Experimental Study On The Mechanics Of Wave Force On The Floating Rope Cage

Deep-sea sea cage aquaculture is developing rapidly in China in recent years. In order to develop various potential of intensification, security and sustainability of cage aquaculture and to produce social, economic and ecological benefits, government at all levels, fisheries production units and scientific institutions attach importance to research on the technology and application development of sea cage, acquiring a great research achievement and technology extension value. Floating rope cage, like the other cages, is gradually prominent in fisheries. The floating rope cages, which have the advantages of storm-resistance over other sea cages, are responded by hydrological factors such as wave and current conditions. Its security and productiveness are correlated with the mechanics of ones under the ocean field conditions, so it is very important to research on its hydrodynamic characteristics and analyze its mechanism of storm-resistance.Two models of the floating rope cage (1m×1m×1m), named cage A and cage C, were designed according to the real dimension of deep-sea cage in the method of pre-tensioned orthogonal mooring. This experiment on hydrodynamic model was carried out to study on the law of variation of the wave force on the mooring lines and ropes of the floating rope cage, in the flume in University of Tokyo, Japan. The regular waves are generated in the experiment, with wave periods varying from 0.8 to 2.0s (0.2s interval) and the wave heights from 50 to 250mm (40mm interval).The conclusions are as follows:Firstly, the wave force on the mooring lines change periodically and asymmetrically, which is similar to the experimental waves, and a small part of wave force of mooring lines is related to the mooring location and the direction of wave.Secondly, the wave force is related to the following parameters: wave heights and wave periods et. At the same period, the max wave force basically grows with the increasing of wave heights, while as to the wave periods, this correspondence is relatively complex, and it is mainly related to the dimension of the cage.Thirdly, the wave heights decrease when they pass by the sea cages. The max losing rate is 19.94% and 17.75% respectively when they pass by cage A and cage C, both of which are below 20%.Fourthly, according to Fourier transform and Fourier integration, the MATLAB was complied to calculate and analyze the ingredients of the wave force. A conclusion on the ingredient of wave force of sea cage can be drawn, which is that wave force on the cage contains drift force and multiple exciting forces. Thereby the variation law of wave force was simulated and calculated,and the simulation results coincided with the experiment relatively.At the same time, when analyzed the wave force on the ropes in frame, it was found that:the peak values of the two ropes which are perpendicular to wave direction are smaller than that of the two ropes which are parallel to wave direction in cage A,and there are difference in cage C. The changes of the wave force of the ropes are not synchronously, with some phase different. However, the frames are made of flexible ropes, so they can take twisting moment and shearing action by wave force of various phases different, which proves the fact that HDPE sea cages are easily to be destroyed while floating rope cages are relatively safe.Finally, the experimental investigation on the hydrokinetics under combined action of wave and current were carried out. The cage will get a increasing impact force, then with the current or increase of flow velocity, hydrodynamic forces of some mooring lines or ropes in frame increase apparently, and the drift force, one of ingredients of hydrokinetics, increase largely, while hydrodynamic forces of some mooring lines or ropes in frame decrease, largely due to the former tension rope which is in an opposite direction becomes slack in currents.