Turbulent wind thrust control to reduce pitch motion of floating wind platforms with improved rotor operation

For the dynamic analysis of floating offshore wind turbines (FOWTs) in realistic operating environment, this paper develops a coupled aero-hydro-mooring-servo model applicable to turbulent wind and irregular sea states with high computational efficiency. A modified rotor control strategy with platform motion feedback is proposed with a novel gain-scheduling technique to mitigate the negative damping effect on platform motions and decouple the rotor dynamics from the platform dynamics for better rotor operating performance. Firstly, the performance for the bottom-fixed wind turbine in steady uniform wind is validated and the turbulent wind tests show that using the rotor-disk-averaged wind speed for control is beneficial for reducing fluctuations of wind thrust and operational parameters compared with using the hub-height wind speed. For the FOWT, the negative damping phenomenon at above-rated wind speeds when using the baseline control is demonstrated through a range of wind and wave scenarios, and effects of control strategy and turbulent wind are investigated. The results indicate that the modified control strategy eliminates the negative damping effect on the platform pitch while maintaining small variations in rotor speed. Though the control including the blade pitch compensation from platform motion feedback with a constant gain can also eliminate the negative damping effect, it produces much larger fluctuations in rotor speed and causes larger overspeed exceeding the safety threshold of 20% at large wind speeds. Compared to steady wind, turbulent wind yields significantly larger low-frequency platform responses and increases the maximum rotor speed by 7.9% to 23.7% for the wind speeds considered.