Comparative of Hydrodynamic Effect between Double Bodies toSingle Body in Tank

C.L.Siow, Jaswar, Efi Afrizal, Hassan Abyn, Adi Maimun, Mohamad PauziDepartment of Aeronautic, Automotive and Ocean Engineering

Faculty of Mechanical EngineeringUniversiti Teknologi Malaysia (UTM)

Skudai, 81300 Johor Bahru, Johor, MALAYSIAPhone/Fax.: +60-7- 5534644

E-mail: jaswar@fkm.utm.my or jaswar.koto@gmail.com

Abstract

Hydrodynamic interaction between floating offshore structures are affecting structures motion. Largemotions between floating bodies would cause the damage on moorings, offloading system and maycolloid to each other. The experiment tests were carried out with and without the influence of TLP inregular waves in the Universiti Teknologi Malaysia (UTM) Towing Tank using semi-submersible GVA4000 type. In this paper, experimental results of the hydrodynamic interaction effect of surge, sway,heave, roll, pitch and yaw motion for Semi-Submersible (Tender Assisted Drilling (TAD)) werepresented. As primarily study, the experiments were conducted without TLP in the system and then withthe exist of TLP. The wave height in full-scale ranges from 5.8 m 11 m and separation distance of themodels is 21.7 m (actual length). The results show that the influence motions response amplitude islocated at a frequency around 0.514 Hz where close to the frequency of the sending wave. (10 TNR)

Keywords: Tension Leg Platform, Tender Assisted Drilling, Floating Semi Submersible,Hydrodynamic Interaction

1. Introduction

Multiples floating structures currently are used inoffshore industries and coastal application.Offshore oil and gas industry, floating structuressuch as a spar, semi-submersible, tensional legplatform (TLP) and ship shape floating productionstorages and off-loading structure (FPSO) is apopular type of structure uses in deep water oil andgas exploration. In coastal application, the floatingstructures applied currently such as floatingterminal, floating airport and floating storages aredeveloped by many countries in the world. Either inoil and gas industry or coastal application, thefloating bodies are often placed near to each otherto complete as a system.

To ensure the floating bodies is arrangingsafely in open water, hydrodynamic interactionbetween floating bodies is one of the importantcriteria must be evaluated to ensure it is safe beforestart operate. Hydrodynamic force and moment due

to wave can cause accidents on floating bodies suchas crashing between each other or causing damageof riser system. One of the noticeable features ofdeep water moored structures is a need of multibody operation which should be paid attention,because it requires more accurate analysis ofhydrodynamic interactions between closely mooredvessels [6].

This research is aimed to study thehydrodynamic interaction between TLP and semi-submersible and characteristic of the multi floatingbodies when placed near to each other in regularwaves. At this situation, the hydrodynamic forcecreated from wave will affect the motion of thefloating bodies by scattering wave causes by thebodies itself. The wave height in full scale rangesfrom 5.8 m 11 m and separation distance of themodels is 21.7 m. The analysis of the floatingbodies motion was focus of this paper.

2. Literature ReviewThe vertical plane motions induced by heaving,rolling and pitching should be kept adequatelylow to guarantee the safety of the floatingstructure, risers and umbilical pipes and otherimportant facilities use in oil production [11].The operability and safety of floating bodiesoperation are greatly influenced by the relativemotions between them. So, the accurate motionprediction of two bodies including allhydrodynamic interactions is important [8].

Normally motions of floating structures areanalyzed by using strip theory and potentialtheory. A number of notable studies were carriedout to solve the problem of hydrodynamicinteractions between multi bodies by Ohkusu(1974) [12]; Kodan (1984) [7]; and Fang andKim (1986) [4]. They used strip theory toanalysis of hydrodynamic interaction problembetween two structures positioned side by side.

Hess and Smith (1964) [5], VanOortmerssen (1979) [14] and Loken (1981) [9]studied on non-lifting potential flow calculationabout arbitrary 3D objects. They utilized a sourcedensity distribution on the surface of the structureand solved for distribution necessary to lake thenormal component of the fluid velocity zero onthe boundary. Plane quadrilateral source elementswere used to approximate the structure surfaceand the integral equation for the source density isreplaced by a set of linear algebraic equations forthe values of the source density on thequadrilateral elements. By solving this set ofequations, the flow velocity both on and off thesurface was calculated. Wu et al. (1997) [15]studied on the motion of a moored semi-submersible in regular waves and wave inducedinternal forces numerically and experimentally.In their mathematical formulation, the mooredsemi-submersible was modeled as an externallyconstrained floating body in waves, and derivedthe linearized equation of motion.

