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NREL is a national laboratory of the U.S. Department of Energy, Office of Energy

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC

Contract No. DE-AC36-08GO28308

Investigation of a FAST-OrcaFlex Coupling Module forIntegrating Turbine and Mooring

Dynamics of Offshore FloatingWind Turbines

Preprint

Marco Masciola, Amy Robertson,Jason Jonkman, and Frederick Driscoll

To be presented at the 2011 International Conference on Offshore

Wind Energy and Ocean EnergyBeijing, ChinaOctober 31 November 2, 2011

Conference PaperNREL/CP-5000-52896October 2011


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NOTICE

The submitted manuscript has been offered by an employee of the Alliance for Sustainable Energy, LLC(Alliance), a contractor of the US Government under Contract No. DE-AC36-08GO28308. Accordingly, the USGovernment and Alliance retain a nonexclusive royalty-free license to publish or reproduce the published form ofthis contribution, or allow others to do so, for US Government purposes.

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INVESTIGATION OF A FAST-ORCAFLEX COUPLING MODULE FOR INTEGRATING TURBINE ANDMOORING DYNAMICS OF OFFSHORE FLOATING WIND TURBINES

MASCIOLA Marco, ROBERTSON Amy, JONKMAN Jason, DRISCOLL FrederickNational Renewable Energy Laboratory

National Wind Technology CenterGolden, Colorado, USA

Abstract

To enable offshore floating windturbine design, the following arerequired: accurate modeling of thecoupled wind turbine structuraldynamics, aerodynamics, platformhydrodynamics, a mooring system andcontrol algorithms. Mooring andanchor design can appreciably affectthe dynamic response of offshorewind platforms that are subject toenvironmental loads. From anengineering perspective, systembehavior and line loads must bestudied well to ensure the overalldesign is fit for the intendedpurpose.FAST is a comprehensive simulation

tool used for modeling land-basedand offshore wind turbines. In thecase of a floating turbine, quasi-static continuous cable theory isused to emulate mooring line

behavior. Improved modeling fidelitycan be achieved through the use offinite element mooring theory. Thiscan be accomplished by the newFASTlink coupling module, whichcouples FAST with OrcaFlex, acommercial simulation tool used formodeling mooring line dynamics.In this coupling, FAST is

responsible for capturing theaerodynamic loads, the turbinecontrol system, the platform globalmotion and flexure of the windturbine and its tower, and OrcaFlexmodels the mooring line andhydrodynamic effects below the watersurface. The FASTlink couplingmodule was developed by Orcina, thecreators of OrcaFlex, incollaboration with industrialpartners. This paper can beconsidered as a third-partyassessment of the coupling which

investigates the accuracy andstability of the FAST/OrcaFlexcoupling operation.

Nomenclature

iapF applied force for iteration i

zK heave buoyant stiffness

m total mass specified in FAST

0

T z-axis tether tension from all

fairleadsiT mean z-axis tether tension from

all fairleads for iteration ii

z mean z-axis displacement foriteration i

AE mooring line axial stiffness mooring line mass per lengthL mooring line unstretched length

Introduction

Although the conventions and

practices used in simulating land-based wind turbines are wellestablished, modeling toolsspecially tailored for floating wind

turbines are still in development.To meet immediate design needs,tools can be assimilated fromvarious disciplines to satisfy thespecialized needs of the offshorewind industry.Conveniently, the resources needed

to model, design and construct suchsystems can be adapted from theoffshore oil and gas industry.

Perhaps most notable, collaborationbetween the wind and the offshoreengineering industry is evident inEuropes North Sea area, where windturbines are attached atop monopilesembedded in the seabed. Fixed towersare perhaps the simplest form ofoffshore wind installationaccessible today. The large moments


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created from wind loads on theturbine, however, require upgradedfoundation designs, likely makingthis concept less cost-competitiveas depths increase, effectivelylimiting the design to shallowwaters.

Floating offshore wind turbinesshow promise for deep waters becausethey: 1) require less constructionmaterial, thereby reducing costs;and 2) can withstand greater sealoads compared to their toweredcounterparts [1]. Floating offshorewind systems can be supported by avariety of platform designs,including a Tension Leg Platform(TLP), a spar buoy, or a simplebarge (Figure 1). Offshore industryinnovations are malleable to meet

immediate offshore wind developmentneeds.In most floating offshore systems,

the mooring lines can alter thevessel response characteristics;therefore, the mooring lines shouldbe included in the design process.The moorings are crucial additionsto ensure safety and stability ofthe platform; however, if designedimproperly, they can havecatastrophic effects, especially ifresonance matching between theplatform and its tethers occurs. Thelikelihood of this happeningincreases as the floating system isinstalled in deeper waters [2,3].For this reason, a rigorous analysisof the mooring line dynamics and itseffects on the offshore wind turbineresponse is important.

