Abstract
The rise in seafood demand as well as the shrinking availability of coastal sites, are forcing fish farming in offshore sites. Despite the risks and costs of operating in open waters, several factors advantage offshore farming. Specifically, the reduced environmental impact of aquaculture, the strong currents and wave conditions, as well as the stable temperatures of open sea naturally benefit fish welfare. Although offshore fish farming systems have not yet achieved commercial maturity, apart from few concepts namely Ocean Farm [1]; Arctic Offshore Farm [2]; Havfarm [3], the feasibility of developing offshore fish cage systems has already been investigated by numerous authors. Indicative studies are [4] – [6] to name a few.
The aim of the present manuscript is to present in a systematic way a frequency-domain analysis approach, along with its experimental verification, for the development of an offshore fish cage system, taking into consideration the prevailing environmental conditions on site. Advanced numerical calculations are conducted to simulate the fish cage’s dynamic response under combined wave and current loading conditions. The method represents an effective design tool for the analysis of offshore fish cages at their first stage of development, offering a fast analysis methodology for the investigation of alternative design concepts.
The proposed fish cage system is consisted of a semi-submersible steel floating collar, i.e., ring type pontoon, located at the water free surface, on which the fish net bag is attached to. The fish cage, which is floating in constant water depth, is moored with a four-line catenary mooring system. The problems of the diffraction and radiation of water waves around the system are formulated within the realm of the linear potential theory, whereas proper boundary conditions on the body’s wetted surfaces (i.e., permeable and impermeable) are applied. Furthermore, climate analysis is performed on a candidate installation site located south of the Evia Island, providing the basis for the corresponding design loads on the fish cage system.
The numerical results obtained through the developed solution are supplemented by corresponding experimental ones in a wave tank. Towards this purpose, a 1/50 scaled-down model was constructed and tested under operational and extreme wave and current conditions. The excellent correlation between the numerical and experimental results reveals that the presented numerical formulation can simulate accurately the porous effect of the fish net bag on the fish cage system hydrodynamics.