Dynamic Responses of a Gravity-Type Fish Cage During Lifting Operations in Waves and Current

Abstract Aquaculture has experienced remarkable growth over the past four decades. The utilization of fish cage systems at sea has played a pivotal role in promoting the growth of seafood production. As technology advances, the gravity-type fish cages become more and more popular because of the low capital cost for the industrial-scaled fish farming. The lifting operation of gravity-type fish cages is an important procedure for the harvest and de-lice of the fish. In the present study, the static and dynamic responses of the fish cage before and during the lifting operation are investigated numerically. A Morison model is adopted to calculate the hydrodynamic forces acting on the net ropes of the aquaculture nets and a screen model is used to calculate the hydrodynamic forces acting on the net twines of the aquaculture net. The irregular waves are generated using the JONSWAP wave spectrum. The structure of the aquaculture nets is modelled using an extended position-based dynamics (XPBD) method, which was first used in the cloth dynamics simulations in games. The proposed method is effectively validated against the published experiment data to ensure its accuracy and reliability. The static response of the gravity-type fish cage at various current speeds, and the dynamic responses of the fish cage in irregular waves before and during the lifting operation are analyzed. Results show that in a smooth sea state, the drag force of the fish cage is slightly higher than that in a uniform flow with the same current speed and the fish cage volume sightly decreases, while the deformation of the fish cage is similar. During the lifting operation, the lifting force mainly results from the weight of the sinkers at the bottom of the side net. The maximum tension of the net is located at the net ropes connected to the centre point of the bottom net. Therefore, the structure of the bottom net should be enhanced for safety lifting operations.