Model Experiment of a Controllable Depth Cage and its Mooring System

A wide variety of submergible cages has been developed mainly for aquaculture in an exposed sea. However, a submergible cage generally positions only at the surface or certain depth. The vertical position of the cage cannot be controlled. Hence, a fish cage installed at various depths (hereafter called controllable depth cage; CDC) is one of the promising cages to cope with environmental variations such as algal blooming, hypoxic water, high waves, etc. CDC consists of a cage, floats and anchors, which are connected by ropes in this order. The vertical position of the cage is controllable by adjusting the buoyancy of floats. In the present study, the effects of waves on motion of CDC were tested in smaller and larger water tanks. A cage model with the scale ratio of 1/100 was made and installed with its mooring system. From the video image of this experiment, the motion of the cage was reduced when the position of the cage was deeper in the same wave condition. The model with the scale ratio of 1/25 was made and installed with the mooring system. The motion of CDC and the tension on the mooring ropes were measured by a motion capture system and load cells, respectively. The range of motion and the tension on the mooring ropes of CDC increased with increasing wave height. When the wave length was around the two times of the size of CDC, the tension of the mooring ropes increased. Under the same wave condition, the displacement and the inclination of the cage, and the tension of the mooring ropes decreased by about half when the cage was installed at 0.6 m below the water surface. However, the effects of wave height and wave period on the motion of CDC and the tension of the mooring ropes could not be analyzed enough, so further analysis will be required in the future.