可変深度型生簀の係留システムに関する研究

－水槽模型実験による解析－

抄録

A submergible fish cage will be useful for avoiding the damage from high waves, attack of polluted water, etc. The current submergible fish cages have the framework for the base of flotation and submersion in the mooring system. However, the vertical position of the fish cage is fixed at certain depth. In the present paper, a fish cage installed at variable depths was proposed for fish to be cultured in suitable environment. The fish cage is moored by four ropes from anchors, and a float is inserted between the fish cage and each anchor. The vertical position of the fish cage can be varied by adjusting the buoyancy of floats. In order to examine the safety of the fish cage, the drag of the fish cage was examined by a towing test in a small water tank, and the mooring system of the fish cage installed at variable depths was tested under various current velocities in Ocean Engineering Basin, Institute of Industrial Science, The University of Tokyo. The model of the fish cage was made according to the similarity law. The scale ratio was 1/100, assuming the actual fish cage for farming tuna with the diameter of 50 m and the depth of 20m. As a result, the drag of the fish cage was proportional to the 1.8th power of the current velocity, which is smaller than 2nd power. This will be attributed to the effects of Reynolds number, which was less than 80 based on the diameter of the twine. In the mooring experiments, the tension of the rope between the float and the anchor increased with increasing current velocity more rapidly than the 1.8th power of the current velocity. The inclination of the fish cage is likely to increase the area which face water current directly. The position of the downstream float went up at the beginning, and then went down with increasing current velocity since the downstream float was pulled by the sinking fish cage. However, the fish cage did not touch to water surface or water bottom in a range of the current experimental conditions. These experimental results provide the benchmark for the validation of a numerical model, which will be developed to design the mooring system as future works.