A fish cage should be submerged to reduce hydrodynamic forces from high waves if the fish cage is installed in an exposed sea area. Usually, the submergible fish cage is suspended from the framework at a fixed depth. The framework is set by floats and anchors at the middle position between water surface and the top surface of the submergible fish cage. The submergible fish cage will be used not only for reduction of hydrodynamic forces but for the other purposes such as choosing the best environment for cultured fishes in the vertical direction, and escaping from the flood with high-level nitrogen or turbidity, harmful algal blooming, and floating ices. In such cases, it is useful for the fish cage to be installed in variable depths. The purpose of the present study is to examine the safety of the fish cage installed in variable depths in current and waves by means of tank model testing. The mooring system consists of a fish cage and four floats. The vertical position of the fish cage is variable by adjusting the buoyancy of these floats. First, the drag of the fish cage was examined by towing test, and the results were compared with the drag estimated by the existing studies. The effects of interaction among twines, the angle of attack, wake, and the top and bottom nets were discussed. Then the fish cage was moored in the water tank, which has the length of 50 m and the width of 10 m. The tank model has a scale of 1/100 of the full-scale model of the fish cage used for tuna farming. The model was made according to Tauti’s similarity law. The water depth was set at 0.68 m by adjusting the position of the variable floor. The motion of the fish cage and four floats, and the tension of the mooring lines between the fish cage, floats, and anchors were measured by the underwater video camera and load cells, respectively. As a result, the drag of the fish cage could be estimated from the experimental results of the drag of a plane net since the results include the effect of interaction among twines. The effects of the angle of attack and the reduction in water current velocity inside the cage were also taken into account. The drag of the fish cage could be estimated well by the above method, while it was underestimated by 10% in comparison with the experimental data. In the water tank testing of the mooring system, the tension of the mooring line increased rapidly with the increase in water current velocity since the drag of the fish cage was proportional to the 1.8th power of water current velocity and increased due to the inclination of the fish cage. The increase in the tension due to wave-induced forces to the fish cage could be negligible when the fish cage was submerged. The safety and the design guideline of the mooring system should be assessed by the simulations using a numerical model, which is being developed by the authors. The experimental data obtained in the present study will be useful for the validation of the numerical model.
Development of Wrapped-Tank Condenser Model (Round Tube and Microchannel), Coupled with Water Tank Model