A comparative study about the effects of linear, weakly and fully nonlinear wave models on the dynamic response of offshore wind turbines

The present work aims at comparing the dynamic response of a fixed–bottom offshore wind turbine subjected to the combined wind–waves action employing different nonlinear irregular wave kinematic models. To this purpose, linear, second–order and fully nonlinear models are implemented in the hydrodynamic module of a global hydro-aero-elastic solver. All the wave models are based on the potential flow assumption. The fully nonlinear wave kinematics is reproduced both on the full simulation time and, in order to save com- putational time, only on some space-time subdomains within a domain decomposition strategy. This approach permits achieving a much higher accuracy in the assessment of the hydrodynamic loads keeping the global computational effort similar to the one required by linear or weakly nonlinear models. The paper represents a preliminary investigation aimed at establishing to what extend the second–order wave model can efficiently capture the system response even when the system is exposed to moderate sea states. Moreover, a comparison between the four wave models seems to reveal that some resonant oscillations of the tower are triggered by nonlinear components higher than the 2nd–order. Hydrodynamic loads associated to the four 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 open-source software FAST.