More and more aquaculture structures are installed in open water and further offshore due to the fact that a variety of conflicts have arisen with coastal water, eco-system, and inhabitants who live nearby. They can additionally benefit from a high energy environment, for example, strong currents and waves could disperse fish farm waste. However, an open water environment accompanies longer or steeper waves and harsher weather conditions where the structures could be exerted by a higher environmental loading and damaged by larger displacements or motions. Therefore, non-linear interaction between wave and structure needs to be investigated to get a better understanding of an accidental phenomenon which might impact on structures.

The collar structure above nets, shaped typically in a torus, consists of either single or double pipes, usually made of high-density polyethylene. Due to its slenderness and the material properties, the structure should be regarded as a flexible body and treated so in the structural analysis. In order to study its non-linearity, in addition, a coupling between structure and fluid surrounding the structure should be taken into account.

In this thesis, a 3-dimensional beam model is used for the collar structure and Newmark method is adopted for the time integration of structural dynamic analysis. To verify and validate a structure solver, which is developed in MATLAB, the results from the solver are examined with reliable sources including literatures and a commercial code, ANSYS Mechanical APDL 19.1. A free floating fluid model is established using an open-source CFD (Computational Fluid Dynamics) solver REEF3D. This fluid solver is capable of solving the Navier-Stokes equations for both water and air. With both solvers this thesis presents the numerical models and these set-ups as a good foundation to tackle nonlinear fluid-structure problem under the assumption the structures are flexible bodies. As a further step from the starting point this structural solver will be integrated into REEF3D to investigate fluid-structure interaction (FSI).