Hydrodynamics Of Net Cage Group In The Open Sea

In the20th century, the main human activity is confined to the land, which takes up one-third of the earth’s area. However, the development of ocean resource has been strengthened recently, due to the population explosion, environmental pollution and shortage of resource. Therefore, it is a new era of ocean economy in21th century. Following the improvement of living standard, the demand of food for fish is increasing. In order to satisfy the demand of food for fish, it is necessary to accelerate the development offish production in the open sea. In recent years, the annual value of the ocean fish production resource has occupied one-third of the total value of marine economy. The traditional development approaches of ocean fish resource include the aquaculture, breeding and fishing. However, the amount of fishing is decreasing due to the ocean pollution and overfishing. It is an important way to develop the ocean fish production by culturing fish in the net cage. Although the aquaculture industry has developed rapidly, the basic research aims at analyzing the hydrodynamic behavior of net cage in the open sea is not adequate. Thus, this research analyzed the hydrodynamic behavior of net cage group and its mooring system in waves and current, with the support of Natural Science Foundation of China.The research scope is described as follows:In the first chapter, the background and significance of this research are introduced, and the advance in the hydrodynamic behavior of net cage is also given. Based on the previous research, the target and content of this study are presented.In the second chapter, the lumped mass method and the rigid body kinematics are used to build the numerical model of net cage and mooring system. In order to validate the numerical model, a series of physical model tests are performed, in which the physical model of gravity type net cage is designed according to the geometrical and mechanical parameters of the most common used net cage in China and the geometrical parameter of the wave tank in laboratory.The third chapter described the spectrum analysis method for the random data. The hydrodynamic response of net cage and mooring system in regular and irregular waves is analyzed by both numerical simulation and physical model test. The numerical result is compared with the test results for validation purpose. The dynamic response of net cage and mooring system in irregular waves is analyzed by using the statistical technique and spectrum analysis method in the time domain and frequency domain, respectively. In addition, the fatigue damage of mooring line for net cage under the action of irregular waves is also analyzed by using the rainflow counting method, the σ-N curve and the Miner-Palmgren theory. The probability distribution characteristics of the range of mooring line forces and the applicability of simplified fatigue analysis method are also discussed.In the fourth chapter, the hydrodynamic responses of multi-cage and grid mooring system in waves, current, and also waves combined with current field are analyzed by numerical simulation and physical model test. The tension distribution of mooring line under the action of waves and current is investigated; and the effect of the depth of submerged grid mooring platform, the number of anchors and the direction of waves and current on the mooring line tension force is analyzed; finally, the effect of net cage arrangements and the number of net cage on the mooring line tension force, the cage motion and the deformation of fish net is also discussed.In the fifth chapter, the submersible net cage with single-point mooring system and multi-point mooring system is proposed and analyzed. The influence of the frontal rigid frame, the depth of connecting point and grid mooring platform on the dynamic response of net cage and mooring system is investigated.