Semiactive Control and Reliability Analysis for Vibration Mitigation of Spar-Type Floating Offshore Wind Turbines under Stochastic Wind and Waves

Floating offshore wind turbines (FOWTs) are subject to structural vibrations caused by wind and waves. Tuned mass dampers (TMDs) provide a promising means of vibration mitigation for turbine structures. However, passive TMDs have nonoptimal performance, since random environmental changes cause a detuning, leading to increased maintenance costs. To address this challenge, a semiactive control for vibration mitigation of the National Renewable Energy Laboratory’s OC3-Hywind spar-type FOWT under stochastic wind and wave loading is proposed in this study using a magnetorheological damper (MRD) to replace the damping unit in the TMD system. Control effectiveness is assessed in terms of reliability metrics. A 15-degree-of-freedom model for a coupled aero–hydro–servo–structure–TMD–MRD is first constructed and validated based on the OpenFAST. Then the TMD–MRD is designed based on standard deviation optimization and the branch-and-bound Hrovat control algorithm. For verification purposes, the extreme-value distribution and the reliability of the platform pitch of the integrated FOWT–TMD–MRD system under stochastic wind and wave loading are addressed using the probability density evolution method (PDEM) followed by a comparative study of uncontrolled and controlled FOWTs with an optimized TMD or TMD–MRD. The results show that semiactive control by the TMD–MRD has good adaptability and significantly improves operational stability and structural safety, bypassing the unscheduled adjustment of TMD parameters.