The Specialist Committee on Vortex Induced Vibrations - LabOceano

Proceedings of 26th ITTC – Volume II561<strong>The</strong> <strong>Specialist</strong> <strong>Committee</strong> on Vortex InducedVibrationsFinal Report and Recommendations to the 26th ITTC1. REPORT FROM THE COMMITTEE1.1 Membership and Meetings<strong>The</strong> members of the Vortex InducedVibrations <strong>Specialist</strong> <strong>Committee</strong> of the 26 thInternational Towing Tank Conference are asfollows:• Halvor Lie, Dept. of OffshoreHydrodynamics, MARINTEK, Norway.(Chairman).• Elena Ciappi, Istituto Nazionale per Studied Esperienze di Architettura Navale(CNR-INSEAN), Italy.• Shan Huang, Naval Architecture andMarine Engineering, University ofStrathclyde, UK.• Sup Hong, Korea Ocean Research &Development Institute (KORDI), SouthKorea• Zong Zhi, Department of NavalArchitecture and Ocean Engineering,Dalian University of Technology, ChinaThree committee meetings were heldrespectively:• CNR-INSEAN, Italy, November 2009• Shanghai (OMAE 2010), China, June2010• University of Strathclyde, Glasgow, UK,February 20111.2 Tasks and Recommendations of the 25 thITTC<strong>The</strong> original tasks of the 26th ITTC VIV<strong>Specialist</strong> committee recommended by the 25 thITTC were as follows.• Task 1. Update the state-of-the-art forpredicting vortex induced vibrations andmotions emphasizing developments sincethe 2008 ITTC in various current profilesat ultra deep water. <strong>The</strong> update shall coverboth small-diameter structures (e.g. risers)and large-diameter structures (e.g. SPARplatforms).• Task 2. Organize, conduct and report theresults of benchmark VIV tests. Cooperatewith OMAE on the benchmark activity.• Task 3. Prepare standard nomenclature forVIV and VIM investigations.• Task 4. Write a procedure for VIV andVIM testing for marine applications.1.3 Brief Description of the Execution of theTasksTask 1. <strong>The</strong> review is presented in thefollowing chapters.Task 2. <strong>The</strong> benchmark study had initiallyseven participating institutes. However duringthe 26 th ITTC period two of them withdrewtheir participation and two others of the

<strong>The</strong> <strong>Specialist</strong> <strong>Committee</strong> on Vortex Induced Vibrations562participants reported unforeseen delay.<strong>The</strong>refore results from only three participantswere reported to the committee in the end ofspring 2011, which was considered to be toosparring for the benchmark study. <strong>The</strong>committee therefore recommends ITTC tocontinue and complete the Benchmark study inthe next ITTC period.Task 3. A nomenclature is included in thereview.Task 4. <strong>The</strong> committee has made a draftversion of a guide line (GL) and recommendsthat the GL should be completed during thenext ITTC period.2. REVIEW2.1 Numerical ModelsMost of the work done in last three yearsconcerning the VIV response of isolated rigidcylinders free to oscillate in 1 or 2 DOF and offlexible cylinders was devoted mainly toimprove the prediction capabilities of wakeoscillator models rather than to thedevelopment of new CFD or semi-empiricalmodels.In fact, a significant number of thepublished papers describe very sophisticatedwake oscillator able to capture the nonlinearmulti-mode dynamics and interactions offlexible curved or straight structuresundergoing VIV and to overcome the limitationof previous models in predicting the amplitudeof oscillations.Much less CFD analyses have been donerecently. All the traditional computationalapproaches have been adopted for the flowdescription including Direct NumericalSimulations (DNS), RANS methods, LESmodel and Detached Eddy Simulation (DES)using finite difference, finite volume and finiteelement scheme. In particular several authorsproposed space-time finite element as a validtool to solve fluid-structure interactionproblems with moving boundaries such as VIVof an elastic cylinder and to improve theconvergence rate in iterative solution of thelarge scale non-linear equation system.Structural analyses are performed usingfinite element, finite difference and modalanalysis.Most of the numerical simulations are twodimensionalor, making use of the strip theory,quasi-three dimensional.Notwithstanding, 3D simulations of theVIV response of flexible risers with highaspect ratio and at moderately high Reynoldsnumber (up to 1.5x10 4 ) in uniform and shearedcurrent can be found and seem to agreesatisfactorily with experimental data.Existing CFD codes have also been appliedto the study of the efficacy of VIV suppressiondevices. For the semi-empirical models, thework recently done is focused on theenhancement of existing codes to overcomesome of the shortcomings of previous methods,on the benchmarking of different tools and ontheir validation with comparisons with modeland full-scale experimental data.On the other hand, considerable interest hasreceived in last years the numerical study of theflow around multiple cylinders usingprincipally DNS for the flow field description.However, up to date, results are available onlyfor low Reynolds numbers (Re

