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• An efficient ANN model for prediction on hydrodynamics of fish cage is proposed. • Structural stress and deformation of fish cage in waves are predicted using ANN. • Accuracy of present model is validated against a well-developed numerical model. Generally, the hydrodynamics of a fish cage are investigated using numerical simulation, physical model experiments, and field measurements. However, these traditional research methods are time consuming and low in efficiency. In this study, an artificial neural network (ANN) model is built such that the hydrodynamic characteristics of a fish cage in waves can be predicted rapidly. The training data of the ANN model are generated by a well-developed numerical model from our previous studies. The parameters in the hidden layer of the ANN model are determined considering the prediction accuracy of the hydrodynamic results of the fish cage. The ANN model is validated against a well-developed numerical model with satisfactory accuracy. Using the proposed ANN model, the hydrodynamic results of the fish cage including the maximum tension in mooring lines, minimum effective-volume ratio, and maximum stress of the floating collar are predicted for various waves. Overall, the predicted results indicate a trend consistent with that of the previous studies. The present model can potentially forecast disasters for an oncoming typhoon, which is important for the hazard prevention of a fish farm. [ABSTRACT FROM AUTHOR]

In this study, waves propagating through a square array of 16 net cages with different levels of biofouling are numerically studied using a three-dimensional computational fluid dynamics (CFD) model. A porous-media fluid model is adopted to simulate both clean and biofouled netting of a cage array in waves. A numerical wave tank is built, and the oscillating-boundary method is adopted to generate waves. The flow motion is solved by the Navier-Stokes equations, and the free water surface is captured using the volume of fluid (VOF) method. The numerical model is validated by comparing the numerical data with corresponding experimental measurements of a net-cage model with clean netting. To analyze wave attenuation, a numerical analysis of wave elevation both inside and around the cage arrays is presented, which considers the effect of biofouling. Based on the results of the present study, the effect of biofouling on wave elevation is noticeable; the damping effect of the cage array increases with increasing level of biofouling. Furthermore, the incident angle of waves has a noticeable effect on the wave field inside and around the cage array. [ABSTRACT FROM AUTHOR]

In this study, waves propagating through a square array of 16 net cages with different levels of biofouling are numerically studied using a three-dimensional computational fluid dynamics (CFD) model. A porous-media fluid model is adopted to simulate both clean and biofouled netting of a cage array in waves. A numerical wave tank is built, and the oscillating-boundary method is adopted to generate waves. The flow motion is solved by the Navier-Stokes equations, and the free water surface is captured using the volume of fluid (VOF) method. The numerical model is validated by comparing the numerical data with corresponding experimental measurements of a net-cage model with clean netting. To analyze wave attenuation, a numerical analysis of wave elevation both inside and around the cage arrays is presented, which considers the effect of biofouling. Based on the results of the present study, the effect of biofouling on wave elevation is noticeable; the damping effect of the cage array increases with increasing level of biofouling. Furthermore, the incident angle of waves has a noticeable effect on the wave field inside and around the cage array. [ABSTRACT FROM AUTHOR]

Information on the length of cultured fish is crucial to evaluate their growth status and biomass. However, existing methods to estimate fish body length are based primarily on manual sampling, which is invasive, time-consuming, and labor-intensive. In contrast, such information can be acquired via noncontact methods relying on images of the animals' centerlines in an image. The centerline extraction method has been widely applied as an efficient approach. However, the greatest challenge in measuring the length of cultured fish is that of accurately extracting the animals' centerlines when fish overlap in an image. To address this problem, we propose a method to obtain the centerlines of overlapping fish by processing images captured from a top-view perspective. First, fish contours are segmented using convex-concave points. Second, the contour segmentations belonging to each fish are identified by an algorithm that matches heads to tails; their centerlines are thus extracted and optimized. We conducted both laboratory and on-site experiments, and results show that the proposed method exhibited excellent performance in extracting fish centerline information, even for fish that overlapped in images captured from a top view. Moreover, it also improved on the performance of existing methods in measuring the length of cultured fish from such images. • The fish centerlines are extracted successfully under the complex culture background. • A convex-concave analysis method is used to segment contours of fish in overlapping images. • An optimization strategy of fish centerlines is done by fish head and tail matching method. [ABSTRACT FROM AUTHOR]