Yilmaz and Incecik (1996) [16] analyzedthe excessive motion of moored semi-submersible. They developed and employed twodifferent time domain techniques due to mooringstiffness, viscous drag forces and damping; thereare strong nonlinearities in the system. In the firsttechnique, first-order wave forces acting on astructure which considered as a solitaryexcitation force and evaluated by using Morisonequation. In the second technique, mean driftforces are used to calculate slowly varying waveforces and simulation of slow varying and steady

motions. Sylemez (1995) [13] developed atechnique to predict damaged semi-submersiblemotion under wind, current and wave. Newtonssecond law of approaching equations of motionwas used in the research to develop numericaltechniques of nonlinear equations for intact anddamaged condition in time domain.

Choi and Hong (2002) [2] applied HOBEMto analysis hydrodynamic interactions of multi-body system. Clauss et al. (2002) [3] analyzedthe sea-keeping behavior of a semi-submersiblein rough waves in the North Sea by numericaland experimental method. They used panelmethod TiMIT (Time-domain investigations,developed at the Massachusetts Institute ofTechnology) for wave/structure interactions intime domain. The theory behind TiMIT is strictlylinear and thus applicable to moderate seacondition only.

An important requirement in determiningdrilling capabilities of the structure is the lowlevel of motions in the vertical plane (motionsinduced by heave, roll and pitch). Matos et al.(2011) [11] numerically and experimentallyinvestigated second-order resonant of a deep-draft semi-submersible heave, roll and pitchmotions. One of the manners to improve thehydrodynamic behavior of a semi-submersible isto increase the draft. The low frequency forcescomputation has been performed in the frequencydomain by WAMIT a commercial BoundaryElement Method (BEM) code. The code cangenerate a different number of meshes on thestructure and calculated pitch forces.

Since demand for oil and gas is growing up,the water depth is becoming deeper and deeper,and chance of multi body operation increasing, soinvestigating reliability of numerical analysismethod for hydrodynamic interaction isworthwhile (Hong et al. 2005) [6].

Zahra Tajali et. Al. (2011) [18] studiedhydrodynamic characteristic of multi-bodyfloating pier under the wave action by numericalmethod. In the research, pontoon of the floatingpiers is connected to each other by hinge. Theresearch found that when the number of pontoonsincreases, peak frequency and peak amplitude forall motion increase. R.C. Zhu et.al (2006) [17]applies numerical methods to study the effect ofgap in the multiple structural system. In thatstudy, wave potential for incident wave andscattering wave were ignored. The motion of thestructures is assumed only affected by radiatedwave. The simulation showed that hydrodynamicinteraction between floating structures can cause

surge, sway and heave motion; however, the onlysway motion shows strong interaction effect incertain resonance wave number. Besides, thatstudy also obtained that the increase of gap widthcaused the resonance amplitude for added massand damping coefficients decrease significantly.

Masashi Kashiwagi (2010) [20] was carriedout a numerical investigation to compare Waveinteraction Theory with the Higher OrderBoundary Element Methods, HOBEM. Incomparison, the wave interaction theory is able tocompute integrated force and pressuredistribution if the condition satisfied the BesselFunction and mathematic limitation of the theory.In addition, the research also found thatreductions of distance between floating bodieswill cause deviation of pressure distributionincrease.

Zhu et.al. (2008) [21] was carried out aresearch on hydrodynamic resonance phenomenaof three dimensional multiple floating structuresby using time domain method. The linearpotential theory in time domain was used todescribe fluid motion. The research found thatpeak force response on floating bodies atresonance frequency is same between frequencydomain technique and time domain technique.This finding proved that the linear potentialtheory in the time domain can be an alternative tosolve a problem related to hydrodynamicinteraction between floating bodies in small gap.

According to Lin Lu et.al (2011) [19],potential theory and viscous fluid theory are ableto predict fluid characteristic in the narrow gapsbetween floating bodies. The objective can beachieved by analyzing the effect of wave forceact two floating bodies. Results from theirnumerical method were showed that increases ofthe gap between floating bodies lead to reducesof wave force. Second, increase of body draughtwill cause the increase of wave force on thebody. Third, larger breadth of the body will leadto higher wave force especially vertical waveforce on the body. Forth, if three floating bodiesin the system, then two peak wave force willoccur in two different frequencies compare to onepeak wave force in two body system.

