Anthony M. ViselliAdvanced Structures and Composites Center,

University of Maine,

Orono, ME 04469

Andrew J. GoupeeAssistant Professor

Department of Mechanical Engineering,

University of Maine,

Orono, ME 04469

Habib J. DagherProfessor

Advanced Structures and Composites Center,

University of Maine,

Orono, ME 04469

Model Test of a 1:8-ScaleFloating Wind TurbineOffshore in the Gulf of Maine1

A new floating wind turbine platform design called VolturnUS developed by the Univer-sity of Maine uses innovations in materials, construction, and deployment technologiessuch as a concrete semisubmersible hull and a composite tower to reduce the costs of off-shore wind. These novel characteristics require research and development prior tofull-scale construction. This paper presents a unique offshore model testing effort aimedat derisking full-scale commercial projects by providing scaled global motion data,allowing for testing of materials representative of the full-scale system, and demonstrat-ing full-scale construction and deployment methods. A 1:8-scale model of a 6 MW semi-submersible floating wind turbine was deployed offshore Castine, ME, in June 2013. Themodel includes a fully operational commercial 20 kW wind turbine and was the first grid-connected offshore wind turbine in the U.S. The testing effort includes careful selectionof the offshore test site, the commercial wind turbine that produces the correct aerody-namic thrust given the wind conditions at the test site, scaling methods, model design,and construction. A suitable test site was identified that produced scaled design loadcases (DLCs) prescribed by the American Bureau of Shipping (ABS) Guide for Buildingand Classing Floating Offshore Wind Turbines. A turbine with a small rotor diameterwas selected because it produces the correct thrust load given the wind conditions at thetest site. Some representative data from the test are provided in this paper. Model testdata are compared directly to full-scale design predictions made using coupled aeroelastic/hydrodynamic software. Scaled VolturnUS performance data during DLCs show excellentagreement with full-scale predictive models. Model test data are also compared directly with-out scaling against a numerical representation of the 1:8-scale physical model for the pur-poses of numerical code validation. The numerical model results compare favorably withdata collected from the physical model. [DOI: 10.1115/1.4030381]

Introduction

A new floating wind turbine platform design called VolturnUSdeveloped by the University of Maine uses innovations in materi-als, construction, and deployment technologies such as a concretesemisubmersible hull and a composite tower to reduce the costs ofoffshore wind. These novel characteristics require research anddevelopment prior to full-scale construction. An offshore modeltest at an intermediate-scale with a wind turbine smaller than 1MW can derisk the development of a commercial-scale Voltur-nUS system by providing scaled global motion data, allowing fortesting of the same material systems used in the commercial sys-tem, and demonstrating full-scale construction and deploymentmethods. Such a test requires careful selection of the test site,wind turbine, scaling methods, model design, and construction toobtain meaningful data.

Model tests of offshore structures are routinely conducted in theoffshore oil and gas sectors in wave basin facilities [1]. A basinmodel test offers a reduced risk and cost venue to accurately evalu-ate the floating system’s dynamic characteristics. Scale model testsof floating wind turbines in a wave basin have been completed byseveral groups. Past wave basin test campaigns include PrinciplePower, Inc., U.S. [2], Hydro Oil & Energy, Norway [3], WindSea

AS, Norway,2 and the University of Maine [4,5]. These experi-ments aimed at validating dynamic global performance of variousfloating wind turbine platform types. These past efforts provideguidance for conducting scale model tests of floating wind turbinesbut do not consider the same materials, construction methods, ordeployment methods as the full-scale system.

Several intermediate-scale floating wind turbines have beendeployed offshore or are in planning stages throughout the world,although few data have been published [6]. There are currently sixknown intermediate-scale testing efforts of floating wind turbinesby commercial and public entities. BlueH3 deployed a 100 kWtension-leg platform off the Italian coast. Floating Power Plant A/S deployed its Poseidon wind/wave prototype with three 11 kWturbines [7]. Sway conducted a 1:5-scale test of a tension-leg spardesign off the Norwegian Coast.4 Kyoto University deployed a1:2-scale 100 kW spar.5 A 1:4-scale testing program is planned byNORCOWE in Norway,6 and a 1 kW vertical axis turbine sup-ported on top of a spar is planned by the Deepwind Consortium inEurope.7 These programs indicate the interest in conductingintermediate-scale tests prior to commercial-scale projects as a stepin the development process for floating offshore wind technology.

This paper presents a unique offshore Froude-scale test pro-gram for an intermediate-scale offshore wind turbine off the coast

1Paper presented at the 2014 ASME 33rd International Conference on Ocean,Offshore, and Arctic Engineering (OMAE2014), San Francisco, CA, June 8–13,2014, Paper No. OMAE2014-23639.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASMEfor publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING.Manuscript received May 1, 2014; final manuscript received April 9, 2015;published online May 19, 2015. Assoc. Editor: Yin Lu (Julie) Young.

