[kurian, 2011] experimental investigation for the response of semi-sumersible platform subjected to...

8/9/2019 [KURIAN, 2011] Experimental Investigation for the Response of Semi-sumersible Platform Subjected to Bidirectional Waves

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978-1-4577-1884-7/11/$26.00 ©2011 IEEE

Experimental Investigation for the Responses ofSemi-submersible Platform Subjected to Bi-

directional WavesC.Y.Ng

Civil Engineering Dept,Universiti Teknologi PETRONAS,

Tronoh, Perak, [email protected]

Abstract - It is rare to find unidirectional wave in the

real seas and only multi-directional waves or short

crested waves are found. The short crested waves are

defined as linear summation of different long crestedwaves propagated to different directions. The wave

forces generated would be randomly varying in

magnitude and direction. Nowadays, designers tend to

use the directional wave statistics for the design of

offshore structures. The main reason is to obtain the

optimum design of the structures for cost and time

effectiveness. As a part of the research focused on the

dynamic responses of the semi-submersible platform

subjected to short crested waves, this paper presents the

results of the experimental investigation for semi-

submersible platform subjected to short crested bi-

directional wave. The experiment was conducted in the

wave tank of the offshore engineering laboratory at

Universiti Teknologi PETRONAS. The model was

subjected to 5 sets of bi-direction waves crossing anglesof 40°, 45°, 55°, 90° and 120°. The direction of the wave

propagating was found to affect the responses. Two

typical trends of the responses were found for wave

crossing angles of 40°, 90° and 120°; along with 45° and

55° respectively. The optimum wave crossing angle 40°

and 55° give the maximum responses.

Keywords- Semi-submersible, bi-directional wave,

response amplitude operator (RAO), experimental

investigation.

I.

I NTRODUCTION

Due to the depletion of shallow water resources,

deep water exploration is active now a day. Thisencouraged the development of the technology for thedeep water exploration platform such as the TLP,spar, semi-submersible and FPSO. Response to theenvironmental forces, consisting predominantly of thewave force, is the most significant challenges for theresearch and development of the deepwaterexploration platforms.

V.J.Kurian1, M.S.Liew2

Civil Engineering Dept,Universiti Teknologi PETRONAS,

Tronoh, Perak, [email protected] [email protected]

Studies on the wave forces have been performedaggressively since last decade. However, most of thestudies focused on the long crested waves. An

analysis of unidirectional wave forces exerted on acylinder in sea waves was performed by Sun [1].Recently, an investigation was carried out by Young[2] for the relationship between component andsystem performance of the offshore foundation andits mooring system of a spar. The investigationfocused on the reliability assessment of the structureunder extreme environment, where unidirectionalwaves were considered. Ran [3] developed anumerical code for coupled dynamic analysis of thefloating structures subjected to unidirectional wavesand currents. A computer program was developed bySeverance [4] to predict the heave, pitch and surge

response of a spar buoy subjected to long crestedocean waves of single frequency and amplitude.Pinkster [5] predicted the low frequency motions and peak mooring loads on semisubmersible bycomputations based on three dimensional potentialtheories in regular and irregular waves. Garrison [6] performed an experimental and theoretical study ofwave forces and overturning moment acting on twogeometrically similar concrete oil production platforms that were subjected to long crested waves.Maeda et al [7] analyzed the time series responseswithout solving the equation of motion in the timedomain for a very large floating structure in irregular

unidirectional waves. Pijfers and Brink [8] presenteda method to obtain the mean and slowly varying driftforce on a semisubmersible due to hydrodynamicloading in regular and irregular waves. A study oncoupled and decoupled analysis on a deepwater sparin varying water depth subjected to long crestedwaves was conducted by Colby et al [9]. Zhang andLi [10] also presented a study on the wave load ofspar and semisubmersible platform subjected tointernal solitary long crested waves.


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Even though many studies were carried outfocused on long crested waves, it is rare that suchunidirectional waves occur in the real sea. The shortcrested waves are the combination of different longcrested waves from different directions that could begenerated from two directions e.g. the bi-directionalwaves and also from more than two directions e.g. the

multi-directional waves. Generally, the realisticwind-generated sea state would provide a betteraccuracy by short crested waves than long crestedwaves. Short crested wave has many different properties compared to a long crested wave [11]. Theappearance of the wave is three dimensional, complexand short crested [12]. Since 1970s, research beencarried out on short crested waves. Until now, theresearch scope has been mainly focused on thevertical circular cylinder, the wave kinematics anddirectional wave spectrums.

Zhu and Satravaha [13] presented a solution inclosed form for the velocity of the nonlinear short

crested waves being diffracted by vertical cylinder.Zhu [14] presented a solution for the diffraction ofshort crested wave incident on a circular cylinder. Ithas been shown that the wave loading obtained byusing plane incident waves would be overestimatedwhen the incident waves are short crested. Jian et al[11] extended Zhu's theory to include the effect of auniform current for different incident angles. Theyderived an analytical solution for the diffraction ofshort crested incident wave along positive x-axisdirection on a large circular cylinder with current. Anexamination on the feasibility of employing an FPSOhull for motion based estimation of the wave spectrum by experimental work was performed by Simos et al

[19]. Zhu and Moule [16] discussed the wave inducedforces due to short crested waves on vertical cylinderswith circular, elliptical and square cross sections.

