Floating Offshore Wind: A Simplified Approach of Aero ...

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]