With the growth of global population, the demand for seafood has been increasing. Aquaculture as a sustainable industry provides a new direction to satisfy the rising demand for seafood. However, typhoons, a common disaster in coastal areas, can cause massive waves that can threaten the safety of aquaculture facilities. Therefore, studying the facilities' failure probability under typhoon waves with different return periods is crucial. The example of offshore high-density polyethylene net cages in the seas near Guishan Island, Zhuhai, China, was examined in this study. The wave conditions for different return periods for these seas were predicted using the SWAN+ADCIRC model and the local wave statistics for the last 30 years. Based on the prediction results, the hydrodynamic loads and stresses of the primary cage structure (floating collar, nets, and mooring lines) in typhoon waves at different return periods were calculated using the lumped-mass and finite element methods. Meanwhile, the failure probability of the cage components was analyzed based on the Monte Carlo method. According to the failure probability and cost of different components, the suitable size of the each of the cage components in the representative seas was studied. This study affords a reference for the design of the cage and improve the aquaculture insurance. • Relation between typhoon waves and failure probability of HDPE net cage components. • SWAN+ADICRC model combined with local hydrographic data to predict typhoon waves. • Lumped-mass and finite element methods for hydrodynamic loads on components. • Quantitative response relationship established using Monte Carlo method. [ABSTRACT FROM AUTHOR]

To enable the optimum design and evaluation of fish farm and mooring performance in the energetic open ocean, a series of physical model experiments were conducted to investigate the hydrodynamic characteristics of a large fish farm containing 1 up to 8 net pens with the model scale of 1:40. In the physical model experiments, the main mooring line tension and flow-velocity magnitude were measured when current flowed through the multiple net cages. According to the experimental data, the upstream anchor lines of the cage will endure most of the current load acting on the multi-cage system. When the net cages are arranged in double columns, the tension force in the upstream anchor lines increases with increasing number of net cage. But this phenomenon is not obvious when the net cages are arranged in single column. There exists obvious flow-velocity reduction inside net cages of the multi-cage configurations; however, there is no statistical difference in flow velocity over varied configurations with different number of net cage. The appropriate multi-cage system in engineering practices should be determined through considering both the mooring line force and the flow-velocity distribution. [ABSTRACT FROM AUTHOR]

A vessel-shaped floating aquaculture platform has numerous advantages over traditional high-density polyethylene gravity cages. Combined with a single-point mooring system, the unique structural form of the vessel shape can significantly reduce the environmental loads transmitted from the bow. This study used potential flow theory and dynamic composite cable theory to simulate the hydrodynamic responses of a single-point moored vessel-shaped floating aquaculture platform in waves. The motion response, mooring force response, and reaction force of the nets were simulated in regular waves and compared with experimental values. The numerical and experimental results agree well. After the validation, the response amplitude operators of the floating frames of the platform were calculated and analyzed in the frequency domain. Then, time-domain calculations were carried out to investigate the hydrodynamic characteristics of the platform. The results show that the heave and pitch responses of the front frame are significantly higher than those of the rear frames of the same platform. The increase in the number of floating frames decreases the motion responses and increases the mooring force response of the platform. The X and Z components of the hinge joint force between the floating front frames are significantly higher than those between the rear frames. Similarly, the reaction force of the nets of the floating front frame is higher than that of the rear frames under the same wave conditions. [ABSTRACT FROM AUTHOR]

Highlights: [•] Interaction between the flow and net cages is simulated using an iterative scheme. [•] Flow through a net cage is studied considering the fluid–structure interaction. [•] Application of the present numerical approach to multiple net cages is presented. [Copyright &y& Elsevier]

Highlights: [•] Statistical analysis of tension force on mooring line for net cage is conducted. [•] The sensitivity of fatigue damage of mooring line to wave height, period is analyzed. [•] Application of simplified fatigue analysis for mooring line is evaluated. [ABSTRACT FROM AUTHOR]

Highlights: [•] Flow-velocity distribution around plane net is studied using PIV and ADV. [•] Flow field around net with different solidities and inclination angles is presented. [•] The effects of spacing distance and net number on flow field are investigated. [ABSTRACT FROM AUTHOR]