The boundary value problem consisting of shortcrested wave diffraction by a vertical circular cylinderwas solved by Scaled Boundary Finite ElementMethod (SBFEM) suggested by Tao et al [15].SBFEM is a novel semi-analytical developed in theelasto-statics and elasto-dynamic areas that has theadvantages of combing the finite element method withthe boundary element method. Studies have also beenfocused on directional wave spectrum, whereHasselmann et al [17] investigated the properties ofdirectional spectrum and the parameterization

proposed previously that applicable to steady windcondition from two systems from JONSWAP. Haver[18] suggested a simple model for spreading function being proper for structural response calculations, andaddressed the uncertainties associated with themodeling of short crested sea. In their study, theydiscussed several issues i.e. the definition of the bestset of input motions, the number of parametersrequired for guaranteeing smoothness of the spectrumin frequency and direction and how to determine their

optimum values that have recently been debatedregarding the advantages of Bayesian inference anddifferent alternatives for its implementation. Otherstudies focused on the very large floating structures,TLP, truss spar and semisubmersible were also found[20] [21] [22] [23].

From the above reviews, it can be seen that thestudies have been generally numerical in nature. Tothe knowledge of the author, no experimental studyhas been performed to investigate the dynamicresponses of the semi-submersible subjected to shortcrested waves. The motion responses for the deepwater platforms subjected to short crested waveswould serve as a very important reference foroptimizing the design of a structure for providing amost cost effective solution.

This paper presents the responses of semi-submersible model subjected to short crested bi-directional waves, which is part of the research work planned to investigate the semi-submersible responses

subjected to short crested waves.

II. WAVE TANK MODEL TEST

Wave tank tests were performed on semi-submersible platform models to investigate thedynamic responses. The tests were conducted in thewave tank in the Offshore Engineering Laboratory ofUniversiti Teknologi PETRONAS.

A.

Wave tank details

The tests were conducted in the wave tank of 22mlong, 10m wide and 1.0m water depth. The waveswere generated by the multi-element wave generationsystem of the wave maker. The wave maker consisted

of sixteen individual paddles that movedindependently to each other. By the forward and backward movement of these paddles waves weregenerated. Also the wave maker could generatewaves at any directional angle instead ofunidirectional wave. Figure 1 shows the wave tankfacilities.

Figure 1 Wave tank facilities


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0

1

2

3

4

5

0.0 0.5 1.0 1.5 2.0

H e a v e R A

O , m / m

Frequency, Hz

Bi-directional Wave Heave RAO

WCA-40° WCA-45° WCA-55°

WCA-90° WCA-120°

0

1

2

3

4

5

0.0 0.5 1.0 1.5 2.0

S u r g e R A O , m / m

Frequency, Hz

Bi-directional wave Surge RAO

WCA-40° WCA-45° WCA-55°

WCA-90° WCA-120°

0

10

20

30

40

50

0.0 0.5 1.0 1.5 2.0

P i t c h R A O ,

d e g /

m

Frequency, Hz

Bi-directional wave Pitch RAO

WCA-40° WCA-45° WCA-55°

WCA-90° WCA-120°

III. R ESULTS AND DISCUSSION

The responses of the semi-submersible modelsubjected to short crested bi-directional wave forfrequency ranging from 0.4Hz to 2.0Hz with 0.2 Hzincrements are shown in Figure 5 to Figure 7.

The surge RAO for semi-submersible modelsubjected to bi-directional regular wave is shown inFigure 5. It was found that the wave crossing angle55° gives the highest surge responses of 4.36 m/m.On the other hand, the wave crossing angle 120° givesthe lowest, 1.27 m/m surge response. The responsesgenerally reduced as the frequency increased. Afterthe frequency 1Hz, the responses were found almostnil.

Heave RAOs for semi-submersible modelsubjected to bi-directional wave is shown in Figure 6.Two trends were found for the heave RAO of thesemi-submersible model subjected to short crestedwave. First, the response due to wave crossing angels45° and 55° were found increased from 0.5Hz to0.6Hz, and gradually reduced as the frequencyincreased. Second, the response due to wave crossingangles 40°, 90° and 120° were found decreaseddramatically from 0.5Hz to almost nil at 1Hz. It isalso found that wave crossing angle 40° gives thegreatest heave response for the semi-submersiblemodel compared to the other wave crossing angles.

As shown in Figure 7 is the pitch RAOs of thesemi-submersible model subjected to bi-directional

wave. Two different trends were also been found forthe pitch RAO of the semi-submersible model. Thewave crossing angle 45° and 55° decreased from30deg/m and 42deg/m respectively to almost nil at1.6Hz. The crossing angle 40°, 90° and 120° increased from 0.5Hz to 0.6Hz, and graduallydecreased to almost nil at 1.6Hz. As compared for thewave crossing angles, the optimum wave crossingangle 55° gives the maximum pitch response.

Figure 6 Heave RAOs for Semi-submersible Model Subjected toBi-directional wave

Figure 7 Pitch RAOs for Semi-submersible Model Subjected toBi-directional wave

Figure 5 Surge RAOs for Semi-submersible Model Subjected toBi-directional wave


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[kurian, 2011] experimental investigation for the response of semi-sumersible platform subjected to...

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