Abstract
Designing large floating wind turbines (FWTs) requires accurately quantifying relevant high-frequency (HF) excitation load and damping sources, and understanding their effect on extreme stresses, fatigue damage, and ultimately costs. Both aerodynamic and hydrodynamic loads contribute at these frequencies, and responses in the hull, mooring, tower, blades, and drivetrain are affected. The present work addresses second-order sum-frequency non-linear wave loads and the associated elastic response of the INO OptiFLEX 22 MW floating wind turbine. Second-order potential-flow calculations are performed using WADAM, and the results of the convergence study are presented. Subsequently, fatigue analysis is performed using time-domain simulations in relevant sea states, employing a new multi-body approach developed in SIMA. The sum-frequency second-order loads excite specific resonant modes of the flexible structure, especially in sea states with short, steep waves. However, the overall impact of the sum-frequency wave loads on the structure’s fatigue life is minimal (< 3%) at the considered site.