External Sea Loads and Internal Hydraulics of Closed Flexible Cages

The main objective for the project has been to increase the understanding of the behavior of floating closed flexible cages in waves and currents. The development of closed cages are motivated by desire to control the water quality inside the cage, and thus eliminate important problems, like lice. To go from a net based cage to a closed cage, there are some hydrodynamic and structural challenges. By closing the cage the behavior of the structure changes significantly, making a flexible closed cage very different from most other floating structures. This is mainly due to the large mass associated with the closed water volume, and deformability of the structure. There is very little available knowledge about how these structures behave in waves and currents, and the focus of the project is to establish such knowledge. When this project was initiated in 2011, there were significantly fewer initiatives within the area of enclosed cages than it is today (2017). Based on the then existing concepts, it was decided to focus on flexible bags in this project. This was mainly chosen because bags was considered the easiest way to connect closed cages to already existing structures and infrastructure of the industry. For instance, existing floaters from conventional cages could be used for the bags as well. The project has been led by SINTEF Fisheries and Aquaculture, and it has had three other research partners: University of New Hampshire, United States Naval Academy (both USA) and Norwegian University of Science and Technology. In addition, the project has had eight Norwegian industry partners. There has also been a PhD student connected to the project. The work in the project has mainly been experimentally founded. Five rounds of model tests have been conducted. Models of cages with different shape and configuration have been tested in waves and current. Measurement of deformation, movement and flow in and around the bags have been made. The results are reported in the scientific publications. At the end of the project, a total of six peer-reviewed articles have been published, and two articles is under review. The project has held two longer (multi-day) workshops, and three one-day meetings with industry partners. This has been coordinated so that industry partners have been key contributors into the experimental design, and at the same time, results from the project have been presented to the partners. The results of the project have shown that the behavior of these structures are complex. A parameter that has been especially investigated is the filling ratio. During normal operation, the bags are usually filled so that there is a slight overpressure inside the bag, and the bag thus have a defined shape. In certain cases (e.g. due to damage or at certain operating conditions), the bag may be under filled. When there is no over pressure in the bag to maintains its shape, and the bag may deform. In current, an under filled bag were seen to experience significant (up to 2.5x) greater drag force than a filled bag. This could be critical in certain situations, and it will be important to avoid simultaneous occurrence of under filled condition and high currents. In waves, however, under filled condition results in improved behavior. Wave experiments shows that the bags can withstand very little waves before the waves run over the floater, causing a transport of water from the outside and into the bag. Subsequently, this leads to an increased filling of the bag, eventually by so much that the floater sinks beneath the surface and there is a critical situation. The reason this happens is the large mass associated with the closed water volume. Due to this, the bag does not follow the wave, and the center of gravity of the structure hardly moves in the vertical direction, and this restricts the floater from moving vertically. Thus, the waves washes over the floater. For an under filled bag, however, the motion of the floater is to less extend coupled with the bag. This makes the floater follow the wave and wave over-running can be avoided. Qualitative observation of local deformation of the bag also shows that an over filled bag may have challenges in the form of snap-loads. This will have less impact when the bag is under filled. The main conclusions of the project is that closed bags will have a complex behavior in waves, and will initially be very susceptible to wave action. It is however possible to influence the bags behavior in waves and one can envision this principle used in future structures. Closed cages represents a great potential for the aquaculture industry, but it is still a way to go before these structures are understood sufficiently to construct safe and robust solutions.

Closed flexible cages (CFC) offers a route for developing closed fish production systems (CFPS) that might be faster than developing new CFPS in rigid material. The rationale behind this is that flexible floating systems are not far from the current net c age floating fish farm systems, and can be used in existing floating fish farms in combinations with net cages. It is important that any new system really do reduce sea lice infestation, reduce release of nutrients and reduce the probability of fish esca ping. The CFCs are structures that differ from most other ocean structures and since there is little previous experience with these kinds of structures, the importance of conducting basic research related to behaviour and operation, to avoid accidents an d unforeseen incidents, cannot be emphasized enough. The flexibility and deformation of the bag is closely coupled to the hydrodynamic forces, making the hydrodynamic load far more difficult to understand than that for a rigid structure. Therefore, techni ques to estimate dynamic movements and internal stresses need to be developed in order to limit the risk of fish escaping due to structural collapse or operational failures. In addition the enclosed body of water must be properly maintained to obtain goo d water quality, fish welfare and high bio-security. This project proposes to establish a better understanding of the internal flow and to look into ways of maintaining good water quality even as the bag deforms. The CFC system to be considered in this pr oject will use technology to remove seal lice and sludge. Reaching the objectives of the project will give the industry a better understanding of the behaviour, operation and reliability of closed flexible cages, and it will give the scientific community a starting point for further work on these systems. The contribution from the project will help the industry to increased reliability, safety and predictability of their CFC products.