Abstract: A three-dimensional (3D) numerical model is established to simulate the flow field inside and around the gravity cages in a current. The realizable k–ɛ turbulence model was chosen to describe the flow, and the governing equations are solved by using the finite volume method. In the numerical model the cylindrical cage is divided into 16 plane nets around the circumference and a bottom net, and the net is modeled using the porous media model. The unknown porous coefficients are determined from the hydrodynamic force on the net under different flow velocities and attack angles using the least squares method. In order to validate the numerical model, the numerical results of the plane nets were compared with the data obtained from two physical model tests. The comparisons show that the numerical results are in good agreement with the experimental data. Using the present model, this paper presents the flow field inside and around the gravity cages with different spacing distances and cage numbers. This study provides information about the flow field inside and around the fishing net cages. [Copyright &y& Elsevier]

Abstract: We conducted experiments and numerical simulations to examine the hydrodynamic characteristics of plane nets in waves. The experiments showed that the effect of K c number (defined in Eq. (3)) on the drag coefficient is insignificant in waves and that the inertia force on the net is much smaller compared to the drag force. We used our experimental results to develop a numerical model to simulate the dynamic behavior of fishing nets in waves. We then compared the simulated and experimental results to evaluate the validity of the numerical model. A comparison of the model and experimental results revealed that the numerical model effectively simulated the hydrodynamic behavior of fishing net in waves. [Copyright &y& Elsevier]

Abstract: In our previous research, the hydrodynamic behavior of a two-dimensional (2D) plane fishing net in current was simulated. Based on the research, a model of a three-dimensional (3D) net is established by using the lumped mass method. To verify the validity of the numerical model, model test results by other authors are cited and compared with the numerical results. The simulated results are in good agreement with experimental ones. In this paper, the 3D net model is applied to investigate the effects of structure size ratio (RDH) and mesh type on the 3D net deformation of the gravity cage in current. The numerical results indicate that the decrease of RDH is practically feasible in improving the cage net deformation. With a sinker system the net deformation with diamond mesh is greater than that with square mesh. When the bottom-collar sinker system is applied, with an increase in current velocity, the net deformation with diamond mesh is less than with square mesh. The results of this study provide a better understanding of the hydrodynamic behavior of the gravity cage. [Copyright &y& Elsevier]

• A vision-based adaptive method for evaluation of fishing net is proposed. • The mesh characteristic gradient method is adopted to realize automatic detection. • Damage state and biofouling degree of fishing net can be simultaneously detected. For offshore aquaculture cages, structural damage can cause serious economic property damage to the aquaculture industry, thus it is necessary to carry out regular inspections of the cage structure. At present, the main method of detecting netting damage is that the staff sneak into the water for manual investigation. This method not only does not guarantee personal safety, but is also labor-intensive and inefficient. This paper proposes a new feature curve method for non-contact underwater netting damage detection, which can effectively recognize damaged netting and analyze the degree of damage for the underwater net and return to its damage position. The new method presented in this paper was designed taking into consideration of effect of seaweed growth: Firstly, an image block within the ROI (region of interest) region was processed by bilateral filter. Secondly, the binary image was obtained via the OSTU (the maximum inter-class variance method) method and connected domain detection was performed. Thirdly, the feature gradient histogram was calculated according to the area of the mesh hole and the local peaks of the curve (named as feature curve) was searched to determine the position of damage in the netting. The proposed method combined the image processing technology with aquaculture engineering seamlessly, and reduced the complexity of the detection system greatly, and significantly improved the efficiency of the netting detection. Finally, the MATLAB program was developed to realize the netting detection process and the proposed method had been verified by the actual underwater netting experiment. The experimental results showed that the netting damage detection method proposed in this paper can successfully detect crack of the netting despite image degradation in water. [ABSTRACT FROM AUTHOR]

