Investigation of multi-DOF WEC S-shaped buoy configurations in the nearshore region of East China

This study proposes a multi-DOF WEC employing S-shaped buoy profiles and a mechanically integrated PTO system tuned for site-specific nearshore wave conditions. The system uses heave and coupled surge/pitch motions, while the proposed PTO mechanism mechanically synthesizes the corresponding modal responses into a unified transmission path and represents their damping contributions through a single equivalent PTO damping coefficient. The hydrodynamic responses were calculated using ANSYS AQWA, whereas the PTO dynamics and absorbed power have been evaluated in Matlab/Simscape. The natural multi-mode periods have been tuned to align the system response with the most frequently occurring peak-period interval at the selected site. Site-specific performance has been subsequently determined using the normalized joint probability distribution to account for the complete location wave distribution, including the high-probability design range and all off-resonance sea states. The results demonstrate that all multi-DOF configurations outperform their corresponding single-DOF heave only models, although the improvement is strongly dependent on buoy geometry since the normalized occurrence weighted mean absorbed-power enhancement ranges from 14.8 to 40.6%. The optimum configuration achieved a mean absorbed power of 4.95 kW and a mean capture width ratio of 23.14%, compared with 3.52 kW and 16.50%, respectively, for its single-DOF counterpart. The findings demonstrate that combining site-specific hydrodynamic tuning, buoy-shape optimization, and multi-mode PTO integration can substantially improve wave energy capture over realistic operating conditions.