• • Platform pitch vibration is reduced by 44.33% and tower fore-aft bending by 46.09% compared to passive TMD, directly lowering fatigue loads and extending structural service life in deep-sea deployments.
• • The fixed-time disturbance observer estimates wind-wave disturbances within a fixed time independent of initial states, enabling precise compensation that eliminates the chattering and slow convergence of sliding mode controllers.
• • Control input remains bounded within ±1.0×10^6 N·m across both operational conditions, confirming actuator saturation margins and practical implementability with commercial TMD hardware.
• • The underactuated system (3 degrees of freedom, 1 control input) is stabilized via an intermediate control input, achieving fixed-time stability proven by Lyapunov analysis, which guarantees convergence time independent of initial conditions—critical for unpredictable sea states.