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Floating breakwaters with a mooring system have been widely applied to protect marine infrastructures (e.g., artificial beach or island, aquaculture farm or marine vessels in harbors) from being destroyed by severe waves. In this paper, an innovative cylindrical dual pontoon-net floating breakwater was developed to enhance the wave attenuation capacity. This dual-module floating breakwater system was constructed as the prototype for on-site testing. A fully nonlinear time-domain model based on the coupled iterative solutions of the fluid integral equation and the pontoon-net dynamic equations was proposed to simulate the interactions between waves and the floating breakwater system. The flow field around the nets was simulated by introducing a porous-media model with Darcy’s law, while the deformation of the flexible nets was solved by using the lumped mass model. The instantaneous free surface was captured using the mixed Eulerian-Lagrangian (MEL) approach which employs an improved moving-grid technique based on the spring analysis to re-mesh the instantaneous water surface and the body wetted surface. On-site tests were also conducted to evaluate wave transmission performance of the floating breakwater system and to validate the numerical model. The comparisons show that the numerical solutions are in good agreement with the measured data. The effects of incident wave direction, wave period, wave height, net height, net number and net porosity on the hydrodynamic performance of the floating breakwater system were emphatically examined.

Freak waves have great peak energy, short duration, great contingency, and strong nonlinear characteristics, and can cause severe damage to ships and marine structures. In this study, numerical simulations in conjunction with experimental tests are applied to study air gap response and wave slamming loads of a semi-submersible offshore platform under a freak wave. A three-dimensional wave tank, which is created based on the computational fluid dynamics (CFD) method, is applied to study the hydrodynamic responses of a semi-submersible platform. The numerical model of the tank and offshore platform system are checked according to the experimental results. A typical freak wave is modelled in numerical wave tanks by the linear superposition method, and its significant wave height is 13.03 m. It is found that the freak wave is closely associated with the wave slamming. The appearance of the freak wave gives rise to a negative air, gap which appears on the side of the back wave surface at the bottom of the deck box, and considerable slamming pressure is generated. Furthermore, the wave run up at the junction of the column and the buoyancy tank is also seen due to the freak wave.