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A numerical study of the dynamics of a nine-module floating aquaculture platform in irregular waves is proposed, to examine the effect of the connector type between net cages and incident wave direction on the hydrodynamic response. A numerical model based on the coupled boundary element method and lumped-mass model was used to simulate the dynamic behavior of the aquaculture platform. Time-domain statistical analysis, and time-frequency analysis based on Hilbert-Huang transform on the motion responses of net cages and the mooring force responses of the platform were then carried out. The results indicate that the mooring system has a decisive influence on the frequency-domain characteristics of surge motion, while the influence of connector can be ignored. The surge motion under flexible hawser connector is less sensitive to wave incident direction, while the wave direction has a greater impact on surge motion under spherical joint connector. The amplitudes of pitch motion under two different connectors decrease significantly under oblique incident waves, compared with perpendicular incident wave condition. The wave incident direction has little effect on the overall mooring force response of the platform. The mooring force responses at different positions show completely different frequency characteristics when the cages are connected by spherical joints. The larger the pitch response of the cage, the smaller the mooring force response of the mooring chain connected to the net cage. There is a positive correlation between the gravity frequency of the mooring force response and the gravity frequency of the pitch response of the net cage. • A numerical model that couples the BEM and lumped-mass model was used to simulate the dynamic behavior of a nine-module aquaculture platform. • The effect of two kinds of connectors between net cages on motion and mooring force responses of the aquaculture platform are analyzed. • The mooring system has a decisive influence on the frequency-domain characteristics of surge motion. • The wave incident direction has a greater impact on the surge response under spherical joint connector. • There is a positive correlation between gravity frequencies of mooring force response and pitch response of net cage connected to the mooring chain. [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]

Computer simulations were conducted to understand the nonlinear dynamics of the cage array to establish an offshore fish farm capable of surviving the severe conditions. To that end, a large fish farm containing 16 net cages in a 2 × 8 configuration was modeled based on the lumped mass model. This numerical model was verified against the physical model tests regarding a single net cage and a net cage array. In particular, three different equivalent mesh groups were compared to each other to balance the numerical accuracy and the computational efficiency. Then, the candidate equivalent mesh group was used to separately model the rigid and the flexible cage arrays subjected to severe conditions. The results show that most trajectory patterns of mooring lines are not as elliptical as the water particle trajectories. This provides evidence to justify the perturbations about the equilibrium of the mooring grid for the full-scale aquaculture cage array system. Alongside this, flexible cages with lower bending stiffness have greater tension forces on the weather side. The bending stiffness has a predominant effect on the surge motion and is also the leading contributor to the nonlinearity of cage motion. The flexible cage with lower bending stiffness is appropriate for more exposed, open ocean fish farming applications, but the increased mooring tension should be taken into account for future design consideration. • Nonlinear dynamics of an aquaculture cage array were examined under severe conditions. • The nonlinear wave was considered by introducing the new Stoke wave solutions. • Flexible cages with lower bending stiffness exhibited greater tension forces on the weather side. • The bending stiffness contributed to the nonlinearity of cage motion had a predominant effect on the surge motion. [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]

Floating aquaculture cages deployed in the nearshore affect the wave field and contribute to the coastal defense by damping incoming waves. Most pioneering works concerning aquaculture cages have concentrated on the hydrodynamics of the structures instead of their influence on the wave field. In order to bridge the gap, this paper examines the wave performance with the interference of floating aquaculture cages via physical model tests. The results show that the existence of cages increases wave height on the weather side. The wave-dissipation performance of the net cage is closely related to the ratio of the wavelength to the structural span, and the minimum transmission coefficients appear in the conditions with the maximum wave number. Moreover, the crests of the free surface elevation inside the floating frames are almost flat, which is particularly obvious in the long-period wave conditions. The surface elevations show nonlinearity in small-wavelength conditions, which is dominantly caused by the pitch motion of the cages. Nevertheless, the presence of the cage does not affect the phase differences in the wave elevation at different positions. The nets have an opposite effect on wave response inside the cage under the conditions of long-period and short-period waves. • The wave performance with the interference of floating aquaculture cages are examined via physical experiments. • The wave transmission coefficients were closely related to the ratio of the wavelength to the structural span. • The wave surface elevations inside the floating frames showed nonlinearity under small wavelengths. • The nets had an opposite effect on the wave response inside the floating frame in long- and short-period waves. [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]