2http://www.windsea.no3www.bluehgroup.com/4http://www.sway.no/5http://www.nawindpower.com/e107_plugins/content/content.php?content.122156http://www.norcowe.no/7http://www.offshorewind.biz/2010/11/11/future-wind-turbines-go-offshore-%E2

%80%93-deep-and-floating-denmark/

Journal of Offshore Mechanics and Arctic Engineering AUGUST 2015, Vol. 137 / 041901-1Copyright VC 2015 by ASME

Downloaded From: http://offshoremechanics.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/journals/jmoeex/933776/ on 05/27/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-use

of Maine, U.S. planned for eighteen months starting in June 2013.This testing program includes the selection of a suitable test siteto apply correctly Froude-scaled wave loads and the selection of acommercial turbine that with the wind speed at the site, applies aFroude-scaled thrust load. The instrumentation plan required tomeasure environmental inputs and model response and the inclu-sion of design and construction methods representative of thoseused for a full-scale system are presented. The usefulness andapplicability of the testing results are then discussed. The testingeffort was developed to meet several objectives:

(1) Validate the VolturnUS design behavior at close to full-scale by conducting a Froude-scale test representative of a6 MW floating turbine deployed far offshore in the Gulf ofMaine.

(2) Design, test, and demonstrate advanced material systems,construction techniques, and deployment methods for theVolturnUS concept.

(3) Collect data for validation of coupled aeroelastic/hydrodynamicnumerical models for floating offshore wind turbines.

(4) Develop deep water offshore wind testing capabilities, pro-cedures, and methods.

The testing was performed near Castine Harbor, ME, as shownin Fig. 1. The model is a 1:8-geometric scale prototype of a 6 MWturbine platform system to be deployed in the Gulf of Maine faroffshore. The turbine has been selected to apply a correctlyFroude-scaled thrust load to the unit. Because the wind speeds aregreater than those required by Froude scaling, a smaller diameterturbine rotor is needed to apply the correct thrust force. The 1:8-model is shown in Fig. 2. The concrete hull and tower weredesigned following the ABS Guide for Building and ClassingFloating Offshore Wind Turbines [8]. The turbine and platformwere constructed onshore and towed to site fully outfitted to dem-onstrate full-scale construction and deployment methods. A com-prehensive instrumentation package monitors key performancecharacteristics of the waves, wind, current, and platform responseto verify the full-scale design behavior and coupled aeroelastic-hydrodynamic modeling software.

Scaling Laws

The model is a 1:8-scale prototype of a 6 MW floating turbinedeployed approximately 23 km offshore Maine. A scale factor, k,of eight was chosen based on the expected wave conditions at the

test site, the selected commercial turbine thrust loading and windspeed at the site, and practical constructability constraints associ-ated with incorporating materials representative of the full-scalesystem. Froude-scale basin model testing techniques commonlyused for offshore structures such as oil and gas and offshore struc-tures have been adapted for this testing program [1]. Particulars ofFroude-scaled testing of floating wind turbines have been incorpo-rated following previous work which highlight the necessity ofproperly scaling the thrust perpendicular to the rotor plane, whichis the dominant wind turbine loading on the floating structure [4].The scaling parameters are shown in Table 1. Unique challengesexist to maintain these parameters in an intermediate-scale off-shore test with competing requirements. For example, the sitewind and wave conditions must occur with the correct magnitude,temporal characteristics, and joint occurrence. Similarly, a scaleturbine is needed to apply the correctly scaled thrust force giventhe wind conditions at the test site. For the hull, the mass anddynamic characteristics must be scaled properly while also beingconstructed of appropriate full-scale construction materials andbeing deployed using representative full-scale deploymentmethods.

Model Description and Deployment

The VolturnUS 1:8-scale model was designed to be con-structed, assembled, and deployed using similar materials andtechniques as the full-scale 6 MW design. The full-scale design ismade of concrete can be fully constructed and assembled dock-side. After assembly, the entire structure is towed at a shallowtransit draft with a single standard tug boat for mooring and elec-trical umbilical hook up at its final installation site.

Table 2 lists the gross properties of the model and the full-scalesystem. A comparison between the ideal target scale factor andthe actual achieved scale factor of the model is presented. Thescale factor compares well with the target scale values for the keyparameters. Some dissimilitude for the turbine exists due to thecompeting goals of the test program. A commercial pitch regu-lated 20 kW turbine modified to produce 12 kW was used for thetest. At this reduced power, the turbine applies the desired peak

Fig. 1 1:8 Castine project test site location. The site produceswith a high probability 1:8-scale wave conditions representativeof the full-scale project site.

Fig. 2 Grid-connected VolturnUS 1:8 scale prototype. Note:The turbine rotor has been reduced to provide correctly scaledthrust loads given the wind conditions at the test site.

041901-2 / Vol. 137, AUGUST 2015 Transactions of the ASME

Downloaded From: http://offshoremechanics.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/journals/jmoeex/933776/ on 05/27/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-use

scaled thrust force at the rated wind speed. The power, turbine Cp,and rotor diameter are not perfectly scaled but were acceptable forthe test given that the thrust load is the primary wind turbine forc-ing on the platform [4,5].

