Effects of Control Strategies on the Coupled Dynamic Responses of a Semi-Submersible Floating Wind Turbine: A Numerical Investigation

Abstract This study presents a numerical investigation into the influences of control strategies on the coupled dynamic responses of a 5 MW SPIC-II semi-submersible floating offshore wind turbine (FOWT). A fully coupled time-domain simulation model is established in the SIMA software, incorporating aerodynamic, hydrodynamic, structural, and control subsystems. The model is validated against scaled physical model test results, demonstrating good agreement in key dynamic responses such as platform motions and aerodynamic loads, thereby ensuring the reliability of the numerical approach. Two control strategies are considered: (1) a baseline ROSCO-designed variable-speed/collective pitch controller and (2) an advanced floating feedback controller incorporating tower-top motion signals into pitch regulation. Sensitivity analyses identified an optimal pitch controller configuration (natural frequency: 0.2 rad/s; damping ratio: 0.7) for robust performance. Simulations under field-measured Guangdong coastal conditions revealed that the floating feedback controller significantly enhances system dynamics under above above-rated wind speeds. Compared to the baseline controller, it achieved reductions of up to 66% in platform pitch motion standard deviation, 17% in tower base bending moment, and 34% in mooring line tension, while maintaining comparable power output. Spectral analysis confirmed its effectiveness in suppressing low-frequency coupled responses. These results underscore the critical importance of platform-aware control strategies for FOWT dynamic design.