Model tests and numerical simulations on the parametric resonance of the deep draft semi-submersible under regular waves

Parametric resonance of the deep draft semi-submersible (DDS) under regular waves are investigated experimentally and numerically. Special focus is put on the occurrence of parametric resonance under weak or failure mooring condition. Firstly, model tests with and without mooring chains are conducted in wave flume. From results of model without mooring chains, it is observed that parametric resonance occurs when wave period is about the natural period of heave and half of the natural period of pitch or roll. The steady amplitude of parametric resonance is larger under higher wave. In heading waves, parametric resonance occurs in roll direction, while parametric resonance occurs in both roll and pitch direction for beam wave. For the model with 12 mooring chains, parametric resonance does not occur under strong mooring restriction. Whereas for the model with four mooring chains representing a failure mooring condition, parametric resonance occurs in pitch direction. Then, numerical simulations under cases same as experiments are conducted using a potential-theory based weakly nonlinear model. Parametric resonance is successfully reproduced numerically. Furthermore, through comparison with experiments, it is found that the numerical model accurately predicts steady amplitudes and occurring wave periods of parametric resonance except for cases with four mooring chains.