The platform also successfully incorporated the same materialsas used at full-scale. The concrete hull design, composite towerdesign, and construction process replicate the full-scale system.The concrete hull is constructed of 15 concrete members and rep-resentative of the full-scale design and construction process. Theconnections, thicknesses, and reinforcements of the concrete hullare scaled. The only difference from the full-scale system is thatthe 1:8-scale model can be fully dismantled for shipping over theroad on a truck. The 1:8-scale composite tower is constructed ofthe same reinforcements and resins as a full-scale tower. Both thetower and concrete hull underwent structural qualification testingto confirm structural designs prior to fabrication. The assembly ofthe hull, tower, and turbine all took place on land at the CianbroModular Manufacturing Facility in Brewer, ME. The model usesthree catenary chain moorings anchored to the sea bed with dragembedment anchors. The average water depth at the test site ismatches the target scaled depth as well. Figure 3 shows the com-pleted unit.

The deployment of the prototype took place on June 2, 2013.The tow route began in Brewer, ME and proceeded down the Pe-nobscot River to Castine, ME. The route was approximately 50km (28 nm), and took about 11.5 hrs. The entire unit, fullyassembled was towed with a single tug boat and an assistance

vessel in such a fashion as to replicate the tow configurationemployed in the deployment of a full-scale 6 MW VolturnUS. Animage of the VolturnUS being towed is shown in Fig. 4.

The tow-out, which was to occur at approximately 3.7 km perhour, was expected to produce a tow line tension of 32 kN as pre-dicted following guidance per DNV-RP-C205 [9]. During theactual tow-out, the line tension measured with an onboard loadcell was very close to the calculations as shown in Fig. 5. Thisdata confirmed the design tow line load prediction methods usedfor VolturnUS system. The measured tow line mean force is about4 kN higher (10% greater) than the predicted value. This differ-ence could be due to uncertainty in the speed of the vessel as thiswas recorded directly from the boat instrumentation. Another rea-son for the difference could be due to uncertainty in tidal and rivercurrent behavior during the tow-out. These were not directlymeasured. Considering these differences and unknowns, the

Table 1 Scaling factors employed for coupled wind/wavemodel testing of floating wind turbines [4]

ParameterScale factor

symbolicScale factor

1:8

Length (displacements, wave height) k 8Area k2 64Volume k3 512Angle 1 1Density 1 1Mass k3 512Time (wave period) k0.5 2.83Frequency (turbine rpm) k�0.5 0.35Velocity (wind speed, current) k0.5 2.83Acceleration 1 1Angular velocity k�0.5 0.35Angular acceleration k�1 0.125Force (waves, turbine thrust, current drag) k3 512Moment k4 4096Power k3.5 1448Young’s modulus k 8Stress k 8Mass moment of inertia k5 32,768Area moment of inertia k4 4096

Table 2 General properties of the VolturnUS 1:8-scale and full-scale systems

Model

Parameter 1:8 Scale as-built Full-scale as-built Full-scale (ideal) k ideal k actual k% Difference

Hull draft 2.9 m 23.2 m 23.2 m 8 8 0%Hub height 12.2 m 97.6 97.6 m 8 8 0%Average water depth 21 m 168 m 168 m 8 8 0%Peak thrust load 1.37 kN 701 kN 700 kN 8 8 0%Turbine Cp at rated wind speed 0.37 0.31 1 1.2 þ20%Rotor diameter 9.6 m 76.8 152 m 8 16 þ100%Rated power 12 kW 17.4 MW 6 MW 8 5.9 þ35%Hull material Concrete Concrete sameTower material Composite Composite sameNumber of mooring lines 3� catenary chain 3� catenary chain sameAnchors Drag anchor Drag anchor same

Fig. 3 1:8-scale model outfitted and ready for deployment.Note: The turbine rotor diameter is smaller than the ideal scalein order to apply the correct Froude-scaled thrust force on thehull given the wind speed at the test site.

Journal of Offshore Mechanics and Arctic Engineering AUGUST 2015, Vol. 137 / 041901-3

Downloaded From: http://offshoremechanics.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/journals/jmoeex/933776/ on 05/27/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-use

predicted versus measured tow-line force compares well and helpsto confirm the design tow-line load prediction methods used forVolturnUS.

Toward the end of the tow, the model encountered wavesapproximately 1.5 m in height (equivalent to 12 m full-scale). De-spite this, the tow-out occurred without incident. Although not adesirable tow-out condition at full-scale, the exercise providesevidence that the stability and towing configuration of VolturnUSduring tow-out is sufficiently robust. Upon arrival in Castine, theunit was hooked to preinstalled chain moorings connected topreset drag anchors.