Y.W. Sun (2012) [22] was studied themethod to simulate wave and flow based on theNavier-Stokes equation and ComputationalLagrangian-Eulerian Advection Remap-Volumeof Fluid (CLEAR-VOF). The Navier-Stokesequation is discretized by using three-step finiteelement method. The study is proposed toestablish the wave and fluid for two dimensional

numerical methods. The regular wavessimulation result has compared to theoreticalwave in the study. From the simulation, temporalcurve of the simulated wave becomes stable afterthree to four waves generated.

3. Concept of interaction floating bodiesThe regular wave acting on floating bodies canbedescribedg by velocity potential. The velocitypotential normally written in respective to theflow direction and time as below:( , , ) = ( , , ) (1)( , , ) = { ( , , ) + ( , , )}

+ ( , , ) (2)Where,

g : gravity acceleration: incident wave amplitude: motions amplitude: incident wave potential: scattering wave potential: radiation wave potential due to motions: direction of motion

From the above equation, it is shown that totalwave potential in the system is contributed bypotential of the incident wave, scattering waveand radiation wave. In addition, the phase andamplitude for both the incident wave andscattering wave is assumed to be the same.However, radiation wave potentials are affectedby each type of motion of each single floatingbody inside system, where the total potential forradiation wave for the single body is thesummation of the radiation wave generates byeach type of body motion such as roll, pitch, yaw,surge, sway and heave.

Also, the diffraction wave potential ( ) mustbe satisfied with boundary conditions as below: ( ) = 0 0 (3)( ) + ( ) = 0 ( = ) (4)( ) = 0 = (5)( )~ 0 (6)

Potential for diffraction wave

Potential for Radiation wave

( ) = (7)The velocity potential for the incident wave

as follows= cosh[ ( + )]cosh ( ) (8)Also, the scattering wave potential due to

the continuous surface of fluid can be explainedby the equation below:( , , )= 14 ( , , ) ( , , ; , , ) (9)

Where the part ( , , ; , , ) isrepresenting Greens function and the part of( , , ) Is the source strength function whichcan calculate by the following equation2 ( , , ) ( , , ) ( , , ; , , )= 4 ( , , ) (10)The field coordinates label as (x, y, z); thecoordinate for source point in the structures labelas (a, b. c)

For radiated wave, the wave potential isrelated to the bodys motion; the velocitypotential for the radiated wave is given asfollows:( ) = ( , , ) (11)2 ( )( , , )+ ( )( , , ) ( , , , , , )= ( )( , , ) ( , , , , , ) (12)

The boundary conditions for the radiatedwave potential same as the boundary conditionsfor incident wave.The radiated wave potentialdue to each floating body can be developed fromequation (12). Where the part ( , , ; , , ) isrepresented the Greens function (x, y, z) is thecoordinate for field point and (a, b, c) is thecoordinate for source point which located on thebody. SB in the equation above is the wet bodysurface of the floating structure.

4. Fourier Transform For Time DomainTo Frequency Domain Transformation

According to sampling theorem, discretelyfrequency (Fs) for signal data must be at leasttwice to the highest continuous signal frequency(F). The continuous signal frequency shoulddiscrete by the rate follow the samplingfrequency, 1/Fs. Let the discrete sample of thecontinuous signal have the magnitude of x(k),k=1,2,3,,n and period between the sample is1/Fs than a function of a continuous signal, f(t)can be reconstructed back from the discretesample by the equation below:( ) = ( ) ( ) (13)

Where,( ) = sin( ) (14)To convert the data in time domain to the

frequency domain, Fast Fourier Transformmethod can be applied. The relationship betweenfunction in the time domain, f (t) and frequencydomain F (f) can be related by the equationbelow:( ) = ( ) ( ) (15)Also, for the variable j, it represents the squareroot of (-1) for the natural exponential function.= cos( ) + sin( ) (16)Therefore, the discrete data can be written incomplex number form as follows:= ( ) + ( ) (17)and,( ) = ( ) cos 2 (18)( ) = ( ) sin 2 (19)

And, i = 2b is the number of data require byFast Fourier Transform method where b can beany integer number larger than or equal to 1.

Finally, the magnitude, phase and frequencyof the signal can be calculated by followingequations:

( ) = ( )= 2 ( ) + ( ) (20)( ) = ( )( ) (21)( ) = (22)5. Model ExperimentalIn this paper, model experiment was carried outto study the hydrodynamics interaction effect forthe floating structures arranged in small gap. Theexperiment was carried out at UTMs towingtank. The experiments were conducted for theconditions where semi-submersible structurealone in the tank and the semi-submersiblearranged behind the TLP structure. Allparameters for wave and semi-submersiblestructures are constant for both the experiments.

