The dynamic response of offshore wind turbines and their sensitivity to wind field models

As the rotor diameter of offshore wind turbines increases, an improved understanding of the structure of the turbulent wind field approaching the rotor is important. Present standards for computing wind loads are resting upon assumptions of neutral atmospheric conditions and a simplistic formulation of the coherence of the turbulence. In the present work, various formulations of the wind field are applied and the dynamic responses of a bottom fixed and a floating wind turbine are computed to investigate the sensitivity to the formulation of the wind field. Focus is on wind situations with above average turbulence intensity as these are expected to have a significant contribution to fatigue damage of the structure. It is observed that choice of wind spectrum, coherence formulation as well as assumptions related to atmospheric stability conditions significantly influences the dynamic loads in tower bending moment, yaw moment and blade flap moment. The differences are significant in particular in the low frequency range. This implies that in particular floaters are sensitive to the formulation of the wind field.