Experimental and numerical studies on successive failures of two mooring lines of a net cage subjected to currents

This study investigates the hydrodynamic changes due to successive failures of two mooring lines on a net cage that are subjected to currents using model tests and numerical simulations. An eight-point mooring net-cage model with two electromagnets, which are used to disconnect its upstream mooring lines, is installed in a water flume. The experiment is conducted to measure the lateral mooring forces, rigid-body motions, and remaining volume of the net cage under successive mooring-failure conditions. To reduce the experimental difficulty, successive failures of the mooring lines are forced to occur at 10 and 30 s after the beginning of measurement. A numerical simulation, which is specifically designed for the model test, is also conducted. The results reveal that as the second upstream mooring line fails, the mooring load of the lateral lines, vertical and horizontal movements of the floating collar and tube sinker, and remaining volume of the net cage obviously increase. The values are much larger than those at the first upstream-line failure. The yaw motion of the floating collar increases as the first failure occurs, but it reverts to its origin after the second failure. Comparison of the simulation and experimental results shows good agreement in terms of the mooring force, especially for velocities less than 20 cm/s. The numerical simulations of the body motions of the floating collar and tube sinker and the remaining volume of the net cage are also similar to the experimental results. However, the simulation results of the motion response of the floating collar and the tube sinker after the second failure are obviously smaller than the experimental results.