Safe operation of CLOSED aquaculture CAGES in WAVES

Floating closed cages have been in the focus of the Norwegian aquaculture industry, and currently there are several working prototypes and even more in the planning phase. The industry's motivation is to have better control over the water quality, and especially to avoid the problem with salmon lice. However, the transition from a conventional net-based cage to a closed cage is not trivial. When the volume is closed, the water inside is forced to move due to the cage motions which can lead to large wave motions on the free surface inside the cage, known as sloshing. This is different from net based cages where the water can move freely in and out of the enclosed volume. The mass of a closed cage will have an effective mass several thousand times larger than a net-based cage. Since great mass often means great forces involved, a closed cage will behave very different than a conventional cage. This is very important to consider in the design of mooring systems for closed cages. Sloshing represents a so-called resonance problem where motions of the cage can cause large waves inside the cage for certain frequencies. This can have a large influence on the dynamic behaviour and induced motions of the cage in a sea-state. Sloshing might also lead to large structural loads that should be considered in the design of closed cages. Closed cages represent a relatively new and extraordinary type of marine structures. Consequently, the knowledge on how these structures behave in the sea is limited. In the present project, scaled model tests have been conducted with generic closed cage models to study dynamic behaviour of the floating cage structure and contained water when exposed to waves. The measurements show that the contained water may lead to severe amplification of the cage motions and mooring line forces for relevant wave conditions. In the model tests, it was also observed that slowly varying wave-loads and resulting cage motions can dominate the mooring loads in irregular sea states. Additional measurements were conducted in the Ocean basin laboratory at SINTEF Ocean, where two elastic models of closed and semi-closed cages were tested in realistic sea-states. Both cage concepts showed similar challenges concerning sloshing and slowly varying wave-loads. A numerical model for the dynamic response and mooring loads of a rigid closed cage in waves was developed. There were also conducted validation studies where numerical predictions obtained with the developed model were compared with the laboratory data. The results confirmed the applicability of the model in cases when the elasticity of the structure can be neglected. Another numerical approach was used to study the combined hydrodynamic and structural response of an elastic cage, focusing on the estimation of coupled fluid-structure resonant frequencies and the estimation of mooring loads and slowly varying motions. These parameters were found important for the design of typical elastic closed cages. To provide good water quality for fish in a closed cage, exchange of the contained water is necessary. Hence, closed cages are equipped with water exchange systems with inlet and outlets to provide circulation. In the project, scaled model tests with forced sloshing in a water-filled circular-cylindrical tank were conducted to study the hydrodynamic interaction between sloshing and rotating flows. The scaled model was equipped with a system for water exchange, where the aim was to represent the typical main features of the circulating (rotating) flow in a full-scale closed cage. The experiments showed that rotation affects the internal water dynamics by modifying the natural frequencies of sloshing. Consequently, resonant sloshing in a cage with a rotating flow can be manipulated (and even suppressed within a narrow range of frequencies) by varying the rotation rate of the liquid. This was also confirmed by utilizing a theory for sloshing in a rotating liquid, which predicted both the natural frequencies modified by the rotation and the reduced sloshing amplitudes at certain frequencies, as observed in the experiments. Thus, it is shown both experimentally and theoretically that resonant sloshing in a cage with a rotating liquid is quite different from sloshing in non-rotating liquids. The difference is more significant at greater flow velocities and in cages with smaller diameters. Several scientific articles published during the project elaborate on the project's results. Here, detailed analyses of the experimental data are presented and applications of the developed numerical models for both rigid and elastic cages are shown. All this can further provide a basis for improving current methods for the design and analysis of closed cages and thus helping make them safer from a hydrodynamic perspective.

The project generated new knowledge important for the development of closed aquaculture systems. An active role was taken in the revision of the technical standard NS 9415, which led to the inclusion of a new chapter on closed-containment systems. The project sought to support the development of new aquaculture technology through collaboration with industry partners. Among the latter were Hauge Aqua Solutions AS with their innovative enclosed fish-farming system "Egget" and Hydra Pioner with their semi-closed concept "Produksjonstank". Both concepts have been awarded development licenses by the Norwegian Directorate of Fisheries and are now either under development or in a building phase. The availability of documented experimental experience with closed cages helps farmers and engineers better understand the typical challenges with such systems. International collaboration between world-leading research communities within experimental marine hydrodynamics was established.

Recently there has been increased interest around aquaculture in floating closed systems. Several concepts have been tested and have shown highly promising results when it comes to fish welfare and growth. A closed cage, however, responds very different to waves than a conventional net based cage. The enclosed water volume represents a large mass (several thousand times the mass of a conventional cage), and this influences the wave response profoundly. Adding to this, sloshing in the enclosed water can represent large, potential problematic forces on the structure. The industry's experience with such cages supports this. Since these closed cages are characteristically different than most other floating structures no adequate simulation tool exists, and the behavior in waves of these structures are difficult to predict and understand. In order to design floating closed cages that can safely operate in waves it is necessary to understand these structures better and in particular establish a good way to model and simulate the wave response. The project will study the wave response of floating closed cages, and with a main delivery to establish models that can be used to better design such structures for safe operation in waves. It is organized into four work packages that are tightly linked: Structural response (WP1), sloshing (WP2) and internal flow (WP3). Laboratory experiments will be an important tool to gain better knowledge of the wave response, and this knowledge will then be used to establish a first version of a numerical model that can estimate the wave response (WP4). The scope of the project will cover all three types of structural principle: i) flexible cages (bags), ii) semi flexible cages (glass-reinforced plastic) and iii) rigid cages (concrete). The industry partners in the project represent initiatives on all three types.