Experimental hydrodynamic study of a novel globe-shaped submersible fish cage system

Zheng, Wen Jie (1991) Experimental hydrodynamic study of a novel globe-shaped submersible fish cage system. Masters thesis, Memorial University of Newfoundland.

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Abstract

In order to meet the increasing requirement of farming fish in more exposed offshore sites, a novel globe-shaped submersible fish cage system with two tension mooring lines has been proposed. In normal sea conditions the cage remains on the surface; during storms the cage can be submerged some distance below the water surface to reduce the wave forces and motions of the cage and hence reduce the forces in the mooring system and the stress on the farmed fish. Since the proposed system is a quite new design concept, few theoretical and experimental results can be found to guide the prototype design. -- To have a better understanding of the hydrodynamic performance of the proposed system, qualitative analyses were conducted and a series of resistance tests and moored tests were carried out with two globe-shaped cage models, one spherical and the other geodesic, to study the resistance forces of the cages in currents and the motion and mooring force responses of the cage system in waves. In the resistance tests, three orientations of the cage axle to the current direction were tested. In the moored tests, three submerged positions and three orientations were tested and the effects of pretension and axial stiffness of the mooring lines were investigated in both regular waves and irregular waves. -- The study shows that the relationship between the current velocity and the resultant resistance force can be interpolated very well with a quadratic equation regressed from the test results. Provided the Reynolds numbers of the cage elements are still within the range of subcritical flow regime, the regressed equation can also be used to extrapolate the resistance force outside the Reynolds number range tested. -- The study also shows that the method of submerging the cage below the water surface to reduce the motions of the cage and the forces in the mooring system is very effective in deep water waves, but becomes less and less effective in intermediate water waves. The pretension of the mooring lines has little effect on the dynamic mooring force and motion responses of the cage system as long as the mooring lines do not go slack. The stiffness of the mooring lines has a significant effect on the responses of mooring force, heave and roll, but has little effect on the responses of surge and sway. When the cage is perpendicular to the wavefront, the vertical displacement and mooring force on one side of the cage may be significantly larger than those on the other side because of the phase difference between the heave and roll motions of the cage.

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