Simulation of nonlinear waves on offshore wind turbines and associated fatigue load assessment

By using a global simulation framework that employs a domain-decomposition strategy for computational efficiency, this study investigates the effects of fully nonlinear (FNL) waves on the fatigue loads exerted on the support structure (monopile) of a fixed-bottom offshore wind turbine. A comparison is made with more conventional linear wave hydrodynamics. The FNL numerical wave solver is invoked only on specific sub-domains where nonlinearities are detected; thus, only locally in space and time, a linear wave solution is replaced by the FNL results as input to the Morison equation used for the hydrodynamic loads. The accuracy and efficiency of this strategy allows long time- domain simulations where strongly nonlinear free-surface phe- nomena, like imminent breaking waves, are accounted for in the prediction of structural loads. The unsteady nonlinear free- surface problem governing the propagation of gravity waves is formulated using potential theory and a higher-order boundary element method (HOBEM) is used to discretize Laplace’s equa- tion. The FNL solver is employed and associated hydrodynamic loads are predicted in conjunction with aerodynamic loads on the rotor of a 5-MW wind turbine using the NREL open-source software, FAST.We assess fatigue loads by means of both time and frequency-domain methods. This study shows that the use of linear theory-based hydrodynamics can lead to significant un- derestimation of fatigue loads and damage.