1-S1548 FINAL

Wind Tunnel Tests on Floating Offshore Wind Turbines:A Proposal for Hardware-in-the-Loop Approach to

Validate Numerical Codes

by

I. Bayati, M. Belloli, A. Facchinetti and S. Giappino

REPRINTED FROM

WIND ENGINEERINGVOLUME 37, NO. 6, 2013

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WIND ENGINEERING VOLUME 37, NO. 6, 2013 PP 557-568 557

Wind Tunnel Tests on Floating Offshore WindTurbines: A Proposal for Hardware-in-the-LoopApproach to Validate Numerical Codes

I. Bayati, M. Belloli, A. Facchinetti and S. [email protected], [email protected], [email protected] [email protected] di Milano, Dipartimento di Meccanica, via la masa 1, 20156, Milano, Italy

Received:August 01, 2013; Revised:August 31, 2013;Accepted: September 01, 2013

ABSTRACTThis paper describes the realization of a 2-DoF experimental rig, performed at Politecnico diMilano Wind Tunnel, aimed at simulating the motion of a floating wind turbine scale model,due to combined effect of hydrodynamic and aerodynamic loads. As floating offshore windfarms are becoming objects of interest among the international scientific community,experimental tests have been increasingly demanded to support dynamic and structuralnumerical simulations. Furthermore measurements on real prototypes are very complexand expensive and it is difficult to use these data to validate prediction models because of theuncertainties in the definition of the environmental data, such as sea state and incomingwind. The motivation of this project is to provide an experimental set-up working in a fullycontrolled test environment (with measurable input/output) in order to validate numericalcodes. The paper describes the features of the experimental rig, the results coming from thevalidation tests on the rig, through a dedicated off-line experimental campaign and,conclusively, a proposal for a Hardware-In-The-Loop real time functioning mode.

KKeeyywwoorrddss:: Wind Tunnel, Wind Turbines, Hardware-In-The-Loop, Imposed Motion..

1. INTRODUCTIONOffshore wind energy is increasingly focusing on deep-water offshore wind turbines andparticularly on floating platforms [1]. In the recent years different floating wind turbineplatforms have been designed in parallel with the study of their influence on the dynamics ofMegawatt Wind Turbines [2], due to the combined effects of wave and wind loads on thestructures. The development of highly complex simulation tools as multibody aero-servo-elastic codes allows to predict loads and natural frequencies of such structures, being asuitable guide for designers of megawatt floating wind turbines [3, 4, 5]. Nevertheless the needof a reliable validation of the above mentioned simulation codes requires the development ofexperimental campaigns in fully controlled test environment, in order to gather consistentinformation on the physics of such complex systems. Therefore scale models of floating windturbines can be studied in facilities such as water tanks [6, 7] or wind tunnels [8, 9]. The latterexperimental approach was chosen in this work (in the 13.84 3.84 m Politecnico di Milano

wind tunnel boundary layer test section) for its advantages in generating high quality andprecisely measurable wind flow; the simultaneous platform displacements, due tohydrodynamic interaction forces, are mechanically generated on the scale model by meansof a dedicated experimental rig.

2. EXPERIMENTAL TEST RIGEven though the actual dynamics of the wave-structure interaction is indeed more complexthan the one that can be replicated by means of a mechanical device, without testing themodel in a water tank [6, 7], in this work the design of a 2-DoF mechanism was performed withthe goal of creating a simple test rig, accounting both for aero and hydro dynamics of the scalemodel. The added value of performing such tests is, in the authors opinion, the ability tocorrelate, in a more precise manner than in other test facilities, the controlled inputs (windspeed, turbulence intensity, shear gradient, displacement and rotation of the platform) to themeasured outputs (aerodynamic forces, generated power). The test rig (Fig. 1) is hence ableto reproduce pitch/roll rotations and surge/sway displacements of a floating wind turbinescale model. The device is mounted on the turntable of the wind tunnel facility that providesthe possibility of rotating the wind turbine model with respect to the fixed wind direction;therefore different yaw angles can be investigated. More precisely yaw angles can be variedstatically during the experimental campaign. In Fig. 2 a sketch of the turntable is reported.Moreover pitch/surge and roll/sway coupled motions can be reproduced simply changingthe configuration of the model with respect to the mechanism (Fig. 3).

