HYBRID MODEL TESTING OF FLOATING WIND TURBINES: TEST BENCH FOR SYSTEM
IDENTIFICATION AND PERFORMANCE ASSESSMENT
Vincent Arnal, Jean-Christophe Gilloteaux, Félicien Bonnefoy, Sandrine Aubrun
LHEEA, Ecole Centrale de Nantes, France
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Outline
1. Context : Wave tank hybrid testing of FWT
2. Specifications of actuators setpoint
3. Test bench presentation
4. System identification and performances
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Wave tank testing of floating wind turbines
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Drag diskGeometry
scaled rotorThrust matched
scaled rotor
100% physical
MeanThrustforce
Unsteadyaerodynamicforce
Test of Control strategies
Actuatorresponsetime
Scalability to large rotor (10-15 MW)
Porous Disk + + - - - - -
Geometry scaled rotor + - - ++ -
Thrust Match rotor ++ - - ++ -
Wind
Reynolds
Wave
Froude Hybrid with actuators
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Wave tank hybrid testing of FWT : principle
Physic → Numeric
Positions and velocities and accelerations Platform and tower
Physic ← Numeric
Rotor Force (Aero, Gyro, Inertia...)
Full scale :Simulation of a wind turbine :Wind + Aero + Servo + Elastic
Model Scale:Waves + Platform + Tower + Moorings+ Actuators
Motions
RNA Force
©SINTEF Ocean (Thys, 2018)
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Numerical tool : Realistic rotor
loads Real-time Similar to fully
coupled models
Actuators :Accuracy Steady state
performance High bandwidth
Physical Numerical
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Wave tank hybrid testing of FWT : actuators
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SINTEF Ocean 6 cables(Chabaud, 2016);(Sauder et al., 2016);...
SOFTWIND Project
Focus on control law influence during wave tank testing campaign
On-land
IH Cantabria 6 on-board fans(Urbán and Guanche, 2019); (Battistella et al., 2018)
On-board
Bandwidth ??
𝟏𝑷 𝟑𝑷
𝑇𝑤𝑟
Blades
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Specifications of actuators setpoints: frequencies
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0 0,04 –0,05 [Hz]Turbulence 0,3
𝑿𝟑
𝑿𝟏 & 𝑿𝟐
𝑿𝟒 & 𝑿𝟓
Wave-induced hub velocity
𝑿𝟔
0,2 1-20,6
Tower shadow
𝑿𝟑 TLP,𝑋4& 𝑋5 𝑇𝐿𝑃
𝑿𝒊 = natural frequency of Platform DoF 𝑖
Highest frequency captured ?
Main questions :• Relative contributions of the different frequencies of
interest?• Required dynamics of the actuators ?• Mean and Max forces ?
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Specifications of actuators setpoints : methodology
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OC4 Semisubmersible OC3 Hywind Spar Triple Spar
NREL-5MW NREL-5MW DTU-10MW
Selected floating wind turbines : Numerical model :OpenFAST v1.0 with:• Rigid blades• Flexible tower (modal)• Active controlerFree access input files
Load cases considered :Norm Design Load Cases of type 1.X (power production) Severe waves Normal Turbulence Model Wind-wave misalignment
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Specifications of actuators setpoints : outcome
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𝐹𝑥Wave frequencies ++Tower bending mode and 3P : +
• Q: Relative contributions of the different frequencies of interest?
𝑂𝑡ℎ𝑒𝑟 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡𝑠 ∶ Spread on the different frequencies
LF X_5 WF Twr & 3P > 3P
f_low [Hz] 5,0E-03 2,5E-02 4,0E-02 3,3E-01 7,0E-01
f_high [Hz] 2,5E-02 5,0E-02 3,3E-01 7,0E-01 1,5E+00
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Inflow, Aero, Servo, Structure
OpenFAST code
On boardcontroler
Actuator
ESC
Actuator command
Host PC
Communication
Mast
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Balance
Motion Capture system
Accelerometer Data acquisition
Data acquisition
Wave tank Dry
Floater
TEST BENCH : Overview
Envisaged system in the wave tank
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Inflow, Aero, Servo, Structure
OpenFAST code
On boardcontroler
Actuator
ESC
Actuator command
Host PC
Communication
Mast
6 DoF Hexapod
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Balance
Motion Capture system
Accelerometer Data acquisition
Data acquisition
TEST BENCH : Overview
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TEST BENCH : Validation of the methodology
Overall validated methodology1. Realistic FWT motions 2. Motions reproduced by Hexapod3. Motions capture4. Force computed in real-time by integrated
numerical code5. Actuator commanded to reproduce this force
Success Indicators :• The real-time computed force corresponds
to the load case we are reproducing.• The actuator reproduces with a sufficient
accuracy the setpoint force.
