NOVOTEL LONDON WEST • LONDON, UNITED KINGDOM • 2‐4 APRIL 2019
Floating Offshore Wind: A Simplified Approach of Aero-Hydrodynamic Coupling to Optimize the Mooring System Design.
Marie Féron, Olivier Langeard, Caroline Le Floc’hDORIS Engineering
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Floating Offshore Wind Turbine Design
• Future commercial farms: Significant need to optimize design.
• Floating Turbine: Highly complex system with coupling effects.
• Trade-off between computing time, calculation accuracy and software expenses.
• Simplified methodology: a very efficient solution.
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NereWind – DORIS Solution
• The results of an innovative optimization process and practical methodologies:
A semi-submersible platform with a reduced draft.
A main steel column to support the turbine and two smaller columns to ensure optimized stability.
• Design and numerical calculations validated by a basin test campaign in 2017.
• Upscaling studies are ongoing for a 10 MW turbine.
• Patent pending.
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Aero-Hydro Coupling Analysis
• To verify a floating turbine platform design:
Need to assess global dynamic performanceof the floater.
Need to check turbine max allowable criteria (max pitch, max accelerations).
• Floating aspect induces an aero-hydro coupling:
Waves, wind and controller action : coupling effects with high impact on motions.
Specific software (FAST-OrcaFlex, Bladed…).
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Spectral Analysis of the platform pitch for three load cases(Hs = 6m, Tp = 10s and u = 11.4 m/s)
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Aero-Hydro Simplified Approach - Theory
• Simplified Turbine : a Thrust and an Aerodynamic Damping.
Thrust integrated (T) on the rotor disk (Karimirad, 2012) and interpolated with the turbine thrust curve regarding the relative wind speed.
o Aerodynamic damping (M) is calibrated based on a fully-coupled model.
• Controller effect: Negative damping is avoided with a filter on the relative wind speed.
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Aero-Hydro Simplified Approach - Application
• Hydrodynamics with OrcaFlex:
Hybrid Model (Potential and Morison theories) calibrated with tank tests.
Allows an accurate mooring modelling.
• Aerodynamics with Python: an external function for Thrust and Damping calculation at each time step.
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Wind Speeduhub(t)
Python FunctionuFiltre(t), Thub(t), MDamping(t)
OrcaFlex Calculation
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Simplified Approach - Equivalent Wind Model• Wind modelling capabilities:
FAST-OrcaFlex Model: 2D wind speed field.
Simplified Model: only a wind speed time series at turbine hub.
• Equivalent Wind based on Smilden (2016):
Sampling of the wind speed on the turbine blades.
Input treatment: no additional computing time.
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Simplified Approach - Validation Process
• Comparison with a fully coupled FAST-OrcaFlex model.
• Three different sets of load cases (LC):
A Limited Batch with 8 LC issued from Karimirad(2012)
A Large Batch with 50 LC (5 wind speeds * 10 seeds)
A Fatigue Analysis Batch with 4410 LC.
• Validation based on statistical study for main parameters (Surge, Pitch, Mooring line Tension…).
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Wind Speeds and Wind Directions of the Fatigue Batch
m/s
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Results 1 – Limited LC - Displacements
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• Good fit between Simplified Models and Fully-coupled Model.
• Use of Equivalent Wind improves the displacement mean values.
• Under-estimation at the rated wind speed (11 m/s): limit of the static thrust interpolation.
• Variability in results depending on the chosen seed.
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Results 2 – Large Batch LC – Mooring Tensions
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• Rainflow algorithm: fatigue assessment for the mooring system.
• Good fit between Simplified Model and Fully-coupled model.
Cycle Amplitude
Num
ber o
f Cycle
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• Calculation Time for a turbulent wind simulation of 4600s (1h + 1000 s transient).
• Comparison for 1 LC calculation:
Results – Limited LC – Calculation Time
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Model Turbulent Wind
FAST-OrcaFlex Fully Coupled Model 40 minutes
Simplified Model 11 minutes
Faster calculation
Cost-effective solutionMore optimisation loops
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Ongoing Improvements
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Integration of the turbine response time induced by the control system and the mechanical damping in the Thrust definition.
Improvement of the aerodynamic damping definition.
Thrust Calculation with Turbine Inertia – Comparison with FAST(kN)
(s)
Pitch Platform Decay Test with Turbine Operating for Wind Speed between 3 m/s and 25 m/s
Thrust Curve Depending on the Wind Speed and on the Blade Pitch
U (m/s)
Blade Pitch (°)
Thrust (kN)
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First Results – Phase II
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To understand the turbine behaviour to improve the simplified model.
(s)
Thrust Calculation with Turbine Inertia – Comparison with FAST
(kN)
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Conclusion
• Floating Wind Turbine needs to be optimized to reduce global costs of commercial farms.
• A Trade-off between computation time and calculation accuracy is necessary.
• DORIS Engineering has developed smart simplified models to optimize each part of the system.
Aero-Hydro analysis simplification allows for instance new possibilities for mooring system optimization.
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Scientific References
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DORIS Engineering
[1] Karimirad, M., & Moan, T. (2012). A simplified method for coupled analysis of floating offshore wind turbines. Marine Structures, 27(1), 45-63.
[2] Jonkman, J., Butterfield, S., Musial, W., & Scott, G. (2009). Definition of a 5-MW reference wind turbine for offshore system development. National Renewable Energy Laboratory, Golden, CO, Technical Report No. NREL/TP-500-38060.
[3] Robertson, A., Jonkman, J., Masciola, M., Song, H., Goupee, A., Coulling, A., & Luan, C. (2014). Definition of the semisubmersible floating system for phase II of OC4 (No. NREL/TP- 5000-60601). National Renewable Energy Lab. (NREL), Golden, CO (United States).
[4] Courbois, Adrien. Étude expérimentale du comportement dynamique d'une éolienne offshore flottante soumise à l'action conjuguée de la houle et du vent. 2013. Doctoral Thesis. Ecole Centrale de Nantes (ECN).
[5] Smilden, E., Sørensen, A., & Eliassen, L. (2016). Wind model for simulation of thrust variations on a wind turbine. Energy Procedia, 94, 306-318.
Marie FÉRONProject Engineer
T +33 (0)1 44 06 10 67E [email protected]
Caroline LE FLOC’H Head of Renewables
T +33 (0)1 44 06 14 33M +33 (0)6 18 55 86 84E [email protected]
