Nonlinear hydrodynamic analysis and optimization of oscillating wave surge converters under irregular waves

To evaluate the mean annual capture width ratio (CWR) of the oscillating wave surge converters (OWSCs) under irregular waves, the time-domain dynamic equation was derived by considering the nonlinear hydrostatic stiffness, drag, and nonlinear power take-off (PTO) system. A Python code was developed to solve the response and capture performance with 4th-order Runge-Kutta integration. The wave spectrum was corrected according to the water depth in shallow water. Unlike the inflexible drag coefficients in steady flow, the drag coefficients in irregular waves (calibrated with OpenFOAM) are strongly affected by OWSC thickness. Multi-objective genetic algorithm (MOGA) optimization of OWSC geometric sizes, internal water filling, and PTO parameters was conducted for two objective functions: maximizing the mean annual CWR and minimizing the structural mass per unit width. The optimized result presents a slender OWSC that neutrally balances these two objectives. The effects and local sensitivities of the width, thickness, axis depth, water filling, PTO stiffness, damping, and friction were comprehensively discussed. The results show that the axis depth has the greatest positive influence on the CWR, and the increase of thickness creates significant economical disadvantage due to a heavier structure. However, inertia adjustment by filling water does not benefit the mean annual CWR.