Proceedings of 26th ITTC – Volume II563periodically forced oscillation and a selfexcitedoscillation both at constant amplitudeand the influence of the Reynolds number onthe vortex-shedding modes. <strong>The</strong> study isperformed by using a 2D LES with aSmagorinsky model to simulate uniform flowspast a circular cylinder oscillating at modulatedfrequency for a Reynolds number rangebetween 500 and 8000. With regards to longriser simulations, He et al. (2010) simulate theVIV of a flexible riser with aspect ratio equalto 250 for Reynolds numbers up to 1.28x10 4using Finite Element for the structure andFinite Volume for the fluid domain. <strong>The</strong> modelis based on strip theory which assumes that theflow is locally 2D due to the correlation oflock-in effects. <strong>The</strong>y found that the in-line andcross-flow motion are strongly coupled andexhibited similar behaviour for particularvalues of the reduced velocity. Moreover,Huang et al. (2009a, 2009b) presented 3Dnumerical simulations of the motion of aflexible riser (L/D=482) in uniform andsheared current, respectively. <strong>The</strong> time domainsimulations are performed using the FiniteAnalytic Navier-Stokes (FANS) code. <strong>The</strong>flow field around the riser is calculated solvingthe unsteady, incompressible 3D Reynolds-Averaged Navier-Stokes (RANS) inconjunction with a large eddy simulation (LES)model for Reynolds numbers between 7.5x10 3and 1.5x10 4 . <strong>The</strong> motion of the riser ispredicted through a tensioned beam equationwith structural damping. <strong>The</strong> numerical resultsagree satisfactorily with experimental data bothfor the in-line and cross-flow motion and forthe sheared current case a fatigue life analysisis performed.Wanderley et al. (2008) used the Roe-Swebe scheme to solve the slightlycompressible 2D RANS equations and the k-turbulence model to simulate the turbulent flowin the wake of a circular cylinder. <strong>The</strong> aim ofthe paper was to duplicate numerically theexperiments of Khalak and Williamson (1996).<strong>The</strong> results achieved agreed very well withexperimental data and demonstrated the abilityof this methodology in predicting correctamplitudes in the entire range of reducedvelocity for the low mass ratio case.Prasanth and Mittal (2008) presentednumerical simulation of VIV of a circularcylinder of low non-dimensional mass in thelaminar flow regime (60

<strong>The</strong> <strong>Specialist</strong> <strong>Committee</strong> on Vortex Induced Vibrations566element program ABAQUS is used to modelthe riser for the generation of modal parameters.<strong>The</strong>y investigated the sensitivity of the resultsto the type of lift coefficients, mode bandwidth,mode cut-off and the Strouhal number usingdifferent version of the program. In additionthey compared the results obtained bySHEAR7 with those by ABAVIV obtaininggood agreement both in terms of fatiguedamage variation along the riser and mostdamaging location. In the paper of Tognarelliet al. (2009) BP's benchmarking of differentversion of SHEAR7 is described. In particularcomparisons are made between predicted andmeasured VIV fatigue damage for several fullscaledrilling risers to demonstrate the efficacyof a calibration for the latest version. <strong>The</strong>yfound that while the prediction, on average, aresimilar, the scatter in predictions which leadsdesigners to large factor of safety is notreduced from version to version. Thus, theyrecommended to obtain more measured, fullscaleVIV fatigue and current data for bettercalibration and development of VIV fatigueanalysis methods and to considerimproved/new analysis models that will reduceprediction scatter.A new stochastic model for prediction offatigue damage from VIV in risers is developedand implemented in VIVANA (Lie et al.,2008). As in the original model the structuralmodel applies a non-linear 3-D FE whichmakes it able to work with an arbitrarydistribution of tension, mass, stiffness,buoyancy and diameter. Response frequenciesare identified while considering a frequencydependent hydrodynamic mass along thestructure. Response amplitudes are calculatedat the discrete frequencies by the frequencyresponse method. <strong>The</strong> excitation force modelincludes a lift coefficient that is a function ofthe response amplitude and the responsefrequency.