• Physical model test of a long-line aquaculture facility with lantern nets is firstly conducted. • Hydrodynamic responses of a long-line aquaculture facility with lantern nets are analyzed. • The influence of layout forms on the mooring line tension, the movement of the mainline and the lantern net is analyzed. To prevent occurrences of structure failure, it is essential to study the mooring line tension and motions of the longline aquaculture facilities with lantern nets. This study describes a physical model experiment that attempts to determine an optimal structure design. The experiment investigated effects of three facility design factors on the mooring line tension and movement of the lantern net and mainline: incident wave angles, lantern net layout depths, and lantern net layout forms (horizontal arrangement/staggered arrangement). The experimental results indicate that the mooring line tension decreases with increasing wave periods. The amplitude and variation of windward mooring line tension are much larger and more complex than that of leeward. The vertical movement amplitude and variation range of the lantern net and mainline are much larger than that of horizontal movement amplitude. Decreasing the incident wave angle, deepening the arrangement depth of lantern nets or adopting staggered arrangement can reduce the mooring line tension in the case of longer wave period. The analysis indicates that increasing the layout depths of the lantern nets and applying the staggered layout form are beneficial in reducing the vertical motion amplitude and elliptical excursion of the mainline. The incident wave angle has a relatively weak effect on the motion of the main and lantern net. Increasing the layout depths of lantern nets will reduce the horizontal and vertical movement amplitudes and elliptical excursion of the lantern nets. Changing the layout forms of the lantern nets from horizontal to staggered will increase the horizontal motion amplitude and decrease the vertical motion amplitude and elliptical excursion of the lantern nets. [ABSTRACT FROM AUTHOR]

Fish farms have moved into the open sea in recent years and the failure risk of fish cages has increased in the harsher environmental conditions. Fatigue failure is an important limit state for the floating system of the fish cage under the long-term action of waves. A fatigue analysis of the high-density polyethylene (HDPE) floating system in waves is presented in this study based on the deterministic method and finite element analysis combined with a hydrodynamic model. As the basis of the fatigue assessment, a stress analysis of the floating pipes is first performed based on the joint use of the finite element model and the hydrodynamic model. The fatigue lifetime is then calculated using the Palmgren – Miner cumulative damage theory together with the deterministic method in which the stochastic sea state is decomposed into a reasonably high number of individual wave scatter diagrams. The results show that the recommended replacement period of the HDPE floating pipes is 20 years and that the fatigue lifetime of the floating pipes is more sensitive to the wave period than to the wave height. In addition, the effects of mooring line arrangements, wave incident angle and typhoons are also investigated. Comparisons of the fatigue lifetimes of the floating pipes for different mooring line arrangements indicate that the fatigue lifetime increases dramatically with increasing number of mooring lines. For a symmetrically arranged four-point mooring fish cage, the most unfavourable wave loading angle is 45°. Typhoons lead to a significant decrease in the fatigue lifetime of the floating pipes despite the short durations of the typhoons. [ABSTRACT FROM AUTHOR]

To obtain the motion state of a school of fish in marine cage culture and improve the automatic detection capability for large areas of marine culture, this paper proposes a fish-motion vector-field extraction method for marine cage culture, which primarily comprises cage region extraction and motion vector calculation. The proposed method uses an unmanned aerial vehicle (UAV) as the carrier, and the interference of marine environment is fully considered. First, blurred images are discarded, and the culture area is obtained by a holistically-nested edge detection (HED) network. Second, the validity of the current frame is determined by the local binary patterns (LBPs) of the feature image. Third, the images are transformed, and the vector-field information of fish swarm is obtained using the particle image velocimetry (PIV) analysis method. This study integrates traditional image analysis and deep network technology while also fully considering the image distortion caused by UAV flight attitude changes in the actual detection process. Finally, images of Takifugu rubripe net cages were used to verify the method. The results show that the proposed method has strong practicability and applicability. • A UAV-based vector field analysis method of fish movement in marine cage is proposed. • The proposed method can detect fish region accurately, steadily and efficiently. • The motion instability of UAV in the air is fully considered in the design process. [ABSTRACT FROM AUTHOR]

Abstract: Large fish farms that include multiple-cages are becoming common in the aquaculture industry. Selecting an appropriate mooring structure and arrangement of cages is necessary to avoid fish cage and mooring grid system failures. A numerical model based on the lumped mass method and the principle of rigid body kinematics is developed to predict the hydrodynamic response of a fish cage and mooring grid system to regular waves. To validate the numerical model, a series of experiments is conducted. The numerical results of this model correspond with those obtained from experimental observations. Then, two cage arrangements are investigated, and the effect of the wave direction is analyzed. The results show that when the wave incident angle is 0°, the maximum tension forces on the anchor lines of the two cage arrangements are close to each other. There is not a significant difference between the effective fish cage volumes of the two cage arrangements. However, if one anchor line is broken, the two cage arrangements have different degrees of risk of having the cage structures swept away. When the propagation direction of the incident wave changes, the tension forces on the anchor lines also change due to different transfer load paths. [Copyright &y& Elsevier]

