Jack Pollack

Passive Tandem Mooring LNG Offloading System

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1. INTRODUCTION

The technology for cryogenic liquefaction and storage of LNG on an offshore floating vessel is known.The safe reliable ship to ship transfer of this LNG in an open sea, possibly harsh, environment hashowever not been possible. SBM having experience in both the supply and operation of mooring systemsput this experience to work in the development of a Tandem LNG Offloading system. The philosophyadopted for this development was to keep the design simple and whenever possible stick to componentsthat had a known track record.

Having over 40 years experience in the design, supply, and operation of Single Point Mooring (SPM)systems and Floating Production Storage and Offloading (FPSO) systems the value of simple robustdesign was well understood and applied to this LNG Arm development. Known technology andoperational experience either directly or indirectly applied to the Arm include soft yoke or gravitymoorings (Fig. 1), articulated fluid transfer arms (Fig. 2), cryogenic swivels (Fig. 3) and tandem mooringsystems (Fig. 4). Additionally experience from designs that must disconnect and reconnect in typhoon orice areas has led to the understanding of structures and mechanical systems exposed to dynamic forcesand motions, as well as the physics of making connection under these conditions. The LNG OffloadingArm detailed herein effectively combines this technology and operating experience to enable the offshoreindustry to effect the safe and reliable offshore transfer of LNG.

2. LOADING ARM DESCRIPTION

The offloading Arm shown in operation and stored (Fig. 5 and 6), is designed for tandem offloading in aharsh environment (up to 5m significant wave height). The Arm is supported from a cantilevered rotatingstructure on the back of a turret moored LNG — FSO or FPSO. The shuttle carrier can be passivelymoored using the gravity restoration of the weighted arm or can also be DP assisted using both the Armand a shuttle carrier thruster system. When connected, the Arm provides for a continuous flow pathbetween the LNG FPSO and the shuttle carrier.The Arm is composed of a central pipe for LNG transfer (up to 24 ) and a 2-m diameter pipe armstructure, which is also used as the vapor return from the shuttle carrier. The Arm support structure andouter structural shell are fabricated from stainless steels compatible with LNG. The inner LNG piping isalso made of stainless steel to minimize the relative thermal deformation between the Arm structure andthe piping.The offloading Arm consists of two long arms, a counter weight, one short arm with connector and sevenswivels. The swivel locations are depicted by the color changes on the arm (Fig. 5). The Arm has beenconfigured in a way that all swivels lie in the same plane. This swivel arrangement minimizes momentsand eliminates secondary bending acting on the articulations. The swivels in the Arm provide for alllinear and angular movements required for the Arm to follow both the FPSO and carrier relative wave andslow drift motions. The lifting of the counterweight at the Arm elbow provides for the mooringrestoration of the shuttle carrier and aids the connection/disconnection procedure.

Each of the loading Arm articulations (Fig. 7) consists of: a large diameter roller bearing with a sealingsystem for the vapor return, and a cryogenic swivel having a sealed LNG chamber and bearing. Thecryogenic swivel and piping is centrally supported to the outer pipe through insulated web structures.Relative axial motions resulting from inner and outer pipe differential thermal or structural strain aretaken by slip joints placed along the inner pipe. The LNG vapor returns through the large diameterstructural pipe on the outside of the central LNG pipe. Heat transfer between the Arm vapor space andouter structural components is controlled by the use of insulation.

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3. LNG LOADING ARM COMPONENTS

The loading Arm described in the previous sections is under development. An elevation view shows theArm overall external arrangement in Figure 8. Components used in this arm, be they mechanical orstructural are well known. The use and combination of some of these components at temperatures rangingfrom ambient to cryogenic is however new. Details of these components will be briefly described. Themain components of the LNG Offloading Arm are as follows:

1. Arm FPSO support structure2. Arm inner and outer pipe3. Arm counterweight4. LNG swivel bearing and sealing system5. Large diameter LNG vapor return swivel bearing and sealing system6. LNG pipe slip joints7. Arm outer to inner pipe supports8. Arm to shuttle carrier connector and regulating valves9. Insulation

