Numerical study of the mooring system failure of aquaculture net cages under irregular waves and current

Abstract

In this study, the dynamic responses of a net-cage system were investigated through a time-domain numerical model based on the Morison equation and lumped-mass scheme. Then, the tension of the mooring lines and the motion of the net cage were studied after the failure of its mooring system. The results showed that the maximum tension in the remaining anchor increased, with large peak spectral values corresponding to the wave principal frequency and a lower frequency corresponding to the mooring force. Although additional sway, yaw, and roll motions of the cage were induced, the floating cage reached a new equilibrium position after several waves with a larger and stronger vibrating characteristic. With an increase in the current velocity, the tension ratio relative to that under the normal condition increased to 1.5. The trace of the cage after the failure showed that the longitudinal drift increased with the current velocity, whereas the lateral shift only reached a certain extent as it was constrained by lateral anchors. With the same number of cages attached, the mooring failure led to an increase in tension. As the number of cages increased, the maximum line tension also increased; however, the tension ratio remained almost constant. Finally, the safety factor of the mooring line tension for engineering applications was found to be substantially reduced after the failure.