Test Site Selection

Selection of a suitable test site for the offshore 1:8 Froude-scalemodel test requires careful treatment. The test site must generatethe correct combinations of wind/turbine thrust and scaled wavesin order to simulate the behavior of the full-scale 6 MW Voltur-nUS farther offshore. Full-scale design conditions for floating off-shore wind turbines specified by the ABS Guide for Building andClassing Floating Offshore Wind Turbine Installations were esti-mated from metocean data obtained far offshore Maine and repre-sent the desired full-scale design environments [10,11]. With thedesired environmental conditions in hand, the Castine site wasthen confirmed to produce the desired environments by deployinga buoy at the Castine site.

There exists over 12 years of wind and wave buoy data at theproposed 6 MW full-scale design deployment site off MonheganIsland, Maine the location of the full-scale system [12]. Buoy datacollected includes significant wave height and peak period, waveenergy spectral parameters and 10 mins average wind speeds at 4m above sea level. Using this data, full-scale DLCs were devel-oped for the Monhegan site for a 6 MW floating wind turbine.Several critical ABS DLCs are shown along with the desired 1:8-scale values in Table 3. DLC 1.2 is an operational type loadingcondition used for fatigue analysis. DLC 1.6 is an extreme turbineoperating condition with an associated 50-yr return period signifi-cant wave height concurrent with the operating wind speed. Thetable shows a critical subset of the DLC 1.6 case, where the tur-bine is operating at peak thrust with a 50-yr return period signifi-cant wave height and wave period associated with the rated windspeed. DLC 6.1 is another extreme case, where the turbine is shutdown and the unit is subjected to a larger 50-yr significant waveheight independent of wind speed. The last case shown is anexample station-keeping survival load case (SLC), where the tur-bine is subjected to a 500-yr return period wave event with the tur-bine parked. These four load cases have been found to control thedesign of the full-scale VolturnUS platform and were used toidentify the proper test site.

Numerous test sites capable of reproducing these load cases inthe Gulf of Maine were considered for this 1:8-scale test. Castine,ME was selected after an analysis of data obtained from an instru-mented metocean buoy deployed in the winter of 2012. The buoydata, including wind, wave, and current measurements confirmedthat the site could readily produce the desired ABS scaled DLCswith a high probability. Figure 6 shows a scatter plot of the meas-ured peak period and significant wave height at Castine. The scat-ter data represents the measured buoy data from the model testsite in Castine. The red trend line is a power law fit of the full-scale peak period v. significant wave height relationship at thefull-scale 6 MW test site scaled by the scale factors in Table 1which represents the mean operational wave conditions requiredfor DLC 1.2. 95% confidence intervals are also included for thetrend lines. Also included on this figure are the peak periods andsignificant wave heights of DLC 1.6, 6.1, and the SLC. Noting theoverlap of the trend line with the measured data at Castine, thereis a high probability of experiencing the desired wave conditionsduring the deployment. The larger scaled extreme wave condi-tions, DLC 6.1 and SLC, have a lower probability of occurrenceas evidenced by the limited data for larger waves collected duringthis short buoy deployment. The peak period as a function of thesignificant wave height, Hs, is also provided. This relationshipwas developed from the buoy record at the full-scale project site.

In addition to maintaining the correct wave peak period and sig-nificant wave height joint occurrence, the correct wave spectrumshape is also highly desirable. Figure 7 shows two images as anexample of the wave spectral characteristics measured during theVolturnUS deployment by a wave staff mounted on the unit cor-rected for the motions of the hull using accelerometer data. Thetop image is a time record of wave elevation over a 1 hr periodwith a significant wave height of 1.6 m and a peak period of 5.2 s.This condition is representative of the SLC conditions. Further,

Fig. 4 (Top) Towing of VolturnUS in the Penobscot River and(Bottom) towing of VolturnUS through 1.5 m waves in Penob-scot Bay

Fig. 5 Measured and predicted tow-line tension at 3.7 km/h instill water were shown to be within about 10%

041901-4 / Vol. 137, AUGUST 2015 Transactions of the ASME

Downloaded From: http://offshoremechanics.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/journals/jmoeex/933776/ on 05/27/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-use

the computed spectrum is very close to the desired scaled 500-yrsurvival case. The bottom image shows the measured wavespectrum along with a superimposed scaled 500-yr JONSWAPspectrum showing good agreement.

With the wave characteristics at the site found to be acceptable,the test site’s ability to produce scaled turbine thrust loads in

conjunction with the associated significant wave height is nowpresented. For the scale test to be meaningful, the wind turbinethrust not only needs to be scaled properly, but also must occurwith an appropriate sea-state (significant wave height and peak pe-riod) representative of a full-scale design condition. To confirmthat the site can produce turbine thrust and sea-states in the correctproportions with a high probability, the scatter plot, Fig. 8, wascreated. This scatter plot shows two sets of data. The desiredscaled conditions are shown as small gray dots and the expectedprototype conditions are shows as larger blue dots. The desiredscaled conditions are representative of various DLC 1.2 type con-ditions and were obtained using 10 mins average wind speed andsignificant wave height data obtained from the buoy deployed atthe full-scale site farther offshore. The wind speed data were ex-trapolated to hub height using a power law wind shear model withan exponent of 0.14 [13] and then used to estimate the 10 minsaverage 6 MW full-scale turbine thrust load thereby generating aturbine thrust v. significant wave height scatter diagram for thefull-scale system. The 10 mins thrust was estimated using FAST