3.1 ARM FPSO SUPPORT STRUCTURE

The main LNG Arm support member consists of a fixed vertical column placed at the stern of the FPSO.A short distance below the top of this column a diagonal support takes longitudinal mooring loads fromthe column into the FPSO. The column houses a centrally located 24 inch LNG line for loading andprovides for vapor return within its outer shell. The LNG line in the column is fitted with a slip joint atthe base and a swivel at the top. Fitted to the top of the column is a bearing and a 2-m diameter horizontalvapor return pipe with a central 24 inch LNG pipe that extends outward to the first articulation of theLNG loading arm. A diagonal support member at this articulation carries the bulk of the Arm verticalloads and also serves as the Arm rotation member. Rotation of the Arm is effected by a motorized rotatingstructure located near the bottom of the fixed vertical column. This rotating structure is fixed to thevertical column by way of a large diameter slewing bearing, which is geared and driven by two hydraulicmotors. These motors are used to deploy the Arm from its storage position to its operating condition whena LNG carrier is moored.Lateral mooring loads from the Arm are resisted by a dual cable arrangement fitted on either side of theloading arm on the stern of the LNG FPSO. These cables attach to the fixed side of the first Armarticulation and pass downward to the corners of the FPSO stern. One of these cables is detachable toallow the rotation of the Arm when moved to its stored position.

3.2 ARM INNER AND OUTER PIPE

The inner pipe of the LNG arm carries the LNG being transferred. The present design uses up to a 24-inchdiameter, stainless steel pipe (Fig. 9). The pipe is fitted with swivels at each Arm articulation and slipjoints handle differential motions with respect to the outer pipe. Fixed and sliding guides keep the LNGpipe centrally positioned in the outer pipe. Short pipe spools or elbows with flanges are located next toswivels and slip joints to allow for removal or servicing of these components. Hatches through the outerpipe shell at articulations provide access to these components.

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The outer pipe of the LNG Arm is the structural member that transmits all loads and carries the weight ofthe Arm as it spans between articulations. The pipe has a nominal diameter of 2 m with a stainless steelshell. At all articulations the pipe transitions into heavy stainless steel rings which are used to house largediameter bearings. At pitch articulations the bearing housings extend as re-enforcement along the shell.This re-enforcement carries the primarily axial pipe load to either the adjoining fixed outer shell, or to anadjoining bearing ring.

3.3 ARM COUNTERWEIGHT

The loading Arm counterweight performs two functions in the offloading system. The primary role of thecounterweight is to provide sufficient weight at the Arm elbow to effect the mooring of the shuttle carrier.A secondary function is to use this weight in the lower horizontal pipe to counterbalance the shuttleconnector causing it to rise when disconnected.The articulated vertical pipe member of the Arm and the counterweight form a mechanism wherebymooring loads are resisted by the horizontal component of this pipe tension as it articulates from vertical.The required size of counterweight largely depends on the length of the vertical pipe, size of shuttlecarrier and environment in which this offloading operation is performed.The counterweight has been positioned behind the Arm elbow to effect the lifting of the arm shuttleconnector when it is not connected. To stop this pipe and connector from full upward rotation, the elbowarticulation is placed in the vertical pipe just above the horizontal pipe. When rotating, this placementcauses a forward shift in the center of gravity of the lower horizontal pipe, which results in a stableintermediate position. This position allows the connecting carrier to connect by pulling the Arm to theconnector, which controls the connection relative motions.

3.4 LNG SWIVEL BEARING AND SEALING SYSTEM

The LNG swivel is housed in the vapor return or structural arm support member. Its function is to rotate,seal and sustain applied loads while passing LNG. The swivel has redundant pressure seals a primary andsecondary and three bushings, which align and connect the LNG pipe as it articulates. The bushingclearance is designed to keep the relative movements of the swivel parts within limits that result in sealdeflections, which will always result in proper sealing.SBM has successfully tested seals for cryogenic LNG use for over 10 years. In 1999 in-house studies andtesting of new materials suitable for seal and bushing use at cryogenic temperature were completed.Based on this work new seals have been designed and are under test. Upon completion of this testing, thebest seals, old or new, will be designed into the 24-inch LNG Arm prototype swivel, which will then betested.The material used for the swivel bushing will be similar to that used for the sealing material. The materialtesting discussed above included materials suitable for this duty. Due to the Arm pipe-in-pipe design, theload carried by the LNG swivel bushings will only be due to pressure, gravity and flow momentum. Theuse of pressure balanced slip joints in the LNG piping eliminates any large shear or bending momentloads acting on these swivels. The bushing material friction, stress and wear properties will therefore givea very long life.Based on testing and past experience, the swivel will require no maintenance for a period of at least 5years. In the unlikely event repair or maintenance is required, there is in-situ access for this and even forswivel replacement.

