Numerical and Experimental Study on the Seakeeping Behavior of Floating Closed Rigid Fish Cages

Abstract

The present paper addresses the seakeeping behavior of a rigid type of floating closed fish cages, with focus on effects of sloshing on the coupled motions and mooring loads. Closed cages have gained much attention recently as a strategy to avoid sea-lice infections in farming of Atlantic salmon. However, closed cages are novel structures and more knowledge is needed on the wave induced motions and coupling effects with sloshing for development of reliable closed cage structures to reduce risk for failure and possible escape of fish. In this paper, dedicated scaled model tests of closed cages in waves are presented and compared with numerical simulations using linear potential theory in frequency domain. The results shows that the influence of sloshing on the rigid body motion is significant. A nonlinear effect of sloshing was observed for a small region of excitation frequencies, where the surge amplitude increased with increasing wave steepness. Mean wave loads were also affected by sloshing. Hence, coupled motions with sloshing is important to consider in design of floating closed rigid fish cages and their mooring system.

Review of cage and containment tank designs for offshore fish farming

“…In a tank without any internal structures, as is the case for most closed aquaculture cages, very little damping resistance to the sloshing, and therefore large responses may occur if the cage is excited with frequencies close to the natural period. The sloshing motion is potentially bad for the fish welfare, and it can also significantly affect the horizontal plane motion of the whole cage, as well as cause stresses in the cage wall (Kristiansen et al, 2018). Therefore, while a conventional flexible net-based cage has few resonance problems, a closed cage has significant challenges with resonance, especially connected to sloshing.…”

Opportunities for floating closed containment systems for fish farming

“…A contained water body with a free surface, like a FCCS, will experience sloshing effects. 6,13,14 At the free surface, sloshing can be observed as chaotic wave motions which go against the need to create a low energy environment for fish welfare. A common way to reduce sloshing is by installing baffles in the tank.…”

“…Other benefits of utilizing sea space are: increasing land prices due to an increasing world population and the immunity to seismic activities since floating tanks are base isolated in the sea. Different types of structures have been investigated 4 and applied in the industry: flexible cages, 5,6 semi-flexible cages and rigid cages. While flexible cages are made out of tarpaulin, semi-flexible cages are made from low stiffness materials like HPDE (high density polyethylene) or GRP (glass reinforced polymer).…”

Coupled Motion and Sloshing Analysis of a Rigid Cylindrical Closed Fish Cage in Regular Waves

“…For the flexible floating cylindrical cage, the responses of the cage in waves have been studied by [8], followed by experimental studies on the wave induced responses [9], where the drag forces on a half ellipsoidal closed flexible fish cage were modeled. Kristiansen et al [10] carried out model tests of a closed fish cage in regular waves, and compared with linear numerical results in the frequency domain. The results showed that the influence of sloshing on the rigid body motion is significant.…”