𝐻𝑠[m]
𝑇𝑝[s]
𝑈𝑤[m/s]
Mean thrust
[kN]
LC1 3 5 11.4 680
LC2 4 5 18 320
LC3 6 10 11.4 680
LC4 7 10 18 320
LC5 7 17 11.4 660
LC6 8 17 18 320
OC3 Hywind Spar – 5MW scale 1:30• Rigid blades and tower• Active controler
Name of FWT OC4_Semi OC3_Spar TripleSpar_10MW
Representative
picture
[14] [17] [15]
Wind Turbine NREL-5MW [18] NREL-5MW [18] DTU-10MW [19]
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STATIC CALIBRATION ACTUATOR
ESC Turbine
Thrust [N]
Du
tyR
atio
[%]
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Throttle or Duty ratio 𝛽[%]
Thrust 𝑇[N]
𝑻 = 𝑲 𝜷 ∗ 𝜷
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TEST BENCH : Performance / static calibration
3P
waves
Load CaseMeanThrust (N)
Error (%)
LC1 25 3LC2 12.5 5LC3 25 5
LC4 12 10
LC5 24 4LC6 12 5
Scale 1: 30
Results OK but higher bandwidth preferred
Rising steps
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𝑻𝒂𝒄𝒕 𝒔
𝜷 𝒔= 𝑯 𝒔 ≈
𝑲 (𝑻𝒎𝒆𝒂𝒏)
𝟏+𝝉 𝑻𝒎𝒆𝒂𝒏 𝒔𝒆−𝑻𝒅𝒆𝒍𝒂𝒚𝒔𝑻𝒎𝒆𝒂𝒏 [N] a working point
TEST BENCH : Identification
Pure delay
White noise
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0
0.05
0.1
0.15
0.2
0.25
0.3
0 20 40 60τ
[s]
Thrust [N]
Time characteristic as a function of mean thrust
𝑻 𝒔
𝜷 𝒔= 𝑯 𝒔 ≈
𝑲 (𝑻𝒎𝒆𝒂𝒏)
𝟏+𝝉 𝑻𝒎𝒆𝒂𝒏 𝒔𝒆−𝑻𝒅𝒆𝒍𝒂𝒚𝒔
𝑻𝒎𝒆𝒂𝒏 [N] a working point
TEST BENCH : Identification
White noise
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TEST BENCH: Dynamic calibration
ESC Turbine
Throttle or Duty ratio 𝛽[%]
Thrust 𝑇[N]
𝑻𝒂𝒄𝒕 𝒔
𝜷 𝒔= 𝑯 𝒔 ≈
𝑲 (𝑻𝒎𝒆𝒂𝒏)
𝟏+𝝉 𝑻𝒎𝒆𝒂𝒏 𝒔𝒆−𝑻𝒅𝒆𝒍𝒂𝒚𝒔𝑻𝒎𝒆𝒂𝒏 [N] a working point
Identification :
𝜷𝒅𝒚𝒏 𝒔 = 𝑻𝒔𝒆𝒕 𝒔 𝑯−𝟏 𝒔
Inverse dynamics
Static Dynamic
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≈ 𝑻𝒔𝒆𝒕 𝒔𝟏
𝑲 𝑻𝒎𝒆𝒂𝒏𝟏 + 𝝉 𝑻𝒎𝒆𝒂𝒏 𝒔 𝒆𝑻𝒅𝒆𝒍𝒂𝒚𝒔 Not
implemented
𝑻𝒂𝒄𝒕 𝒔
𝑻𝒔𝒆𝒕 𝒔≈ 𝟏 ?