Abstract: Compared with net cage used in sheltered area, the net cage in open sea will resist more severe wave loading. But the open sea area is more suitable to keep the environment condition to avoid pollution. For this reason it is necessary to understand the behavior of net cages as they are exposed to large sea-loads from waves and current. In this paper, the hydrodynamic behavior of gravity cage in waves is systematically analyzed by numerical methods. The lumped-mass method and the principle of rigid-body kinematics are used to establish the governing equations of motion, which are solved by the Runge–Kutta–Verner sixth-order method. In order to validate the numerical model, a series of physical model tests in regular waves are conducted. The results of numerical simulation show a good agreement with the experimental observations. Thereafter, the irregular wave fields are generated according to the random phase method, and then the hydrodynamic behaviors of gravity cage in irregular waves are simulated and compared with the results of regular waves. The statistical approach and Fourier transform are applied to analyze the cage motion response and the mooring-line tension response in both time and frequency domain. The numerical results indicate that the mooring-line forces decrease as the wave frequency increases. At low frequencies, the heave motion response is synchronized with the wave elevation. [Copyright &y& Elsevier]

The hydrodynamic performance inside the recirculating aquaculture tank is critical for the welfare of aquaculture. It is undoubtedly a preferable alternative to achieve a conducive flow environment inside the aquaculture tank through optimizing the geometry structure of the aquaculture tank. In this study, combining the computational fluid dynamics (CFD) approach and multi-objective genetic algorithm (MOGA), these two parameters, namely α (R / L) and β (C / L), are assessed and optimized based on the average flow velocity and flow velocity uniformity inside the square arc angle aquaculture tank. Physical experiments are set up to verify the accuracy of the numerical models and predicted results. The results indicate that the variation of both α and β affects the flow velocity and vortex distribution. However, the sensitivity of flow velocity uniformity and average flow velocity to α (74.49% and 91.14%) is much higher than that of β (−24.12% and −23.68%). It demonstrates that increasing α can significantly improve the average flow velocity and flow velocity uniformity inside the aquaculture tank compared to β. In addition, according to MOGA optimization results, taking the maximum flow velocity uniformity and average flow velocity as the optimization goals, the final optimization scheme is α = 0.396 and β = 0.074. Finally, combining CFD and MOGA methods can optimize the aquaculture tank structure accurately and efficiently in a comprehensive and systematic approach with relatively minimal computational cost. • Combining CFD and MOGA methods optimize the aquaculture tank structure accurately and efficiently. • Flow velocity distribution and vortex region are investigated in detail and comprehensively. • The sensitivity of hydrodynamic performance to structural parameters is thoroughly discussed. [ABSTRACT FROM AUTHOR]

Moored fish cage structures are forced to move into the offshore area due to the shrinking availability of sheltered near-shore sites and increasing environmental impacts of aquaculture, and this provokes a significant challenge for the reliability design of mooring system. The uncertainties involved in the predictions of applied loads and structural strength are analyzed for calculating the reliability level of mooring system. In this study, a probability corrosion model is used to describe the uncertainty of corrosion depth and the joint probability density of significant wave height and spectral peak period is selected to depict the randomness of sea states. The non-linear finite element model is developed to investigate the minimum breaking strength of mooring chains and is validated through comparing with the formulas providing by the International Association of Classification Societies (IACS) Unified Requirements concerning for Materials and Welding. A validated numerical model is applied to calculate the tension force of mooring system for both the single-cage system and the multi-cage system at several sea states, and the response surface method is utilized to display the limit state function to calculate the reliability level in terms of the uncertain metocean variables. The results indicate that the reliability level decreases significantly with the increasing corrosion depth and the correlation of corrosion model for mooring chains has a remarkable impact on the failure probability of mooring system. In addition, the failure probability of mooring system for the four-cage system is much higher than that for the single-cage system. [ABSTRACT FROM AUTHOR]