Wave and Current Effects on Floating Fish Farms

Abstract

The paper is partly a review on hydrodynamic and structural aspects of fish farms. In addition, new numerical results are presented on the stochastic behavior of bending stresses in the floater of a realistic net cage in extreme wave conditions. The behavior of traditional-type fish farms with net cages and closed fish farms in waves and currents is discussed. Hydroelasticity can play a significant role for net cages and closed membrane-type fish farms. The many meshes in a net cage make CFD and complete structural modeling impracticable. As an example, a hydrodynamic screen model and structural truss elements are instead used to represent the hydrodynamic loading and the structural deformation of the net. In addition, the wake inside the net due to current plays an important role. The described simplified numerical method has been validated by comparing with model tests of mooring loads on a single net cage with two circular elastic floaters and bottom weight ring in waves and currents. It is discussed which parts of the complete system play the most important roles in accurately determining the mooring loads. Many realizations of a sea state are needed to obtain reliable estimates of extreme values in a stochastic sea. In reality, many net cages operate in close vicinity, which raises questions about spatial variations of the current and wave environment as well as hydrodynamic interaction between the net cages. Live fish touching the netting can have a non-negligible influence on the mooring loads. It is demonstrated by numerical calculations in waves and currents that a well boat at a net cage can have a significant influence on the mooring loads and the bending stresses in the floater. The latter results provide a rational way to obtain operational limits for a well boat at a fish farm. Sloshing has to be accounted for in describing the behavior of a closed fish farm when important wave frequencies are in the vicinity of natural sloshing frequencies. The structural flexibility has to be considered in determining the natural sloshing frequencies for a membrane-type closed fish farm. Free-surface non-linearities can matter for sloshing and can, for instance, result in swirling in a certain frequency domain for a closed cage with a vertical symmetry axis.

Reliability of multi-purpose offshore-facilities: Present status and future direction in Australia

“…The Australian aquaculture industry commonly uses the HDPE made floating collars and longlines which are flexible and deform under wave action ( Gagnon and Bergeron, 2017 ; Bi et al, 2020 ; Bore et al, 2017 ). Unlike the common practice in floating structures of the oil and gas industry, the dynamic analysis of such integrated system, must consider the hydroelastic nature of the structure because of the considerable impact of structural deformations ( Fu et al, 2007 ; Leble and Barakos, 2016 ; Faltinsen and Shen, 2018 ). One of the main areas of research that accounts for the hydroelastic behaviour of flexible floating systems involves discretising the structure into several rigid modules (plates or beams) which are connected by Euler-Bernoulli beam elements ( Lu et al, 2019 ).…”

Fish welfare in offshore salmon aquaculture

“…Powerful waves are expected at offshore sites and their impact is likely to impose a serious challenge to many aquaculture operations (Bjelland et al 2015;Faltinsen & Shen 2018). However, unlike water currents very little research has so far been made on the impact of waves on farmed fish.…”

Finite Element Analysis of the Effect of Currents on the Dynamics of a Moored Flexible Cylindrical Net Cage

“…Particularly, designing floating net cages and mooring systems [1,2] is of great concern to the marine aquaculture industry. The consideration of current loading on the dynamic response of aquaculture cages is necessary when designing fish farms, enabling the estimation of operational limits of a particular design system component [3].…”

“…The behaviour of traditional-type fish farms with net cages and closed fish farms in waves and currents was discussed in [3]. The effects of waves and currents on the behaviour of a gravity-type cage system were investigated in [26], which used a tube-sinker to replace the common weights at the bottom of net cages to overcome the deformation.…”