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TEST BENCH: Dynamic calibration performance
OC3 Hywind Spar – 5MW at scale 1:30Waves conditions : Severe Sea State, 𝐻𝑠 = 8.6𝑚 𝑇𝑝 = 13𝑠;
Wind conditions : Normal Turbulence Model, 𝑈𝑤 = 18𝑚. 𝑠−1; 𝑇𝐼 =14.6%;
3P
Waves
Name of FWT OC4_Semi OC3_Spar TripleSpar_10MW
Representative
picture
[14] [17] [15]
Wind Turbine NREL-5MW [18] NREL-5MW [18] DTU-10MW [19]
30 ms delay Highly encouraging resultsOMAE 2019-96374 Vincent Arnal - 12/06/2019
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Conclusion
Parametrical study with numerical simulations 1. Identification and quantification of relative importance of frequencies of interest. It is important to have 3P and 1st tower mode correctly reproduced
Test Bench identification2. Validation of communication protocol and overall methodology Real-time execution, motion and force observers design
3. Identification of actuator transfer function, delays, ... Good match between white noise and response to steps
4. Improvement of actuator command The inverse dynamics / dynamic calibration methodology gave significantly betterresults compared to static only
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Next steps
• Wave tank tests with 1 actuator in 09/2019 on 1:40 multipurpose platform(Blue Growth Farm project) and in 10/2019 on a 1:40 10MW SPAR with 1 actuator
• Calibration and identification of a more advanced emulation system• Wave tank tests with advanced emulation system in ~ 05/2020 on a 1:40
10MW SPAR Focus on control law influence
• Flexible blades modelling• Improve quality of dynamic calibration• Use braking of the turbine to improve decelerating performances
THANK YOU FOR YOUR ATTENTION
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Specifications of actuators setpoints: background
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𝐹𝑠𝑒𝑡 =
𝐹𝑎𝑒𝑟𝑜 𝑥𝐹𝑎𝑒𝑟𝑜 𝑦𝐹𝑎𝑒𝑟𝑜 𝑧0
𝑀𝑎𝑒𝑟𝑜 𝑦
𝑀𝑎𝑒𝑟𝑜 𝑧
+
000
𝑀𝑥 𝐿𝑆𝑆
00
+
0000
𝑀𝑔𝑦𝑟𝑜 𝑦
𝑀𝑔𝑦𝑟𝑜 𝑧
+
𝐹𝐼𝑛𝑒𝑟𝑡𝑖𝑎 𝑓𝑙𝑒𝑥 𝑥
𝐹𝐼𝑛𝑒𝑟𝑡𝑖𝑎 𝑓𝑙𝑒𝑥 𝑦
𝐹𝐼𝑛𝑒𝑟𝑡𝑖𝑎 𝑓𝑙𝑒𝑥 𝑧
𝑀𝐼𝑛𝑒𝑟𝑡𝑖𝑎 𝑓𝑙𝑒𝑥 𝑥
𝑀𝐼𝑛𝑒𝑟𝑡𝑖𝑎 𝑓𝑙𝑒𝑥 𝑦
𝑀𝐼𝑛𝑒𝑟𝑡𝑖𝑎 𝑓𝑙𝑒𝑥 𝑧
Aerodynamics Free rotor speed Inertia Rotor -rotating
Inertia Rotor - flexible
In the followinganalyses
Rotor Nacelle Assembly
RNA Physical model Numerical model
Gravity
Inertial | 6 DOF platform
Inertial | flexible tower
Inertial | flexible blades
Inertial | rotating blades (gyroscopic)
Aerodynamics
Free rotor speed
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Specifications of actuators setpoints : outcome
OMAE 2019-96374 Vincent Arnal - 12/06/2019
• Q: Required dynamics of the actuators ?
𝐿𝑜𝑤 𝑝𝑎𝑠𝑠 𝑓𝑖𝑙𝑡𝑒𝑟 ∶𝐹𝑟𝑜𝑡𝑜𝑟 𝑎𝑐𝑡𝑢𝑎𝑡𝑒𝑑, 𝑖 𝑠
𝐹𝑟𝑜𝑡𝑜𝑟 𝑠𝑒𝑡𝑝𝑜𝑖𝑛𝑡, 𝑖 𝑠=
1
1 +1
2𝜋𝑓𝑐𝑠
𝜖𝑅𝑌 < 5% 𝑓𝑐 > 0.3 𝐻𝑧 Full scale1.8 Hz scale 1:30
𝜖 ሷ𝑋𝑁𝑎𝑐< 5% 𝑓𝑐 > 0.5 𝐻𝑧 Full scale
2.7 Hz scale 1:30
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Specifications of actuators setpoints : outcome
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• Max
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TEST BENCH : Coupling of numerical model
• Wind Inflow• Aerodynamics• Servo• Structure
OpenFAST code
On boardcontroler
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Methodology and time stepping ?