<strong>The</strong> new method overcomes some of theshortcomings of previous methods. A fully 3Dmodel is implemented, 3D (non-uniform)current conditions can be analyzed, “crossflow”(CF) and “in-line” (IL) response arepredicted and the stochastic nature of theresponse is accounted for. This is done byintroducing a time varying envelope functionof the response combined with “time sharing”between dominating response frequencies.DNV has issued a new recommendedpractice on riser interference (DNV-RP—F203). <strong>The</strong> RP recommends methodology forengineering analysis, design criteria andguidance for assessment of riser interface(Rustad et al., 2009). Wu et al. (2008) present amethod for calculating 3D riser wakeinterference. <strong>The</strong> numerical model take intoaccount the drag reduction on a downstreamriser due to being in the wake of and upstreamriser, and the lift force on the down stream risertowards the centre of the wake of upstreamriser.2.2 ExperimentsA survey of the papers presented in OMAE,ISOPE, international journals of OffshoreMechanics and Arctic engineering, AppliedOcean research, Ocean engineering in recenttwo years shows that the majority of thereported work are concerned with model tests.<strong>The</strong> typical scaling factors are within the rangeof 1:40-1:75.It is known that Reynolds number andFroude number scaling are the two relevantscaling parameters for model testing. However,simultaneously satisfying Reynolds and Froudescaling for the model and prototype conditionsis practically impossible in the oceanengineering. <strong>The</strong>refore, full scale testing isnecessary.2.2.1 2D testForced oscillation tests of rigid cylinderhave been extended from pure cross-flow (CF)oscillation tests and pure in-line (IL) oscillationtests to combined forced oscillations.

Proceedings of 26th ITTC – Volume II567Soni (2008) performed extensive modeltests to investigate the realistic interactionbetween IL and CF responses. Long flexiblepipe model tests were conducted in uniformcurrent conditions, and the couple VIVdisplacements at each cross-section wereanalyzed. <strong>The</strong> cross-section trajectoriesresulted from the 3D tests were used as inputconditions for motion-controlled tests of rigidcylinder. Based upon the test measurements,the hydrodynamic coefficients for empiricalVIV prediction models have been renewed(Soni et al., 2009). Soni and Larsen (2009)showed the higher order harmonic componentsof the forces were correlated with vortexshedding patterns observed by PIV method.Yin and Larsen (2010) performed motioncontrolledtests of rigid cylinder in order todetermine VIV coefficients under shear flows(NDP riser). <strong>The</strong> IL excitation forcecoefficients have much higher values thanfound from pure in-line test.However, Reynolds number is still in thesubcritical regime even for experiments on theReynolds number effects on hydrodynamiccoefficients (Szwalek and Larsen, 2009).Practical concern on reduction of VIV responsewere continued by experimental investigationsusing strakes (e.g. Hao et al., 2010, Korkischkoand Meneghini,2010) and rotating splitterplates (Assi et al., 2009).2.2.2 3D testVarious conditions have been undertaken in3D VIV tests such as hybrid risers, freespanning pipeline, locally strong shear flowcondition, tension riser with internal flow, CFVIV in current and waves, VIV in linearlysheared flows, etc. Fibre optic strain sensorshave been widely used for 3D VIV tests oflong flexible pipes (e.g. Vandiver et al., 2009;Hagatun et al., 2010; Tang et al., 2010; Wu etal., 2010a; Hong et al., 2011).Liu et al. (2009) experimentally studied theVIV of free standing hybrid riser.Measurements show that buoyancy and risercan both contributes to the system VIVresponse. VIV response of buoyancy candominates at all current speeds. However, thecontribution of riser VIV response to thesystem increases with current speed.Hagatun et al. (2010) conducted VIV testsof a neutrally