Numerical and experimental investigations on mooring loads of a marine fish farm in waves and current

Abstract

A realistic aquaculture fish farm system in both regular and irregular waves is investigated by numerical simulations and model tests. The main purpose is to develop a reliable numerical tool and in this respect to investigate the survival conditions of the system. The structural and hydrodynamic modelings of the system are briefly introduced. Numerical sensitivity analysis is performed to investigate which physical parameters are dominant when modeling the system.The considered fish farm comprises a floating collar with two concentric tubes, a flexible net cage including a cylindrical part and a conical part with a center point weight at the bottom, and a sinker tube attached directly to the net. The system is moored with a complex mooring system with bridle lines, frame lines and anchor lines, supported by buoys.The mooring loads in the front two anchor lines and bridle lines are investigated in detail. Numerical results are first validated by the experimental data. Both numerical and experimental results show that one of the bridle lines experiences larger load than the rest of the mooring lines, which is surprising. Then a sensitivity analysis is carried out. The mooring loads are not sensitive to the majority of the parameters. The flow reduction factor in the rear part of the net is the most important parameter for the anchor loads. Modeling the floating collar as a rigid body has a small effect on the anchor loads but not for the bridle lines as it will alter the force distribution between bridles. The mooring loads are not sensitive to the wave load model for the floating collar in both regular and irregular seas and modeling the floating collar as elastic with zero frequency hydrodynamic coefficients is enough to give reliable results.Finally, the survival conditions of the fish farms with different set-ups is studied. Numerical results indicate that the dominant limitation to move the conventional fish farms to more exposed sea regions is the large volume reduction of the net cage. The existing mooring system can be applied in offshore regions as long as the bridle lines are properly designed. The maximum stress in the floating collar is moderate compared with the yield stress.

Wave and Current Effects on Floating Fish Farms

“…The mooring loads are rather insensitive to frequency-dependent added mass/damping and non-linear Froude-Kriloff and hydrostatic restoring loads on the floater. Shen et al (2018) also documented the latter facts.…”

“…Also, the fish farm including the mooring system is simplified compared with a realistic setup. Shen et al (2018) presented comparison of numerical calculations with model test results of mooring loads of a marine fish farm in waves and currents. The physical model is representative for a single cage commonly used in Norway.…”

“…The sinker tube was attached directly to the net in the present study, without vertical chains between the floating collar and the sinker tube to avoid chafing between the chain and the net cage. Shen et al (2018) used the curved beam equations with axial tension to describe the radial and vertical deformations of the two concentric floating tubes similarly as Li et al (2016) did for an isolated elastic torus. The deformations were expressed as Fourier series.…”

“…The hydrodynamic radiation loads for different Fourier modes were represented in terms of convolution . Two springs were inserted in the front two anchor lines (Shen et al 2018) integrals with retardation functions. The effect of current was described by an empirical viscous drag formulation.…”

“…The numerical investigations by Shen et al (2018) have been extended by studying the stress distributions of maximum stress along the floating collar due to horizontal and vertical deformations. The corresponding results are given in Fig.…”