Offshore Wind Modeling Tools

The purpose of this work is toassess the accuracy and stability ofa coupling module used to combine

the capabilities of two softwaretools to enable high-fidelitymodeling of a floating offshore windturbine. This coupling module is tobe used exclusively with FAST [4]and OrcaFlex. FAST is a modelingtool used to compute the response ofa land-based or offshore windturbine. While FAST has manysophisticated features for modeling

the aerodynamics, hydrodynamics,turbine structure and controller, itcurrently employs a simplisticquasi-static cable model toapproximate the mooring forces on anoffshore floating wind turbine.Quasi-static models use continuous

cable theory to estimate tetherforces by assuming the cable is instatic equilibrium at every timestep; hence, this model accounts forcable weight, but not its inertia[5,6]. Improvements can be made byreplacing the quasi-static cablemodel with a dynamic cable model sothat cable inertia, drag, and fluid

added mass can be modeled. This isdone by linking FAST with OrcaFlexusing the appropriately namedFASTlink coupling module.

FASTlink was assembled by Orcina,the originators of OrcaFlex, incollaboration with industrialpartners. The National RenewableEnergy Laboratory chose to performan independent third-partyevaluation to assess numericalstability, perform model-to-modelcomparisons, and verify the accuracyof the coupling module.This paper focuses on the

usability and numerical stabilityaspects of FASTlink by comparingrudimentary simulations in FAST withresults generated using theFAST/OrcaFlex coupling tool,FASTlink. The analysis is performedusing a 5-MW OC3-HyWind sparplatform deployed in a water depthof 320 meters. The reader isdirected to the study in Robertsonand Jonkman [7] for a detailedsummary of the OC3-HyWind spar, butthe essential attributes of thesystem are listed in Table 1 below.

Table 1: OC3-HyWind spar Properties

Spar + Turbine Mass 7.466x106

kgFluid Displacement 8,029 m

Water Depth 320 m

Spar Draft 120 m

Spar Diameter 6.5 9.4 m

Mooring Mass per Length 77.7 kg/m

Mooring Axial Stiffness 3.842x10 N/m

Mooring Line Length 902.2 m


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motions. As an added benefit,OrcaFlex also provides a graphicaluser interface (GUI), whereas FASTdoes not; therefore, OrcaFlex isuseful in the visualization process.OrcaFlex is designed to be robustfor a variety of cable

configurations, whether the cable isslack or taut. Similar to FASTsHydroDyn, WAMIT input files can beloaded into OrcaFlex to capturevessel reactions to waves.

FASTlink Capabilities

FASTlink is a coupling modulebeing developed by Orcina inconjunctions with industrialpartners to exploit the strengths ofFAST and OrcaFlex to create a high-

fidelity modeling tool for offshorewind turbines. The wind turbine, itsaerodynamic loads, control system,tower and the six degree-of-freedomrigid-body platform motion ismodeled in FAST. The subseacomponents, such as the mooringlines and support platformhydrodynamics, are modeled inOrcaFlex. Therefore, when FASTlinkis being utilized, HydroDyn is nolonger performing calculations.Naturally, as part of this couplingprocess, the quasi-static cablemodel in FAST is disabled. Mooringforces would then be represented inOrcaFlex.In the FASTlink coupling module,

FAST passes the vessel position andvelocity vectors into OrcaFlex; theline tension and hydrodynamic addedmass matrix and non-accelerationdependent hydrodynamic forces arecalculated by OrcaFlex; theresulting added mass matrix andtotal force and moment on theplatform is passed back into FAST;

then the resulting platform motionis solved in FAST. There is aninconsistency between FAST andOrcaFlex regarding the way platformstatic equilibrium is calculated,and this will be discussed next.