buoyant hybrid riser model toinvestigate VIV during towing operations.Fatigue calculations were performed withrespect to current speeds and angles, and backtension. It was shown that the reduced backtension gives increased fatigue damage.Vandiver et al. (2009) investigated field testdata (Miami II). <strong>The</strong>y have shown that, intension dominated condition, travelling wavesat high mode numbers occur. It was suggestedto investigate if bending stiffness effectscausing wave dispersion, will reduce VIV atthe fundamental CF frequency and/or at thehigher harmonics.Cunha et al. (2009) suggested a closed-formanalytic solution of pin-pin conditioned beamfor symmetrical response mode (1 st , 3 rd , 5 th …),and carried out model tests to evaluateliterature data.Experiments of long free-span pipeline withsag were carried out by Soni and Larsen (2010).Moreau and Huang (2010) performed modeltests on CF VIV in combined IL current andoscillatory flows. <strong>The</strong> VIV amplitude responseis dependent upon velocity ratio and muchreduced compared to the steady flow only. Wuet al. (2010a) experimentally investigated thesuppression of VIV of long flexible riser byfour control rods, running parallel alongquadrant of the riser. Internal flow effects onriser VIV was experimentally investigated byZhang et al. (2009).Wavelet transform has been applied toinvestigate time-frequency response of longflexible riser in uniform and sheared flows(Larsen et al., 2010 and Wu et al., 2010b). Wuet al. (2010b) estimated the hydrodynamiccoefficients using inverse force analysis based

<strong>The</strong> <strong>Specialist</strong> <strong>Committee</strong> on Vortex Induced Vibrations568on VIV response data of deepwater riser insheared current (Hanøytangen test program,MARINTEK). Shi et al. (2010) employedwavelet transforms and weighted waveformanalysis (WWA) to estimate fatigue damage.<strong>The</strong>y show that, using WWA, it is possible toestimate reasonably accurate fatigue damage, ifproperly placed, with limited number ofsensors.Kim et al. (2009) studied the effects oflocally strong sheared flow on VIV response.Hong et al. (2011) performed VIV model testsof long flexible pipe in linearly sheared flows.It was shown that the peak VIV frequencyequals to the Strouhal frequency correspondingto approximately 70% of the maximum flowspeed, and it was confirmed that the VIVresponse is multi-mode. Drag amplification oflong flexible riser models undergoing multimodeVIV in uniform flow were studies byHuang, et al. (2011). <strong>The</strong>y used measurementdata to estimate the drag amplification due toVIV and compare the results with existingempirical expression.Vortex and wake-induced vibrations of atandem arrangement of two flexible cylindershas been investigated in a model test programby Huere-Huarte and Bearman (2011). <strong>The</strong>dynamic response for small centre-to-centredistance was studied. Planar digital particleimage velocimetry (DPIV) was used tovisualise the wake. One main observation wasthat both cylinders response showed classicalVIV resonance whenever the reduced velocitiesare near to those typical at lock-in,independently of the gap distance between thecylinders. When the reduced velocity is higher,their behaviour is different depending on thegap separation and hence on the existence ornot of vortex shedding between the cylinders.2.2.3 Field measurementsIrani et al. (2008) considered VortexInduced Motion (VIM) of a Spar platform.Field measured data for VIM of BP’s HornMountain Truss Spar, presented in this paper,shows that the Horn Mountain Truss Spar VIMresponse is within original design assumptions.<strong>The</strong> maximum VIM measured in the field A/D≈ 0.26) is well below the acceptable designvalue of A/D = 0.5. <strong>The</strong> field data is analyzedand results presented in the form of Spar VIMresponse time traces, Spar VIM responseversus the reduced velocity and variation ofperiod of VIM oscillation with Spar offsets.