Linear wave response of a 2D closed flexible fish cage

Abstract

Closed flexible fish cages (CFFC) are proposed as a new concept in marine aquaculture, replacing the conventional net cages in order to meet ecological challenges related to fish lice and escapes. A linear mathematical model of a freely floating 2D CFFC in waves have been developed. It was found that the wave induced rigid body motion responses of a flexible CFFC in sway, heave and roll are significantly different from the responses of a rigid CFFC. Large ratios between free-surface elevation amplitudes and incident wave amplitude are predicted inside the tank at the first and third natural sloshing frequencies. It implies that non-linear free surface effects must be accounted for inside the tank in realistic sea conditions. The dynamic tension in the membrane of the CFFC must be smaller than the static tension in the applied structural method. For the analysed case with 25 m between the centre of the floaters, the most probable largest dynamic tension is larger than the static tension, for significant wave heights larger than 0.5 meter. The effect of scaling of elasticity on the rigid body motion have also been investigated. The non-dimensional response of the CFFC versus non-dimensional frequency, and based on Froude scaling using an elasticity available in model scale have been compared to the response of the CFFC using the elasticity for full scale. These responses were found to deviate to a large extent, showing limitations of model tests of a CFFC.