Two-stage stiffness–damping tuning of a passive PTO for a heaving point absorber in irregular seas

This study investigates an S-shaped point absorber floater with a passive spring–damper power take-off (PTO) under JONSWAP irregular-wave conditions using a three-dimensional CFD numerical wave tank coupled with a single-degree-of-freedom heave model. A two-stage passive tuning framework is proposed: stiffness is first adjusted to modify the effective response band, and damping is then tuned to balance energy extraction and motion suppression. The numerical model reproduces the target wave heights and spectral energy distributions with good accuracy. Results show that, under fixed baseline PTO parameters, the mean absorbed power differs markedly among sea states because of differences in spectral matching between the dominant wave band and the device response band. Within the tested parameter range, the mean absorbed power obtained after two-stage tuning increases by 5.4%–53.8% for the five representative sea states. The results indicate that coordinated passive tuning of stiffness and damping can improve irregular-wave energy capture in a sea-state-dependent manner.