Wave and Current Effects on Floating Fish Farms

Abstract

The paper is partly a review on hydrodynamic and structural aspects offish farms. In addition, new numerical results are presented on the stochasticbehavior of bending stresses in the floater of a realistic net cage in extreme waveconditions. The behavior of traditional-type fish farms with net cages and closedfish farms in waves and currents is discussed. Hydroelasticity can play asignificant role for net cages and closed membrane-type fish farms. The many meshesin a net cage make CFD and complete structural modeling impracticable. As anexample, a hydrodynamic screen model and structural truss elements are instead usedto represent the hydrodynamic loading and the structural deformation of the net. Inaddition, the wake inside the net due to current plays an important role. Thedescribed simplified numerical method has been validated by comparing with modeltests of mooring loads on a single net cage with two circular elastic floaters andbottom weight ring in waves and currents. It is discussed which parts of thecomplete system play the most important roles in accurately determining the mooringloads. Many realizations of a sea state are needed to obtain reliable estimates ofextreme values in a stochastic sea. In reality, many net cages operate in closevicinity, which raises questions about spatial variations of the current and waveenvironment as well as hydrodynamic interaction between the net cages. Live fishtouching the netting can have a non-negligible influence on the mooring loads. It isdemonstrated by numerical calculations in waves and currents that a well boat at anet cage can have a significant influence on the mooring loads and the bendingstresses in the floater. The latter results provide a rational way to obtainoperational limits for a well boat at a fish farm. Sloshing has to be accounted forin describing the behavior of a closed fish farm when important wave frequencies arein the vicinity of natural sloshing frequencies. The structural flexibility has tobe considered in determining the natural sloshing frequencies for a membrane-typeclosed fish farm. Free-surface non-linearities can matter for sloshing and can, forinstance, result in swirling in a certain frequency domain for a closed cage with avertical symmetry axis.

Keywords

Wave / Current / Aquaculture / Net cage / Closed fish farm