Design of a novel tower damping system for semi-submersible floating offshore wind turbines considering fatigue and ultimate limit states

This study proposes and analyses a novel tower damping system to enhance the structural performance of the NREL 5MW semi-submersible wind turbine under operational and ultimate loading conditions. First, the tower damping system's working principles and structural distribution are introduced, which describes the integration of a magnetorheological damper with pulleys, steel cables, and the wind turbine. Afterwards, a detailed description of how the new system effectively reduces vibrations and external force impacts on the wind turbine. For the comparative analysis, the environmental load dataset from the Norwegian MET Centre is used to derive the extreme loading for the ultimate limit state through the Inverse First-Order Reliability Method (IFORM). The same dataset is also used to identify sea states describing the operating loading condition for the fatigue limit state. Fully coupled aero-hydro-servo-elastic simulations are performed to analyse the dynamic behaviour of the semi-submersible wind turbine. The presented novel damping system demonstrates superior vibration reduction and flexibility across fatigue and extreme loading conditions compared to traditional damping technologies. The results of the coupled aero-hydro-servo-elastic simulations indicate that under operational sea states, the displacement at the top of the turbine tower is reduced by 60-70%, while the acceleration decreases by 30-40%, which improves the tower stability significantly. Under extreme loading, the tower top acceleration decreases by 5-7%, and the displacement decreases by 6-8%. Moreover, the tower damping system significantly reduces fatigue damage by up to 72% and extends the remaining useful fatigue life of the tower base by 140%.