Dynamic response analysis of a semi-submersible wind turbine with power cable under swell conditions at intermediate water depth

Swell waves generated by distant wind systems are characterized by long wave periods that can induce significant motion responses in floating offshore wind turbines (FOWTs). Semi-submersible platforms are particularly vulnerable to these conditions because their natural periods in heave and pitch lie close to typical swell wave periods, heightening the risk of near-resonant responses. Simultaneously, this platform concept is widely deployed at intermediate water depths. This study investigates the platform dynamics, mooring line tensions, and dynamic power cable responses of a 15 MW semi-submersible FOWT deployed at an intermediate water depth of 70 m under diverse swell and wind-wave conditions. Fully coupled aero-hydro-servo-mooring simulations are performed in OrcaFlex by utilizing the Ochi-Hubble bimodal spectrum to capture double-peaked sea states. The findings demonstrate that swell-dominated spectra trigger heave resonance, enhancing heave and pitch oscillations, power cable curvature, and touchdown point (TDP) fatigue, whereas wind-dominated spectra govern surge motion and upstream mooring tension. Under parked conditions, increasing wave periods elevate dynamic responses in swell-dominated environments, but reduce responses in wind-dominated environments. Under operating conditions, wave period sensitivity remains minimal. These findings advance coupled FOWT dynamics under swell conditions, providing a benchmark for future real-sea validation and robust design guidelines.