krilov assumption a majorload factor as wave-diffraction is neglected. But the forces calculated are most likely veryconservative in the wave situation that is considered.

A simplified load calculation is also done for the case when the catamaran is placed on deck ofthe heavy lift vessel for transportation to installation site.

8.4.1 Load case 1: Horizontal bending of box-girder

A load case of perpendicular wave direction to the pontoons with a wavelength of exactly twotimes the distance between the centerlines of the pontoons, and a wave height equal the extremeexpected wave height under preparation for installation, are considered. This load case isexpected to represent the absolute maximum horizontal bending moment that could be induced inthe box-girder. Figure 8.3 and 8.4 shows the case.

The horizontal Froude-Krilov forces acting on the pontoons are calculated. These forces will laterbe used for calculation of a bending moment in the box-girder.

Figure 8.3: Horizontal Froude-Krilov forces

Figure 8.4: Top view of catamaran with load case 1


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8.4.2 Load case 2: Vertical bending of box-girder

The same load scenario that results in the extreme horizontal bending moment, load case 1 is alsoused to estimate an extreme vertical bending moment in the box-girder, Figure 8.5. The onlydifference is that in this case the entire force along the pontoon is effective.

Figure 8.5: Vertical bending of the box-girder

8.4.3 Load case 3: Catamaran and Spar-deck on heavy lift vessel

When the catamaran, with Spar-deck, is carried on the heavy lift vessel some parts of it will behanging outside on each side of the heavy lift vessel. The Spar-deck and the weight of thepontoon itself will induce a moment in the pontoon. A simplified model is used to calculate theinduced moment. To estimate a bending moment in the pontoon, only the parts of the deck andthe catamaran hanging outside of the heavy lift vessel deck is considered.

Figure 8.6 Catamaran on HLV


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Figure 8.7 shows the model used for stress calculations when the catamaran with Spar-deck isresting on the HLV. The two uniform distributed loads represent the pontoon weight and theSpar-deck weight.

25 m11 m

Figure 8.7: Beam model of pontoon outside HLV deck


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8.5 Structural Analysis

The structural analysis of the catamaran is divided into four different parts. First an analysis ofthe global forces on the pontoons are calculated, that is global bending moment and global shear

forces. These forces are then used to calculate bending stresses and shear stresses in the pontooncross-section. Then the stresses in the box-girder cross-section are calculated from global box-girder forces. The forces are based on some extreme load cases described in section 8.4.Last, a plate analysis of the bottom shell plating is done by use of plate-strip theory and finiteelement analysis.

8.5.1 Global forces

The pontoon is considered to be a beam in equilibrium. It is exposed to a positive buoyancy forceand a negative force from the load and its own steel-weight. The buoyancy force is a uniform

distributed force acting upward, while the steel weight is assumed uniform distributed over thepontoon beam acting downward. The Spar-deck weight is also a uniform distributed force actingdownwards directly on the skid rails.When the forces acting on the pontoon beam are established the resulting shear forces andbending moment is calculated which then give the maximum bending stresses and shear stressesin the pontoon cross-section. If the stresses are too high, exceeding yield criteria, the cross-section area has to be increased by making the plating thicker or by adding more or largerlongitudinal beams.

The calculations are shown in Appendix 1 and the main results summarized in Table 8.6

Deckload

Pontoon weight

Buoyancy

M V V M

Cross-section

Figure 8.7: Global forces on pontoon beam


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Global load case 1:The catamaran is floating with the Spar-deck on top in calm water with a normal draft of10 m. That is, no ballast water is added to lower the deck onto the Spar.

Global load case 2:

The catamaran is floating with maximum draft under lowering, 20m, and the deck is still totallyloaded on the pontoons. The tanks in the ends have no ballast water, while the ballast tanks usedfor lowering is filled with water and compressed air. This is not a realistic scenario, since theSpar-deck load is gradually transferred to the Spar. But the case represents a worst case, so if thepontoon is able to carry these loads, it will be able to take the load occurring in the transfer phase.

