Dynamic analysis of flexible gravity-based fish cages under accidental failures in the mooring system

Abstract

A growing population needs an increasing amount of high-protein and nutritious food. Having acknowledged that, there is a substantial increase in seafood production in the world today. Due to spatial and environmental concerns, the marine aquaculture is moving further towards more exposed seas, motivating a lot of research in this field. An accidental failure of a critically loaded cable in the mooring frame would contribute to additional loads in one of the mooring lines which might affect the overall structural integrity of the fish farm.

In this thesis, a conventional gravity-based fish cage is analysed using a numerical tool called FhSim, which is an in-house software developed by SINTEF Ocean AS. The focus is on the effect of accidental failures on the mooring line loads, which represent the global loads acting on the system. These structures are built for farming the fish. Therefore, its welfare should be the primary concern. Thus, this thesis also focuses on the extent of volume reduction in the net structures caused due to deformations under severe environmental loading. The sensitivity of the mooring loads and net deformations of the cages to the environmental parameters and net solidities is also presented in this thesis.

Two different models of the fish cages are numerically analysed. Firstly, a preliminary study is performed to understand the response of a single cage configuration exposed to the environmental loads. The same net structure is then used to develop the second model with multiple cages in a 1x3 configuration, representing a fully functional fish farm. The response of this model is studied in detail under different currents and waves approaching from different directions. For both of these models, the critically loaded mooring lines and cables are identified. The loading condition is defined corresponding to an accidental failure in a particular cable.

It is found that the failure in the critical frame cables and bridles does not significantly affect the mooring line loads for the model with a single cage. The loads are instead transferred onto the neighbouring cables which may trigger the rupture of other cables if not designed to withstand these additional loads. This could start a chain of failures in these cables which would eventually lead to adverse consequences. For the second model, the accidental failure in one of the critically loaded frame cables leads to a momentous increase in the mooring line loads. Therefore, the design and dimensioning of the critical mooring line will depend on this frame cable of the first fish cage in the direction of the currents.