Arne Fredheim

Exposed Aquaculture in Norway

Farming of Atlantic salmon in exposed areas poses unique challenges to operations, structures and equipment due to severe and irregular wind, wave and current conditions, and sheer remoteness. Many of the operational challenges seen at present sheltered sites are likely to be amplified when moving production to more exposed locations. There is, however, a strong Norwegian industrial interest in utilizing such areas. A new research center, the Exposed Aquaculture Operations center has been initialized to develop competence and technology to address the challenges. Six core research areas are identified that will be crucial to address the challenges with exposed farming, with a focus on the industrial status in Norway. Four areas target technological innovations that will enable safe and reliable exposed aquaculture operations: 1) Autonomous systems and technologies for remote operations, 2) Monitoring and operational decision support, 3) Structures for exposed locations and 4) Vessel design for exposed operations. Two areas represent core requirements for sustainable production: 5) Safety and risk management and 6) Fish behavior and welfare. This paper describes the research needs and the research strategy planned for the Exposed Aquaculture Operations center.

Numerical Investigation of the Flow Through and Around a Net Cage

Structure and design of fish cages can be improved by the knowledge of the flow pattern around and inside the net cages. To address this problem, commercially available computational fluid dynamics (CFD) software is used to analyze this problem by calculating the drag and the flow velocity distribution around cylinders with different porosities. The results of these simulations are compared with the data from experiments which have been previously published. Aquaculture cages are very large structures that consist mainly of netting, which can be approximated by small cylinders connected at knots. But due to the large number of these cylinders (millions for a single salmon farming cage), it is computationally expensive to model the exact geometry. Bio fouling is another factor which is of particular interest as fouled nets (lower porosity) can significantly reduce flow of well-oxygenated water reaching the fish during normal rearing conditions. Therefore the numerical approach used to simulate the flow through and around the net cage is to consider it as a circular cylinder with a porous jump boundary. Drag coefficient and flow pattern are compared with available experimental data. Vertical cylinders are used for this study. Different porosities have been used for the simulations as for the experiments (0%, 75%, 82% and 90% open area) in order to simulate the impact of the fouling on the load of the net structures and the flushing of the cage. The results show that a porous jump with a pressure drop proportional to velocity squared has the best agreement with measured data.Copyright

Numerical Modeling of Wake Effect on Aquaculture Nets

Accurate modeling of drag forces on net cages due to water current is important when designing floating fish farm systems. These drag forces give a major contribution to the total environmental forces on a fish farm, especially mooring line forces. When subjected to current, the net cage will deform. High current velocities can result in large deformations and lead to collapse of the net cage. For circular fish farms with a flexible floating collar, large deformations may induce contact between the weighting system and the net, resulting in abrasion that can cause tearing of the net material and consequently failure that will lead to fish escape.The motivation for this paper is to obtain a better understanding and more accurate model for drag forces and corresponding deformations of circular net cages due to water current. Calculation of drag forces on a net cage is complicated due to the porous nature of the net, geometry and flexibility of the system. Adding to the complexity is the wake effect, or reduced velocity, behind each individual twine which will have a significant effect on the forces and deformations of the net cage. This wake effect will result in reduced inflow velocity on parts of the net being downstream.A method for estimating wake effects acting within an aquaculture net structure was developed and implemented in a numerical code taking net deformation into account. Numerical simulations of a cylindrical net cage were compared with experimental results. Comparison between simulations with and without wake effect revealed a reduction in total drag up to 22% when wake effect was applied. Although the model consistently overestimated drag forces on the net cage (average deviation of 25%), simulation results compared well with measurement data, particularly for low current velocities where deviations were as low as 7%. This indicates a consistent wake effect and drag model that produces conservative estimates of drag forces on net cages.Copyright

Structural Analysis of Aquaculture Nets: Comparison and Validation of Different Numerical Modelling Approaches

