Experimental and numerical investigations of a mooring line failure of an aquaculture net cage subjected to currents

Abstract

This study investigates the impact of a mooring line failure on an aquaculture net cage through a laboratory experiment and numerical simulations. An eight-point mooring net-cage model with two load cells in its upstream mooring lines was installed in a water flume. An experiment was conducted to measure the upstream mooring force acting on the net cage exposed to currents in the intact and failed mooring conditions. A device equipped with a pneumatic cylinder and a blade was designed to cut the mooring line. The obtained results revealed that as one of the upstream mooring lines was cut, the mooring load in the other line increased greatly. However, the net-cage system reached a new force equilibrium within a few seconds. The mooring line tension ratios between the failed and intact states observed in this study were between 1.5 and 2.0. In addition to the strong mooring tension response, a significant yaw rotation of the floating collar of the net-cage system was also observed, but the motion stopped within a second after the failure occurred. In addition, a comparison of mooring forces between the experiment and numerical simulations exhibited good agreement for current velocities less than 25 cm/s. The simulated results showed that after the failure of one upstream line, the mooring load in selected lateral mooring lines increased substantially, and the net-volume change showed a sharp peak at the time of failure. Moreover, when the cage system regained balance after failure, the net-cage volume returned to its pre-failure value. Finally, the reduction in the safety factor resulted from the mooring line failure was discussed for practical engineering applications.