Table 8.6: Global forcesGlobal case 1 Global case 2

Mmax 5150 MNm 7200 MNm

Vmax 20.5 MN 27 MNI 181 m

2 181 m2

Aw 1.2 m

2

1.2 m

2

max 284 MPa 397 MPa

max 17 Mpa 22.5 Mpa

8.5.2 Box-girder

The box-girder is probably the most critically loaded part of the catamaran. Depending on thesea-state it could be severely loaded in vertical bending, horizontal bending and torque. At thesame time it is also to some degree limited how large forces it will encounter. This because thetransportation phase is done on a HLV and the potential large sea states under transportation do

not affect the box-girder at all. While under installation the maximum allowed wave height is 2m,which decreases the critical stresses significantly. The only real critical phase is the waiting onsite for the weather window. And while waiting the catamaran can easily be turned so that the seais either meeting or following, which will remove the most critical load cases.

To be sure the box-girder can take a worst-case load scenario, some critical load cases areassumed and the stresses induced are calculated. The load cases and stress calculations are veryconservative.If the box-girder can withstand the loads used in the calculations it should be able to withstandalmost all other load cases. The use of such conservative stress calculation proves to some extentthat the catamaran could be used in more severe weather areas of the world, but a site-specificcalculation should off course be conducted in each case. On the other hand a too conservativestress analysis will lead to use of too extensive strengthening, which will increase the cost.

The three load cases considered represent extreme vertical bending, horizontal bending of thebox-girder. From the loads a maximum stress value is calculated and compared to yield criteria tosee if the cross-section area is enough to take the stresses. The load cases are described in section8.4. The calculations are shown in Appendix 1 and the results are summarized in Table 8.6.and8.7.


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Table 8.6: Resulting forces and moments on box-girder

Force Moment arm MomentLoad case 1

(Vertical moment)

124 MN 12 m 1488 MNm

Load case 2

(Horizontal moment)

94 MN 38 m 3581 MNm

Table 8.7: Box-girder stressesLoad case 1 Load case 2

I 185 m4

18m4

y 12 m 3 my 380MPa 380MPa

max 327 MPa 350 MPa

8.5.3 Plate Analysis

A local plate field on the bottom shell plating is analyzed to check that it can withstand the localhydrostatic pressure. This analysis can be done either by plate-strip theory, plate theory or finiteelement analysis, FEA. Plate-strip analysis is only accurate when the length of the plate is longcompared to the width, and will generally give a too large stress since the short end sides areassumed free. On a ship structure the shell plating is exposed to hydrostatic pressure, and when asingle plate, bounded by stiffeners and transverse support, is removed form the shell it will havefixed boundary conditions. And an exact solution to the fixed thin walled plate does not exist.

Therefore a finite element analysis will give the best solution to the problem.

A finite element analysis is conducted for the plate, and a plate-strip analysis is done to verifythat the FEA results are reasonable. The plate strip analysis and a figure of the stress distributionare showed in Appendix 1. The finite element analysis is done in MCS/NASTRAN and someFEA properties and the results from the two analyses are listed in Table 8.6.

Table 8.8: Plate analysis resultsMethod Stress

Max plate stress, strip theory 111 MpaMax plate stress, FEA 81 Mpa

The FEA results show that the plate stresses are well inside yield criteria.


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9 Risks For Severe Or Catastrophic Damage of Deck, Vesselor Substructure

The float over deck operation can essentially be broken down in to 6 subsections, which can be

qualitatively ranked with respect to the highest risk.

1. Deck Installation on Substructure2. In field waiting3. Transport to site4. Deck load out onto the catamaran5. Catamaran offloading6. Deck load out onto the catamaran

Efforts should be made to mediate the risks of the top three operations to bring them in line withthe acceptable risk level of the system. Codes, marine warranty surveyors and finances will

dictate the acceptable risk for the system. The three failure outcomes that should be consideredfor an installation are:

1. Catastrophic failures were the deck or substructure is damaged beyond feasible repair orrecovery.

2. Damage that would lead to a lengthy repair and/or a temporary abandoning of theinstallation process.

3. Damage that would require a small time to repair and would result in a small delay,leaving the possibility of missing an installation weather window, or being damaged whilein use for installation and requiring a longer weather window.

9.1 Deck Load From Quay

The largest risk during the d

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