Numerical study on structural responses of gravity-based fish cages

The location of a fish farm is moving towards more exposed locations due to spatial and environmental concerns. As the fish farm moves to open seas, the structure of the fish farm would undergo increased environmental loads induced by a larger wave and faster current compared to the sheltered areas. The increased loads may cause several problems, such as a reduction in cultivation volume and increased tension in mooring lines. The cultivation volume of a fish cage is essential for fish welfare since the reduced volume of a cage stresses the fish leading to high mortality. Moreover, the increased tension in mooring lines may cause accidental failure. Therefore, the accurate estimation of the loads and the structural behavior of the fish farm should be obtained to verify the performance and to secure the integrity of the fish farm structure. In this thesis, the focuses are made on mainly two issues to estimate loads and structural behavior of the fish farm structure. Firstly, the structural responses such as drag force of a cage, cultivation volume, and mooring line tension are investigated considering the wake effect of a permeable net structure. The wake effect is essential to accurately estimate the hydrodynamic forces of the system since it greatly alters the flow field after a net structure. Moreover, most of the environmental loads come from the net structure due to its largest volume among all components of the fish cage. 4x2 multi-cage fish farm model under current load is investigated, using a well-validated numerical tool called FhSim. The wake effect of a net structure is divided into three regions to realize the flow field inside and outside of a net structure, i.e., (i) twine-to-twine wake effect, (ii) net-to-net wake effect and (iii) cage-to-cage wake effect. A comparative study is used to determine and quantify the influence of the wake effect on dynamic responses of the fish farm structure. The results from the numerical simulations suggest that the drag force of a fish cage can be overestimated up to 76% without the wake effect. Secondly, the accidental failure of a cable in the mooring grid is considered. As a cable in the mooring grid fails, the load is transferred to neighboring cables leading to a possible rupture of another cable. Thus, it is imperative to discern the loads in cables of the mooring grid under a failure condition to secure the integrity of the structure. Two numerical models are investigated under pure current condition. One is the single-cage model, and the other is the 4x1 multi-cage model. A built-in function of FhSim is utilized to control the occurrence of failure. The cables which trigger thelargest tension in a cable are identified. Furthermore, the most significant increases of tension in different types of cable, i.e., mooring line, frame cable, and bridle, are found by comparison between intact and failure mode models. The results indicate that the maximum tension in a mooring line can be increased by up to 31% and 53% for the single and multi-cage systems, respectively