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For complex flexible structures such as nets, the determination of drag forces and its deformation is a challenging task. The accurate prediction of loads on cages is one of the key steps in designing fish farm facilities. The basic physics with a simple cage can be addressed by the use of experimental studies. However, to design a more complex cag...
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... For the structural methods, various methods were proposed to assess the structural responses of different components of the fish cage system. A mass-spring model [11] or a truss model [12] was used to calculate the motions and deformations of the aquaculture nets, mooring lines and bridles. A rigid body model [13] or beam model [12] was adopted to model the motions and deformations of the floating collars and the sinker tubes. ...
Submersible fish cages are designed for installing in the open sea sites that are subjected to violent sea conditions to reduce the hydrodynamic forces acting on the fish cages. To assess the dynamic responses of a single point mooring submersible fish cage under the rough sea conditions, a numerical model including hydrodynamic and structural methods is proposed. The waves and current are modelled using the airy wave theory. For the floating collars and sinker tube, the Morison model is adopted to calculate the hydrodynamic force and the modal superposition method is proposed to calculate the structural responses. For the aquaculture net, the screen model is adopted to calculate the hydrodynamic force and the extended position-based dynamics (XPBD) method is proposed to obtain the structural deformations. The hydrodynamic dynamic forces and the deformations of the fish cages are compared between the fish cages on water surface and in deep water. Results show that when the fish cage is submerged 20m from the water surface, the average horizontal force is reduced by 30% and the variation of the horizontal force is reduced by 81%. Significant reductions in the deformation of the floating collars are observed. Therefore, using submersible fish cage can help reduce physical stress and avoid structural damage for the fish cage system. In addition, the coupling of XPBD and MS can provide a quick solution for the structural modelling of the fish farm system, which can be utilized at the initial stage of fish farm design.
... A numerical method is adopted to study the dynamic response of the fish cage during the lifting operation in the present study. For numerical simulations, there are a few approaches and methods that can be used, such as finite element method (FEM) [6], OrcaFlex [7], and mass-spring model [8]. Rui [2] to model and analyse a semi-submersible fish cage. ...
... Rui [2] to model and analyse a semi-submersible fish cage. Cifuentes and Kim [7] calculated the current load acting on a fish cage using a Morison-force model applied at instantaneous positions of equivalent-net modelling using Orcaflex [7]. Lee et al. [8] used the mass-spring model to simulate the flexible structures' behaviour to understand the movements and design an appropriate system. A new efficient method called extended position-based dynamic (XPBD) method [9][10] was proposed to simulate the cloth dynamics in games. ...
... Rui [2] to model and analyse a semi-submersible fish cage. Cifuentes and Kim [7] calculated the current load acting on a fish cage using a Morison-force model applied at instantaneous positions of equivalent-net modelling using Orcaflex [7]. Lee et al. [8] used the mass-spring model to simulate the flexible structures' behaviour to understand the movements and design an appropriate system. ...
The dynamic behaviours of the lifting operation of a gravity-type fish cage under calm sea conditions are investigated in this study using an extended position-based dynamics (XPBD) method to obtain the structural deformations of the aquaculture nets. The original XPBD is improved to accurately predict the tensions of the aquaculture nets by applying correction forces. The present XPBD is validated by comparing the experimental results of a flexible horizontal net. The time-step sensitivity is verified for the case of the lifting operation of the fish cage. Results show that the lifting force increases rapidly resulting from the weight of the sinkers at the bottom of the side net. The maximum tension of the net is located at the net ropes connected to the centre point of the bottom net. The structure of the bottom net should be enhanced for safety lifting operations.
... There are several studies reporting drag coefficients on nets, even including biofouling [3][4][5][6][7][8][9][10]; however only a couple of those studies look at the hydrodynamic response of metallic nets. Research on metallic nets applied to aquaculture shows the advantages of this kind of nets, e.g. ...