software for a representative 6 MW turbine for steady winds [14].The thrust and significant wave height data were then scaled downusing Froude scaling laws to generate the desired scaled condi-tions. The expected 1:8-scale prototype thrust and significantwave height scatter data were obtained using the same methodexcept that the buoy data gathered at Castine during the winter of2012 and 12 kW turbine thrust v. wind speed relationship wasused. Also shown as a red diamond is the ABS DLC 1.6 case,which is an extreme load case. The maximum turbine thrust forboth desired and expected is about 1.37 kN based on the operatingcharacteristics of the turbine.

Fig. 6 Measured and desired scaled wave peak period v. sig-nificant wave height

Fig. 7 Wave elevation time series (Top); measured and idealscaled 500-yr JONSWAP wave spectrum (Bottom)

Fig. 8 Measured and desired scaled 10 mins average turbinethrust v. significant wave height

Table 3 Full-scale (far offshore in the Gulf of Maine) and model scale metocean parameters for key ABS DLCs

ABS DLCs Required metocean design parameter Full scale 1:8 Scale

DLC 1.2 Operational load case significant wave (m) 0–6.0 0–0.75Associated wave period (s) 6.1–11.5 2.2–4.1

DLC 1.6 50-yr significant wave height associated with turbine peak thrust (m) 8.0 1.0Associated wave period (s) 12.7 4.5

DLC 6.1 50-yr significant wave height (m) 9.8 1.3Associated wave period (s) 14.2 5.050-yr 10 mins wind speed at hub height (m/s) 40 14.1

SLCs 500-yr significant wave height (m) 12.0 1.5Associated wave period (s) 15.3 5.4500-yr 10 mins wind speed at hub height (m/s) 45 15.9

Journal of Offshore Mechanics and Arctic Engineering AUGUST 2015, Vol. 137 / 041901-5

Downloaded From: http://offshoremechanics.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/journals/jmoeex/933776/ on 05/27/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-use

The overlap of the two sets of data shows that the site fre-quently produces turbine thrusts with the correctly matched signif-icant waves. Further, extreme cases like DLC 1.6 are also likely tohappen. This is an advantage of an intermediate-scale approach asmany extreme return period events can be witnessed in shortamount of time as compared to a full-scale deployment.

Sample Test Results: Dockside Hydrostatic Stiffness

and Free Decay Testing

Several tests were completed to confirm the model’s hydrostaticand hydrodynamic characteristics dockside prior to deployment.As a confirmation of the static stability characteristics of the 1:8-scale model, a fully assembled unmoored heave and pitch hydro-static static stiffness test was completed by adding known weightson the structure and measuring the displacements. The results areshown in Fig. 9 and are compared to the estimated response. Thetesting confirmed the expected properties of the model. The heavemeasured has some variability due to local waves created by windand small craft nearby during the test. This issue was not experi-enced in the pitch stiffness test.

As a confirmation of the dynamic characteristics of the model,a combined heave/pitch decay test was completed using a cranepulling and then releasing from one of the outside columns.Onboard instrumentation measured pitch and heave motions andcorrected for angular displacement. Figure 10 shows a powerspectral density plot created using the free decay data. The pitchand heave natural frequencies matched closely the estimatedscaled values of 0.16 Hz and 0.12 Hz, respectively, as predicted

using FAST [14]. Hydrodynamic damping of the system was deter-mined based on this data for use in a quadratic damping modeldiscussed later in this paper.

Sample Test Results: Operational and Extreme Seas

Preliminary installed testing results are presented in this sectionas well as an overview of the onboard instrumentation systems.Figure 11 outlines the types and locations of sensors onboard themodel, as well as the remote sensors. Data collected include windspeed and direction, wave height, wind turbine power and rotorspeed, platform angular position, platform translational and rota-tional accelerations, tower loads, platform loads and mooring lineloads. Sample rates vary for the instruments and range from 10 toover 60 Hz depending on purpose. For example, the inertial mea-surement unit used for measuring the motions of the platform aresampled at 60 Hz while turbine power output is 10 Hz. In total,over 60 channels of data are being collected from the floatingwind turbine. A Lord microstrain 3DM-GX3-45 inertial measure-ment unit was used to measure the motions of the platform. Thedevice follows a right-handed coordinate system with positivesurge defined as the centroid of the turbine tower to Leg C andheave along the central axis of the turbine tower, positive upward.Also shown are the labels for the three legs of the platform as wellas the coordinate system utilized for expressing the accelerationresults. Further, an instrumented buoy provides a separate waveheight measurement, wind speed, atmospheric, and current pro-files from the surface to the seafloor. The buoy was used as ameans of quality checking the data measured directly from theVolturnUS floating platform. This comprehensive set of data per-mits careful comparisons between predicted performance andmeasured performance, and as noted earlier, can also be used toaccurately predict performance for a full-scale 6 MW VolturnUSfloating wind turbine located farther offshore.

