A Physical Model Approach to Mooring Forces and Motion Responses of a Closed Fish Tank Including Internal Free Water

Abstract

Abstract In marine aquaculture farming, currently used flexible open-net cage systems face many problems and challenges, such as sea lice and other parasites, fish escape, and water pollution. The development of closed fish cage systems for fish farming has been driven by such problems. Closed fish cage systems interact with sea forces when subjected to waves and currents. The wave-induced motions of the closed fish cage would result in sloshing problems, i.e., internal fluid oscillations. Knowledge and experience on the internal sloshing issue are very limited, as closed fish cages are still a novel concept. In the present study, a series of physical model tests were performed to investigate the mooring forces and motion response characteristics of a circular closed fish cage. A decay test in calm water and regular wave experiments were performed for the closed fish cage in a wave basin. In addition, we set three different netting panels with different mesh sizes in the closed fish cage and investigated their inhibitory effects on internal sloshing flow under different configuration conditions. The results indicated that the general trend of maximum mooring forces was the same in both upstream and downstream mooring lines: first increased, then decreased, and finally increased again as the wave period continues to increase. The global motion responses of the fish cage system in the surge (pitch) directions exhibited similar trends with mooring forces. When the incident wave periods closed to the first mode of the sloshing period, the surge (pitch) response was almost zero. For internal water sloshing, it clearly showed that the second or higher harmonic components cannot be negligible, but had little effect on the global motion of the cage. Furthermore, the inhibition effect of the netting panels on the free water surface oscillation was confirmed, especially when the netting panel was horizontally placed on the inner free water surface, the most obvious inhibitory effect could be observed. Overall, these findings are expected to provide references for the design and operation of the closed fish cage system.