The determination of mooring tension on a prototype cage for salmon production on exposed areas in southern Chile is obtained using experiments at laboratory scale, numerical simulations as well as on site measurements using a submersible load cell. The 40m diameter prototype cage used in this study is constructed out of HDPE pipe and has a ballast tank attached to the bottom to control the operational depth in case of high waves. The cage uses a copper nickel alloy net cancelling the need for a predatory net as in traditional systems. First, drag coefficients for the net were obtained at the Wave/Towing tank at Universidad Austral de Chile (UACh), plus the characterization of dynamic response and deformation of two cage models. These results were used as input for numerical simulations of the cage using the commercial software AquaSim. The last stage of the campaign was the measurement of tension on the prototype cage, using a load cell on a mooring line along with measurements of tides, wind, waves, and currents. The results are the base for a more complex analysis, since the net, due to its geometry, stiffness and wet weight, has a particular behavior when compared to traditional fiber-based nets.
... Some commercial software packages, such as FhSim (https://fhsim.no/, accessed on 11 November 2022), Orcaflex [79], AquaSim (https://aquasim.no/, accessed on 10 February 2023), and Ansys AQWA [44], can be used for analysing the mooring system of a fish farming structure. ...
While moving fish farms to offshore sites can be a more sustainable way to expand farmed fish production, the fish pens have to contend with a harsher environment. Thus, it is necessary to draw on offshore engineering competences for designing and analysing the offshore fish farming infrastructure. This paper reviews existing design and analysis guidance from maritime classification and national/international authorities that can be applicable for offshore fish farms. Based on the existing design guidelines, a review of design criteria for offshore fish farms under the following subtopics is provided: design life, design environmental loads, combining environmental loads, and miscellaneous load conditions. This review on the global performance analysis procedures and methods is presented based on practices used for neighbouring industries, such as offshore oil and gas and wind energy production, under the following subtopics: hydrostatic analysis, hydrodynamic analysis, and mooring system analysis with introducing theoretical background and modelling techniques. This paper also highlights limitations and cautions when using these design and analysis methods. Providing this comprehensive information, as well as commentary on their applications, will help engineers and designers to develop offshore fish farming infrastructure with confidence.
... A truss element is introduced as a consistent net element model in order to reduce the number of elements and improve computational efficiency (Tsukrov et al., 2003). The mass-spring or lumped mass model regards the net structures as mass points interconnected by massless springs (Cifuentes and Kim, 2017;Turner et al., 2017;Reite et al., 2014). In order to simplify net structures modelling and improve computational efficiency, the mesh grouping method (Hou et al., 2017;Xu et al., 2012;Yang et al., 2019) has been introduced to the fish cage simulation. ...
In this paper, we propose an implicit Eulerian–Lagrangian model to resolve the fluid–structure interaction of submerged nets. The Reynolds-averaged Navier–Stokes equations are solved on an Eulerian grid using OpenFOAM and the geometry of the flexible net structures is tracked using Lagrangian points. The motion and deformation of the flexible net structures are advanced using an implicit scheme to avoid excessively small time intervals. The force acting on the net structures is calculated based on a mass–spring system. The screen model and the immersed boundary method are implemented to capture the interaction between the net structures and the surrounding fluid. We present a new drag coefficient formulation as based on a piecewise function for different net solidities to obtain a much improved drag force prediction. The numerical results are compared with a series of existing experiments showing excellent agreement and improved drag force predictions.
... The lift force can be calculated by the same drag force term in the Morison equation, but the lift coefficient, instead of the drag coefficient, must be introduced [36]. Considering that the mass of the nets accounts for a small proportion of the whole cage mass, the inertia force of the nets may be ignored [5,37,38]. Moreover, when the nets are attached to the cage sides and bottom, the wave and flow directions are almost 0 • or 90 • to the normal direction of the net plane where the lift force is almost zero [15,39]. ...