A Fourier analysis of select data from the tower bottom inertialmeasurement unit is now presented for an operational type event(i.e., DLC 1.2), where the turbine was producing power in a seastate of 0.52 m which is equivalent to a 4.2 m full-scale significantwave height. Data considered herein include angular rotations forwhich a Fourier analysis is performed to demonstrate that the datasets are capturing the expected motion behavior of the VolturnUSfloating wind turbine system. The results are shown in Fig. 12. Asdepicted in the figures, the expected behavior observed includesslowly varying motions associated with second-order difference-frequency wave diffraction forces, motion in the wave energy

Fig. 9 Hydrostatic heave (Top) and pitch incline (Bottom)testing

Fig. 10 Power spectral density plots heave (Top) and pitchangle (Bottom)

041901-6 / Vol. 137, AUGUST 2015 Transactions of the ASME

Downloaded From: http://offshoremechanics.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/journals/jmoeex/933776/ on 05/27/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-use

frequency range and wind turbine excitation. The pitch motion ishigher than the roll because the primary loading is in the swaydirection. As the wave direction is not aligned perfectly with thesway direction, the asymmetry in the wave loading causes some

appreciable yaw motion. With regard to the wind turbine excita-tions observed, specifically the once-per-revolution (1P) turbine ex-citation is present.

During this testing campaign several storm events occurred thatproduced the desired extreme scaled conditions required by theABS guide. This intermediate-scale test offers a unique opportu-nity to study floating turbine behavior when exercised to designlimits at near full-scale. In this section, one such event, occurringon November 1, 2013 is discussed. On this day the model experi-enced a scaled DLC 1.6 which considers the turbine operating atrated wind speed, applying the peak overturning moment to theplatform while experiencing 50-yr waves. Key maximum recordedmeasurements collected during the event are shown in Table 4. Themaximum recorded wave height and peak thrust of the turbine werewithin 3% of desired DLC 1.6 scaled conditions. Also shown is thepredicted behavior from a coupled aeroelastic/hydrodynamic simu-lation of the full-scale system subjected to ABS DLC 1.6. The float-ing platform performed markedly well and exhibited accelerationsvery close to the predicted results used for design of the full-scalesystem. Direct comparisons of key parameters of the scale modeltest data to the numerical models of the full-scale system show veryfavorable results and give confidence for full-scale implementation.These favorable direct comparisons of the model to full-scale pre-dictions also indicate that the model test program is performing asintended and produced model basin quality test data. Figure 13shows the prototype in this extreme environment.

Fig. 11 Instrumentation systems

Fig. 12 Power spectral density plots of bottom tower angularmotions

Table 4 Comparison of measured and predicted key loads and motions for scaled and full-scale ABS DLC 1.6 50-yr extremeconditions

1:8 scale (full-scale)

Parameter VolturnUS data FAST simulation % Difference

Maximum wave height (m) 2.69 (21.5) 2.64 (21.1) 1.9%Maximum turbine thrust (kN) 1.4 (717) 1.37 (700) 2.2%Maximum accelerations at the tower base (g) 0.165 (0.165) 0.177 (0.177) �6.8%Maximum platform pitch angle (deg) 5.91 (5.91) 5.81 (5.81) 1.7%

Journal of Offshore Mechanics and Arctic Engineering AUGUST 2015, Vol. 137 / 041901-7

Downloaded From: http://offshoremechanics.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/journals/jmoeex/933776/ on 05/27/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-use

Sample Verification of Numerical Model

Validation of the results of coupled aeroelastic-hydrodynamicnumerical modeling software FAST [14] against experimental per-formance data collected is now presented. Detailed validationstudies are presented by Allen et al. [15]. FAST is a coupled aeroe-lastic and hydrodynamic model for simulation of floating windturbine global performance and has been validated through com-parisons with tank testing data [16–19]. The date of the event usedfor this model correlation was November 27, 2013, from 12:51:54pm to 1:51:58 pm. The mean and maximum wind speeds were15.4 m/s and 23.6 m/s, respectively. The significant wave heightwas 1/6. The peak period was 5.2 s and the maximum wave heightwas 2.6 m which is equivalent to a scaled 500-yr event for the 6MW VolturnUS. The simulated time was 1 hr. To facilitate com-parison between the data and model, a custom version of FAST wasutilized that computes wave diffraction forces from the measuredwave elevation time series record collected during this event. Thestandard version of FAST can only simulate regular waves or ran-dom waves based on a user defined spectra. Wind speed and direc-tion were also input directly. This approach allowed for areasonable modeling approximation of the measured environmentand followed the work of Coulling et al. [16]. Figure 14 showscomparison between the data and FAST for the tower base swayacceleration. Here, the sway acceleration and pitch angular accel-eration are compared because the waves and wind came primarily

from this direction. The numerical model results compare wellwith the measured performance capturing the maximum ampli-tude and frequency of the acceleration. Similarly, tower base rollacceleration is shown in Fig. 15 again confirming the ability of thenumerical code to accurately predict the peak performance of theVolturnUS system. Table 5 lists maximum recorded values forselect variables. The other directions not reported were very smalland are omitted from the comparisons. The FAST model comparesvery well with measured performance for these directions and fur-ther improvements are expected as a more refined study isunderway.

