Experimentally validated simplified prediction model of unloaded spar-buoy wave energy converter motions' responses in waves

The main objective of this study is to develop a fast and validated model to predict the motions' responses of a spar-buoy wave energy converter based on oscillating water column principle. Experiments and former investigations highlighted the strong effect of the spar-buoy heave motion on the performance of the OWC whereas water oscillations slightly affect the spar-buoy motions. Therefore, a single degree of freedom model was adopted to simulate the structure's surge, heave, and pitch responses separately. The prediction of the structure’s heave and pitch responses is a straightforward task and results were experimentally validated and compared to previous studies. Linear and quadratic damping coefficients were introduced for better agreement between measured and predicted pitch responses. In contrast, surge response prediction task was more complicated as it requires the consideration of the mooring system installation properties, while the mooring configuration was not experimentally investigated since the reduced-scale model was floating freely (un-moored). Therefore, the predicted surge response obtained from the proposed model was validated using the finite element method. Two different approaches were used to solve the equation of motion in surge direction considering multi-catenary mooring system. Static approach where constant mooring-lines stiffness was used and a quasi-static approach where reaction curves developed were used to estimate the instant resultant mooring line tension at different offsets. Load excursion and reaction curves of the full-scale device developed by the proposed model were finally validated.