Fixed-Time Vibration Mitigation Control for Underactuated Offshore Floating Wind Turbines Under Coupled Wind-Wave Excitation
Offshore floating wind turbines (OFWTs) suffer from severe wind-wave-induced vibrations that degrade power quality and accelerate structural fatigue. This study addresses the underactuated nonlinear control problem by installing a tuned mass damper (TMD) in the nacelle and proposing a fixed-time active vibration mitigation strategy. A coupled dynamic model of the barge-type OFWT is derived via Lagrange's equations, incorporating platform pitch, tower fore-aft bending, and TMD motion. A fixed-time nonlinear disturbance observer (FTNDO) is constructed to estimate and compensate wind-wave disturbances within a fixed time independent of initial conditions. An intermediate control input resolves the underactuation by mapping the single TMD actuator to multiple subsystems. Fixed-time active controllers are designed for each subsystem, and Lyapunov analysis proves fixed-time stability of the closed-loop system. Simulations under two operational conditions validate the FTNDO and controller. Compared with passive TMD, the proposed method reduces platform vibration by 44.33% and tower vibration by 46.09%. The control input remains bounded within ±1.0×10^6 N·m, demonstrating practical feasibility. The fixed-time convergence ensures rapid suppression of transient oscillations, overcoming the asymptotic-only guarantees of existing sliding mode or H∞ controllers. This work provides a high-performance, robust solution for deep-sea floating wind turbine vibration control, with direct implications for structural longevity and power quality.