Code Coupling Considerations

One challenge with couplingsoftware packages together isensuring program compatibility ismaintained. The purpose of FASTlinkis to ensure compatibility issuesare settled between FAST andOrcaFlex. A unique feature ofFASTlink is that it selects theunits in OrcaFlex to match FASTsunits automatically. This featureprevents an obvious mismatch betweenthe models, which often times isoverlooked. There are, however,considerations the end user mustaccount for when using FASTlink toavoid double counting of forces,continuity between reference framesand consistent time-stepping between

programs. The following steps aretaken to ensure uniformity betweenmodels:

The mooring lines should bearranged in OrcaFlex relative tothe same reference frame originidentified in FAST, which islevel with the mean surface line.

To avoid double counting of thegravitational forces, all massquantities (both submerged andwind turbine components) are

specified in FAST, and the vesselmass is set to zero in OrcaFlex. In this work, the OrcaFlex

integration time-step is equal tothat of FASTs to ensureconsistent resolution between thecoupled and uncoupled data sets.

The last point identified above isnot ideal if the goal is to run thesimulation quickly. Instead, it isacceptable to increase OrcaFlexstime-step by an integer multiple ofFASTs.Issues may arise in the coupling

process because of how the platformweight and buoyancy in handled inthe FASTlink exchange. This can leadto a system not beginning at staticequilibrium. In the next section,corrective measures are prescribedto account for this particular


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disparity occurring between the twomodels.

Resolving the Static Solution with FASTlink

The platform static configurationmust first be determined before thesimulation is initiated. This isdone to ensure the ensuing platformmotions are not caused by animbalance in the initial conditions.Although there are several ways toforce the platform into a staticconfiguration, this correction isapplicable to circumstances wherethe total mass (platform and wind

turbine combined) is specified inthe FAST input file. To begin, anapplied force equal to theundisplaced buoyancy is specified at

the center of buoyancy in theOrcaFlex model.

01ap TmgF += (1)

The simulation is run for asufficient amount of time to ensurethe initial start-up transients areconverging toward a steady-statevalue. If transients still occur,the solution for Fiap at anysimulation iteration i can be

refined using:

( )i0iziap1iap TT-zK-FF =+ (2)

Equation (2) can be repeated asnecessary until the static errorsare sufficiently small. This processresolves the static equilibrium forthe heave direction because the

applied force is inserted in the zdirection only.

Preliminary Results and Stability Assessment

Several simulations with varying

conditions were performed; however,for brevity, only subsets of resultsare reported to illustrate accuracy,stability, and usability ofFASTlink. All simulations areperformed over an 800-second timeinterval with a constant time stepof t = 0.01 seconds in both

OrcaFlex and FAST. The results beingpresented include:

A single degree-of-freedom free-decay simulation to ensureaccuracy of the FAST/OrcaFlexcoupling by comparing platformmotions to results generated withthe standalone FAST program

OC3HyWind spar response toregular waves for the purpose ofillustrating multi degree-of-freedom motion and to ensuresolution stability to wave loads

The spar platform in calm waterwith a below-cut-off wind speedof 8 m/s. This is done to ensurewind load calculations are being

carried out correctly in thesimulation process.

Each simulation is performed withthe rotor in a parked arrangementand with turbine control turned off,even when wind velocity isprescribed. This ensures aerodynamicloads are computed, but controlforces are not introduced into thesystem.CASE I: Single Degree-of-FreedomOffset, z0 = 0.2 meters

A total of six free-decaysimulations (one for each platformdegree-of-freedom) are carried outto ensure consistency between theFAST/OrcaFlex coupling tool and thestandalone FAST program. Figure 2shows the free decay response forthe heave (z) direction with theplatform initially displaced z0=0.2 meters. The platform naturalfrequency is used to assesscontinuity between FAST (using thequasi-static cable model) andFASTlink (labeled as FAST/OrcaFlexin the figures). This step serves asa check to ensure the time-step isproperly selected, to ensure similartransient characteristics arepresent and assess simulationstability.Spectral analysis is an effective

means to compare time series databecause subtle phase shifts in the


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data may skew results. Differentmooring forces are calculated inFAST versus OrcaFlex, which may leadto different platform displacements.This, in turn, may lead to differingwave forces because the platformwill be positioned differently in

the wave field. Hence, there is noguarantee the time series will beidentical between the two models.In the figure below, the time

series z(t) displacement plots arenearly identical, except for theexponential decay appearing in theFAST/OrcaFlex model. This is likelycaused by tether damping, which isnot present in FASTs default quasi-static model. The frequency responseplot, however, shows exceptionalagreement, and it is concluded that

