Effects of Floater Equivalent Stiffness and Damping on the Dynamic Responses of a 22 MW Two-Body Floating Wind Turbine

The upscaling of floating wind turbines and the lightweight design of floating support structures make floater flexibility increasingly important for the dynamic response and load transfer of two-body floating platforms. This study investigates the influence of upper-floater flexibility on Tsemi, a previously proposed suspended-ballast two-body floating platform supporting the IEA 22 MW reference wind turbine, using a fully coupled multi-body flexible-joint model. The floater is discretized into multiple rigid bodies connected by damped six-degree-of-freedom joints, which represent equivalent flexibility and local energy dissipation. The model is evaluated through member-level response comparisons, global modal analysis, and nearly rigid limiting-case simulations. Results show that equivalent floater stiffness strongly affects wave-frequency responses. Reducing the stiffness shifts the platform pitch period from 25.6 s to 30.6 s and significantly amplifies structural and tendon loads under extreme environmental conditions. The standard deviations of tower-top acceleration and tower-base bending moment increase by 75.8% and 72.1%, respectively, and the maximum effective tension in the upwave tendon increases from 19.93 MN to 32.50 MN, exceeding the tendon minimum breaking load. For the considered low-stiffness case, equivalent damping reduces this tension to 21.77 MN. These results suggest that neglecting floater flexibility may underestimate tower responses and extreme tendon loads, while equivalent damping can mitigate wave-frequency amplification under low-stiffness conditions.