Numerical modelling of water ballast. Application to fish cages.

Abstract

. To withstand harsh sea states, fish cages can be immersed during critical sea state conditions. In this paper, we present the numerical modelling of water ballast in flexible pipe. According to our results, we consider this modelling to be easy to implement, easy to use, and numerically efficient. We use the finite element method, in which the pipes are modelled using bar elements. The volume of the ballast tank is distributed over the extremities of the bar elements and is represented as cubes. Once the volume of water ballast has been determined, the altitude of the free surface of the ballast is calculated by dichotomy in the cubes. The force on the extremities of the bar elements is calculated using the resulting altitude. Comparison of the present numerical modelling with an analytical method shows that for a vertical tube, the present modelling is robust and realistic. Through the modelling we have assessed the behaviour of a fish cage using ballast: We analysed the shape of the cage during the different phases (immersed, emerged, ballasting, unballasting), the tension in mooring lines, and in the floating part of the cage.