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3.5 LARGE DIAMETER VAPOR RETURN SWIVEL BEARING AND SEALING SYSTEM

This large diameter swivel system has a multitude of functions. The bearings must provide for rotation,alignment and structural loading resulting from thermal, gravity, dynamic, pressure and mooring loads.This swivel consists of four seals and three sets of rollers. Two of the seals will provide for thecontainment of the LNG vapor and two seals will close in the bearing cavity. A leak detection system isused to monitor the integrity of the vapor seals. The roller bearing is configured as a normal three-raceroller bearing and has sufficient strength to take the required loading while maintaining an alignment,which will insure proper sealing.The seals used in this large diameter swivel will be of similar construction to those used in the innercryogenic swivel. The seals will also be capable of sealing cryogenic LNG liquid should the inner pipesealing system fail. The three-race roller bearing will have the geometry of normal bearings used atambient temperatures; it will however be modified to be able to operate at lower temperatures. Use ofinsulation on the inside of the outer shell will however insure that the bearing operating temperaturesalways remain close to ambient.Maintenance of this swivel would only involve bearing lubrication at specified intervals. This will beautomated and designed once the bearing selection is finalized. Servicing of these articulations wouldinvolve the disassembly and separation of the inner piping and outer bearing flange. This is regarded to bean unlikely occurrence, however this can be carried out on board the LNG FPSO.

3.6 LNG PIPE SLIP JOINTS

The LNG loading Arm uses the outer structural pipe to support the inner LNG pipe. Thermal differencesbetween these pipes can approach 200… C. Differential elongation of these pipes due to thermal expansionwould result in high loads and stresses should they be rigidly tied together. To account for this relativeelongation slip joints are placed along the inner cryogenic pipe.The slip joints used are axially pressure balanced to maintain the longitudinal pipe tension due to pressureeffects. A non-pressure balanced slip joint would induce compressive loads in the piping system. Theseloads would be taken by supports and swivel bushings complicating the overall design.To minimize bending across the slip joints they are placed near pipe supports. This helps to ensureminimal eccentricity and wear. The slip joints do not require any maintenance for the life of the arm.Should service be required, the joint can be removed by disconnecting the adjacent flanges and pullingthe joint out of the piping for disassembly.

3.7 ARM OUTER TO INNER PIPE SUPPORTS

The inner LNG piping is entirely supported from the outer structural pipe. Supports are fixed to the outershell wall and have an open radial web terminating in a central ring that surrounds the inner LNG pipe.This central ring is fitted with segmented blocks having good insulating and friction properties. Theseblocks will either have a close sliding fit to the inner LNG pipe, or they will be grooved to engage aflange from the inner piping or swivel. The open radial web in these supports allows for LNG vaporreturn flow and manned access should servicing be required.The supports are located throughout the arm to both secure the inner pipe and minimize thermally inducedstresses. The support arrangement that best accomplishes this is shown in figure 10. The workingprinciple of this arrangement is that all supports and slip joints lie in the same plane when the Arm is inits equilibrium position. Three key supports are those at the Arm pitch articulations as these articulationsare orthogonal to the plane and thus arrest both the axial and rotational movement with respect to theouter pipe. Swivels having articulations within the plane are fixed to the outer shell and are turned by the

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inner piping as it articulates with the Arm. The rotating pipe sides of these swivels pass to a slip joint,which minimizes swivel loading.

3.8 ARM TO SHUTTLE CARRIER CONNECTOR AND REGULATING VALVES

The connector (Fig. 11) of the LNG arm to the shuttle carrier is a mated unit, half of which is permanentlyinstalled on the carrier bow. The connector design incorporates the following functions:• 1) - Connector winch, alignment and locking system• 2) - LNG pipe connector and valve system• 3) - LNG vapor connector and valve system• 4) - Arm purging systemThese functions are further elaborated below:1) - A single shaft, double drum, 50 tons winch is used for the pull-in of the LNG arm to the shuttle

carrier bow connector. The connector’s central and azimuthal alignment is obtained through the use oftwo stab guides. Pull-in cables attach to Arm mounted male stabs that are pulled through and intocarrier mounted female stab guides. Once the connectors have been pulled into contact, a hydrauliclatching system locks and seals the connector and outer vapor barrier from the atmosphere. The rateof connection and the differential motion between shuttle and arm is controlled by the speed of theshuttle pull-in winch

2) - Once the arm has been secured to the carrier and the mooring is established, the LNG pipeconnection is made by hydraulic actuation of closure rods, which lift a carrier based flange and sealagainst the Arm LNG flange. Lowering of these closure rods opens this flange connection. Shouldthis occur without draining of the Arm during an emergency disconnect, LNG between flangeconnection and Arm valve spills into carrier plenum. This avoids LNG spill when Arm disconnects.Prior to startup of LNG transfer valves on both sides of this connection flange are opened, whichallows flow from the LNG FPSO to commence.

