Nonlinear waves influence to a large extent the coupled hydro-aero-elastic response of offshore wind turbines. Higher-order contributions in the hydrodynamic forcing are responsible for resonant springing-like vibrations of the tower causing an increase of stress cycles and amplitudes. The present study investigates the effects of these amplifications in terms of fatigue load. Equivalent fatigue loads are estimated by means of both time and frequency domain methods. A comparison between linear, second-order and fully nonlinear wave models is proposed and it is shown that the weakly nonlinear model, widely used in the state-of-the-art simulations, may significantly underestimate the actual fatigue load. Hydrodynamic loads associated with the different wave models are coupled with aerodynamic loads acting on the rotor of a 5-MW wind turbine (fixed-bottom).
