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.

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The behaviour and characteristics of a turret-moored FPSO subjected to loading from waves, wind and current are investigated. Of particular importance is to find out if fishtailing instabilities may occur, and if such instability can be disclosed by simple criteria involving basic parameters of the system’s mathematical model. Eight cases from model tests are chosen for theoretical study and time domain simulation. Four of the cases involve heading control with thrusters. For the stability study, a simplified linear model in sway and yaw is formulated. It is shown that the inherent characteristics of the model depend on the strengths and relative directions of the metocean processes. The eigenvalues of the sway-yaw model are computed for the eight selected cases to check the stability. A simple approximate criterion for heading stability is derived from the sway-yaw model. It is assumed, but not proven, that the criterion is a sufficiency criterion for stability. Both the experiments and the simulations show that the eight cases are stable. This is also confirmed by the eigenvalues of the sway-yaw model, while the simple criterion wrongly deems several cases unstable. The simple stability criterion is therefore probably conservative, at least when there is significant damping in the system. In one additional hypothetical case with only wave excitation and weak or lacking stability, the simplified criterion agrees well with the model test and simulation. Heading control is necessary when a heading different from the natural weathervaning heading is wanted. The controller used in the experiments and simulations is of simple SISO PID type. With control, the heading variations are reduced significantly.