Fish farm operators worldwide are planning to move offshore due to lack of available nearshore production sites in heavily utilized coastal zones, where there is increasing community opposition to coastal development and conflict with other usages such as shipping, fishing, tourism, conservation and recreation. Moreover, offshore sites provide more sea space and generally better water quality, which are needed to increase the production of healthy fish. This review paper begins with the definition of offshore for fish farming based on a unified viewpoint and proceeds to highlight the challenges faced by going offshore. Next, the paper presents a review of designs of fish cages from conventional nearshore fish farms to nextgeneration offshore fish farms, which have to contend with a high energy environment. The fish cages may be divided into the open net cage system and the closed containment tank system. The open net cage system can be categorized further into 5 types. The advantages and disadvantages of the various fish cage designs will be discussed. Further, different types of cage designs are compared with the view to guide feasibility of offshore fish farming. Colocation with other synergetic industries is discussed as a possible example of future offshore fish farms.

This paper is concerned with the hydrodynamic response of a novel offshore fish farm that combines a floating spar wind turbine and a fish cage (named as COSPAR for brevity). The open net steel cage is octagonal in shape with a partially porous wave fence at its top end to attenuate wave energy for a calm fish farming environment as well as to keep predators out. The deep draught spar is made from concrete for its bottom half and from steel for its top half. The spar carries a control unit and a 1[Formula: see text]MW wind turbine that provides the required power to operate the offshore salmon fish farm. The COSPAR fish cage has four catenary chains as mooring lines attached to mid length of the spar (outside the fish cage) so as to mitigate tension force in the mooring lines and to reduce the benthic footprint. ANSYS Design Modeler and Aqwa are used to perform the hydrodynamic response analysis of free-floating condition of COSPAR in the frequency domain and coupled analysis involving COSPAR and the mooring lines in the frequency domain and time domain. Environmental conditions, representing 5-year, 20-year and 50-year wave return periods with a constant current flow at an exposed fish farming site in Storm Bay of Tasmania, Australia, are adopted for the analyses. A comparison study is made against having a floating fish cage only (i.e. without the bottom half concrete of the spar) with four catenary chains attached to side vertical columns of the cage so that the fish cage behaves like a semi-submersible cage. Based on the comparison study, the COSPAR fish cage shows enhanced hydrodynamic responses in the following respects: (1) more stable motion responses in heave and pitch against wave and current forces, (2) less susceptible to the viscous damping when it is assumed by a linearized drag force of Morison elements in the frequency domain and (3) reduction of tension forces in the mooring lines. Interestingly, the pitch motion response of COSPAR fish cage in the frequency domain is in close agreement with the time domain result due to its greater pitching stiffness that reduces nonlinear effects from viscous drag and mooring interaction.

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Presented herein is a design concept for an offshore fish farm that combines a spar platform and a partially porous wall fish cage, named COSPAR fish cage. The design features a cylindrical cage with an open net for its bottom portion and a partially porous wall (with specially designed internal section to attenuate strong surface waves) for its top portion. A hybrid concrete and steel, deep draft spar platform carries a wind turbine and a control unit for power supply for remotely controlled fish farming operations. The central spar is connected to the top and bottom of the cylindrical fish cage by frame structures. The top frames are also used as walkways to access the central spar and the bottom frames as ballast compartments for adjusting the draft. By combining fish cage and spar, the meta centre height can be adjusted to induce a low frequency motion response. Moreover, an inherently unbalanced mooring tension problem of a mega-scale cage alone can be resolved by a 4 mooring line system attached to the central spar just below the bottom of the cage. This novel concept of a combined spar and fish cage design is able to overcome challenges of a high energy environment, lack of power supply, and stable mooring system for an offshore fish farm.

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