Dynamic analysis of gravity based fish cage

Abstract

With the development of economy and growth of population, we are constantly facing the challenge of decreasing fish population and increasing needs in the fish market. Aquaculture is a practical way to fulfill this demand. Due to limited coastal area for fish farming, the fish farms are moving to deeper sea with harsher wave and current condition. This thesis focuses on the numerical analysis on the responses of the most commonly used gravity based fish cage under various wave and current conditions using ABAQUS program. Firstly, the behavior of a simple beam element and the single circular floater in waves and currents are investigated before studying the complex responses of the fish cage. The purpose is to understand the dynamics of the simple fish cage components and lay a foundation for modelling of the whole fish cage system. Secondly, the responses of a model scaled fish cage are studied using two commonly used numerical models for fish cage modelling: truss model and mass spring model. Convergence study and sensitivity study are conducted and responses in terms of volume reduction and hydrodynamic forces are compared using the two models. These numerical results are also validated with the numerical and experimental results from previous study. Thirdly, truss model is further applied to the full scale fish cage. The interaction of the net and the floater are studied. More sensitivity studies related to solidity ratio, bottom weight and wave-current condition are performed with the improved truss model to study the responses of volume reduction, hydrodynamic force, mooring tension, and critical stresses. The conclusions are that the current velocity is the dominating factor which affects the responses of the fish cage. The hydrodynamic force and deformation are closely influenced with each other, so the hydrodynamic force in reality may have some deviations with that calculated from Morison’s equation. The interaction of the floater to the net should not be overlooked when studying the responses of the whole fish cage. The solidity ratio, bottom weight, wave condition can also influence the responses of the fish cage.