measurements of hydrodynamic loading on

8/13/2019 Measurements of Hydrodynamic Loading On

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easurements of hydrodynamic loading on alarge scale jacket structure under waves andcurrentA. C. ~ e n d e s ' , . hev2, J. A. ~ o l o d z i e j ~Universidade da Beira Interior, Laboratory of Fluid Mechanics,CovilhSi6200 Portugal~ u l ~ a r i a nhip Hydrodynamics Center, Varna 9000 Bulgaria.Institute of Applied Mechanics, Poznan University of Technology ,60-965Poznan, Poland.

bstractThis paper reports on wavetank measurements of hydrodynamic loading on amodel of offshore structure in waves and current. Th e model is a large sca le steelstructure, 3.2 m high, geometrically similar to a real four-legged jacket platformbottom-standing in the ocean floor. Two series of tests were conducted. Themodel is firstly mounted on a fixed 3-DOF force balance and tested underregular incident waves. O n a second stag e the force balance is removed to the topof the structure, which is then suspe nded from the carriage of the tank. The entireexperimental apparatus is advanced in still water and in waves, in order toimitate the wave-current conditions. In both cases measurements of water-particle kinematics and overall loading acting on the structure have beenrecorded, for a number of wave heights and periods, and for different values ofthe speed of towing. The experiment has been repeated incorporating at each s tepadditional members to the structure, here represented by the risers of theplatform. Horizontal force and overturning moment have finally been assessed inorder to put in evidence the fluid-structure interactions and wavelcurrentblockage effects.


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4 Fluid Strxcture Interaction

IntroductionA number of problems have arisen in relation with the development of the oiland gas fields in the ocean where the extremely severe sea states demand thatthe environmental loading process be well understood see Vugts [l ]. Apart frommajor hazards the ocean platforms conceived for offshore operation have so farbeen able to withstand the severe sea conditions. However we know that theseplatforms are sometimes very complex in shape; Faltinsen [2] gives acomprehensive review on different concepts and the corresponding mechanismsthat are relevant in calculating the wave forces. Nevertheless more research onthe flow around their structural components has been stimulated to a large extentby challenges of conceiving new offshore units. Their reliability constitutes infact one of the main goals of design and construction. Chakrabarti [3] focuses onsome of the techniques utilized in engineering applications in this field. Pressurefor further progress is also generated by the need for periodic re-certification ofexisting structures. The costs during non-operating periods and those caused bydamage and repair of the structure are very high and therefore should be kept toa minimum.All these aspects are quite relevant for fixed offshore jackets bottom-standing in the sea floor. Efthymiou Graham [4] make with this respect anextended analysis of the survival conditions of such tubular structures inconnection with the common practice used in their design. The design of suchstructures is usually based on studies involving computer modelling andexperim ental tests carried out in situ or in wavetank. A number of completemodel tests have indeed appeared in the literature since the middle seventies.How ever owing to very important scale effects the flow around components oftubular structures and the resulting loading cannot be usefully studied inconventional laboratories. According ly emphasis has been placed on carefullydesigned field experiments and on the use of large scale facilities. Recentprogress has been achieved using both approaches see Chaplin [5 6].Chakrabarti [7] has evaluated the contribution of analytical modelling andfield studies to the design of offshore structures. In-situ experiments are in factdesirable but expensive. An alternative approach is to make use of largepremises. One of the solutions found by the research community has been tomake use o f the Delta Flume a very large flume at Delfi Hydraulics in theNetherlands. Chaplin [ ] discusses some recent experiments there where forceshad been calculated from decay measurements with a cylinder of 0.75m indiameter oscillated in still water and in current. When interpreted by methodsused in conventional design practice the experimental results available seem toindicate that present techniques are on the whole slightly conservative.The purpose of the present work is to follow a methodology to correctlyassess the hydrodynamic loading exerted upon offshore jacket structures due tocombined action of waves and current in order to put in evidence the fluid-structure interactions and wave/current blockage effects. The investigation


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luid Structure nteraction 5

constitutes a part of an European consortium project on design of offshorestructures under extreme wave loading.Structural analysis of the physical model

The physical model is a steel tubular tower built at the Varna shipyardISRDD inBulgaria. The model was scaled down from a realistic offshore jacket designprovided by Atkins Science Technology at a scale of 1:28. Fig. shows aschematic view of the model structure. The purpose of this section is to calculatethe natural frequencies of the structure as well as maximal deflections andmaximal stresses under prescribed loading conditions.

