Drag Forces and deformation of aquaculture cages – full-scale towing tests in the field

Abstract

Fish cages can experience strong loads due to hydrodynamic forces in the sea. Numerical models are often used to estimate drag forces on net cages, and the development and validation of such models is mostly based on laboratory tests that can be performed under controlled conditions. However, several environmental factors are difficult to account for in a laboratory. Experiments using full-scale fish cages in the sea could produce valuable data and new insights on the fluid-structure interaction between sea-cages and ambient flows, given sufficient control over environmental factors. Today very little field data is available on the forces on full scale fish cages in the sea.

In this study, an Atlantic salmon cage (12 m diameter, 6 m depth) was towed in a fjord environment at 5 different speeds to induce a relative water current past the net between 0.1 ms−1 and 1 ms−1. Drag on the cage was measured using a load shackle attached to the towing rope and net deformation and cage volume were calculated based on the positions of pressure tags mounted to the net cage.

The towing method produced consistent results on deformation in the range from 0.2–1 m/s, and the volume of the net pen decreased almost linearly from 86% (0.2 ms−1) up to 33% (1.0 ms−1). Measured drag forces and their relationship to flow speed were consistent with existing literature. Drag calculations for net cages generally consider flow speed reduction inside the cage due to blockage effects. However, there are large differences in the flow reduction inside net cages found in few laboratory and field studies, which calls for better descriptions of the flow past net cages. This is illustrated by the comparison of drag calculated by a simple, deterministic model, using a static flow speed reduction of 20% inside the cage and a variable flow speed reduction that depends on the ambient flow speed. The results from this study provide valuable information about the interplay of flow speed, net deformation and drag on a full scale fish cage at different flow speeds and underline the need for a better description of the flow past net cages.