To maximize the utilization of ocean resources, shorten the return period of investment and directly supply energy to the fishing cage, this paper performs a preliminary study for a state-of−the−art concept integrating a floating offshore wind turbine with a fishing cage. An octagonal semisubmersible rigid fishing cage with a slack catenary mooring system is designed to match the NREL 5 MW offshore baseline wind turbine. Combined with the blade pitch controller, fully coupled aero-hydro-elastic-servo-mooring simulations are performed through FAST and AQWA to explore the dynamic performance of the integrated system. Free decay conditions, uniform wind with irregular and regular waves, and turbulent wind with irregular waves are tested. The results showed that the integrated system works normally at the operating conditions and exhibits different dynamic characteristics for various scenarios. Additionally, the study on the influence of mooring line length indicates that the increasing line length can significantly affect the cage surge motion and the maximum and mean values of the upwind line tension at fairlead. Specifically, the maximum surge motion with a 924-m−long line is 404.8% larger than that with an 880−m-long line. When the line length increases by 5%, the maximum and mean line tensions decrease by 45.7% and 47.7%, respectively, while when the line length increases by 10%, the maximum and mean line tension decrease by 52.9% and 54.2%, respectively. It should be noted that the main purpose of this work is to conduct a preliminary study on this integrated system, aiming to provide an idea for the conceptual design, modeling and simulation analysis of this integrated system.
... With regard to normal and parallel drag coefficients, the results obtained by the present numerical model and those obtained experimentally showed good similarity with the relative errors lower than 3.5%. These relative errors are 6.2%, 25%, 6.5%, and 4.2% lower than those obtained by Patursson et al. (2010), Chen and Christensen (2016), Cifuentes and Kim (2017), andTang et al. (2017b), respectively. The numerical simulation results obtained by Cifuentes and Kim (2017), Tang et al. (2017b), and this study revealed that the drag force of netting increases with increasing flow velocity. ...
Nettings are complex flexible structures used in various fisheries. Understanding the hydrodynamic characteristics, deformation, and the flow field around nettings is important to design successful fishing gear. This study investigated the hydrodynamic characteristics and deformation of five nettings made of polyethylene and nylon materials in different attack angles through numerical simulation and physical model experiment. The numerical model was based on the one-way coupling between computational fluid dynamics (CFD) and large deflection nonlinear structural models. Navier-Stokes equations were solved using the finite volume approach, the flow was described using the k-ω shear stress turbulent model, and the large deflection structural dynamic equation was derived using a finite element approach to understand the netting deformation and nodal displacement. The porous media model was chosen to model the nettings in the CFD solver. Numerical data were compared with the experimental results of the physical model to validate the numerical models. Results showed that the numerical data were compatible with the experimental data with an average relative error of 2.34%, 3.40%, 6.50%, and 5.80% in the normal drag coefficients, parallel drag coefficients, inclined drag coefficients, and inclined lift coefficients, respectively. The hydrodynamic forces of the polyethylene and nylon nettings decreased by approximately 52.56% and 66.66%, respectively, with decreasing net solidity. The drag and lift coefficients of the nylon netting were approximately 17.15% and 6.72% lower than those of the polyethylene netting. A spatial development of turbulent flow occurred around the netting because of the netting wake. However, the flow velocity reduction downstream from the netting in the wake region increased with increasing attack angle and net solidity. In addition, the deformation, stress, and strain on each netting increased with increasing solidity ratio.
... The lift force can be calculated by using the same drag force term, but it has to introduce a lift coefficient instead of the drag coefficient [86]. By considering the mass of a net element to be small and its motion is not violent, the inertia force may be neglected [73,87,88]. Besides, the net (especially a rigid EcoNet) is attached over the sides and bottom where wave and current flow directions to the net normal vector are almost 0° or 90° where the lift force of the net is near zero [89,90]. ...
In response to public and environmentalists’ oppositions towards expansion of current nearshore fish farms, fish farmers have begun to explore offshore sites that provide more sea space, better waste dispersion and less contests with other sea space users. This thesis begins with the definition of offshore for fish farming by studying various attempts presented in the literature, and proceeds to highlight the challenges faced by going offshore. Next, a review of various fish cage designs is presented. The fish cage designs may be divided into two main groups: open net cage system and closed containment tank system. The open net cage system is further categorized into six types: floating flexible, floating rigid, semi-submersible flexible, semi-submersible rigid, submerged, and bottom-resting cages. The closed containment tank system is categorized into two types: floating rigid containment and flexible bag containment tanks. The advantages, disadvantages and application for offshore sites are discussed for each type of cage. Types of mooring system for fish cages are presented and their pros and cons are discussed.