5.1 Models ParticularsIn this experiment, semi-submersible model andtension leg platform was selected to studyhydrodynamics interaction between multifloating bodies system. The semi-submersiblemodel was constructed based on GVA 4000 typemodel. Both the semi-submersible and TLPmodel were scaled down with ratio 1:70.

Table 1 Principal particular of ModelsCharacter TLP Semi unitLength 57.75 66.78 mWidth 57.75 58.45 mDraft 21 16.73 m

Displacement 23941 14921 m3Water Plan Area 715 529.6 m2

Number of Columns 4 4Pontoon length 31 66.78 mPontoon depth 7.28 6.3 mPontoon width 9.73 13.3 mPontoons centerlineseparation

- 45.15 m

Columns longitudinalspacing (centre)

- 45.58 m

Column diameter - 10.59 m

Upon the model complete constructed, fewtests were carried out to obtain the modelparticulars. Firstly, inclining test, swing frametest, oscillating test, decay test and bifilar testwere carried out to identify the hydrostaticparticular for both the semi-submersible andTLP. The results obtained from inclining test,

swing frame test, oscillating and bifilar tests areGM value, KG value and gyration radiuses atplaner (horizontal) and vertical axis. Besides,natural periods for the motions also obtainedfrom decay test. The dimension for the modelswere summarized as in table 1.

5.2 Motion Tests5.2.1 Instrumentation for motion testThe floating bodies were assumed to experiencesix degrees of freedom during the experiment.The six DOF motions of the models whenmoored on springs are measured by opticaltracking system (Qualisys Camera) that uses a setof infrared cameras attached to the carriage tocapture the positions of the reflective opticaltracking markers placed on the model.

Water-proof load cells are attached to thesprings at the model fairlead locations to measureapplied tension force on the model from themooring springs directly. The purpose of thissetup is to avoid any losses in force. Lightweightring gauge load cells used here are sufficientlysensitive to provide a good signal for smallmooring line tensions. The measured mooringline tensions are recorded by Dewetron DataAcquisition System (DAQ).

Data recorded from different data systemswere synchronized to obtain phase information.For this purpose, the optical tracking system wasused as the master. The external sync pulse isrecorded on the DAQ thus enabling synchronizedsimultaneous data recording on both systems.

5.2.2 Springs and connectors for mooringsystem simulationSoft lateral springs are attached to the TLP andSemi-submersible to give horizontal restoringforce to prototype TLP tendons and Semi-submersible moorings. One Side of the softlateral spring was clamped to the mooring postsattached to the carriage and other side of the endswas connected to load cells at model side tomeasure the spring tension forces at the model.Anchor locations for the springs were properselected to ensure mooring lines of the modelmake 45 degree angle with respect to the fairleadattachment points on the model. The springpretension and spring stiffness applied in the testwas same as horizontal stiffness required for thesystem to match the natural periods of thehorizontal motion (surge, sway) for the TLP andSemi-submersible. The TLP and Semi-submersible are connected between each other by

two connectors to control the gap between thefloating bodies.

Due to the limitation of this tank, thetendons, risers and moorings are not actuallypresented in the model tests. Therefore, thismodel setup was expected had less dampingcompared to the prototype and caused largermotion amplitude at model scale compared to theprototype. However, it is common practice toneglect damping from mooring, tendons andrisers in floating structure tests in order to obtainconservative response estimates at the designstage. A similar philosophy is followed here aswell.

5.2.3 Experiment setupAs mentioned, the water depth is less thanrequired depth to include full length of thetendons and mooring for the experiment. Hence,almost horizontal springs set considered forcompensation of horizontal forces (Figure 1). Iftruncated tendons were used by following themodel scale, 1:70th, the set-down would begreatly exaggerated. An alternative option wouldbe used a very small 1-200th scale model withouttruncation, but this would impose significantscale effects because when Reynolds number, Reless than 10,000, the vortex shedding patternaround the body may be changed and undulyaffected the results. For bluff bodies, if Re biggerthan 10,000, the vortex shedding is mostlyindependent of Reynolds number since the flowseparates close to the column corners at bothmodel scale and full scale (Magee et al. 2011).