I 6

Figure 1 Model of the offshore jacket structure.


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6 Fluid Structure Interaction2.1 Model s tructu reThe geometrical dimensions of the model structure are as follows. It is composedof 158 structural members welded at 48 junction nodes; the total height of thestructure is 3.2m, of which 2.4m is underwater. The base square is 1.2 X 1.2mand the square at the top is 0.6 0.6m. The four legs have an external diameter D0.057m and 4mm wall thickness, and the bracing members have an externaldiameter d=0.032m and 2.5mm wall thickness. A group of 11 vertical risers ofdiameter 0.032m extend from top to bottom, attached by three horizontal plates.The model is painted and has a slightly rough surface. Table 1 shows detailedinformation related with material properties and cross-section areas used in thecalculations.

Table 1. Data used in the calculation of deflections, stressesand natural frequenciesProperties Steel

Modulus of elasticity: = 2 . 1 x 1 0 ~ aPoisson ratio: v 0.3Mass density: p 7 8 5 0 ~ gm3Cross-section area:members A 0.2317x10 m2legs A 0.666 X 10-' m

2 2 Natural frequenciesFor calculating the natural frequencies of the structure a preliminary analysis bydirect application of the stiffness-matrix method has been made Kolodziej [9]In this case the model is treated as a space truss, each node having three degree-of-freedom with the exception of fixed nodes. According with the plan of theexperiment the model structure can be fixed at the base or at the top. In this waywe will obtain two different sequences of natural frequencies for the samegeometry and material properties, each sequence corresponding to a differentattachment of the structure. In the calculation of natural frequencies four caseswere still considered, giving rise to the following physical situations: 1) thestructure vibrates in vacuum and there is no water inside the pipes; 2) thestructure vibrates in vacuum but with water inside the pipes; 3) the structurevibrates in water but there is no water inside the pipes; 4) the structure finallyvibrates in water and the tubes are also fill of water. In practice for generatingthe mass matrices we use the same geometrical parameters of Table 1, but withappropriate mass densities which take into account the added mass and thedensity of water inside the pipes. The effective mass densities used in thesecalculations are given in Table 2 .


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luid Structure Interaction 17Table 2. Values of effective densities in [ ~ ~ r n l ]sed in calculation

of natural frequencies1) Vacuum inside and outside the tubes; 2) Water inside and vacuum outsidethe tubes; 3) Vacuum inside and water outside the tubes; 4) Water inside andoutside the tubes.

Situation 1 Situation 2 Situation 3 Situation 4Members 7850 10320 11320 13790Legs 7850 10680 1 1680 14510

Two cases of fastening the model were then considered. In the first one thestructure was fixed at the top nodes 41, 43, 45, and 47). In the second one thestructure was fixed at the base nodes 1, 3, 5, and 7). The values of principalfirst) natural frequencies that were obtained are summarised in Tables 3 and 4.

Table 3. Values of the first natural frequency in [Hz] for the structurefixed at the top1) Vacuum inside and outside the tubes; 2) Water inside and vacuum outsidethe tubes; 3) Vacuum inside and water outside the tubes; 4) Water inside and

outside the tubes.Situation 1 Situation 2 Situation 3 Situation 426.634449 23.036573 22.01 1616 19.847197Let us point out that a realistic situation with this respect is when the structurevibrates in water, and its elements are equally filled with water.

Table 4. Values of the first natural frequency in [Hz] for the structurefixed at the base1) Vacuum inside and outside the tubes; 2) Water inside and vacuum outsidethe tubes; 3) Vacuum inside and water outside the tubes; 4) Water inside andoutside the tubes.