3) - The LNG vapor path is sealed by a seal carried in the flange on the LNG shuttle carrier side of theconnector. The vapor return flow from carrier to LNG FPSO passes through two large diameterspring-loaded valves located in the sealed bottom deck of the LNG arm. Opening of these valves iseffected by the action of the same closure rods that lift and close the LNG flow connection. Oncethese valves are opened, the LNG vapor path to the plenum under the arm connector is established.The carrier vapor return piping also connects to this plenum. A valve on the carrier outside theplenum is opened prior to commencing LNG loading.

4) - After offloading is completed the Arm is LNG gas freed by purging with nitrogen. This purging canbe carried out from the FPSO with use of a cross over between the LNG piping and vapor return atthe carrier end of the arm. This cross over consists of a small diameter pipe and valve, which allowsnitrogen to flow from the LNG piping into the vapor return path by way of a ring manifold.

3.9 INSULATION

A study has been carried out on the Arm heat transfer. This study indicated that for the rate of LNG liquidand vapor flow, heat gain was not a problem and insulation was not required. This study also indicatedthat with normal sort of ambient air movements the outer uninsulated shell remained at near ambientconditions. Without air movement the outer shell however remains at near vapor return temperature.These temperature differentials require the Arm inner to outer piping design to handle almost 200 degreesC of thermal deformation. Having the capability to handle these deformations, it was found beneficial toinsulate the inside of the outer pipe as it eliminates thermal cycling of the outer shell and its mechanicalcomponents.

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4. ARM SAFETY SYSTEMS

The LNG loading arm has two functions:- To transfer the mooring loads between the LNG carrier and the FPSO- To transfer the LNG and the vapor return between these vessels

The safety system monitoring and controlling these function will be detailed below.

4.1 MOORING SAFETY SYSTEMS

The Arm mooring function is designed for a 5-meter significant wave height threshold. This weathercondition will result in a certain maximum excursion for the moored LNG carrier. If this excursion issurpassed, the carrier needs to be readied for disconnection.A safety system monitors this mooring function. The safety system consists of a hawser, winch andhawser load monitor. After connection of the loading arm to the LNG carrier, the hawser winch adjuststhe length of the mooring hawser so that it will only have a tension of 30 tons when the maximumexcursion between the two vessels is reached. Should this excursion be reached the load monitoringsystem on this hawser will signal the Emergency Shut Down (ESD) system of the arm. When this signalis received, the following procedure is initiated:

1. Start ESD 1 procedure2. Wait a certain time continuing to monitor the mooring hawser tension

- Should loads remain at normal level, the mooring master may choose to resume loading.- If the loads continue to increase, the mooring master can initiate the arm disconnection- Should mooring hawser loads reach a higher predetermined level triggering ESD 2, the

Arm is automatically disconnected.3. Disconnection procedure

- Open the main connector- Disconnect the mooring line

4.2 FLUID TRANSFER SAFETY SYSTEMS

The fluid transfer system is designed in accordance with the requirements of the OCIMF (Design andconstruction specification for Marine Loading Arms). The fluid transfer safety system monitors andcontrols valves, Arm sealing systems and flow pressure.The safety system is composed of:

• 1 ESD valve for the LNG before the loading Arm• 1 ESD valve for the vapor return on the carrier before the connector• 2 SD hydraulically activated LNG valves; one at the end of the loading Arm and one after the

connector on the carrier• 1 LNG flow connector and vapor return self-closing system at the end of the Arm, which will

automatically operate when the Arm to carrier connector is activated.• LNG pipe and vapor return pressure detection sensors.• Leak detection system for vapor passing the Arm outer vapor seals.• LNG liquid detection in the carrier plenum chamber.