<strong>The</strong> field data shows VIM responsecharacteristics consistent with model testobservations.BP has conducted several full-scale VIVmeasurement program on drilling risers, conferTognarelli et al. (2008), Beynet (2008) andSrivilairit (2009). <strong>The</strong> first paper presentsresults from some of the measurements, withparticular emphasis on fatigue results which iscompared to predicted fatigue results. For fullscale drilling risers without suppression, thedata show that state-of-the-art analysis methodsare, on average, inherently 30% conservativeon a maximum fatigue damage basis. <strong>The</strong> dataalso reveal that VIV does not occur nearly asoften as it is predicted. Further the resultsshowed that both in-line and higher harmonicresponse were often observed during themonitoring of risers. However, theircontribution to the total VIV fatigue damage tothe riser was found to be relative small. <strong>The</strong>second paper presents results from full scaleVIV response measurements of a drilling pipein GoM loop currents. <strong>The</strong> test set-up is a freehanging system that represents non-connectedrisers hanging from the platform duringtemporary operation. <strong>The</strong> paper presentsobservations of cross-flow VIV, in-line VIVand additional response at higher frequencies.<strong>The</strong> response was found to be predominantlystanding wave behaviour up to 17 th mode. <strong>The</strong>last paper presents results of current and riserVIV response analysis applying the properorthogonal analysis technique, which is foundto be an efficient numerical technique forcharacterizing the spatial coherence in arandom field.

Proceedings of 26th ITTC – Volume II5693. TECHNICAL CONCLUSIONS<strong>The</strong> ITTC VIV <strong>Specialist</strong> <strong>Committee</strong> hasproduced report describing the four tasks.3.1 Review3.1.1 General• <strong>The</strong> offshore oil and gas industries stillhave a strong interest in VIV of marinerisers, free spanning pipelines, tethers andfloating vessels. Recent accidents of theoffshore platforms demonstrated the needof an accurate design of all the platformcomponents. Moreover, offshore oil andgas industries are the proponents ofbenchmarking between differentprediction tools and of full scale VIVmeasurement campaigns.• <strong>The</strong> huge number of technical papers onVIV, published in last three years ininternational journals and presented atconference indicated the great attention ofthe research community on this subject.• Despite the tangible improvements madein last years in the understanding anddescription of VIV of an isolated cylinder,this complex fluid-structure interactionproblem has not yet been fully understoodand there is not a consolidated procedurefor its analysis.• Less well studied and understood are thechanges to the flow around two or morecircular cylinders placed in closeproximity to one another.3.1.2 Prediction methods<strong>The</strong> prediction methods comprise a largevariation in methods where the three maingroups are CFD, semi-empirical methods andwake oscillator models. <strong>The</strong> status of thesegroups is:• CFD codes are nowadays able to simulatethe 3D VIV response of long flexiblerisers at Reynolds number up to the highsub-critical regime (Re=1.5x10 4 ). Severalquestions remain open issues e.g. the useof 2D or 3D models for flow description,the accuracy of less time consumingsimulation methodologies (LES, RANS,DES etc) with respect to DNS. CFDremains a research tool for multiplecylinder configurations.• Semi-empirical models are still thetechnique currently used in the design ofmarine risers. Large scatter betweendifferent codes in the fatigue damageprediction is observed leading thedesigners to adopt extremely large factorsof safety. <strong>The</strong>re is a demand for systematiccomparisons with full scale data.• Among low order models, the wakeoscillator has received a renewed attention.Sophisticated wake oscillator models havebeen developed in last three years butmost of the results obtained show only aqualitative agreement with experimentalobservations.3.1.3 Experimental studies• <strong>The</strong>re is still a lack of high Reynoldsnumber model test and full scalemeasurement data devoted to thedetermination of the coefficients used inthe semi-empirical codes.• For validation of prediction tools and forfurther research of VIV new experimentsare needed for both single and multipleflexible cylinders at moderate and highReynolds numbers. <strong>The</strong>se tests should beperformed for both bare cylinders and forcylinders with suppression devices, suchas strakes and fairings.• Some flow field measurements performedby PIV technique are available forvalidation of numerical codes. However,new experimental campaigns should beperformed specifically devoted to CFDvalidation at high Reynolds number for theisolated cylinder and at low and high Refor the multiple cylinder configurations.