Situation 1 Situation Situation 3 Situation 460.336655 52.153023 49.835350 44.9 186972 3 Maximum stresses and displacementsIn our calculations of maximum displacements and stresses we have againassumed that each node has three degree-of-freedom, i, e, three components ofdisplacement. The method used in the determination of displacements andinternal forces for the space truss is once more the stifhess-matrix method.Based upon the results of the hydrodynamic model for standard environmentalconditions, the structure has been subsequently loaded in node number 4 by a


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18 Fluid Structure Interactionforce of components F = -850.9 N F = 0 , F = 1094N and at node number8 by a force of components F = 0, F = 0 , F = -1094 N Force F = 1094 innode 4 and force F = -1094 in node 8 originate together a coupleM = 13 12.4 Nm The loading forces in other nodes were taken equal to zero.Nodes number 41, 43, 45, and 47 were assumed fixed for calculation purposeswhen the structure is fixed at the top, which is the most unfavorable situationfrom the experimental point of view. The values of displacements and stressesobtained are shown in Table 5.

Table 5. Maximum displacements mm) and stresses MP a) for the structurefixed at the top, under prescribed load conditionsStress Displacement2.8060 0.211819

3 Fluid-structure interactionsAn extensive experimental programme was undertaken in order to determine theenvironmental loading on the model structure see Mendes et al. [10]. Toconduct the experiment we have chosen to use a deep-water wavetank200x16x6.5m) in operation at the Bulgarian Ship Hydrodynamics Center, inVarna. This facility is equipped with a towing carriage of 1.5 tonnes andhydraulic wavemakers suitable to our purposes.3.1 Experimental ap pa ra tu s an d test conditionsTwo series of tests were undertaken. The model is firstly mounted on a fixedthree degree-of-freedom force balance and tested under regular incident waves.A supporting underwater platform was used to serve as the base for the model atthe appropriate elevation. On a second stage the force balance is removed to thetop of the structure, which is then suspended fiom the carriage of the tank Fig.2); the entire experimental apparatus is in this case advanced in still water and inwaves, in order to imitate the wave-current conditions. Measurements were madeof the wave profile, the overall loading horizontal force and overturningmoment), water-particle velocities and speed of towing.The test conditions for regular waves cover a range of wavelengths h=2.0 to10.0m, at model scale, and wave steepness Wh=l/lO to 1/50. Differentconfigurations of risers were considered in successive runs: a) 11 risers presentconfiguration 1); b) 2 to 5 risers spread over the cross-section configuration 2 ;c) 3 to 5 risers concentrated in the mid-section configuration 3 ; d) no risers atall configuration 4). The test conditions were set for three speeds of towing:V,=O.O, 0 .3, 0.6 rnlsec.The measuring equipment consisted of a 3-DOF force balance, a perforated-ball flow velocity meter PVM) Chaplin Subbiah [ l l ] and two wave probes


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luid Structure Interaction 19that recorded the undisturbed wave passing alongside the model and the distortedwave behind the model. A Pentium PC collected and processed all the data.

Figure 2: M odel structure suspended from the carriage of the wavetank.3 2 Water-part icle kinematicsTw o com ponen ts of the undisturbed water-particle velocity were measured. Alsoa complete set of data on the water-particle kinematics and wave motion hasbeen acquired at the close vicinity of the model (see Kishev et al. [12]). Thevalues measured were the wave profile at two points alongside and behind themodel structure and the water particle velocities at a point 0.6 m below the stillwater surface and at different locations in the horizontal plane: (a) 0 6 m directlyin front of the model (at 0.6m, and y Om); (b) 1.2 m behind the model (at-1.2m, and y 0, 0.3, 0. 6 ,0 .9 , and 1.2m).3 3 Hydrodynamic loadingThe measured overall loading on the model was separated into components dueto the current, a constant drift added by waves and wave-current interactions, andan oscillating contribution due to the waves. The wave drift forces and momentwere estimated as the mean readings of the horizontal load records. Themeasuring uncertainty of the force gauges was estimated to be 1.5 for thehorizontal force and 2 for the overturning moment.The forces measured when the model was towed in still water are plotted inFig. 3 as a finction of the speed of the carriage (reverse current), forconfigurations 1 (all risers present) and 4 (no risers present). As expected, thecurrent loading is parabolic with respect to the velocity of the flow. Inclusion of11 additional members, here represented by the vertical risers, can increment thecurrent loading by up to 35 in the range of current speeds up to 0 . 6 d s e c . Their


