Numerical study on the mooring force of a gravity-type fish cage under currents and waves

A numerical model of an entire fish cage under currents and waves is built using the finite element method. All the components in a fish cage are slender structures, thus, the hydrodynamic load is modelled by the Morison equation. The collar and the bottom ring are represented by pipe and beam elements. The net, ropes and mooring lines are modelled by link elements, which are uniaxial tension-compression elements. The initial shape of mooring lines is calculated based on mooring dynamics, using catenary equations. Because link elements are used, the weight and hydrodynamic load on the mooring lines are considered. The counterweight on the tapered bottom is modelled by a mass element. With this numerical model, the mooring forces and deformation of the cage are assessed for the cage in different sea states. The relationships between mooring forces, and current velocities, wave heights and periods are investigated. Both the mean value and the amplitude of the mooring forces increase with the current velocity, while the wave height and period mainly affect the amplitude rather than the mean value.