Numerical study of closed rigid fish cages in waves and comparison with experimental data

Biao Su,Andrei Tsarau,Per Christian Endresen,David Kristiansen,Pål Furset Lader

doi:10.1016/j.oceaneng.2021.109210

Biao Su, Andrei Tsarau + Show 3 more

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https://doi.org/10.1016/j.oceaneng.2021.109210

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Abstract

Closed fish cages have gained increased interest in marine aquaculture. However, knowledge on the seakeeping behaviour of a floating closed cage and the influence of the contained water inside the cage are still limited. In this paper, a coupled numerical model is developed for the simulation of closed rigid cages in waves. Numerical studies are conducted both in the frequency domain and in the time domain, and compared with scaled physical experiments. Special attention has been drawn to the coupling effects of sloshing on cage response and the resulting mooring line forces. The comparative analyses show that sloshing of the contained water has large influence on the coupled surge and pitch motions of the cage. Sloshing is also found to have significant effect on the mean-drift forces in regular waves. In the tested/simulated irregular waves, the mooring forces are found to be dominated by the slow-drift motions, which indicates that the slowly-varying wave drift forces need to be considered in the design of the mooring system for a floating closed cage.

Highlights

Fish farming in floating closed cages has gained increased interest in recent years as a strategy to mitigate the problems with sea lice and the negative environmental impact, which are often associated with the open net cage systems that are widely used for sea-based production of Atlantic salmon (Salmo salar)
As a continuation of the previous work (Kristiansen et al, 2018a), this paper presents an integrated numerical model for analysing the seakeeping behaviour of closed rigid cages and the mooring forces in regular and irregular waves
The nonlinear viscous damping was considered in the timedomain simulations (FhSim) by using a drag coefficient of 0.6, which was evaluated based on the model test data of a similar cylindrical spring constants from the physical experiment

Summary

Introduction

Fish farming in floating closed cages has gained increased interest in recent years as a strategy to mitigate the problems with sea lice and the negative environmental impact, which are often associated with the open net cage systems that are widely used for sea-based production of Atlantic salmon (Salmo salar). The existing concepts of closed fish cages can be categorised as flexible membrane structures (fabric), semiflexible structures (glass fibre) and rigid structures (steel or concrete), based on the construction materials being used and to what degree the cage can deform (Kristiansen et al, 2018a). The coupling effects of internal hydrodynamics on cage response and structural deformation, are important design consider ations for floating closed cages. Knowledge on and experience with these problems are quite limited, as sea-based fish farming in closed cages is still a novel concept

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