The fluid transfer safety system continuously monitors the above valves and sensors. Should any faults bedetected the system will automatically go to an ESD 1 or ESD 2 condition. When an ESD condition istriggered, the safety system automatically starts a sequence of actions. These actions are described in thetables below.When the ESD system is activated, the following procedure is initiated

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Timing ESD 1 INITIATES THE FOLLOWING SEQUENTIAL ACTIONSAct. Ti dT Tf FPSO LNG carrier

1 0 0 0 Send shut-down signal to the shuttle Send shut-down signal to the FPSOvia the FPSO/LNG carrier link

2 0 1 1 Stop loading pump Stop vapor return compressor3 0 15 15 Close ESD valve Close ESD valve4 0 20 20 Close SD valve 1 Close SD valve 2

5 20 5 25 NA Open LNG flow connector whichdrains lines and also closes vaporreturn

6 Hold Assess conditions causing ESD1 Ready to open arm connector

Ti: Initial time of the action, starting from ESD signaldT: Duration of the actionTf: Final time for action to be completed

Timing ESD 2 INITIATES THE FOLLOWING SEQUENTIAL ACTIONSAct. Ti dT Tf FPSO LNG carrierIf under ESD1:

1 0 0 0 NA Disconnect the mooring line andopen LNG Arm connection

If not under ESD1 condition:1 0 Send shut-down signal to LNG

carrierSend shut-down signal to the FPSO

2 0 1 1 Stop loading pump Stop vapor return compressor3 0 15 15 Close ESD valve Close LNG ESD valves4 0 20 20 Close SD valve 1 Close SD valve 25 20 5 25 Open LNG arm drain line Open LNG flow connector which

drains line and also closes vaporreturn

6 25 5 30 Open arm connector

5. LOADING ARM ANALYSIS

The components described above when assembled and connected to LNG vessels will moor and transferLNG. To facilitate sizing of this system a fully dynamic computer simulation tool was developed. Todemonstrate this capability a sample analysis is shown. Results of this analysis will provide values for themotions and reaction forces at each articulation during a design storm.Environment conditions for the simulation are:• Waves: Pierson-Moskowitz spectrum, Hs=5m, Tp=12s,• Wind: Mean velocity U10=25 m/s,• Surface current: Uc=1.3 m/s.For directionality, two cases have been considered:• All parallel: wind, waves and current come from the same direction,• Transverse: wind and waves parallel while current is at 60¡.The LNG FPSO and shuttle tanker hulls are modeled as finite element meshes, which are input and runusing diffraction software.

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As variations of draught are very small for both the FPSO and the shuttle, the models use the averagedraught for both vessels. The main particulars are listed in the table below:

Units FPSO Shuttle

L.O.A. m 328 290Lbp m 320 275Beam m 58 48.1Depth m 30 27Draught m 15 11.3Displacement t 288 000 124 500Area exposed tolongitudinal wind

m_ 2 320 1 700

Area exposed totransverse wind

m_ 11 200 8 500

Position of the turret(from bow)

m 68 N/A

The overall model of the loading arm, FPSO and shuttle tanker is shown in figure 12. The FPSO has aturret mooring near the vessel bow, and the shuttle tanker is tandem moored to the FPSO using the LNGloading arm. The arm in this model consists of 6 tubular sections of 2.0-m diameter. Each section isconnected to the next via a hinge articulation allowing only one rotational motion. The masses, centers ofgravity and moments of inertia have been derived from the detailed design and entered in the model.The articulated Arm has a 150-ton counterweight, which gives the required restoring properties to themooring system. Figure 13 gives the load versus displacement curve of the Arm. The arm equilibriumposition is at 51.8 m. and positive load values indicate tension when the carrier is pulling away from theLNG FPSO.

Parallel Storm AnalysisA 3-hour time domain simulation was performed for the parallel storm. The statistics of the relative surgeand heave motion between the LNG arm attachment points on the deck of the two vessels are listedbelow:

Mean Max MinRelative surge (m) 61.4 69.9 52.9Relative heave (m) -0.6 6.2 -4.8

This table shows the range of excursion of the vessels relative motions are 17 m in surge and 11 m inheave and that the minimum surge always stays above 51.8 m that shows the carrier always pulls on theFPSO. The statistics of the pitch angle for the long vertical and horizontal LNG Arm components arelisted below:

