DNV Software’s Riflex is a tailor-made advanced tool for static and dynamic analysis of slender marine structures, primarily used offshore. But Barlindhaug Consult has successfully used Riflex to model fish farms, beneficiating from experience gained while simulating the impact of ice blocks on a riser.
By automating input processing, modelling, Riflex calculations and reporting, Barlindhaug Consult AS was able to success-fully overcome the constraints of a low-margin market. The company was able to deliver high-quality structural analyses of fish farms. The main challenges were in the complexity and size of the models, along with simulation of particular coastal environments.
New requirements for fish farm design
The Norwegian Standard Organisation issued a standard (NS 9415) in 2003 regulating the design, dimensions and operation of marine fish farms. Since 1 January 2006, all existing fish farms must comply with this standard, created to prevent the escape of fish. This means that the structural integrity of existing fish farms must be checked in relation to a 50-year storm. The response of the entire farm under storm conditions must be modelled and calculated.
Modelling a fish farm using Riflex
The primary area of use for Riflex is the modelling of risers or hawser for offshore installations. It is part of SESAM’s DeepC package. Originally, fish farms could not be modelled using Riflex as they include large net panels. But studies on the response of fish cages to waves and current (Ormberg, 1991; Løland, 1991) enabled Riflex to include the modelling of net panels. In addition, Riflex’s capabilities were extended to elements floating at the surface.
With the latest upgrades, Riflex includes all necessary elements to build the model of a plastic fish farm. It requires using a very large number of lines, as one fish cage may have to be modelled by 500 lines, one farm may include more than 20 cages, and the whole model can include thousands of lines. This is possible today due to increased computational speed and memory.
Non-linearity and coupling effects
The main focus is on plastic fish farms where the floating structure is made of polyethylene tubes. These farms are usually built around a main rope frame moored via anchor lines to the sea floor and the shore. The fish cages hang in plastic rings that are moored on the main rope frame. These structures present a high degree of flexibility and non-linearity, concerning for instance:
Ÿ | The wave and current forces on the net |
| Ÿ | The response of the net and the current shading effects |
| Ÿ | The plastic ring response to the net, wave and anchoring forces |
| Ÿ | The response of the anchoring system to environmental and structural forces. |
Such a complexity requires dynamic simulation of the whole system. In addition, the entire farm must be described by one model – including coupling effects between the different elements.
Model of fish farm anchored to the shore and the fjord bottom
One example is the consequence of an anchor line rupture. This will possibly cause disequilibrium of the anchoring system and deformation of the main frame. The disequilibrium is dependent on the anchoring properties and the number of fish cages moored on the frame. This could lead to large deformations and the rupture of some of the floating structures. The design of the floating structure is thus dependent on the mooring design.
Response of the vages to waves and current
Calculated stresses in a farm plastic ring from waves and current
Typical 50-years storm sea state spanning over four fish farm cages
Coastal challenges
Fish farms are usually located in areas protected from the open sea. The environmental design conditions are different from offshore conditions. Examples are:
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Challenging market
In order to be competitive, the price for such analysis must be kept very low. This requires a minimum of man-hours for the structural analysis of each fish farm. On the other hand, the complexity of the system requires high-quality calculations. Thanks to a combination of Microsoft Excel, the MathWorks Matlab and DNV Software’s Riflex, the pre-processing (reading input data, model-building and writing of Riflex input files), calculation (simulation of the 50-year storm in ultimate and accidental limit states), and post-processing (data analysis and reporting) is automated.
The terrain and practical knowledge expertise is ensured through a collaboration with Barlindhaug Norfico, a company with many years of experience in consulting services for the aquaculture industry.
Successful technological development
The success was due to the flexibility of Riflex to model a large variety of components, effective automation of the modelling and calculation process, and the combination of aquaculture and marine technology knowledge at the heart of Northern Norway.
The technological developments successfully opened new markets to Barlindhaug Consult.
Authors: Basile Bonnemaire/Arnor Jensen
Barlindhaug Consult