Subharmonic excitation and nonlinear coupling in submerged wave energy converters

As power systems integrate higher shares of renewable energy, wave energy is increasingly viewed as a complementary and predictable resource. However, efficient energy extraction remains challenging, particularly for submerged wave energy converters (WECs) as wave energy density decays exponentially with depth. Many submerged devices are multi-modal, harvesting energy across multiple degrees of freedom (DoFs), but inter-modal coupling can introduce nonlinear dynamics that degrade performance. This study experimentally investigates how nonlinear excitation in a submerged WEC varies with wave period and influences power production. Results show strong nonlinear behavior, with pronounced subharmonic excitation in surge and pitch, particularly at short wave periods and near heave resonance. To address this, a partially nonlinear state-dependent hydrodynamic model (SDHM) is developed, incorporating large-amplitude motion effects and coupled PTO–mooring dynamics. This is combined with commensurate natural frequency analysis (Heave T n ≈ 15 s , surge T n ≈ 7.8 s , and pitch T n ≈ 2.5 s ), which drives the period-dependent DoF-pair coupling between DoFs. Together they provide a mid-fidelity framework to assess the design space for the severity and impact of nonlinear coupling on device performance.