A Study on Air-Controlled Floating Breakwater

Abstract

Floating breakwaters are required to protect offshore structures and fish breeding facilities on the coast from the damage due to heavy sea. In order to meet this requirement, a study has been conducted to develop a new breaking-water system of better performance. The new system is based on the principle shown by Bessho, "which utilizes the interaction of the diffracted and radiated waves through a proper contol of the motion of the floating body to attenuate the transmitted wave." The air-controlled floating breakwater shown in this paper adapts a method which, instead of controlling the motion of the floating breakwater itself, regulates the motion of the water mass in the tank of the air bell type by controlling the pneumatic pressure which acts on the free surface of the water mass. Tank tests with small scale models were carried out in order to confirm the effectiveness of the system. In the tests with fixed models, the influence, on the breakwater performance, of dimensionalratios such as the beam draft ratio etc., the opening degree of valves that serve to regulate the air flow, and so on were investigated by varying these parameters. It has been shown through these experiments that the air-controlled floating breakwater can well attenuate the transmitted wave as compared with the pontoon type floating breakwater of the similar size. In the tests with floating models, the tests were carried out by varying the mooring conditions to investigate their effect on the breaking-water performances and to evaluate the tension of the mooring lines. Thus it has been clarified that the tension-leg mooring system has difficulties from a practical point of view because of the too big tension of the mooring lines, and a small transmission coefficient is obtained for incident waves of the considerably long wave-length even in the case of slack moorring. A full-scale field test was carried out on a sea surface off Uwajima City, Ehime Prefecture. Through measurements of the incident and transmitted waves, the wave transmission coefficient for irregular waves was obtained. Also the tension on the mooring chain was measured. These results are reported in this paper.