Hydrodynamic interaction between floatingstructures model test between TLP and Semi-submersible was set up as shown in Figure 2. Inthe arrangement, progressive wave firstlyattached TLP model before semi-submersible.

Figure 1 Model test set-up in available waterdepth

The models were attached to tow carriage onsprings and regular waves generated by wave-maker at the end of towing tank (Figure 3). At the

start and end of these tests, the model wascarefully held to prevent large offsets due tosudden wave exciting forces which could damagethe mooring springs. Measurement datacommenced when the model had settled at aconstant incident wave was coming. The tanklength was sufficient to assure enoughoscillations were recorded for each tested beforereflection occur.

Figure 2 Layout TLP and semi-submersiblemodel experimental set up (Dimension is inmodel scale).

Figure 3 TLP and Semi-Submersible set up intotowing tank.

Due to limitation in generating wave heightand period by wave making system, some periodswere chosen to cover natural period of modelsand also wave slope are considered 1/20, 1/40and 1/60 to get an acceptable motion to record[1]. The setup is generally unique to a particulartype of floating system and may not beappropriate for others. For this paper, thediscussion will only focus on the wave slop 1/40and the separation distance of the models is 21.7m in full-scale or 300mm in model scale. Waveparticulars for the experiment were shown inTable 2.

SemisubmersibleTLP

Table 2 Incident wave particulars

Wave Particular Semi-submersible

Semi-sub.With TLP

Wave Period (s) 1.85Wave Length (cm) 534

Wave Slope 1/40

Wave height (cm) 13.4Wave Direction 1800

6. RESULT AND DISCUSSION

Figure 4 EreTransUTM Software interface

Data are collected from the experiment byusing an optical tracking system (QualisysCamera) with sampling frequency 120frame/s.The data were recorded in time domain by themeasurement device before it converted tofrequency domain for further analysis. Sameresult can be obtained from the frequency domainanalysis and time domain analysis according toZhu et.al. [21]. However, in this research, thefrequency domain analysis is preferred becausethis method is easier to compare the differentbetween the motion of semi-submersible whenalone in the system and interact with other body.By using the equation from 13 to 22, a set ofDiscrete Fourier Transforms programming codename as ExReTransUTM (Experiment ResultDomain Transformation UTM, 2012) wasdeveloped to convert the data to frequencydomain. The software will able to read discretedata in time domain and return in frequencydomain with a simple click on execution bottom.The interface of this software was shown infigure (4)

Figure 5 to figure 10 shows a comparisonbetween the motions of semi-submersible when itis alone in the tank without interaction betweenother floating body and effect of existing of TLPin the tank. The motions detected by the motioncaptive camera are surge, sway, heave, roll, pitchand yaw.

The figure 5 and 6 were shown thecomparison of the surge and heave motion for thesemi-submersible when it alone in the tank andexisted of TLP. The effect of hydrodynamicsinteraction between the floating structures causedboth the heave and surge motion for semi-submersible increased. This is because the waveforce contributes to the surge and heave motion isinfluenced by the TLP and then increase the forcemagnitude. From the potential equation (2), it isexplained that the extra wave force acting to thesemi-submersible can be generated by radiationwave from the TLP. For the single body system,the motion of the structure will only contributeby the force of incident wave. The radiated wavegenerated by the structures itself only separateout from the system and no influence to the bodymotion in single body system.

In addition, the gap between the structuressetup for this experiment is around 30cm wherethe ratio of gap to length of TLP and semi-submersible are 0.037 and 0.032. In this situation,it was predicted that the hydrodynamicinteraction between the structures is large. Fromthe experiment, the surge and heave motion areshown to increase significantly. For the surgemotion, the magnitude of the motion is increasedfrom 55.3mm to 77.0mm where the magnitude ofthe motion increased around 39% compared tothe single body system. Besides, the magnitudeof heave motion also increases around 80% from30.7mm for single body system to 55.2mm formultiple body system. The experiment result alsoagrees with the numerical result which studied byR.C. Zhu [17]. The same result shown that theradiation wave force in small gap can causesignificant effects for the motion.

Figure 7 shows the comparison of pitchmotion for semi-submersible when interact withTLP and when semi-submersible alone in thesystem. The amplitude for pitch motion alsoincrease in interaction situation where the pitchmotion at single body system measured is 0.71rad/s and increase 22% to 0.86 rad/s wheninteract with TLP. However, the effect of the

hydrodynamic interaction for pitch is lowercompare to surge and heave motion. In this case,the reason for lower effect for pitch motion maybe caused by the arrangement of semi-submersible behind TLP in the experiment. Thescattering wave which can contribute to themotion may not have enough distance separateafter passing through the TLP in two bodysystem. Besides, the design of semi-submersiblewhich has small water plane area at column mayreduce the wave load for pitch motion.