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20 luid Structure Interactioninfluence on the drift force is illustrated in Fig. 4. The specific weight of water

y ), the wave amplitude 5 , ) and the width of the model at water line t ) werehere taken as reference parameters. In waves and a current there is a net increasein mean values. The blockage effect on wave forces is also clearly shown in Fig.5, where the horizontal force is represented for configurations 1 and 4, and acurrent velocity of 0.3dsec. In the presence of waves and a current, the risersrepresent up to 37.5 of the total wave force, steadily increasing with thewavelength, and 25 to 30 of the wave drift for h=2.0-8.0 m. Th e influence ondrift decays afterwards to insignificant values for longer periods. A similartendency could be observed on the overturning moment. Fig. 6 exhibits the non-linear dependence of the drift forces on wave height. For a bottom-fixedstructure having all risers on it, tested under incident regular waves o f A=4 m andno current, the drift force represents 12 to 32 of the total wave force inhorizontal direction, for wave heights ranging from 0.05 to 0.15m. As is shownin Fig. 7 at wave steepnesses less than 1/20 the wave loading remains linear.The presence of all l I riser components is responsible for up to 30 of the totalwave drift and for about 25 o f the base-shear force in the previous range ofwave heights.4 Discussion and conclusionsThis paper reports on extensive experiments with the model o f an offshore jacketin waves and current, at a scale 1:28. The total height of the prototype platform is89.6 m, from which 67.2 m underwater, and the width of the structure at waterline is 16.8 m. Horizontal force and overturning moment were assessed in a widerange of wave heights and periods, and current velocities, for different structuralconfigurations. The center of application o f the resultant force is located between1.9 and 2 3 m from the base of the model (53 to 65 m in full scale). Thecalculated natural frequency of the structure is above 20 Hz in realisticconditions. On the other hand, due to the stiffness of the model very smalldeformations take place under standard loading conditions. A complete set ofdata on the water-particle kinematics and wave motion has been acquired at theclose vicinity of the m odel, in order to investigate the fluid-structure interactions.The main contribution to the velocity of the flow arises from the current.Wave distortion behind the model was visible but remained small. A discreteblockage influence on regular waves elevation has been observed. However, atshorter wavelengths significant changes in wave height behind the model mayoccur, especially in the presence of a current. In frontal position the velocityprobe did not feel the presence of the structure and no reverse flow has beentraced. Behind the model the wake is well developed in the presence of thecurrent.The oscillatory wave forces and moments increase with wave height andwith coupled influence of waves and current. For small amplitude waves thewave loading remains proportional to the square of the wave height. On the otherhand, the structure with no risers experiences a steady increment in the


