Effects of a Moonpool on Steady Wave Drifting Forces on a Floating Pontoon

A pontoon-type floating structure with a moonpool has been proposed as the base floating structure for floating offshore wind turbine systems. By utilising the resonance phenomenon of the water mass in the moonpool, it is expected to reduce wave forcing and motion response. On the other hand, there is little discussion on wave drift forces, which are nonlinear wave forces important for mooring design. In particular, the potential theory, which neglects viscosity, may overestimate the resonance of the water mass of the moonpool, and its behaviour and wave drift force characteristics should be well informed. Therefore, this study was undertaken to investigate the relationship between the steady wave drift force and the behaviour of the water surface in a moonpool on a pontoon-type floating structure with a single moonpool. A hybrid boundary element method, in which the boundary integral equations are discretized by second-order isoparametric elements, was applied to calculate the wave drift forces. The wave drift forces were calculated from the perturbation expanded second-order nonlinear differential frequency component wave forces by the direct pressure-integral near-field method. The influence of the water surface behaviour of the moonpool on the steady wave drift forces is discussed from the calculation results. The frequency bands in which the wave drift force is increased and those in which it is drastically reduced by the moonpool compared to the wave drift force acting on a typical pontoon. It was shown that the wave run-up term had a significant influence among the terms that make up the wave drift force. To confirm this, the state of the water surface fluctuations around the object was investigated, and it was shown that the water level fluctuations around the floating object changed in a complex manner due to the moonpool. These results indicate that a more accurate understanding of the behaviour of the water surface in the moonpool is needed.