558 WIND TUNNEL TESTS ON FLOATING OFFSHORE WIND TURBINES: A PROPOSAL FORHARDWARE-IN-THE-LOOP APPROACH TO VALIDATE NUMERICAL CODES

Figure 1: Wind turbine 1/25 scale model on the test rig in the wind tunnel BL test section.

2.1. FeaturesThe design choice of reproducing certain rotations and displacements instead of others is toplace emphasis on the most influential movements of a floating wind turbine in terms ofstructural, dynamic and power aspects [10]. The physics of wind-induced marine wavessuggests that when wind appears over the free surface, water waves and turbulence aregenerated by shear stresses. More precisely turbulent diffusion promotes significantly massand momentum transport beneath the interface between the water and air, significantlycontributing on the generation of the wave itself. Consequently, the generated waves aretypically co-aligned with the prevailing wind direction [11]. This physical explanation can be


yaw

Yaw

Surg

e

Sway

Heav

e

Roll

Pitch

Win

d

Figure 2: Top view sketch of wind tunnel turntable.

Distance laser meter 1

Distance laser meter 2

Hydraulic actuator 2Hydraulic actuator 1

Slider

Slider-crank mechanismSliding pair

Surge/sway

Pitch/roll

Figure 3: 2-DOF mechanism.

consistently transposed to offshore environments where the floating wind turbines operate.These aspects have greatly influenced the design of the 2-DoF mechanism, so that by settingthe wind turbine model with the rotor shaft along the sliders lengthwise direction, as shown inFig. 1, the pitch and surge motions can be reproduced.

As previously said also different yaw angles can be investigated by rotating wind tunnelplatform with respect to wind direction. Analyzing different yaw angles means consideringparticular environmental conditions where wave and wind directions are not perfectlyaligned with respect to each other.

The experimental rig also allows to simulate the hydrodynamic forcing responsible forsway and roll coupled motions, simply setting the wind turbine model perpendicularly withrespect to the above mentioned configuration, see Fig. 3.

In Fig. 3 an overview of the main components of the mechanism is shown. The platformmainly consists of a long slider able to move on a rail fixed on the ground. The surge/swaymotion is provided by a hydraulic actuator (denoted as hydraulic actuator no. 2, Figs. 3 and 4)connected to the slider by a coupling bar fixed on the slider plate, as shown in Fig. 4.

Another hydraulic actuator (no. 1, Figs. 3 and 5) is mounted over the slider itself andconnected to it by a fastening plate, suitable for preventing the relative motions due toinertial forces. The function of the latter actuator is to provide the rotation (pitch or roll) tothe wind turbine model. In order to convert the linear motion of the actuator into rotation, aslider-crank mechanism connected to the actuator and mounted over the slider wasdesigned (Figs. 5 and 6).

The motion of this mechanism is given by the actuators stem, linked to the crank by ball-socket joints and a connecting rod (Fig. 6), to overcome the unavoidable misalignmentbetween the slider and the crank. This obviously involves non-linearities in the kinematicchain of transmission that can be however considered negligible in comparison with thedisplacements of the wind turbine nacelle. Moreover, as shown in Fig.6, the excursion of thecrank is limited by an end stop bar, in order to prevent extreme rotations of the wind turbinemodel in case of an accidental wrong actuator maneuver; a similar safety solutions isimplemented for the linear stroke of actuator no. 2.


Laser distance meter 2

Hydraulic actuator 2

Hydraulic actuator 1

Figure 4: Details of the test rig: surge/sway actuation.

2.2. Measuring systemThe wind turbine model is fixed to the mechanism through a 6 components load balance thatis connected to the mechanism itself by an appropriate support, as shown in Figs. 3 and 6. Thecoupling between the wind turbine tower and the balance is allowed by a holed steel plate,ensuring the possibility of mounting the model in different configurations to providesurge/sway and pitch/roll motions, as previously mentioned. Another 6-component balanceis installed in the nacelle (Fig. 7) in order to gather the aerodynamic forces acting on the rotor.Moreover the test rig is equipped with two laser distance meters, as can be seen in Fig. 3 and


Ball-socket joint

Crank

Laser distance meter 1Connecting rod

Hydraulic actuator

Figure 5. Details of the test rig: roll/pitch actuation.

Balance support

Crank

Connecting rod

Actuator stem

Ball-socket joints

Crank end stop

Figure 6: Details of the test rig: slider-crank and balance support.

