Main expected result: Create and improve existing cage systems, feeding systems, nets and mooring as one submersible unit in order to be used offshore.
Background A detailed survey of the status of current submersible cage technologies has been undertaken. Several different technological solutions for submersible cages exist but none have been particularly successful, mainly due to poor control of the submerging and lifting processes, insufficient cage volumes for commercial culture and poor integration between all the necessary farm components. Most of these existing solutions have been trialed and abandoned with the result that farmers still have to use conventional surface-based cages. The survey also identified the key technological requirements for the success of a submersible cage system. These can be classified in three parts:
1) Nets: Depth, twine and mesh size, requirements for access, submerged and in surface position, positioning of the net roof in surface position, system to maintain the net volume, cleaning system2) Environmental information: Wind, current, waves3) Choice of mooring solutions: Limitation of area, depth of seabed, method of submerging, type of mooring, single or in a cage-frame system
MethodsThe SubCage project is being developed in two phases: 1) development of a system for short-term submergence at offshore sites to avoid bad weather, poor temperatures or other suboptimal conditions e.g. a system that can be submerged for 1 to 5 days without the need for specialised feeding and surveillance systems; this will done using conventional cages and development of a simple yet controllable system to sink the cage; and 2) development of a next generation submersible cage that satisfies all design requirements listed above.
Results and discussion Development of a system to sink conventional cages: This system will be used when bad weather, poor surface temperatures or other conditions negative to production in surface waters occur at a farm. A detailed experimental study in the SINTEF FISHERIES AND AQUACULTURE Hirtshals flume tank using a scale model of a conventional commercial cage has been undertaken. The Egersund group has delivered the nets used and cage has been designed by AKVA group. In parallel a simulation tool has been developed to reproduce the behaviour of the submerged cage.
Experiments: A cage sinking and lifting concept was tested in the Hirtshals lab (Figure 1). Different cage positions were obtained by moving the loads (black dots) along the mooring lines and the behaviour of the cage was studied by increasing the flow velocity. Stresses were measured along each line and cage deformations due to currents were recorded with a 3D camera system.
The combination of the load positions and the current flow velocity produced different strains in the anchoring lines in the front and back of the cage and resulted in different angles of rotation of the cage (Figure 2)
Figure 1. Concept of the system to sink conventional cages: a) floating and b) submerged cage.
FIgure 2. Example of submerged cage behaviour in the flume tank.
Simulation tool: Mathematical models of three different submersible cage concepts are being develop to allow simulations to performing stability analyses of the models. All models are fully dynamic and in 3D, based on a common basic model of the cage, but with different concepts for the submergence process. These three concepts are: 1) movable point mass on the (only) top bridle line; 2) fixed mass on the bottom bridle line at various positions, connected to the bottom ring; and 3) variable buoyancy devices attached to the bottom ring. This model is still under development.
Next generation submersible cage concepts: Phase two of the project will explore alternatives for next generation cages, with a focus on fish welfare. The concept aims to address the critical aspects for cage performance and innovate new solutions. Ideas and concepts are under development. The final concept is taking shape and is expected to be ready for testing before the mid-2009.
Published July 8, 2009
Alf Kristian Fjelldal (AKVA group ASA)
AKVA GroupEgersund Net ASErling Haug ASSINTEF Fisheries and Aquaculture