OC3 Hywind Spar – 5MW Waves conditions : Severe Sea State, 𝐻𝑠 = 8.6𝑚𝑇𝑝 = 13𝑠;
Wind conditions : Normal Turbulence Model, 𝑈𝑤 =18𝑚. 𝑠−1; 𝑇𝐼 = 14.6%;
Name of FWT OC4_Semi OC3_Spar TripleSpar_10MW
Representative
picture
[14] [17] [15]
Wind Turbine NREL-5MW [18] NREL-5MW [18] DTU-10MW [19]
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Motions RAO of the 3 floaters, for 2 incident wave directions. (a)Velocity RAO of the different floaters at origin point (b) Velocity and acceleration RAO at hub height for the different floaters
OC4 Semisubmersible OC3 Hywind Spar Triple Spar
NREL-5MW NREL-5MW DTU-10MW
Freewheel
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Inverse dynamics ?
Sachant 𝑻 𝒔
𝜷 𝒔= 𝑯 𝒔 ≈
𝑲 (𝑻𝟎)
𝟏+𝝉 𝑻𝟎 𝒔𝒆−𝑻𝒔
𝜷𝒂𝒅𝒂𝒑𝒕𝒆𝒅 𝒔 = 𝑻𝒔𝒆𝒕 𝒔 ∗ 𝑯 𝒔 −𝟏
Table 1 : Limits of the frequency bandwidths at full scale
Aerodynamic « engineering » models
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BEM DBEMGDW
FVW
CFD
Physics
CPU Time
Hypotheses Data required Main limitations CPU Time
BladeElementMomentum
Blade elements + momentum (induction) theoryPotential FlowStationnary
𝑪𝒍 and 𝑪𝒅 of each airfoil section.Each empirical correction has a proper set of inputs.
No vorticity and quasi-static assumption dynamic inflow need to be modelledNo viscous effects dynamic stall need to be modelled2D blades effects only 𝑪𝒍 𝒂𝒏𝒅 𝑪𝒅 .
short
GeneralizedDynamicWake
Potential FlowDynamic inflow partly modelled
𝑪𝒍 and 𝑪𝒅 of each airfoil sectionEach empirical correction has a proper set of inputs.
No vorticity dynamic inflow approximatedNo viscous effects dynamic stall need to be modelledDon’t converge with high induction factor
short
DynamicBladeElementMomentum
Blade elements + momentum (induction) theoryPotential FlowDynamic inflow partly modelled
𝑪𝒍 and 𝑪𝒅 of each airfoil section.Each empirical correction has a proper set of inputs.
No vorticity dynamic inflow approximatedNo viscous effects dynamic stall need to be modelled2D blades effects only 𝑪𝒍 𝒂𝒏𝒅 𝑪𝒅 .
short
Free-Wake Vortex Model
Potential flowlifting line theory
𝐶𝑙 and 𝐶𝑑 of each airfoil sectionEach empirical correction has a proper set of inputs.
Dynamic inflow and proper calculation of modified induction due to blades –wake interactionDynamic stall and other viscous effects need to be modelled
Medium
Modelling assumptions
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Summary of hypotheses
Module Hypotheses Limitations
Hydrodynamics
HydroDyn - Potential 1st and 2nd order- Morison elements
- Nonlinear excitation forces and wave kinematics (steep waves)- Complex viscous loads not well represented by Morison in severe sea states
Aerodynamics
AeroDyn v15
Blade Element Momentum Theory- ~ Dynamic Stall- ~engineering models for Hub and Tip loss, skewed wake induction factor calculation-~Tower influence
- Highly Unsteady Aero in severe sea states and turbulent wind- Glauert correction for induction below rated (induction factor ++)- In skewed wake: interaction with the near wake
Control
ServoDyn and associated .dll
- Collective Blade Pitch Control- Generator Torque Control- Fixed Nacelle Yaw
Relevant control strategies include :- Individual Blade Pitch control - Nacelle Yaw control (not really dynamic)- Control adapted to disturbance rejections with
additional inputsNo blade pitch actuator dynamic
Moorings
MoorDyn -Dynamic Lumped-mass model
- Approximated drag + added mass- no VIV- no wave-induced kinematics
Structural dynamics
ElastoDyn
- Tower flexible- Modal description fitted to testing specifications- Rigid blades
- No blade pitch actuators modelling
- Assumptions of small angles with correction for orthogonality for platform rotations and tower deflections.
Summary of hypotheses for FAST simulations run for the parametrical studies
Modelling assumptions
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[1] Courbois A 2013 Étude expérimentale du comportement dynamique d ’ une éolienne offshore flottante soumise à l’action conjuguée de la houle et du vent.PhD Thesis, École Centrale de Nantes [2] Azcona J, Lemmer F, Matha D, Amann F and Botasso C L 2016 INNWIND Deliverable 4.2.4 : Results of wave tank tests