With accumulated evidences and comparison studies on the fish cages and mooring systems, it is suggested the future of offshore fish farms in a way of collaboration between offshore fish farming and renewable energy industries that has synergetic advantages in sharing a substructure and mooring system, better utilization of sea space, and reduction of service and maintenance costs. In this thesis, a novel integrated fish cage and floating spar wind turbine, named as COSPAR, is proposed. The fish cage design comprises a semi-submersible cage and a partially porous collar barrier at the top perimeter of the cage. A wind turbine is mounted on a hybrid (concrete-steel) spar platform that is connected to the fish cage. The porous collar barrier attenuates wave energy for calmer water in the cage as well as it keeps predators and debris out, and serves as a working platform. The wind turbine provides the necessary power for operation, monitoring and maintenance of the fish farm. COSPAR is held in place by four mooring lines (catenary chains) attached to the spar in order to gather mooring lines within a small distance so as to leave a small benthic footprint.
Hydrodynamic response analyses were performed for COSPAR by using ANSYS Design Modeler (modelling) and AQWA (solver) under a free-floating condition in frequency domain, and a coupled COSPAR with the mooring lines in frequency and time domains. For the analyses, three environmental conditions (representing 5-year, 20-year and 50-year wave return period with a constant current speed and water depth) at an offshore fish farming site in the Storm Bay of Tasmania, Australia, were adopted. A comparison study was conducted against having a semi-submersible fish cage only (i.e. without the spar platform), and COSPAR shows better hydrodynamic responses in the following respects: (1) more stable motion responses in heave and pitch against wave and current forces, (2) less susceptible to the viscous damping when it is assumed by a linearized drag force of Morison elements in the frequency domain and (3) reduction of tension forces in the mooring lines.
A design concept of the porous collar barrier for COSPAR is presented. The key-dimensions of the porous collar barrier were decided by performing a series of numerical analyses based on the linear potential wave theory and the eigenfunction expansion method. Single and double collar barrier designs corresponding to single net and double net cages were considered, and the wave transmission, reflection and energy-loss coefficients of the barriers were analyzed in order to obtain an optimum collar barrier design. It was found that a double collar barrier with underwater height of h=4m and porosity combination of ε1=0.25, ε2=0.5 for inner and outer layer respectively, is recommended for COSPAR as it yields competitive wave scattering performance and saves construction material. COSPAR with the porous collar barrier performs better heave and pitch responses, and less mooring tension and anchor uplift forces than COSPAR with a non-porous collar barrier so that these features will maximize the efficiency of wave attenuation.
A frequency domain (FD) approach is presented for analyzing the motion responses of COSPAR under wind and wave actions. Wind loads were estimated by using pre-defined thrust coefficients. Wind force coefficients were specified in order to incorporate the thrust force and moment into the hydrodynamic response analysis that can be performed by AQWA. It was found that the AQWA-FD approach can furnish wind and wave induced motion responses that are comparable with published experimental and numerical results of the DeepCwind floating foundation. In order to investigate COSPAR with a 1MW wind turbine, a hydrodynamic response analysis by the time domain method was employed together with the AQWA-FD approach for wind loads consideration. From the results obtained, it was evidenced that COSPAR can provide a stabilized heave response for onboard working, keeping the porous collar barrier underwater for wave dissipation, acceptable pitch angles for a stable wind turbine operation and permissible mooring tension forces. For a preliminary mooring redundancy test, an accidental case (i.e. the least favorable mooring orientation with three environmental conditions) was considered to investigate the mooring redundancy of COSPAR. In spite of a loss of one up-wind line, the maximum tension forces of adjacent mooring lines were observed within the breaking tension force with 15% marginal value.
