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Offshore salmon fish farming

The aquaculture company SalMar, based on the island of Frøya, has through its development company Ocean Farming, proposed a new design for an ocean salmon fish farm on exposed sites.

Offshore cage model

Until now the standard fish farm designs have been based on flexible rings floating on the sea surface with the net suspended beneath the rings.

Such structures will follow the movements of the waves and the net changes its shape with the current. This has been a source of wear and tear on the sea-cage, and increases the risk of escapes. The production volume is also affected.

The development from conventional flexible fish farms in sheltered waters has been brought a major step forward with the new ocean fish farm design, which is a piece of pioneering work in the aquaculture sector. It is made of steel, and is formed as a circular semisubmersible rig with a diameter of 110 m. The design is based on experience gained in the offshore industry. The net is made up of a fence-like grating that is stretched over the whole volume of the structure up to a height of 50 m. The floatation elements are located at the bottom below six vertical main columns that support the entire structure. The ocean fish farm is moored to the seabed by eight mooring lines, as used on oil platforms. The water-line area is small, which means that the fish farm is relatively unaffected by waves. Its volume is eight times as great as that of a conventional sea-cage.

MARINTEK has performed model tests of the ocean fish farm in the Ocean Basin at Tyholt. It was tested under realistic sea-states with waves, current and wind conditions similar to those experienced at Frohavet Sea. The model tests showed that the structure behaved very well in heavy seas, with significant wave heights of up to 5 m, a sea-state including maximum wave heights of almost 10 m.

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