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The volume of a gravity cage is greatly reduced under a current due to the flexible structure, which affects the growth and health of the fish. Thus, an accurate assessment of cage volume is essential to determine the number of fishes in the cage. In this study, firstly, a numerical model was built to study the cage volume reduction of gravity cages due to the flexible net deformation when subjected to uniform flow. The remaining volume was calculated and compared with earlier experiments. Even though the flow velocity reductions were considered according to the data from previous experiments, the differences between the results from the numerical calculation and the towing tests are still significant. The physical model tests were treated as the reference value to investigate the uncertainty of the model results. Both the velocity-independent model error and velocity-dependent model error were calculated. With the help of the error models, the uncertainty of the remaining volume can be predicted. In addition, the velocity-dependent model error performs better in evaluating the uncertainty of the numerical calculation of the remaining culturing volume. Overall, the results show that the numerical model assisted by the model errors can calculate the cage volume accurately. [ABSTRACT FROM AUTHOR]

A numerical model is developed to compare the performances of mooring materials for a gravity cage system. At first, a small-scale model is built and it is validated by the previous experiments. With the same method, a full-scale gravity cage model is built to study the performance of different mooring materials. Link elements are used to model the wire ropes to consider the weight, and nonlinear spring elements are used to model the composite fibre ropes to consider the nonlinear stiffness. High-tenacity polyester multifilament fibre has the best performance. It can reduce the average value and amplitude of the mooring forces, which also contributes to reducing the stress concentration on the collar. At last, the mooring forces at specific locations in Portugal are calculated and the distributions of mooring force amplitudes are analysed. • A numerical model is developed to compare the performances of mooring materials for a gravity cage system. • A small-scale model is built and it is validated by the previous experiments. • With the same method, a full-scale gravity cage model is built to study the performance of different mooring materials. • Wire ropes and composite fibre ropes are studied and are modelled considering the nonlinear stiffness. • High-tenacity polyester multifilament fibre has the best performance as it can reduce the mooring forces. [ABSTRACT FROM AUTHOR]

A numerical model is developed to calculate the mooring forces and cage volumes for a gravity cage system. The collar, nets, bottom ring and mooring system are all modelled by line elements due to the slender structure. The effect of structural parameters on the cage capacity and mooring forces is evaluated in a sensitivity analysis, to provide knowledge in the design phase of a fish cage. Heaver sinkers and nets with smaller solidity are preferred to prevent the volume reduction of fish cages but they lead to a larger mooring force or an increase in the probability of fish escaping. Linear wave theory is applied. Short waves increase the mean value of the mooring force and higher waves result in a larger amplitude of the mooring force. It is also found that short waves can change the shape of the nets while the shape of the cage subjected to long waves almost keeps constant as the one under uniform flow, which obtains a good agreement as previous experiments. • A numerical model to calculate the mooring forces and cage volumes for a gravity cage is developed and validated. • Sensitivity studies determine the influence of various governing parameters on the dynamic response. • Effects studied are the elasticity of the net, weight of the bottom ring and the net solidity. • Short waves reduce the cage volume and the amplitudes of mooring forces, but increases the the mooring force. • The average value of mooring forces and cage volumes approach those in uniform flow when the wave period increases. [ABSTRACT FROM AUTHOR]

Experiments are conducted in a flume to study the mooring forces and motions of a gravity cage in regular waves. Cage models are built with hollow pipes and nets. Mooring forces and cage motions including surge, heave and pitch are measured. With the increase of the wavelength, the mooring force, surge and heave increase while the rate of increase gradually slows down until stops. On the other hand, under short waves, the pitch increases with the decrease of the wavelength. In addition, the cage does not finish a cycle of motion before the next peak arrives, which leads to a reduction in mooring forces and motion in all directions. With the observation of overtopping, it can be concluded that the motion of the cage goes through three phases with the growth of the wavelength. Under short waves, the cage does not have enough time to complete a cycle of motion and overtopping occurs. With the increase of the wavelength, the motion of the cage starts to follow the waves and overtopping disappears. When the wave is long enough, the cage is pulled into the water by the mooring line and overtopping occurs again. • Experiments are conducted in a flume to study the mooring force and motion of a gravity cage subjected to regular waves. • A light and strong frame is designed to fix a motion sensor above the collar. Mooring forces and cage motions are measured. • Under short waves, the cage does not have enough time to complete a cycle of motion and overtopping occurs. • With the increase of the wavelength, the motion of the cage starts to follow the waves and overtopping disappears. • When the wave is long enough, the cage is pulled into the water by the mooring line and overtopping occurs again. [ABSTRACT FROM AUTHOR]

Experiments are conducted in a flume to study the motions and the wake behind a circular gravity cage under linear waves. The gravity cage model is built with high-density polyethylene pipes and nylon nets. Wave heights and periods are measured by wave gauges both upstream and downstream. Supports are used above the flume to fix the gauges and to connect the cage model. It is found that the cage behaves differently under short and long waves. The relative motion between the floating cage and the water surface is large under short waves. It can be observed clearly how the short waves pass over the collar. On the other hand, the collar just rides on long waves without apparent relative motion. The wave height is reduced when short waves pass the cage, while the wave height reduction under long waves is too small to be measured. The effect of the cage diameter, the collar cross-section, the bottom weight and biofouling is also studied. Generally speaking, larger and heavier cages have a greater impact on waves. [ABSTRACT FROM AUTHOR]