Conclusions

A unique offshore intermediate-scale model testing approachhas been developed and successfully produced scale global per-formance data as well as demonstrated the full-scale design, mate-rials, construction techniques, and deployment methods. Scalemodel test techniques developed for floating wind turbine testingin wave basins were adapted for this offshore test. The Castinetest site was selected to readily produce scale ABS DLCs repre-sentative of the full-scale project site far offshore in the Gulf ofMaine. The ability of the test site to produce these conditions wasconfirmed through a comparison of measured metocean data col-lected at the model test site with far offshore data scaled by theappropriate factors. At 1:8-scale, this test site reliably producesscale extreme wind/ wave events (e.g., 50-yr return period) in ashort period of time offering a unique venue for understandingfloating turbine behavior in extreme events at near full-scale. Themodel was designed and constructed using full-scale structuralmaterials, arrangements, and methods while maintaining desiredhydrostatic and dynamic characteristics confirmed through dock-side testing. A commercial grid-connected turbine was used andmodified to apply the desired scaled dominant wind forcing,thrust. The wind turbine was the first grid-connected offshore

Fig. 13 VolturnUS 1:8 subjected to scaled extreme 50-yrdesign conditions of ABS DLC 1.6 on November 1, 2013

Fig. 14 Comparison of tower base sway acceleration predictedby numerical modeling in FAST and model test data collected atCastine on November 27, 2013

Fig. 15 Comparison of tower base roll acceleration predictedby numerical modeling in FAST and model test data collected atCastine on November 27, 2013

Table 5 Comparison of peak values calculated using FAST andthose measured on November 27, 2013

Variable Units Data FAST % Difference

Blade pitch angle deg 31.7 29.7 6%Tower base sway acceleration g 0.121 0.116 4%Tower base heave acceleration g 0.102 0.091 10%Platform roll acceleration rad/s2 0.119 0.108 9%

041901-8 / Vol. 137, AUGUST 2015 Transactions of the ASME

Downloaded From: http://offshoremechanics.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/journals/jmoeex/933776/ on 05/27/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-use

wind turbine in the U.S. Over 60 channels of instrumentationwere onboard the model. A scale mooring system consisting ofthree chain catenaries and drag anchors was implemented.

Since the experiment has been carefully crafted to match thescale system properties and environmental loads, the generateddata can be used to predict the behavior of a full-scale 6 MW Vol-turnUS. This extensive data set has confirmed design calculationsestablishing confidence in the principles of the VolturnUS designmethodology as well as the concrete hull and composite towerstructural designs. The VolturnUS 1:8-scale floating turbine hasbeen successfully subjected in service to scaled 50-yr extremeABS design conditions. This experiment represents a major mile-stone in the design of the VolturnUS technology and floating tur-bines in general as there is little data for floating wind turbinessubjected to their extreme design conditions in a real ocean envi-ronment. Based on current observations, the 1:8-scale VolturnUSexhibits responses in line with coupled aeroelastic-hydrodynamicmodel design predictions and helps to derisk the design and con-struction of a full-scale floating offshore wind turbines utilizingthe VolturnUS platform technology. Numerical codes for offshorefloating wind turbines have also been partially validated as part ofthis work providing, another benchmark for development andunderstanding of floating wind turbines. Numerical models of thephysical model show good agreement.

In summary, the major lessons learned from this experimentinclude the following: (1) the test plan and procedures includingsite selection, prototype design, and instrumentation designallowed for a successful Froude-scale model test in a real offshoreenvironment, (2) numerical design codes for floating offshorewind turbines are suitable for modeling a near full-scale floatingturbine subjected to extreme design environments in a really off-shore environment, and (3) an intermediate Froude-scale test cannot only produce scaled motion data useful for predicting full-scale response but also serve as a proving ground for new materi-als, construction and deployment methods. This testing approachsignificantly derisks new technology and could be directly appli-cable to other new floating structure development efforts. Addi-tional data from this test will be released through publishing infuture peer reviewed journals and conference proceedings.

Acknowledgment

The authors would like to acknowledge the financial support ofthe U.S. Department of Energy EERE Grant No. DE-EE0003278.001, the National Science Foundation Partnership forInnovation Grant No. IIP-0917974, the State of Maine, theUniversity of Maine, and the support of the members of theDeepCwind Consortium including Cianbro, Ershigs, and VryhofAnchors.