Fig. 4, and two capacitive accelerometers mounted on the wind turbine nacelle (one set alongthe shaft lengthwise direction, the other one perpendicularly). The laser distance metersconsist in laser triangulation sensors to measure respectively the rotational and lineardisplacement. The signals coming from the laser acquisitions are fundamental for a correctphasing between the forces measured with balances and the displacements themselves, in thetime domain, in order to avoid errors in the post-processing.

3. WIND TUNNEL TESTS FOR THE VALIDATION OF THE EXPERIMENTAL RIGIn this section some preliminary results for the validation of the experimental setup arereported, considering pure-surge sinusoidal motion of the wind-turbine towers base. Asketch regarding the configuration for pure-surge motion tests is shown in Fig. 7.

The experimental session was carried out considering zero yaw angles between the windtunnel platform and wind direction (see Fig. 2), so that wind direction was parallel withrespect to the direction of the translational motion (surge). The rotor angular velocity was setconstant to 16 rad/s. Tab. 1 summarizes the performed tests, in terms of considered frequenciesf and amplitudes A for the imposed surge motion, and of considered wind speed W. For thesake of completeness, the wind turbine model has a 1/25 scale factor, corresponding to rotorblade length of 0. 93 m and nacelle height of 1. 75 m.


Table1: Test parameters of the pure-surge experimentalvalidation campaign

Frequency (f) [Hz] 0.2 0.4 0.6 1Amplitude (A) [mm] 10 20 40 80Wind Speed (W) [m/s] 4.7 5 5.5 6

Balance

Balance

Wind

Flat ground

Actuator

Actuator x, x. , x

..

, ,...

Figure 7: 2D-sketch of wind tunnel tests.


1

0.8

0.6

0.4

0.2

0

1.2

4.4 4.6 4.8 5 5.2 5.4 5.6 6.2

Wind speed (m/s)

(Nm

)

TM

5.8 6

Figure 8: Motor torque for static inflow conditions.

1.4

1.2

1

0.8

0.6

0.4

0.2

04.4 4.6 4.8 5 5.2 5.4 5.6 5.8 6 6.2

Apparent wind (m/s)

TM

(Nm

)

80 (mm)40 (mm)20 (mm)10 (mm)

Figure 9: Motor torque as function of apparent wind for a sinusoidal imposed surge motion of 0.6 Hz atvarious amplitudes.

As a term of reference Fig. 8 shows the rotor torque (motor torque) corresponding todifferent wind velocities, without any imposed motion of the tower base. Fig. 9 shows the samequantity obtained with an imposed sinusoidal surge motion, considering different amplitudes

A and fixed frequency (0.6 Hz). In this case the rotor torque is reported as a function ofapparent wind v, considering the contributions of both wind velocity w and surge platformvelocity x. : v = w + x. .

What can be easily appreciated from Fig. 9 is a non-linear relationship between theapparent wind acting on the blades and the loads acting on the structures due to thedisplacement imposed to the platform. Moreover, considering Fig. 9, when a relative motionbetween the nominally laminar inflow and the generator itself occurs, the torque motor showshysteretic areas centered at the related static load condition points (Fig. 8). These non-linearities are both significantly connected, in terms of shape and slope, to the frequency andto the amplitude of the relative motion and seem to be ascribable to the added-mass effect dueto the motion of the system inside the wind tunnel section.

A similar behavior can be observed also when considering the aerodynamic loads actingon the whole machine, Fig. 10. In this case, measured forces were conditioned by subtractingthe forces coming from the related inertial no-wind measurements. This subtraction wasperformed in post processing by re-phasing the wind force signal (aerodynamic + inertialforces) with respect to the related no-wind force signal (inertial forces), through the firstharmonic contribution given by the frequency of the imposed motion. Therefore the reportedresults represent only the aerodynamic forces acting on the scale model.

In Fig. 11 the frequency responses of the motor torque are represented, in terms ofamplitude of the torque component synchronous with the applied motion and of phaserelative to the applied sinusoidal motion, as functions of reduced velocity V *:

(1)V WfA

* =


18

16

14

12

10

8

6

44.4 4.6 4.8 5 5.2 5.4 5.6 5.8 6 6.2

Apparent wind (m/s)

(N/m

)

My - Tower

10 (mm)20 (mm)40 (mm)80 (mm)

Figure 10: Torque at the base of the tower generated by drag force on the rotor as function of apparentwind: sinusoidal imposed surge motion with 0.6 Hz frequency and various amplitudes.

where W, f and A are respectively the wind speed, the frequency and the amplitude of thesinusoidal imposed surge motion.