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Fluid Structure Interaction 1horizontal wave force due to the current. The load increment appears to increaselinearly with the velocity of the current. Depending upon the wave period, puredrag may represent as much as 30% increase in the wave force, for an averagewave height of 2.8 m in full scale. At higher current velocities the overallloading is essentially dominated by drag.The maximum drift force in regular waves occurs for wavelengths of about6.6 times the width of the structure at waterline. A substantial contribution is alsoadded to the drift force in the presence of a current. A similar tendency could beobserved on the overturning moment. The contribution of the drift force to thetotal horizontal force is significant. Increasing the complexity of the structurebrings an additional contribution to the total drift force and to the base shear.However, for long periods its influence on drift decays to insignificant values.AcknowledgementsTh e results here presented summarise part o f the research carried ou t on the jointresearch project ERB CIPA CT-940150 Design of Adaptive Offshore StructuresUnder Extreme W ave Loading , funded by the European Commission (1995-98).This Project was coordinated by Universidade da Beira Interior (Laboratory ofFluid Mechanics and Hydrodynamics) within the framework of theCOPERNICUS initiative.The authors wish to acknowledge the valuable contribution of all individualcollaborators of the consortium, and in particular the collaboration received fromBSHC.References[ l ] Vugts, J. A review of hydrodynamic loads on offshore structures and theirformulation. Proc.. of the Int. Con on the Behaviour of Offshore Structures:London, 1979.[2] Faltinsen, O.M. Sea Loads on Ships and Offshore Structures, CambridgeUniversity Press: New York, 1990.[ ] Chakrabarti, S.K. Offshore Structure Modeling, World Scientific: NewJersey, 1994.[4] Efthymiou, M. Graham, C.G. Environmental loading on fixed offshoreplatforms. Environmental Forces on Offshore Platforms and their Prediction,Kluwer Academic Publishers: London, Vol. 26, pp.293-320, 1990.[5] Chaplin, J.R. Loading on a horizontal cylinder in irregular waves at largescale. Int. J. Offshore and Polar Engn g., 1 4), pp.247-254, 1991.[6] Chaplin, J.R. Hydrodynamic damping of cylinders in still water, in currentsand in oscillatory flow, Proc. of the Int. Workshop on Environm ental Loading onOffshore Structures, Universidade da Beira Interior: C ovilh l, Portugal, pp.6-15,1997.[7] Chakrabarti, S. Inpact of analytical, model and field studies on the design ofoffshore structures, Proc. of the Int. Symp, on O ESH : Gothenburg, 1980.


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22 Fluid Structure Interaction[ ] Chaplin J.R. Hydrodynamic damping of a cylinder at p=106 Proc. of theTMRIALF End-of-Programme Workshop: Delfi 1999.[9] Kolodziej J.A. Calculation of forces and moments acting on offshorestructures subjected to wave loading Progress report on Project ERB CIPA CT-940150 Institute of Applied Mechanics Technical University of Pom anPoland 1995.[l01 Mendes A.C. Kishev R. Chaplin J.R. Tomchev S. Experimentaldetermination of the hydrodynamic loading on a model of offshore platform inwaves and current Proc. of the 1 lhInt. Offshore and Polar Engineering C o n jISOPE: Seattle vol. I pp.196-203 2000.[ l l ] Chaplin J.R. Subbiah K. Velocity measurements in multi-directionalwaves using a perforated ball velocity meter Applied Ocean Research 16pp.223-234 1994.[l2 1 Kishev R. Radev D. Rakitin V. Mendes A. Chaplin J. Modelinvestigation on wave kinematics in vicinity of a jacket structure Rep. of the ndITTC Loads and Response Committee: Seoul-Shanghai 1999.

J C K E T P L T F O R MSTILL WATER TOWING

Figure 3 Current drag force


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luid Structure Interaction 23J A C K E T P L A T F O R M

TOWING BY T H E T O P IN REGULAR WA VES

j Confi ration

02 j i i i i i i i i i i i i i i i i i ii......iiiii iiiiiiiii 8f6

0.00 10 3 6 ml l

Figure : Wave drift force

I J A C K E T P L A T F O R M

0 3 6 [m1 ' 2Figure 5: Horizontal fo rce at waves and current

TOWING B Y THE TOP IN REGULAR WAVESV 0 3 m sFX Configuration

@ ; @0.2

0.00

@


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4 Fluid Structure Interaction

F J C K E T P L T F O R M

0 00 0 .01 0 .02 0 , 03 k:[m21 0.04

Figure : Variation of wave drift with squared wave amplitude

BOTTOM FIXED IN R E G U L AR W A V E SVC 0 m/s

J C K E T P L T F O R MBOTTOM-FIXED IN R E G U L AR W A V E S

IN150

5

0 .00 0 .05 0 .10 OI5 kaIm1 0.20Figure 7: Variation o f wave force with wave amplitude

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measurements of hydrodynamic loading on

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