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.
Highlights
Norway has become the World’s largest producer of Atlantic salmon through the use of open net structures in the sea
It was found that the wave induced rigid body motion responses of a flexible Closed flexible fish cages (CFFC) in sway, heave and roll are significantly different from the responses of a rigid CFFC
The response of sway and roll rapidly decreases to a small value in a narrow frequency range around k1R and k3R, as can be seen from Fig. 5. This is different from the coupled response of the rigid CFFC where it was found that this cancellation did not happen
Summary
Norway has become the World’s largest producer of Atlantic salmon through the use of open net structures in the sea. One reason is that viscous loads dominate on a net cage in waves and current (Kristiansen and Faltinsen, 2012, 2015), while potential flow loads with body–wave generation is significant for a CFFC in waves Model experiments for both still water (Strand et al, 2014), current (Lader et al, 2015; Strand et al, 2016) and waves (Lader et al, 2017) have been performed, for various filling levels and geometries. To predict the response of a 2D CFFC in waves three coupled sub-problems must be solved: the membrane dynamics, the external problem with incident waves and the effect of the structure on the flow, and the internal problem, which is an internal sloshing problem. At last conclusions with future suggested needed investigations are given
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