Examples of cage constructions
Below, examples of designs used for sea cages are shown to give some idea of the dimensions.
Plastic collar cages made either of PE (actually high density polyethylene (HDPE)) or PP are often circular, but may also be made quadrangular and be used as a system farm in less exposed sea areas.
In circular cages it is normal to have two pipes of diameter 200–315 mm. Both may be filled with PS, or one filled with PS while the other is air filled. A wide range of circumferences are available, commonly between 30 and 120 m. Between the two pipes some type of fitting is used, either of plastic or steel, to ensure strength and make a base for the walkway. Some also use a chain all round the circumference to improve the strength. Examples of required buoyancy are from 40 to 120 kg/m depend-ing on the cage dimensions.
Steel cages are constructed with pontoons to ensure buoyancy, while the steel framework gives strength and stretches out the net bag. The steel construction is normally galvanized but can also be painted. Typically there is a 2–3 m wide walkway that eventually can be used by a small forklift truck. Normally the buoyancy is in the range 800–4000 kg/m2, the highest value being for walkways for driving. The walkways around the cages are smaller (up to 1 m wide) and have lower buoyancy of around 500 kg/m2. Between the centre gangway and side-ways there are special movable hinges. The pontoons are normally made of PE infilled with expanded PS.
One type of cage suitable for large waves is made of rubber pipes with a typical exterior size larger than 400 mm. The cages are made as a quadrangle, hexagon or octagon. Steel pipes are used in the corners and to connect the parts made of rubber pipes. The rubber pipes will follow the wave motion very well. The cages are reported to tolerate very rough weather conditions, such as wave heights of up to 8 m.
Ocean spar technology is another technology available for ocean cages, and these have no typical cage collar. In one system vertical cylinders (spars) are placed in each corner of a quadrangular cage bag to stretch it out; the horizontal areas affected by the waves are thus reduced. Another type includes a central spar and rim held together with tension stays. It forms a cube-like construction that is only partly above the water surface which will be dragged below the surface when there is much wave activity or strong currents.