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 timedomain simulations where strongly nonlinear free-surface phenomena, like imminent breaking waves, are accounted for in the prediction of structural loads. The unsteady nonlinear freesurface 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 equation. 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- Corresponding Author and frequency-domain methods. This study shows that the use of linear theory-based hydrodynamics can lead to significant underestimation of fatigue loads and damage.