Nomenclature and Abbreviations

ABS ¼ American Bureau of ShippingCp ¼ turbine power coefficient

DLC ¼ design load caseHs ¼ significant wave height

IMU ¼ inertial measurement unitSLC ¼ survival load case

k ¼ scale factor

References[1] Chakrabarti, S. K., 1994, Offshore Structure Modeling, World Scientific

Publishing, Singapore, Chap. 2.[2] Roddier, D., Carmelli, C., and Weinstin, A., 2009, “A Floating Foundation for

Offshore Wind Turbines, Part I: Design Basis and Qualification Process,”ASME Paper No. OMAE2009-79229.

[3] Skaare, B., Hanson, T. D., Nielsen, F. G., Yttervik, R., Hansen, A. M., Tho-mesn, K., and Larsen, T. J., 2007, “Integrated Dynamic Analysis of FloatingOffshore Wind Turbines,” European Wind Energy Conference and Exhibition,Milan, Italy, pp. 1–3.

[4] Goupee, A. J., Koo, B. J., Kimball, R. W., and Lambrakos, K. F., 2014,“Experimental Comparison of Three Floating Wind Turbine Concepts,” ASMEJ. Offshore Mech. Artic Eng., 136(2), p. 020906.

[5] Koo, B. J., Goupee, A. J., Kimball, R. W., and Lambrakos, K. F., 2014, “ModelTests for a Floating Wind Turbine on Three Different Floaters,” ASME J. Off-shore Mech. Artic Eng., 136(2), p. 020907.

[6] Thiagarajan, K. P., and Dagher, H. J., 2014, “A Review of Floating PlatformConcepts for Offshore Wind Energy Generation,” ASME J. Offshore Mech.Artic Eng., 136(2), p. 020903.

[7] European Wind Energy Association, 2010, The European Offshore Wind Indus-try: Key Trends and Statistics for the 1st Half of 2010, Brussels, Belgium.

[8] American Bureau of Shipping, 2013, Guide for Building and Classing FloatingOffshore Wind Turbine Installations, Houston, TX.

[9] Det Norske Veritas AS, 2013, Design of Floating Wind Turbine Structures,DNV-OS-J103, Hovik, Norway.

[10] Pettigrew, N. R., Xue, H., Irish, J. D., Perrie, W., Roesler, C. S., Thomas, A. C.,and Townsend, D. W., 2008, “The Gulf of Maine Ocean Observing System:Generic Lessons Learned in the First Seven Years of Operation (2001–2008),”Mar. Technol. Soc. J., 42(3), pp. 91–102.

[11] Viselli, A. M., Forristall, G. Z., Pearce, B., and Dagher, H. J., “Gulf of MaineExtreme Wave and Wind Design Parameters for Offshore Wind Turbines,” J.Ocean Eng. (in press).

[12] University of Maine, 2013, “Mooring E0130: Central Maine Shelf, Gulf ofMaine Moored Buoy Program,” http://gyre.umeoce.maine.edu/buoyhome.php

[13] International Eletrotechnical Commission, 2009, IEC 61400-3:2009 Wind Tur-bines – Part 3: Design Requirements for Offshore Wind Turbines, Geneva,Switzerland.

[14] Jonkman, J. M., and Buhl, M. L., Jr., 2005, “FAST User’s Guide,” NationalRenewable Energy Laboratory Wind Technology Center, Golden, CO, Techni-cal Report No. NREL/EL-500-38230.

[15] Allen, C. K., Goupee, A. J., Viselli, A. M., and Dagher, H. J., 2015, “Validationof Global Performance Numerical Design Tools Used for Design of FloatingOffshore Wind Turbines,” ASME Paper No. OMAE2015-41437.

[16] Coulling, A. J., Goupee, A. J., Robertson, A. N., Jonkman, J. M., and Dagher,H. J., 2013, “Validation of a FAST Semi-Submersible Floating Wind TurbineModel With DeepCwind Test Data,” J. Renewable Sustainable Energy, 5(2),p. 023116.

[17] Prowell, I., Robertson, A., Jonkman, J., Stewart, G. M., and Goupee, A. J.,2013, “Numerical Prediction of Experimentally Observed Behavior of a ScaleModel of an Offshore Wind Turbine Supported by a Tension-Leg Platform,”Offshore Technology Conference, Houston, TX, Paper No. OTC 24233.

[18] Browning, J. R., Jonkman, J., Robertson, A., and Goupee, A. J., 2012,“Calibration and Validation of a Spar-Type Floating Offshore Wind TurbineModel Using the FAST Dynamic Simulation Tool,” Science of Making TorqueFrom Wind Conference, Oldenburg, Germany.

[19] Roald, L., Jonkman, J., Robertson, A., and Chokani, N., 2013, “The Effect ofSecond-Order Hydrodynamics on Floating Offshore Wind Turbines,” J. EnergyProcedia, 35, pp. 253–264.

Journal of Offshore Mechanics and Arctic Engineering AUGUST 2015, Vol. 137 / 041901-9

Downloaded From: http://offshoremechanics.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/journals/jmoeex/933776/ on 05/27/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-use