Fig. 11 shows that as the reduced velocity increases, the rotor torque becomes almost inquadrature with respect to the input surge sinusoidal motion. In this conditions, the forcesdepending on the velocity of the fluid are mainly contributing to the aerodynamic forces,whereas the in-phase and push-pull components become negligible. This is consistent withthe fact that increasing the reduced velocity deals with reducing the inertial mass-addedeffects.

On the other hand, for low reduced velocities, the aerodynamic forces, and thus the rotortorque, appear significantly affected by the platform motion. It is therefore desirable toaccount for both wind and platform motion, possibly considering their combined effects. Tothis end a HIL testing approach is proposed in the following.

4. PROPOSAL FOR AHardware-In-The-Loop(HIL) functioning mode of the rigDuring the HIL test the hydraulic actuators of the system are driven on the basis of theaerodynamic forces, measured by the balances, and on the hydrodynamic forces, evaluatedby the numerical real time model of the hydro-structure interaction.

The HIL procedure is summarized in Fig. 12. At a given time step hydrodynamic Morrisonsforces on the submerged platform are calculated on the basis of the sea time history (waterparticle velocities), imposed to be consistent with the speed of the wind blowing inside thewind tunnel [11], and of the state of the turbine and platform motion variables.


1.5

1

0.5

00 100 200 300 400 500 600 700 800 900 1000

(Nm

)(

)

TM

V*

0 100 200 300 400 500 600 700 800 900 1000V*

100

105

110

115

120

A: 80 (mm); W: 4.5, 5, 5.5, 6 (m/s)

A: 20 (mm); W: 4.5, 5, 5.5, 6 (m/s)

A: 40 (mm); W: 4.5, 5, 5.5, 6 (m/s)

A: 10 (mm); W: 4.5, 5, 5.5, 6 (m/s)

Figure 11: Motor torque: amplitudes and phases as functions of reduced velocity, for a sinusoidalimposed surge motion of 0.6 Hz.

The aerodynamic forces acting on the wind turbine scale model are gathered by thebalance installed at the base of the model, and subtracted from the inertial forces byconsidering the actual accelerations and the inertial properties of the scale model. Differentlyfrom the validation experimental campaign previously commented, in the HILimplementation, the subtraction of the inertial from the measured forces has to be performedin real-time.

The two contributions, i.e. the virtual hydrodynamic loads and the measured aerodynamicforces, are applied to the numerical model of the wind turbine and floating platform andintegrated in real-time. The resulting motion is finally physically applied to the wind turbinescale model, placed inside the wind tunnel chamber, by the two hydraulic actuators, thusclosing the loop.

To this end, the displacements, in terms of translational and rotational motion of the model(surge/sway-pitch/roll), coming from the real-time software calculations, are multiplied byconsistent gains and conversions in order to provide the proper commands to the hydraulicactuators.

The numerical model, developed in Matlab/Simulink environment and running in real-time on dSPACE hardware environment, is reported in Fig. 13, and takes the Lagrange form ofa 2 DoF mechanical system (translation and rotation, consistently with the rig):

(2)

where the mass, stiffness and damping matrices have contributions coming from the scalemodel and contributions coming from the real hydrodynamic added matrices, consistentlyscaled by the wind turbine model scale factor 1/25 (for example coming fromnumeric/experimental studies on specific platform, e.g. [12]).

In the current model the hydrodynamic contribution is taken into account only by meansof Morrisons forces, but different, and more complex, modelling options can be adopted forthe same purpose or as a means to include more complex phenomena [14].

+ + = +M x C x K x F F x x[ ] [ ] [ ] ( , )eq eq eq Measuredaero

Morrisonhydro


Figure 12: Hardware In The Loop scheme.

5. CONCLUSION AND FUTURE DEVELOPMENTSThe work presented in this paper shows the possibility of using a 2-DoF experimental rig tosimulate the wave forcing on scale floating wind turbine platform within a wind tunnel facility.The experimental tests have been carried out in a wind tunnel boundary layer section ofPolitecnico di Milano, with the ability of a reliable control in the quality of the wind flowgenerated and to perform precise correlations between measured inputs and outputs.

