Modelling of Loads and Responses of a Permaskirt on a Flexible Net Cage Fish Farm

Abstract

As the numbers of fish farms are increasing, the problems with lice are becoming of greater importance. The lice cause diseases and death, leading to loss in profit for the fish farms as well as reduction of the wild stock. To deal with this problem, many solutions have been introduced. For example, placing the fish farms on land or make the fish farms as floating concrete structures. However, these suggestions are costly. Another idea is to put a watertight skirt, a Permaskirt, around the upper part of the net cage. However, the introduction of such a skirt would increase the loads and responses on the structure. This thesis shows how to calculate these forces and displacements, and how they are increased when a watertight skirt is placed around the net cage, by using the finite element analysis software AquaSim by Aquastructures. The program is used to calculate the drag and net displacements when current forces influence the structure. Results are compared to results from model experiments and calculations done by the program FhSim.The introduction of such a skirt will also influence the pressure and flow distribution around the net cage. An investigation regarding the flow distribution, to see if the flow travels under the skirt, will therefore be of importance to evaluate the effect of Permaskirt. In order to investigate these effects, computational fluid dynamic (CFD) calculations has been performed, using the Ansys software. The analysis shows that the AquaSim program overestimates the forces and displacements, compared with experimental values. Possible solutions to obtain correct calculations in AquaSim, by using varying solidity for the skirt, has been presented and shows good results compared with experimental values.CFD analysis shows that a skirt covering the upper part of a net cage would reduce 99.61% of the water particles, from the upper part of the water layer, to enter the upper part of the net cage, leading to reduction of lice contamination on the fish in the net cage.