Gravity-type fish cages are playing an increasingly important role in fishery aquaculture. As the main load-bearing component of gravity-type fish cages, the floating collar supports the whole cage and undergoes large deformations. In this paper, a numerical method is developed to study the motions and elastic deformations of an HDPE floating collar in waves. The governing equations of motions are established by Newton׳s second law, and those governing the deformations are obtained according to the curved beam theory and the modal superposition method. Damping of the elastic deformations is introduced and analysed in detail to improve the numerical model. To validate the numerical model, a series of physical model tests are conducted. Good agreement between the numerical and experimental data is obtained. The numerical results indicate that the in-plane and out-of-plane deformations calculated using the first four modes yield satisfactory precision. The positions of the maximum deformations in different wave propagation directions and the effect of the forms of the mooring line arrangement have also been investigated. The numerical results show that the four-point-mooring cage is more reliable than the single-point-mooring cage. The endpoints of the diameters that are parallel or perpendicular to the wave direction have the largest deformation. [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]

• A new method combining the porous media and rigid wall is applied to analyze the flow field. • The blocking effect of the semi-submersible aquaculture platform on water flow is analyzed. • A phenomenon including low flow velocity region and vortex generation are studied. The study of flow velocity distribution inside and around a semi-submersible aquaculture platform is extremely important for the living environment of marine fish, especially in the open sea. Therefore, a method combining porous media and rigid walls is established to analyze the flow patterns inside and around a semi-submersible aquaculture platform. In the present study, the shear stress transport k–omega turbulence model is chosen to describe the flow, and the governing equations are solved by using the finite volume method. The fish nets and main frame, including pontoons, columns, and braces, are modeled by porous media and rigid wall conditions, respectively. The accuracy of the numerical model is verified, and the results show that the experimental values agree with the calculated values. By comparing the flow field of aquaculture platforms with and without fish nets, it can be concluded that fish nets play a vital role in the attenuation of flow velocity. In addition, vortices exist near the wall of the aquaculture platform. The risk of resonance will increase when the frequency of the vortex shedding is close to the natural frequency of the structure, which should be avoided in practical engineering. [ABSTRACT FROM AUTHOR]

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]

A series of physical model experiments were performed to investigate the characteristics of the motion responses of a multi-module aquaculture platform. A decay test in calm water and regular wave experiments were performed for the platform in a wave basin. The natural period, damping coefficients of the platform, horizontal stiffness of the mooring system in calm water, and motion responses of the platform modules in regular waves were obtained. Owing to the central enclosed cabin and greater mass, the living module of the platform has a larger natural period and damping coefficient than the aquaculture module. Furthermore, the existence of the nets has a significant effect on the motion characteristics of the modules, and the influence of the nets on the damping of the heave motion is larger than that of the pitch motion. The motion responses of the entire platform system in the surge (sway) and yaw directions exhibit obvious low-frequency characteristics. Under regular waves with two incident angles (45° and 180°), the three-degree-of-freedom motion responses of the two types of modules exhibit the same trend as the wave frequency increases. The surge and heave responses have low-frequency characteristics, whereas the pitch response exhibits wave–frequency characteristics. • An innovative offshore multi-module aquaculture platform was proposed. • Hydrodynamic responses of both the single module and platform were conducted. • Transfer functions of motion responses for various modules in waves were analyzed. [ABSTRACT FROM AUTHOR]

Extreme wind and waves can easily destroy aquaculture facilities in the open sea. Therefore, it is essential to analyze the hydrodynamic responses of aquaculture facilities based on a numerical method. To ensure the accuracy of calculation and avoid the huge computational resources required by computational fluid dynamics-CFD, this study adopts a new method that combines the boundary element method based on the potential flow theory with the Morison equation to simulate the hydrodynamic response of a semisubmersible aquaculture facility in pure waves. The hydrodynamic response, including mooring line tension and motion response, is simulated in pure waves and compared with experimental values. The results show that the numerical data agree well with the experimental data. Based on the above work, the numerical model presented here can be used to analyze the hydrodynamic response of a semisubmersible aquaculture facility. The effects of wavelength and wave steepness on the mooring line tension and motion response are studied. Additionally, with increasing draught, both the force on the mooring line and the motion response decrease, and thus, an effective way for an aquaculture facility to avoid damage due to severe waves is through properly increasing draught. • A new method combines the boundary element method with the Morison equation is applied. • Hydrodynamic responses of a semi-submersible aquaculture facility are analyzed. • Influence factors including wave steepness, wavelength and draught are analyzed. [ABSTRACT FROM AUTHOR]

Biofouling of netting adversely impacts aquaculture operations and the structural safety of fish cages. In this study, the effects of the hydroids on the hydrodynamic performance of a plane net in waves were determined experimentally. A series of plane nets with various levels of biofouling was obtained via field sampling from a fish farm. The hydrodynamic force and motion response of both clean and biofouled nettings in waves were measured in a wave-current tank. The corresponding drag and inertia coefficients of the nettings were then calculated. In the studied waves, the wave force acting on the biofouled netting was as high as 3.88 times that acting on the clean netting. The maximum amplitude of the horizontal motion for the most severely biofouled netting was 3.84 times that for the clean netting. For various wave conditions, the biofouled netting drag coefficients and inertia coefficients were 3.30–7.72 and 5.04–10.83, respectively. The results indicated that netting biofouling by hydroids increases the drag force more substantially than it increases the inertial force. • Force and motion responses of hydroid-fouled nets in waves is experimentally studied. • The drag and inertia coefficients of the hydroid-fouled nets are analyzed. • Ratio of inertia force to drag force of both clean and biofouled nets are discussed. [ABSTRACT FROM AUTHOR]

