Numerical Simulation Of The Fishing-net Gravity Cages Group System On Anti-current Characteristics

With the decline and aging of the traditional fisheries resources in the world, the fishery is gradually changed from fishing industry to aquiculture industry, from exploiture to management. So the culture in net cage develops rapidly and is becoming a new economical increasing point of the ocean fishery.Even so, however, there still exists a disadvantage that the aquiculture areas are mostly in the bay where the culture in net cage is put in the shallow sea. This usually leads to the deterioration of aquiculture environment, which subsequently causes the decline of the aquiculture quality. Therefore, the best way to the further development of the fishery is to develop the deep-sea fish cages. However, these kinds of cages, which are put in the open ocean areas, are vulnerable to the strong sea waves and currents. So the key point of further development of the fishery is to develop the anti-current and anti-storm fish cages system.Firstly, this paper, which deals with the numerical simulation of the various dynamical responses of the fish cages system( formed in the fishing-net gravity cages) to the different circumstances(such as the flow velocities, the flow directions, the depth of the float-line frame, the interval between the fish cages, the length of the mooring lines, and the anchor types) , offer a best theoretic help to design and apply the fish cages in various practical ocean environments and operation conditions. The numerical simulation, which is based on the finite element method in sea current and whose results are worked out by means of Newton-Raphson methods and can be represented by 3D computer technology, is proved to be most practical and stable.Secondly, the Grouped Mesh method, which is used to simplify the numerical stimulation, can reduce the operational time. We all know that the accuracy is very important to stimulation, while it depends on the number of the elements--- that is to say, the more number the elements have, the more accurate the stimulation is. And the elements number is restrained by the speed of the operational calculation. So in order to speed up the numerical simulation, the Grouped Mesh method is used.Thirdly, considering the various characteristics of the stiffness matrix, such as symmetry, strangeness, sparseness and so on, Matrix Partitioning method and the direct solution, which can calculate the linear algebra equations, are used to decrease the elements number, and consequently save the EMS memory and reduce the operational time, because in the process of calculation, the large elements number are apt to enlarge the stiffness matrix and cause the instability of the arithmetic.Fourthly, the simulation results show that the mooring type is very important for the cage system safety. Using the"井"and"米"style, the No.11 mooring line in the current have the highest tension and therefore tend to break up, so we should be cautious about the materials. There are some other important points that we should pay much attention to. The deeper the underwater float-line frame are, the less tension the mooring lines take. The larger the interval between the cages are, the higher tension the float-line frame take. The proper depth of the mooring lines are essential to the safety of the fish cages system. However, the longer mooring lines have little effect on the tension of them. In the"井"style, the sideways mooring lines take a higher tension but can contribute to the good shape of the whole system. In the"米"style, the mooring lines can be saved much but the float-line frame that are against the current tend to change their shapes. The final simulation results show that the effect of the to-and-pro current is the most important point to be considered in the safety of the cages system.Fifthly, the simulation results show that the best way to enhance the anti-current characteristics of the fish cages system is to set the float-line frame at the place 50 percent in height of the fish cages, the interval between the fish cages a half of the length of its diameter, and the mooring lines 2.5 times the depth of the sea. At the same time, the mooring type can be various according to our need.