Coordinated control of tower loads and platform pitch in a deep-water semi-submersible floating offshore wind turbine using a nacelle–platform dual TMD

Deep-water semi-submersible floating offshore wind turbines exhibit pronounced coupling between platform motions and tower loads under combined wind–wave–current loading and mooring-line constraints. This study proposes a nacelle–platform dual tuned mass damper (D-TMD) configuration for the NREL 5-MW wind turbine mounted on the OC4-DeepCwind semi-submersible platform to achieve coordinated mitigation of tower loads and platform-pitch motion. With the tower-base fore-aft bending moment and platform-pitch response adopted as the control objectives, the single-TMD configurations are optimized using the sparrow search algorithm, whereas the dual-TMD configuration is optimized using the non-dominated sorting genetic algorithm II. The optimized configurations are evaluated through fully coupled OpenFAST simulations under free-decay and representative wind–wave–current conditions. Under free-decay conditions, the dual-TMD reduces the standard deviation (STD) of tower-base bending moment and the peak platform pitch by 39.10% and 12.18%, respectively. Across LC1–LC4, the corresponding reductions range from 0.09% to 26.31% for the bending-moment STD, from 3.18% to 43.12% for the damage equivalent load, and from 4.58% to 8.55% for the peak platform pitch. Under the higher-wind conditions of LC2–LC4, the standard-deviation reductions are only 0.41%, 1.11%, and 0.09%, respectively, indicating limited fluctuation suppression. Under the LC1-based conditions, mitigation is maintained across the considered current velocities and load directions.