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 harsh natural condition such as typhoon attack. Actually, in the current aquaculture system, the offshore fish cages are submerged every time around 10m below the sea surface to escape from high waves and strong currents. However, the safety of the sea cage against the incident wave has not been examined, while that against the water current has been analyzed by tank model test and numerical simulation. In the present study, therefore, the hydrodynamic forces on the submerged sea cage from the incident wave were investigated by the tank model test, and the motion of the submerged sea cage was observed. As a result of the forced oscillation test, the numerical values of the added mass and damping coefficient of the sea cage scatter according to the wave amplitude for the same frequency of the forced oscillation. The difference in the added mass may be attributed to the effects of ladder and the bending of the net. The current analysis is focused on modeling of the bending of the net. Then the difference was also found in the damping coefficient at the same frequency of the forced oscillation. The damping coefficient based on viscosity was separated by the equivalent linearization using the equation of Morison’s drag force. As a result, the scattering of the damping coefficient was reduced, adjusting the drag coefficient. It is a future study to examine the relationship between each mode of the motion of the sea cage and the adjusted drag coefficient. As for the experiment of the mooring system, the larger mooring force worked on the rope between the cushion float and anchor than that between the cushion float and frame rope of the sea cage. The motion of the submerged sea cage was suppressed so that the mooring force between the cushion float and frame rope was reduced. However, the mooring force depends on the initial tension of the mooring rope, therefore the relationship between the initial tension and the mooring force should be examined as future studies.

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