Biofouling on netting is an important problem, which can increase hydrodynamics and reduce water exchange across the netting. In this study, drag effect of netting panels with artificial hard and flexible fouling was investigated by physical model tests. The results showed that fouling can cause a rapid increase in netting drag, and that the drag amplification factor of a netting caused by severe hard fouling can be up to 30 times higher than that of a clean netting. With increasing fouling, the drag coefficients for artificial hard fouling can up to 54% higher than those flexible fouling. The formulae proposed to predict the drag coefficients of netting panels with severe hard fouling, can provide a conservative drag coefficient for netting cage design to evaluate the cage safety. • The hydrodynamic effect of netting with different types of biofouling is studied. • The blocking effect of netting with different levels of biofouling is analyzed. • The prediction formulae for drag coefficient of netting compensates for the inadequacy of the existing empirical formulae. [ABSTRACT FROM AUTHOR]

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]

Investigation of the embedded chains in soil starts to play an important role in understanding the structural performance of mooring system, when the embedded anchors will be employed to sustain large loads with the gradually growth of installation depth of offshore aquaculture farm. The aim of this study is to investigate the dynamic response of mooring line considering the influence of embedded chains in clay soil for net cage system. Lumped-mass method is used to establish the numerical model for evaluating the performance of mooring line with embedded chains. To validate the numerical model, comparisons of numerical results with the analytical formulas and the experimental data are conducted. A good agreement of the profile and the tension response is obtained. Then, the effect of embedded chains on the static and dynamic response of mooring line is evaluated, and the dynamic behavior of mooring system considering embedded chains for net cage system is investigated. The results indicate that the soil resistance on embedded chains should be included to predict the mooring line development and the load on the embedded anchors in the numerical simulations. An appropriate safety factor should be included if employing the simplified model Case C at the initial design phase. And the effect of embedded chains on the holding capacity of embedded anchors in single-point mooring system for single net cage cannot be negligible during the design and operation phases. Consequently, it is profound to take into account the interaction of embedded chains and soil for accurately predicting the reliability of mooring system for fish cage. [ABSTRACT FROM AUTHOR]

This study aimed to examine the nonlinear dynamics of aquaculture platforms using the Hilbert–Huang transform (HHT). First, a numerical model that couples the boundary element method and lumped-mass model was used to simulate the dynamic behavior of a multi-body floating aquaculture platform in irregular waves. Second, a time–frequency analysis method based on the HHT was used to analyze the dynamic responses, including motion and mooring force responses. The results highlight that the surge motion of the net cage is dominated by the low-frequency components, whereas the heave and pitch motions are dominated by the wave-frequency components. The pitch response of the net cage has an obvious energy concentration near the peak frequency region of the incident wave. The surge motion of the net cage in the middle of the platform has a greater response to wave components with lower frequencies than that of the other two cages. The inherent characteristic vibration form of the mooring force response is affected by both the low-frequency characteristic of surge motion and the wave-frequency characteristic of motions in the vertical plane. The amplitudes of the wave-frequency components of the mooring force are much larger than those of the low-frequency components, which is like the characteristics of heave and pitch motions. The gravity periods of the wave-frequency mode components of the mooring force response were very close to the gravity period of the surge motion. [ABSTRACT FROM AUTHOR]

The primary purpose of the aquaculture facility is to provide a comfortable living environment and achieve welfare aquaculture for cultured fish in the open sea. Nevertheless, it has been a challenge to accurately assess the suitable flow region of the cultured fish inside the aquaculture facility for the severe ocean environment, which is a pressing concern for aquaculture farmers. First, the maximum domed swimming speed (MDSS) is used as an indicator to assess the swimming ability of Sciaenops ocellatus in this study, which can be measured through a vertical circulation water channel. Subsequently, the porous media theory is adopted and applied to predict the flow field inside the shielding device of the submersible aquaculture facility based on the shear stress transport k-omega turbulence model using the computational fluid dynamics (CFD) approach. In addition, combining the swimming ability of Sciaenops ocellatus and current speed distribution inside the shielding device of a submersible aquaculture facility, the suitable flow region is quantitatively assessed. The results indicate that when middle height, the inclined angle and net solidity are 0.4 m, 3π/12 and 0.65, respectively, the current speed inside the shielding device attenuates below the maximum domed swimming speed of Sciaenops ocellatus - 0.76 m/s and meets the survival of Sciaenops ocellatus for the actual sea condition with an incident current speed of 1.5 m/s. In addition, a suitable flow region for Sciaenops ocellatus, which is 204.512 m3 and accounts for 17.76% of the total aquaculture volume, is formed and located inside the shielding device, close to the downstream sides, and gradually extends towards the middle of the shielding device. Based on the swimming ability of Sciaenops ocellatus, using numerical means to describe the suitable flow region in detail is to provide clear and macroscopic guidance for aquaculture farmers. • Quantitative evaluation on the suitable flow region inside the shielding device is carried out. • The swimming ability of Sciaenops ocellatus is assessed through a vertical circulation water channel test. • The flow field distribution inside the shielding dvice is predicted through CFD model based on porous media. [ABSTRACT FROM AUTHOR]

