Coupled motion and nonlinear sloshing of an upright circular container floating in irregular waves

Rigid upright circular fish cage is one of the aquaculture structures with good development prospects as it may reduce the risk of fish escaping and keep the fish safe from parasites. The internal liquid sloshing, which is excited by the cage motion, has a great effect on the motion response of closed cages and may potentially damage the cage structure. Moreover, the liquid sloshing in floating cages exposed to waves can be quite different in comparison to sloshing due to forced harmonic motions. In this paper, the coupling between external wave hydrodynamics, nonlinear internal sloshing and rigid-body dynamics of a rigid upright circular cage is studied numerically. Different from uncoupled sloshing analysis with forced tank motions, this study suggests that it is practically very difficult to trigger nonlinear swirling-type sloshing responses under unidirectional regular or irregular waves. This is attributed to the external wave-radiation damping and the  possible cancellation effects between surge/sway and roll/pitch motions.  he influence of the mooring system on the responses of the coupled system is also discussed.