Abstract. The computational effort for wind turbine design load calculations is more extreme than it is for other applications (e.g., aerospace), which necessitates the use of efficient but low-fidelity models. Traditionally the blade element momentum (BEM) method is used to resolve the rotor aerodynamic loads for this purpose, as this method is fast and robust. With the current trend of increasing rotor size, and consequently large and flexible blades, a need has risen for a more accurate prediction of rotor aerodynamics. Previous work has demonstrated large improvement potential in terms of fatigue load predictions using vortex wake models together with a manageable penalty in computational effort. The present publication has contributed towards making vortex wake models ready for application to certification load calculations. The observed reduction in flapwise blade root moment fatigue loading using vortex wake models instead of the blade element momentum (BEM) method from previous publications has been verified using computational fluid dynamics (CFD) simulations. A validation effort against a long-term field measurement campaign featuring 2.5 MW turbines has also confirmed the improved prediction of unsteady load characteristics by vortex wake models against BEM-based models in terms of fatigue loading. New light has been shed on the cause for the observed differences and several model improvements have been developed, both to reduce the computational effort of vortex wake simulations and to make BEM models more accurate. Scoping analyses for an entire fatigue load set have revealed the overall fatigue reduction may be up to 5 % for the AVATAR 10 MW rotor using a vortex wake rather than a BEM-based code.

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Abstract. The computational effort for wind turbine design loads calculations is more extreme than it is for other applications (e.g. aerospace) which necessitates the use of efficient but low-fidelity models. Traditionally the Blade Element Momentum (BEM) method is used to resolve the rotor aerodynamics loads for this purpose, as this method is fast and robust. With the current trend of increasing rotor size, and consequently large and flexible blades, a need has risen for a more accurate prediction of rotor aerodynamics. Previous work has demonstrated large improvement potential in terms of fatigue load predictions using vortex wake models together with a manageable penalty in computational effort. The present publication has contributed towards making vortex wake models ready for application to certification load calculations. The observed reduction in flapwise blade root moment fatigue loading using vortex wake models instead of the Blade Element Momentum method from previous publications has been verified using a numerical wind tunnel, i.e. Computational Fluid Dynamics (CFD) simulations. A validation effort against a long term field measurement campaign featuring 2.5 MW turbines has also confirmed the improved prediction of unsteady load characteristics by vortex wake models against BEM based models in terms of fatigue loading. New light has been shed on the cause for the observed differences and several model improvements have been developed, both to reduce the computational effort of vortex wake simulations and to make BEM models more accurate. Scoping analyses for an entire fatigue load set have revealed the overall fatigue reduction may be up to 5 % for the AVATAR 10 MW rotor using a vortex wake rather than a BEM based code.

For the design of a floating wind turbine it is necessary to take the loading due to the wind, wave and current in equal consideration. The PHATAS computer program from ECN (Energy research Centre of the Netherlands) is a time-domain aero-elastic simulation program, that accounts for the complete mutual interaction of unsteady rotor aerodynamics, structural dynamics of the rotor blades and tower, and interaction with the turbine controller under influence of turbulent wind and wave loading for fixed wind turbines. The aNySIM computer program from MARIN is a multi rigid body time domain model that accounts for wave loadings, current loadings, wind loadings, floating body dynamics, mooring dynamics. The coupled computer program aNySIM / PHATAS accounts for all loadings acting on a floating wind turbine and its response whereas PHATAS can only be used for fixed wind turbines onshore and offshore. This paper reports on the dynamic coupling between PHATAS and aNySIM. As a typical case study, the controller for floating offshore wind turbines is evaluated. This new tool has been used to repeat phase IV of the Offshore Code Comparison Collaboration (OC3) within IEA Wind Task 23, regarding floating wind turbine modelling. The results of these simulations are presented in this paper.