Numerical Modeling of the Mooring System Failure of an Aquaculture Net Cage System under Waves and Currents

In this article, a numerical model based on the Morison equation and lump-mass method is developed to simulate the failure of an aquaculture net cage system, by changing an upstream anchor from a fixed node to a free node. Current-only and wave-current conditions are employed to investigate the mooring line tension and volume reduction coefficient of a net cage after a failure. The results show that both mooring line tension and volume reduction coefficient increase after a failure. The failure causes the cage system to drift downstream and move aside. Remaining mooring lines twist to rotate and deform the net cage. The maximum mooring line tension for the current-only cases increases with the current speed. However, the tension ratio only increases up to some certain value, i.e., 1.91 instead of 2. Beyond this, the cage system is completely collapsed resulting in a smaller minimum volume reduction coefficient compared to its counterpart under the normal state. When examined under wave-current conditions, the cage system exhibits oscillatory motion, and a large excitation of the mooring line tension is induced. The corresponding minimum volume reduction coefficient is larger than under the normal state, due to the twisting deformation of the net cage. Different instances of failure time are also examined. It is found that the results at later times (steady-state region), including the mooring line tension, the volume reduction coefficient, and the body motion of the floating collar, are not affected by the failure time. Different wave heights, wave periods, and current speeds are also simulated. The results show that the tension ratio increases with the wave height and the current speed but it decreases with the wave period.