FASTlink operates reasonably wellunder the prescribed conditions.Appearing to the right of the z-

axis displacement plot is the tethertension time history and itsassociated spectral response. TheOC3-HyWind spar has three mooringlines, and the one being representedin the plot is aligned with the

platform x-axis. Observing themooring line tension is a conciseand effective way to monitor theplatform response, health, andcondition, because a displacement inany degree-of-freedom will influencethe leash tension. As expected, themooring line tension is heavilyinfluenced by the heave naturalfrequency. This is a characteristicshared by both models. In theFAST/OrcaFlex model, several highfrequency vibration components arepresent. This is because of the windturbine and mooring line axialnatural frequency. The mooring linenatural frequency is easilyidentifiable from [13]:

AE

2L

n

fn

u = (3)

where n is the nth vibrations mode.The first three modes, which agreewith the Power Spectral Densityplot below, are fu

n= {1.23 ; 2.46 ;3.70} Hz.

Figure 2: Simulation results for Case I: free decay with z0= 0.2 meters.

CASE II: Regular Waves,H = 5 meters, T= 20 seconds

A test case is performed with theOC3-HyWind platform in regular wavesto evaluate FASTlinks stability toenvironmental loads. In Figure 3,the time series for surge (x), heave(z), and pitch () directions, aswell as the mooring line tension at

the platform, is given. Because thewaves are progressing in a directionaligned with surge, there is nomotion in sway (y) and roll ();hence, they are not shown alongsideFigure 3. The Power Spectral Densityplot for each degree-of-freedomcomplements the time-series plots.Once analyzed in the frequencydomain, the data sets reveal strong


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agreement between FAST and theFAST/OrcaFlex coupling tool.

Although the surge time historieslook uncoordinated between FAST andFAST/OrcaFlex, the surge PowerSpectral Density plot, X(f),illustrates that both data sets are

in agreement. Equally satisfactorydata are reported for the remainingdegrees-of-freedom. In reference tothe tension Power Spectral Densityplot T(f), large differences betweenthe two models occur at frequenciescoinciding with the wave-bandfrequencies; however, it is stressedthat the purpose of this exercise isnot to investigate why differenceoccurs, but rather to show that thecoupling tool reports comparableresults to its FAST counterpart.

Differences between the two modelscan be attributable to:

Longitudinal and transverse cableexcitations;

Ringing or springing; Mathieu instabilities; Resonance matching between the

wind turbine and tethers; Internal cable damping, cross-

flow cable drag and fluid addedmass.

Naturally, differences in mooringline tension are expected becausethe two cable models arefundamentally different. This model-to-model comparison projects theappearance that FASTlink isoperating as designed, the resultsgiven by its solution arereasonable, and the coupling module

has a high degree of usability.

Figure 3: Simulation results for Case II:

Regular Waves H = 5 meters, T= 20 seconds.


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CASE III: Below Rated Wind, 8 m/s

The final case is initiated toensure the aerodynamic calculationsbetween FAST and the FAST/OrcaFlexcoupling modules are consistentunder wind excitation. The platform

is initially at its equilibriumposition with zero translational androtational displacement. A uniformunsheared wind of 8 m/s isprescribed in a direction alignedwith the platform x-axis, with nowaves or currents acting on the

platform. Similar to Case II, modelsymmetry and the wind directionconfine the platform motion toprimarily the surge (x), heave (z),and pitch () directions. Theresults from this simulation aregiven in Figure 4. Once again, the

model-to-model comparison suggestseach program is executing itsassigned calculations correctly andthe coupling process between FASTand OrcaFlex appears to be operatingproperly.

Figure 4: Case III: Uniform, unsheared wind. The turbine is operating at a

below-rated capacity with the wind speed set to 8 m/s.

Other Simulations Considered

Although the authors havepresented a limited number of testcases in this manuscript, otherstarting conditions were examined toensure consistency between modelsand ensure functionality of theFAST/OrcaFlex coupling tool. Similarto Case I, a free-decay numerical

experiment was performed on theremaining degrees-of-freedom. Thistest revealed that the FAST/OrcaFlextool obtains platform naturalfrequencies comparable to thestandalone FAST program. This testwas essential to make certain theplatform and/or wind turbine weightwas not counted twice in thesimulation.


