Response sensitivity of a semisubmersible floating offshore wind turbine to different wind spectral models

Previous research on the OC3 spar floating offshore wind turbine (FOWT) has shown the sensitivity of the yaw and side-side modes' load and motion responses to different atmospheric conditions. Using the same baseline turbine of the OC3 spar wind turbine for a semisubmersible floater (OC4), this study investigates the load and motion responses of such offshore wind turbine for neutral and unstable atmospheric conditions. The effect of different levels of wind spatial coherence associated with two different wind spectral models for neutral conditions (Kaimal and Mann) are studied for the same turbulence intensity levels. An increase of 18% in the tower torsional moment fatigue damage equivalent load (DEL) is observed for the wind inflow with the weakest coherence (Mann spectral model), compared to the DELs under turbulent wind inflow with the highest coherence (Kaimal spectral model). Unstable atmospheric conditions are also simulated based on the Pointed-Blunt spectral model derived from FINO1 wind measurement. The yaw mode of the semisubmersible wind turbine is found to be the response component most affected by the variation in atmospheric stability conditions. A 28% higher fatigue DEL for the tower torsional moment is observed for very unstable atmosphere than the DELs under neutral atmospheric conditions.