TANK MODEL TESTING OF A FISH-CAGE FLOTATION/SUBMERSION SYSTEM USING FLEXIBLE HOSES

摘要

A fish-cage flotation/submersion system using flexible hoses is proposed to achieve horizontally stable floating and sinking motions. Waterproof flexible hoses are inserted into polyethylene pipes installed at the top of the frame of the fish cage. These hoses flatten when they are devoid of air and water as the fish cage is submerged. The injection of high-pressure air regenerates buoyancy and enables the fish cage to rise in the water. The advantage of this system is the uniform, circumferential generation of buoyancy at the top of the frame, which suppresses inclination of the fish cage and concomitant deformation of the flexible chemical fiber nets. Tank model testing was carried out to examine the inclination and the floating velocity of the fish cage. Tauchi's similarity law was applied to make a 1/30 model of the full-scale fish cage. The tank model of the fish cage was installed in the ocean engineering basin at the University of Tokyo, and it was made to float and sink in water with and without currents. The inclination and position of the fish cage were measured using video camera images. As a result, the proposed fish cage was observed to float stably in still water in contrast to systems based on the existing method. When subjected to water currents, the new fish cage inclined by a maximum of 18° just after leaving the bottom; the inclination was reduced with further ascension. The ratio of buoyancy to gravity, the rate of air injection, and the arrangement of the flexible hoses should be optimized to achieve a more stable motion. The floating velocity for the rising fish cage in still water was then analyzed. The drag coefficient of the fish cage, as calculated from experimental data, corresponded to that estimated from a structural analysis of the fish cage. Analysis projected accelerated motion for 0.02 s after the fish cage rose from the bottom, while acceleration lasted a few seconds in the tank model test. This is because uniformly accelerated motion was assumed in the analysis, while the acceleration actually varies from zero to a constant acceleration, because of the difference between gravity and the varying buoyancy of the flexible hoses.