Coupled dynamic simulations of offshore wind turbines using linear, weakly and fully nonlinear wave models: the limitations of the second-order wave theory

The present work investigates the dynamic response of a fixed–bottom offshore wind turbine subjected to the combined wind-waves action employing different nonlinear wave kinematic models. Linear, 2nd-order and fully nonlinear models are imple- mented in the hydrodynamic module of a global hydro-aero-servo-elastic solver. All the wave models are based on the potential flow assumption. A first study of the structural response is performed in regular waves with increasing steepness considering the turbine both in parked condition and in power production. A more realistic simulation is then carried out with irregular waves and turbulent wind. Hydrodynamic loads associated to the three wave models are coupled with aerodynamic loads acting on the rotor of a 5-MW wind turbine. Hydro-aero-elastic calculations are performed using the NREL software FAST. The paper shows that from wave steep- ness ka = 0.1 on the 2nd-order model becomes inaccurate. It underestimates the structural loads as well as the resonant oscillations of the tower caused by the higher-order components.