The results concerning the validation of the experimental rig have shown non-linarities inthe aerodynamic forcing on floating wind turbines due to hydrodynamic loads acting on thefloating platform itself. This aspect can be investigated in further studies on the topic of thedesign of control system oriented to the minimization of dynamic effects of the floatingplatform and to the maximization of the generated power. Certainly specific studies on theaerodynamics of such a floating systems can be carried out considering this rig.

The added value of integrating this set-up with a hardware-in-the-loop real time system,able to reproduce the platform motion consistently with wind and wave loads, lays in thepossibility to investigate more realistic test conditions. The proposed experimental set up canbe usefully adopted to validate numerical codes, also carrying out experimental tests by aero-elastic scale models. Moreover the proposed rig can be used for specific wind turbine scalemodels to be tested in advance with respect to their final implementation, with theremarkable advantage of reproducing in a wind tunnel test session realistic sea and windcharacteristics, based on the statistical properties of the operating environment.

REFERENCES[1] S. Butterfield, W. Musial, and J. Jonkman, P. Sclavounos, Engineering Challenges for

Floating Offshore Wind Turbines, Tech. rep. NREL/CP-500-38776, 2007.

[2] D. Matha, Model Development and Loads Analysis of an Offshore Wind Turbine on aTension Leg platform with a Comparison to Other Floating Turbine Concepts, Tech.rep. NREL/SR-500-45891, 2010.

[3] J. Jonkman, Dynamics Modeling and Loads Analysis of an Offshore Floating WindTurbine, Tech. rep. National Renewable Energy Laboratory, 2007.


Omega + t + alpha

t

Frequency and randomphases (Jonswap spectrum)

Water particles dynamics Morrison forces

In 1

u

up

w

u

up

w

xp

Fx F_hydro

F_hydro 2

Hydrodynamic forces

Hardware in the loop

Mechanical system

Platform dynamics

Terminator

Disp.

xgThetaFhydro

Faero

F_tot

X

Xp

Xpp

Translational & angular velocity

In 1Out 1Platform velocity

Clock

Enable

Figure 13: Matlab/Simulink implementation of the HIL procedure.

[4] J. Jonkman, S. Butterfield, W. Musial, and G. Scott, Definition of a 5-MW Reference WindTurbine for Offshore System Development, Tech rep. NREL/TP-500-38060, 2009.

[5] J. Jonkman, Definition of the Floating System for Phase IV of OC3, NREL/TP-500-47535, 2010.

[6] B. Skaare, T.D. Hanson, F.G. Nielsen, R. Yttervik, A.M. Hansen, K. Thomsen, and T.J.Larsen Integrated Dynamic Analysis of Floating Offshore Wind Turbines, EuropeanWind Energy Conference, Milan, Italy, 2007.

[7] D. Roddier, C. Cermelli, A. Aubault, A. Weinstein, Windfloat: a floating foundation foroffshore wind turbines, Journal of Renewable and Sustainable Energy, 2(3), art. No.033104, 2010.

[8] J.-P. Xiao, J. Wu, L. Chen, Z.-Y. Shi, Particle image velocimetry (PIV) measurements oftip vortex wake structure of wind turbine, Applied Mathematics Mechanics (EnglishEdition), 32(6), 729738, 2011.

[9] L. P. Chamorro, F. Port-Agel, A Wind-Tunnel Investigation of Wind-Turbine Wakes:Boundary-Layer Turbulence Effects, Boundary-Layer Meteorology, 132, 129149, 2009.

[10] J. Jonkman, Loads Analysis of a Floating Offshore Wind Turbine Using Fully CoupledSimulation, Tech rep. NREL/CP-500-41714, 2007.

[11] Wind turbines - Design requirements for offshore wind turbines. BS EN 61400-3:2009

[12] S. Butterfield J. Jonkman W. Musial E.N. Wayman P.D. Sclavounos, Coupled DynamicModeling of Floating Wind Turbine Systems, Tech. rep. National Renewable EnergyLaboratory, 2006.

[13] G. Diana, F. Resta, A. Zasso, M. Belloli, D. Rocchi, Forced motion and free motionaeroelastic tests on a new concept dynamometric section model of the Messinasuspension bridge, Journal of wind engineering and industrial aerodynamics, 94(5),341363, 2006.

[14] C. Lugni, E. Marino, C. Borri, Influence of wind-waves energy transfer on the impulsivehydrodynamic loads acting on offshore wind turbines, Journal of Wind Engineeringand Industrial Aerodynamics, 99(67), 767775, 2011.


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