Figure 5: The magnitude of surge motion forsemi-submersible with and without influence ofTLP.

Figure 6: Magnitude of heave motion for semi-submersible with and without influence of TLP.

Besides, frequency of peak magnitudefor these three motions is same either the semi-submersible alone in the tank or existed of TLPand semi-submersible in the tank. For the heave,pitch and surge motion, the peak magnitudeoccurs at frequency 0.541 Hz or period 1.85sec.The period for peak motion magnitude is same tothe wave period. This is shown that the motion ofthe semi-submersible is following the incidentwave frequency. Another finding from the

experiment is the wave resonance is occurring atthe incident wave frequency and this causes themaximum motion magnitude occur at the samefrequency.

At single body system, motion for roll, swayand yaw is relatively small and can be ignoredcompared to interaction condition. As shown infigure 8 (roll motion), figure 9 (sway motion) andfigure 10 (yaw motion), the motions which noexist on single body systems had increased andexisted in interaction condition. These wereproved that the hydrodynamic interaction occurswhen the structures placed close to each other asstudy by other researchers by using numericalmethods.

Figure 7: Magnitude of pitch motion for semi-submersible with and without influence of TLP.

In general, the change of motions amplitudeand the behavior of semi-submersibles motionscan be caused by the scattering wave andradiation wave generated from TLP which placednear to the semi-submersible. In addition, theequation (10) and equation (12) also explainedthat both the scattering wave and radiation wavegenerated are affected by the hull form design.For the scattering wave, the direction of thiswave travel after it passes through the structureswill effect by the shape of hull according toequation (10). Different travelling direction ofthe scattering wave causes the semi-submersibleto roll, sway and yaw although the structure isheading the incident wave. Besides, the radiatedwave causes by the structures motion can alsocontribute to roll, sway and yaw. The radiationwave generated will travel in the direction normalto the structures surface and it magnitude is in thefunction of structures motion. The combinationof the scattering wave and radiation wavegenerated by TLP become the disturbance to thethree motions in interaction condition.

-200

20406080

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Interaction Single Body

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-0.20

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Mag

nitu

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nitu

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Figure 8: The magnitude of roll motion for semi-submersible with and without influence of TLP.

Figure 9: Magnitude of sway motion for semi-submersible with and without influence of TLP.

Figure 10: Magnitude of yaw motion for semi-submersible with and without influence of TLP.

Another observation obtained from theexperiment result is all peak motion for roll, swayand yaw are occurring at 0.541 Hz where themotions periods are same as the incident wavefrequency. These three motions are mostlyinfluenced by scattering wave and radiation wavecompared to incident wave and this phenomenalshow that that the scattering wave and radiationwave frequency were same as the frequency forincident wave. This assumption can be logically

accepted because the scattering wave generatedwhen the incident wave pass through the TLP andthe speed of travel is constant before and afterscattered. Second, the radiation wave is assumedgenerated by the motion of neighbor floatingbody and take the frequency of the motion. Inthis experiment, the TLP places in front of thesemi-submersible should vibrate with the samefrequency as the incident wave frequency. As theresult, the radiation wave generated from TLPand progress to semi-submersible was expectedhave the same frequency as the incident wave.

ConclusionsThis paper was presented the comparison

study for semi-submersible motion in single bodysystem and interaction condition by usingexperimental methods. From the comparison,exist of second body in the system will causes thehydrodynamic interaction to happen in thesystem. This phenomenon will cause amplitudeof all six types of structures motion increase.

In addition, the hydrodynamic interactionbetween the structures can cause safety problemsto the system because amplitude for each type ofmotion was increased significantly if thestructures are placed close to each other as thearrangement in this experiment. Large increase ofsurge motion and exist of sway and yaw motionin the interaction condition will increase thepossibility of the floating body to crash with thenearest floating body. However, the largeincrease of pitch, heave and roll motion will givethe significant problem to riser system andincrease the possibility of system damage orleakage.

ACKNOWLEGMENTThe authors are very grateful for guidance of

Dr. Allan Magee from Technip during the testand also would like to gratefully acknowledgetheir gratitude to the Marine Technology Centerstaff for their assistance in conducting theexperiment.

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