With the increasing world demand for seafood and environmental problems in coastal aquaculture, offshore area has been increasingly expected to be utilized for aquaculture. The offshore aquaculture system has fewer effects on the surrounding marine environment through the rapid diffusion of organic wastes from the cultured fish than the coastal one. The offshore area then provides clean waters for cultured fish. On the other hand, the offshore aquaculture system is subject to the severe natural condition such as typhoon attack. Actually, in the current aquaculture system, the offshore fish cages are always submerged around 10m below the sea surface to avoid the effects of high waves and strong currents. However, the safety of the sea cage against the incident wave has seldom been examined, while that against the water current has been analyzed by model tests in tank and numerical simulation. In the present study, therefore, we investigated hydrodynamic property of a heaving sea cage as the first step. Forced oscillation tests and wave exciting force tests have been carried out, and numerical modeling have also been made to estimate hydrodynamic characteristics theoretically and to estimate the drag and mass coefficients. Results of the forced oscillation tests show that the added mass and damping coefficient of sea cage models depend on forced oscillation amplitude. This dependence may be mainly attributed to the deformation of net, and we successfully reduce the dependence on the forced oscillation amplitude in the result of reanalysis which takes into account the deformation of net. Results of measured wave exciting force show that wave exciting forces are not linear in wave amplitude. This may be due to the viscous drag effects as well as the deformation of net. On the other hand, we calculated the flow around a sea cage. This is based on velocity potential and supplemented the effect of viscosity by equivalent linearization. In the boundary condition on the sea cage surface, the vertical velocity to the surface is not equal to zero, but determined by the permeate coefficient of the surface. This permeate coefficient is a function of wave amplitude, wave period and porosity of the net. In the future works, deformation of net should be also taken into account in this calculation.

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