<strong>The</strong> <strong>Specialist</strong> <strong>Committee</strong> on Vortex Induced Vibrations5703.2 Benchmark study<strong>The</strong> committee recommends ITTC tocontinue and complete the Benchmark study inthe next ITTC period. <strong>The</strong> benchmark studyhas been initiated and is undergoing in somemember institutes. Given the criticalimportance of the subject to offshoreengineering and the challenging nature of thesubject to many towing tank organisationsaround the world, continuing efforts in this areaare imperative. It is recommended that ITTCshould establish cooperation with OMAE onthe benchmark activity, where ITTC canprovide valuable experimental data to OMAE.3.3 NomenclatureA nomenclature is included in the review.Submergence depthCharacteristic size of roughnessStability parameter,Keulegan-Carpenter number,Cylinder lengthHydrodynamic mass per unitlength,. For acircular cylinder,Mass per unit length (includinginternal fluid)Effective mass per unit length,Mass ratio,Abbreviations2D3DCFCFDILVIVVIMTwo dimensionalThree dimensionalCross-flowComputational fluid dynamicsIn-lineVortex induced vibrationVortex induced motionFroude number,Reynolds number,Strouhal number,TensionOscillation periodFlow velocityMaximum of oscillatory flowvelocityRoman symbolsDisplacement amplitudeExternal cross-section areaAdded mass coefficientExcitation coefficient, i.e. forcecoefficient in phase with cylindervelocityDrag coefficientLift coefficientCylinder diameterModulus of elasticityBending stiffness of a cylinderNatural frequency in still waterOscillation frequencyVortex shedding frequencyFrequency of the oscillatory flowNon-dimensional frequency,Reduced velocity,Reduced velocity (oscillating1cylinder),fˆGreek symbolsϛDamping ratioµ ViscosityυKinematic viscosityDensity of fluidФMode shape3.4 Guide Line<strong>The</strong> committee recommends that the guide lineshould be completed during the next ITTCperiod.Acceleration of gravity

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<strong>The</strong> <strong>Specialist</strong> <strong>Committee</strong> on Vortex Induced Vibrations574Wu, J., Larsen, C. M. and Lie, H. (2010b):“Estimation of Hydrodynamic Coefficientsfor VIV of Slender Beam at High ModeOrders”, Proceedings of 29 th ConferenceOffshore Mechanics and Arctic Engineering.Wu, H., Sun, D. P., Lu, L., Teng, B., Zhang, J.Q. and Xie, B. (2010a): “SuppressingVortex-Induced Vibrations of Long FlexibleRiser by Multiple Control Rods”,Proceedings of 29 th Conference OffshoreMechanics and Arctic Engineering.Wu, W., Saint-Marcoux, J., Blevins, R. D.,Quiggin, P., 2008, “Innovative 3-DImplementation of Riser Wake InterferenceAssessment”, Paper No. ISOPE-2008-MWU10. Proceedings of 18 th ISOPEConference, Vancouver, Canada.Xu, W.H., Zeng, X.H. and Wu, Y.X., 2008,“High aspect ratio (L/D) riser VIVprediction using wake oscillator model”,Ocean Engineering, Vol. 35, pp. 1769-1774.Xu, W.H., Wu, Y. and Yu, J., 2010, “A newwake oscillator model for predicting vortexinduced vibrations of a circular cylinder”,Journal of Hydrodynamics, Vol. 22(3), pp.381-386.Yin, D. and Larsen, C. M. (2010): “OnDetermination of VIV Coefficients underShear Flow Condition”, Proceedings of 29 thConference Offshore Mechanics and ArcticEngineering.Zhang, Y. B., Meng, F. S., and Guo, H. (2009):“Experimental Investigation of Vortex-Induced Vibration Responses of TensionRiser Transporting Fluid”, Proceedings of28 th Conference Offshore Mechanics andArctic Engineering.