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A test combining the attributes ofCase II and Case III was alsoperformed. In this case, a steadywind of 8 m/s and a regular seadefined by H = 5 meters and T = 20seconds were introduced into themodel concurrently. This simulation

was performed with the seaspropagating in the same direction asthe platform x direction, as well asone with the sea offset by 20relative to the x direction. Threesimulations which integrated windturbine control algorithms were alsoperformed to further verifystability of the coupling module.These simulations introducedirregular sea conditions using theJONSWAP spectrum; the sea wasdefined by a significant wave height

of Hs= 6 meters and an averageperiod of Tavg= 20 seconds. The windconditions were generated using anormal turbulence model, and thethree mean wind speeds consideredwere Um= 10, 12 and 18 m/s. Ananalysis of the simulation resultsupports the same conclusionspresented in this paper.While only results for the OC3-

HyWind spar are presented in thismanuscript, the authors have alsoperformed limited analysis on theMIT/NREL TLP, and our preliminaryfindings reveal the MIT/NREL TLPtends to be less stable compared tothe OC3-HyWind spar. The source ofinstability is likely attributableto the mooring implementation ratherthan the platform design itself.Instability issues are more likelyto occur with taut leg systems (suchas the MIT/NREL TLP) as opposed tosystems using slack mooring lines(such as the OC3-HyWind spar) sincethe cable can alternate betweenslack and taut conditions. This

swing in operation mode canintroduce large jumps in cabletension, even if the platformdisplacement is small. Theseinstability issues are often curedby simply decreasing the time stepresolution. A further investigationis needed to identify failure modes,track the source of instability and

offer remedies to correct thisissue.

Remarks and Conclusions

In this work, the stability,usability, and functionality ofFASTlink are assessed. The resultsreported with the FAST/OrcaFlexcoupling tool are consistent withthose obtained while using FAST withits default quasi-static cablemodel. The FAST/OrcaFlex couplingmodule reports promising results,which warrants a further study toidentify the mechanisms that causethe mooring lines to influence thewind turbine response.

As expected, the simulation run-time is extended when using the

FASTlink coupling module; however,high-fidelity modeling almost alwaysrequires simulation speed to besacrificed. Earlier, it was notedthat that simulation speed can beincreased by simply increasing thesimulation time-step. In this work,the time-step used in FAST equalsthat of OrcaFlexs. However,recalling that FAST calculates thetower and rotor blade aerodynamicsand deflection, the size of FASTstime-step must be balanced to ensurethe tower/blade dynamics resolutionis acceptable between integrationsteps. On the same note, OrcaFlexstime step must be small to ensurethe cable does not go unstable.Increasing the integration time stepmay not be a viable option for somesystems, such as a TLP.

The method used to select time-steps between FAST and OrcaFlex isup to the user, and no clear rulesare stipulated. One area needingfurther investigation involvesdefining a method to select time-

step length. In terms of stability,the time-step has a profound effecton the solution, and in general, asmaller time-step implies greatersimulation stability.

In terms of usability, OrcaFlexsGUI allows easy management ofplatform hydrodynamic properties andcable configuration. Once thesimulation is run with FASTlink, the


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simulation file can be opened inOrcaFlex, allowing the user to gainan appreciation of the wind turbinemotions through three-dimensionalrendering, although the turbinedynamics are not animated inOrcaFlex.

In terms of applicability, theFASTlink tool appears suitable for amultitude of offshore wind turbinedesigns, including semi-submersiblesand TLPs, whether the mooring linesare taut or slack. Our analysis ofFASTlink suggests that the resultsare reasonable, the solution methodis stable, and the overall couplingis meaningful. By combining the bestattributes of FAST and OrcaFlex, ahigh-fidelity offshore wind turbinemodel is garnered.

Acknowledgements

This work was performed at NREL insupport of the U.S. Department ofEnergy Wind and Water Power Program.The authors would like to extendgratitude to Peter Quiggin ofOrcina, Antoine Peiffer of Principle

Power, and Pierre Coulombeau ofIDEOL Offshore for their assistancewith FASTlink.

References

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Computer Software AeroDyn.Windward Engineering LLC., SaltLake City, UT, USA.

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10.Lee, C.H., and Newman, J.N.,2006. WAMIT Users Manual,Version 6.3, 6.3PC, 6.3S, 6.3S-

PC. WAMIT, Inc. Chestnut Hill,MA, USA.

11.Walton, T., and Polacheck, 1960.Calculation of Transient Motionof Submerged Cables.Mathematics of Computation, Vol.14, No. 69.

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