Nonlinear motion simulations of a combined wind and wave energy converter concept in survival conditions considering water-entry and exit phenomena

A spar torus combination (STC) concept is an energy converter concept for both wind and wave. A torus shaped wave energy converter (WEC) is installed on a spar floating wind turbine to absorb wave power. Model tests were performed on the survivability of the concept with the focus on different survival modes, and water entry and exit phenomena of the WEC were observed for one survival mode during the tests under extreme sea conditions. A hybrid numerical solver is proposed to simulate the dynamic responses of the STC in extreme sea states conditions involving water entry and exit of the WEC, as well as water on deck phenomena. The method combines a weakly nonlinear potential-flow solution for the wave-body interactions, a local Wagner-type model for the bottom slamming phenomenon and a shallow water approximation for water on deck events. The viscous damping loads acting on the different component of the STC system are modelled using empirical formulas based on the involved damping mechanisms and the local Keulegan-Carpenter number. Comparisons between the numerical predictions and model test results in mild and steep regular waves are presented in this paper, and show reasonable agreement for the induced body motions and occurrence of water shipping and slamming events. The hybrid solver is then used to investigate the effects of green-water and slamming loads on the motions of the platform, showing important influence of the former.