The performance of three different numerical methods were compared and evaluated against data from physical model tests. A parameter study of a simplified net cage model subjected to a steady flow was performed by all methods, varying the net solidity and the flow velocity. The three numerical methods applied models based on springs, trusses or triangular finite elements. Hydrodynamic load calculations were based on the drag term in Morison’s equation and the cross-flow principle. Different approaches to account for wake effects were applied. In general, the presented numerical methods should be able to calculate loads and deformations within acceptable tolerance limits for low to intermediate current flow velocities and net solidities, while numerical analyses of high solidity nets subjected to high current velocities tend to overpredict the drag loads acting on the structure. To accurately estimate hydrodynamic loads and structural response of net structures with high projected solidity, new knowledge and methods are needed.Copyright

Simulation and Validation of a Numerical Model of a Full Aquaculture Net-Cage System

Numerical simulation models are useful tools for the design and capacity analyses of cage-based fish farm systems. To ensure that such tools produce realistic estimates on forces and deformations experienced by fish farms, it is important to validate the models through comparison with experiments. A recent experiment investigated the response of a scaled model of a full aquaculture net cage placed in a mooring system when exposed to waves and current. In this study, a numerical model of this system containing the main components used in the physical experiments was set up and simulated. After simulations the tension in anchor lines, bridles and buoys were compared to the corresponding data series obtained in the experiments. The comparison indicated that FhSim was able to reproduce the main dynamics and responses of the physical model when exposed to currents and waves. Furthermore, a sensitivity analysis was conducted, aimed at investigating how much model output is affected by variations in the stiffness of the mooring system.Copyright

An Optimum Design Concept for Offshore Cage Culture

There is an overwhelming support to move aquaculture cages into offshore waters. Some of the key drivers for moving into offshore waters are limitation of available space near the coast, conflicts within as well as other coastal users, prospect of limitless expansion in offshore sites, the potential for optimum growth conditions and the need to reduce the production cost by increasing the scale of operation. Therefore, by using a set of requirements derived earlier by the authors, the paper looks into the current offshore cage designing concepts in order to propose an optimum design concept for offshore aquaculture. With the help of an expert panel, representing various disciplines which are important for fish farm development, the assessment point towards a single point mooring cage concept as the best option for offshore aquaculture farming. This concept is demonstrated by the use of simple geometric relations and graphs, showing the relation between the total horizontal forces, FH, vertical force component, FV, as the cage submerge. While the contribution from the current, FHC, to the total horizontal force, FH, is kept constant, the contribution from the waves to the total horizontal force, FH, (as assumed to be 33%) is subjected to a reduction proportional to the factor e2kz, representing the reduction in water particle velocity as a function of depth. Further, in light of the requirements derived through the major stakeholders, this paper also propose an alternative classification of cages into two major categories, i.e. systems that are intended to resist and dissipate environmental forces and system that are designed to avoid environmental forces.Copyright

Tensegrity Structures in the Design of Flexible Structures for Offshore Aquaculture

Aquaculture is the fastest growing food producing sector in the world. Considerable interest exists in developing open ocean aquaculture in response to a shortage of suitable, sheltered inshore locations. The harsh weather conditions experienced offshore lead to a focus on new structure concepts, remote monitoring and a higher degree of automation in order to keep the cost of structures and operations within an economically viable range. This paper proposes tensegrity structures in the design of flexible structures for offshore aquaculture. The finite element analysis program ABAQUS™ has been used to investigate stiffness properties and performance of tensegrity structures when subjected to various forced deformations and hydrodynamic load conditions. The suggested concept, the tensegrity beam, shows promising stiffness properties in tension, compression and bending, which are relevant for development of open ocean aquaculture construction for high energy environments. When designing a tensegrity beam, both pre-stress and spring stiffness should be considered to ensure the desired structural properties. A large strength to mass ratio and promising properties with respect to control of geometry, stiffness and vibration could make tensegrity an enabling technology for future developments.Copyright

Technological Approaches to Longline- and Cage-Based Aquaculture in Open Ocean Environments

