Advancing insights into the structural load effects in a semi-submersible floating wind turbine tower considering sum-frequency wave excitation

There is a lack of a fundamental understanding of the wave-induced high-frequency (WIHF) dynamic responses in semi-submersible floating wind turbines (FWTs). However, experiments carried out by the Ocean University of China show that sum-frequency wave load effects might have a significant contribution to the extreme loads in the semi-submersible FWT tower. This paper primarily addresses the WIHF dynamic responses under combined wind and wave environmental conditions that may induce extreme tower structural loads relevant for the ultimate limit state (ULS) design check. Ten typical combined wind and wave conditions are determined using the environmental contour method and considerations wind-wave scenario that cause critical tower response. The sectional structural loads of the DTU-10-MW wind turbine mounted on a semi-submersible platform are obtained by a fully coupled time domain dynamic analysis. Two numerical models—a linear (LHM) and a nonlinear hydrodynamic model (NHM)—are developed for analysis. The NHM includes second-order sum-frequency wave excitation and is validated against experiments. Tower structural loads are examined using both models, and the most critical condition is selected for ULS assessment of the tower’s critical section. The study shows that the LHM significantly underestimates high-frequency responses and extreme tower loads, particularly for moderate sea states with wave periods near twice the tower’s first-order bending natural period, as well as for critical ULS conditions (extreme conditions). This highlights the importance of considering sum-frequency wave load effects, and accurately modeling the tower’s natural frequency and structural damping for reliable structural design of semi-submersible FWTs.