Mooring Analysis of a Closed Fish Cage

Abstract

As of today, Norway is the second largest seafood exporter in the world after China, and Norwegian seafood is currently exported to more than 130 countries (Laksefakta, n.d.). In conjunction with the present growth of the aquaculture industry in Norway, there is a rising skepticism regarding the sustainability of the industry. Major concerns are fleeing, fish welfare, salmon louse and impact onmarine life in close proximity to the fish farms. The aim of this thesis is to investigate the possibility of exchanging a fish net, at an operational fish farm, with a newly proposed closed fish cage concept without altering the existing mooring system. The benefit will be reduced downtime and expenditure during setup, since there is no need to install a new mooring system. A conventional fish net is analyzed to obtain comparable results to that of the fish cage, with identical mooring used by both systems. A numerical, and a simplified analytical method is used. AquaSim, which is a finite element analysis software, is used to perform the numerical comparison of both systems exposed to regular waves, while an analytical approach is used to derive static values, such as; wave and current forces, and static mooring line data. The focus is on mooring line loads, since these loads would be the governing factor regarding the interchangeability. Drag loads of the fish net with varying amounts of marine fouling are calculated. The results show that there is an approximately linear increase in drag loads for current velocities between 0 m/s and 1,5 m/s. These drag loads are compared to the estimated loads of the fish cage, calculated by hand and through AquaSim. By introducing a combination of waves and current in the numerical analysis, the results shows that the wave loads are within the same range for both solutions. The results also show that the mooring line loads are dominated by current forces, even for the largest wave conditions. Additionally, the results related to the fish net are higher than that of the fish cage for all simulated conditions, suggesting that it would be possible to exchange the fish net with the closed fish cage. Although the simulations converge, there is some uncertainty related to the validity of the values obtained in the simulation of the fish cage, when exposed to the largest wave conditions. The fish cage picks up a slight rolling motion, and waves overtops the upper part of the structure on one side only, resulting in an unloading of the mooring lines, located in the direction opposite of the waves propagation. A comparison to experimental results obtained through model testing would be beneficial, but are not included since no experimental data is available.