This paper numerically studies the dynamic behaviors of a hinged multi-body floating aquaculture platform under regular waves. A coupled scheme that integrates the boundary element method (BEM) and the lumped-mass model is used to model the frames and the moorings/netting simultaneously. The effect of the hinge-joint rotational stiffness on the motion responses, mooring and hinge-joint forces at different wave steepnesses are examined. The results show that both the wave period and the hinge-joint rotational stiffness affect the pitch response of the multi-body aquaculture platform. However, the hinge-joint rotational stiffness has a predominant influence on the maximum pitch response. Unlike the pitch response, the stiffness has little effect on the heave response. Moreover, with the increase of stiffness, these two individual cages also show distinctive dynamic behaviors. The mooring force decreases with the stiffness and a 14.5% reduction has been obtained at the maximum stiffness. Under the same wave period, the y-direction rotation moment increases alongside the stiffness. When the wavelength is less than 0.9 times the structural span, the increased wave period decreases the y-direction rotation moment. In contrast, the stiffness rarely affects the z-direction hinge-joint force. • An innovative hinged multi-body aquaculture platform is investigated numerically. • A coupled scheme that integrates the boundary element method (BEM) and the lumped-mass model is applied. • The effect of the rotational stiffness on the dynamic responses at different wave steepnesses are examined. • Rotational stiffness has a predominant influence on the pitch response but it rarely affects the z-direction hinge-joint force. [ABSTRACT FROM AUTHOR]

Predicting the distribution of flow velocity inside a semi-submersible aquaculture platform and taking measures to optimize the flow field are critical to provide a dependable inhabit for farmed fish. This paper examines the hydrodynamics of a semi-submersible fish farm under current using the porous media method combined with a rigid wall based on the shear stress transport k-omega turbulence model. The results show that the presence of nets expands the low flow area and provides a better inhibit for farmed fish in the severe currents. The increase of net solidity enhances the damping effect on the flow, and it also simultaneously increases the current loads acting on the net. The attenuation caused by different net solidities produces a 66.10 % deficit of the flow velocity in maximum inside aquaculture platform. A maximum increase of 36.82 % for the drag force acting on the aquaculture platform is obtained. Furthermore, the distribution of drag force acting on the aquaculture platform in current is also assessed. Although the drag force of the frame-only is approximately twice compared to the net-only case, the improvement in the strength of the net structure under the action of flow should not be ignored. • A novel dependable habitat of farmed fish: a semi-submersible aquaculture platform is researched. • A new method combining the porous media and rigid wall is applied to analyze hydrodynamic characteristic. • The influence of the existence of the net and the net solidity on the flow field and drag force is studied. • The drag force acting on the net-only and frame-only for the aquaculture platform are discussed. [ABSTRACT FROM AUTHOR]

The gravity-type fish cage is extensively applied with the increasing demand for fishery products. The flotation ring is its main load-bearing component and supports the whole cage. So it is essential to study the hydroelasticity of the flotation ring for the safety of a fish cage. An analytical method is proposed to study the elastic deformation of a simplified flotation ring subjected to water waves. The equations governing in-plane deformations are obtained according to curved beam theory, in which the modal superposition method is used to represent the in-plane deformation of an element of the ring. Then, the motion equations of the ring are built up coupled with deformation equations. The correlation between the predicted results and the experimental data is acceptable to validate the numerical modeling. Then the effect of Young's module, radius of the ring, and wave conditions on elastic responses is discussed in terms of the prototype scale of a flotation ring. It is concluded that the deformations over the ring in the direction of waves' propagation are the largest, and that the mooring point in the head-on direction of the waves is critical for reliability of the ring. Large deformations of the flotation ring may induce the failure of the fish cage when the storm covers it. So more attention to the hydroelasticity of the flotation ring should be paid in the design for a fish cage. [ABSTRACT FROM AUTHOR]