In summary, COSPAR is a promising solution for offshore fish farming as it performs stable hydrodynamic responses under wave, current and wind actions, and provides a stand-alone power system with small carbon footprint, a capability to create calmer water for fish well-being, a simple and stable mooring system with small benthic effect. The presented method of analysis and specifications considered for uncertain structure components (e.g. net, collar barrier and mooring lines) can guide fish cage developers as they are based on commercially proven software packages and credited offshore structure rules & standards.
... Also based on a mass-spring model, Xu et al. (2013) and Zhao et al., 2007a; presented a series of simulations on fish nets under different wave and current conditions, as well as various structural characteristics and forms. Cifuentes and Kim (2017) conducted the simulation of fish nets in currents by employing a Morison force model to calculate the current load and by modelling the nets as lines and buoys in the commercial software OrcaFlex. Chen and Christensen (2017) developed a fluid-structure interaction model involving a porous media model and lumped mass structural model to analyse the flow through and around a fish cage and the deformation of cage nets. ...
Owing to heavy criticisms of nearshore fish farming for causing environmental pollution and encroaching on sea space used for shipping, boating, recreational sea activities and marine eco-tourism, offshore fish farming has now being seriously considered. Moreover an offshore site provides more pristine water and greater space for increased fish production. However, offshore fish farming poses challenges such as a more energetic sea environment. A higher sea current can lead to large deformation of fish net and hence a net volume reduction which compromises fish welfare. With the view to identifying the effects of various important parameters on net volume reduction of a gravity-type open-net fish cage, this paper adopts a mass-spring model for the dynamic analysis of current-induced net deformations of cylindrical fish nets with discrete weights hanging at the bottom edge of the nets. In this model, the net mesh comprises knot nodes and bar nodes connected by tension-only massless springs. The spring stiffness is determined from the net bar diameters and material properties. The current-induced loads are applied to each node and calculated based on Morrison’s equation. The governing equation system for nodal motions can be established according to Newton’s second law, and solved by using the Runge-Kutta method for the real-time net deformations. The effects of net string reinforcements, weight distributions and net shapes on the net volume reduction are studied with the view to shed insights into how one may improve fish cage designs to effectively mitigate net deformation under high sea current speeds in offshore fish farming sites.
... There is a limited amount of experimental research focused especifically on the influence of cages over flow: Li et al. [2005], Patursson [2008], [Rasmussen et al. [2015], Zhao et al. [2015], and a small number of numerical studies: Huang et al. [2006], Patursson et al. [2010], Zhao et al. [2013a], Cornejo et al. [2014], Bi et al. [2014], Herrera et al. [2018], Winthereig-Rasmussen et al. [2016]. Another example may be found in Cifuentes and Kim [2017], whose aim was to improve cage design considering structural and environmental conditions, integrating nonlinear effects such as deformations and viscous drag forces, using Orca Flex code based on a Morison-force model. ...
This paper presents a numerical study to assess the hydrodynamic effects of different levels of biofouling in fish cage aquaculture netting. The methodology adopted includes high resolution Large Eddy Simulation (LES) coupled to a regional model implemented within the framework of the Coastal and Regional Ocean Community Model (CROCO). The physical coherence of this approach has been previously verified and published by the authors. Three levels of biofouling were considered to describe a clean fish cage netting with no biofouling, medium biofouling and high biofouling growth levels. Clean and biofouled fish cage netting were described using a porous media model and the porous media coefficients were taken from the literature. A salmon farm located in the Estero Elefantes Channel in Patagonia (45°39′16.50 S 73°35′59.40 W) was used as study case. A change in the biofouling level in aquaculture fish cages netting can lead to drastic changes in the local velocity field. The high biofouling level produces the largest change in velocity direction and magnitude not only locally in near-cage areas, but also changing features in the overall fjord circulation. Velocity reduction in the shadow zone of fish cages reached values between 30% and 10% of the incident current for null and high biofouling levels, respectively. Simulations also showed dramatic effects of biofouling on flushing time. The predicted water availability of the salmon farm shows decreases of 27% and 36% considering medium and high levels of biofouling. This decrease in the available water is also related to a reduction in the amount of available oxygen and accumulation of waste products, affecting not only the health of farmed fish but also the environment. These and other environmental and productive implications are discussed.
