Numerical simulation of hydrodynamic behavior of gravity cage in irregular waves

Compared with net cage used in sheltered area, the net cage in open sea will resist more severe wave loading. But the open sea area is more suitable to keep the environment condition to avoid pollution. For this reason it is necessary to understand the behavior of net cages as they are exposed to large sea-loads from waves and current. In this paper, the hydrodynamic behavior of gravity cage in waves is systematically analyzed by numerical methods. The lumped-mass method and the principle of rigid-body kinematics are used to establish the governing equations of motion, which are solved by the Runge-Kutta-Verner sixth-order method. In order to validate the numerical model, a series of physical model tests in regular waves are conducted. The results of numerical simulation show a good agreement with the experimental observations. Thereafter, the irregular wave fields are generated according to the random phase method, and then the hydrodynamic behaviors of gravity cage in irregular waves are simulated and compared with the results of regular waves. The statistical approach and Fourier transform are applied to analyze the cage motion response and the mooring-line tension response in both time and frequency domain. The numerical results indicate that the mooring-line forces decrease as the wave frequency increases. At low frequencies, the heave motion response is synchronized with the wave elevation.