Mean Max MinVertical (¡) 20.2 43.7 2.0Horizontal (¡) 0.4 14.2 -23.3

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The basic physics of the Arm is such that the pitch of the vertical leg accommodates surge while thehorizontal leg pitch accommodates heave. The most loaded articulation of this loading arm is the firstarticulation between the FPSO and the Arm. This articulation is one that allows for relative roll whilesupporting the bulk of the LNG arm weight. For the in-line environment, the reaction at this articulationis:

Mean Max MinSurge (kN) 932 3,511 87Heave (kN) 2,856 4,087 2,663Pitch (kN.

m)11,710 16,757 10,918

Transverse Storm AnalysisA 3-hour time domain simulation was also performed for this storm. The statistics of the relative motionof the Arm connector attachment on the FPSO and shuttle vessel are given below:

Mean Max MinRelative surge (m) 50.7 62.3 39.4Relative sway (m) 31.9 43.7 16.4Relative heave (m) -0.6 5.9 -4.8

As a combination of sway and surge determine the separation between vessels, a time history plotshowing these motions is found in figure 14. This run again shows the Arm not to go into compression,meaning the vessels never come closer than their equilibrium position. The statistics of the pitch angle forthe Arm vertical and horizontal legs is listed below:

Mean 2↔RMS Max MinVertical (¡) 17.7 11.3 41.8 0.8Horizontal (¡) 1.2 8.6 14.6 -17.6

The transverse environment causes all Arm articulations to be loaded. The most loaded articulation isagain the first at the top. The statistical and extreme values of the loads in this articulation during the 3-hour storm are given below:

Mean 2↔RMS

Max Min

Surge (kN) 655 557 2,671 -5Sway (kN) 487 410 1,741 -49Heave (kN) 2,827 202 3,867 2,602Pitch (kN.m) 11,590 828 15,855 10,668

The above bearing loads and rotations found in the Arm analysis do not present any problem for the Armcomponents.

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6. SUMMARY

The LNG Offloading Arm described has been developed to carry out the transfer of LNG in sea states upto 5 meters. The components and the particular arrangement thereof have been chosen to create a passivetransfer system as experience indicates automation in these systems leads to down time. The Armmooring is based on a well-proven gravity principle, and the design of the Arm mechanical components isrobust for a long life. The use of a pipe-in-pipe arrangement for transferring LNG liquid protects theliquid line from external shock and provides a secondary containment should this pipe have a containmentproblem. The sealing of LNG liquid and vapor is known and is based on seals that have been tested for along operational life. Access for maintenance, service or repair is provided to all mechanical componentsto assure minimal down time. The Arm connect/disconnect system can perform in seastates giving quickcarrier turnaround and is controlled by safety systems assuring quick disconnect in the event ofemergencies.

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Figure 1

Soft Yoke System

Figure 2 - Articulated transfer arm

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Figure 3

Cryogenic swivel

Main characteristics of the prototype swivel are:

¥ Fluid passage diameter : 16 (400 mm)

¥ Temperature range : ambient to -200¡C

¥ Pressure range : 0.50 bar to 30 bar

¥ Double sealing arrangement

¥ Main body is made of stainless steel, with high resilience at low temperature

¥ Standard bearing materials

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Figure 4 - Tandem offloading systems

Figure 5 - LNG offloading arm connected

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Figure 6 - LNG offloading arm stored

Figure 7 - LNG arm swivel articulation

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Figure 8 - LNG offloading arm arrangement

Figure 9 - Inner and outer pipe arrangement

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Figure 10

Arm support arrangements

Figure 11 - Arm to carrier connector

bc

d e

A B C D E F

γδr

C

Sliding pipeguide

Swivel anchored to outerpipe

Slip joint

Tension/compression rod to outershell

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Figure 12 - Offloading system model

Figure 13 - LNG arm load excursion curve

Horizontal distance FPSO-Shuttle tanker (m)

35 39 43 47 51 55 59 63 67 71

-200

-100

0

100

200

300

400

Hor

izon

tal M

oori

ng L

oad

(t)

Arm compression

Arm tension

Equilibrium position

51.8m

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Figure 14

Shuttle/FPSO relative position transverse environment

60¡

Hs= 5m

Surge (X) distance from FPSO Stern at center line

Wind25 m/s

44.0

40.0

36.0

32.0

28.0

24.0

20.0

16.0

-64.0 -60.0 -56.0 -52.0 -48.0 -44.0 -40.0 -36.0

Sway (Y)distance fromFPSO Stern atcenter line

Current1.3 m/s

2