As the worldwide exploitation rate of capture fisheries continues, the development of sustainable aquaculture practices is increasing to meet the seafood needs of the growing world population. The demand for aquatic products was historically satisfied firstly by an effort to expand wild catch and secondly by increasing land-based and near-shore aquaculture. However, stagnation in wild catch as well as environmental and societal challenges of land-based and near-shore aquaculture have greatly promoted efforts to development farming offshore technologies for harsh, high energetic environments. This contribution thus highlights recent technological approaches based on three sample sites which reach out from sheltered near-shore aquaculture sites to sites with harsh wave/current conditions. It compares and evaluates existing technological approaches based on a broad literature review; on this basis, we then strongly advocate for presently available aquaculture technologies to merge with future offshore structures and platforms and to unveil its added value through synergetic multi-use concepts. The first example describes the recent development of longline farming in offshore waters of New Zealand. New Zealand has designated over 10,000 ha of permitted open ocean water space for shellfish farming. The farms range from 8 to 20 km out to sea and a depth of 35–80 m of water. Research has been ongoing for the last 10 years and the first commercial efforts are now developing in the Bay of Plenty. New methods are being developed which should increase efficiency and reduce maintenance with a particular focus on Greenshell mussel (Perna canaliculus) and the Pacific Oyster (Crassostrea gigas), Flat Oyster (Tiostrea chilensis) and various seaweeds. The second case study involves a long-term, open ocean aquaculture (OOA) research project conducted by the University of New Hampshire. During the course of approximately 10 years, the technological aspects of OOA farming were conducted with submersible cages and longlines, surface feeding systems and real time environmental telemetry. The grow-out potential of multiple marine species such as cod (Gadus morhua), haddock (Melanogrammus aeglefinus), halibut (Hippoglossus hippoglossus), blue mussel (Mytilus edulis), sea scallop (Placopecten magellanicus) and steelhead trout (Oncorhynchus mykiss) were investigated at a site 12 km from shore. The last study presents a multi-use aspect of aquaculture for an open ocean site with fish cages attached to existing offshore wind energy foundations. Technological components such as mounting forces and scour tendencies of two different cage structures (cylindrical and spherical) were investigated by means of hydraulic scale modeling. The cages were pre-designed on the basis of linear theory and existing standards and subsequently exposed to some realistic offshore wave conditions. The wind farm “Veja Mate” in German waters with 80 planned 5 MW turbines anchored to the ground by tripiles is taken as the basis for the tested wave conditions. Based on findings stemming from the three example approaches conclusions are drawn and future research demand is reported.

FHSIM — Time Domain Simulation of Marine Systems

Numerical time domain simulations have proven applicable for analysing marine systems and operations, but available tools often target specific sub-problems or applications associated with a system or an operation. Such tools are also often limited in terms of extensions and usage. This has motivated the development of FhSim at SINTEF Fisheries and Aquaculture (SFA). FhSim is a software framework aimed at simulating especially marine systems in the time domain, using models described as ordinary differential equations (ODEs).In this paper, we present the architecture and core functionality of the FhSim framework, including modelling, integration and 3D-visualisation. We also present a series of simulation cases which illustrate the different core properties of FhSim, including numerical simulations of aquaculture structures, model-based estimation of trawl nets and optimisation of energy systems in ships.Copyright

Experimental Study on Interaction Between Waves and Netting: Damping Effects and Forces

One of the possibilities to expand sea-based fish farming is to move the aquaculture installations away from the conflicts of the coastal zone, and into more open ocean locations. However, open ocean aquaculture puts other demands on the structures than aquaculture in sheltered locations, and in this context it is necessary to understand the behaviour of the aquaculture structures as they are exposed to large sea-loads from waves and current. Flexible netting is a main part of most sea-based aquaculture structures, and in this paper the interaction between waves and netting is studied. Experiments were conducted at the narrow wave flume facility at the University of Oslo, Norway, where several different regular wave cases were run through netting with different solidity. The wave energy was measured after the wave had passed through the net and compared with the energy of an undisturbed wave to assess the wave damping properties of the net. The vertical and horizontal forces were also measured. The findings show that the damping effects of the netting are not necessarily correlated with the wave forces, indicated complex nonlinear processes contributing to the fluid-net interaction. The amount of nonlinear energy in the wave and force waveforms is also investigated, and it is shown that the nonlinear energy in the incoming wave results in an even higher level of nonlinear components in the forces experienced by the net.Copyright