"Nytt fra HAVBRUK" newsletter no. 2/2010:
Knots that hold
Powerful marine forces pound away at aquaculture netting. Now, basic research at SINTEF in Trondheim has calculated just how strong these forces can get.
Greater knowledge about the wave and current loads on aquaculture cage nets will lead to more specialised nets that are up to the challenge.
Previous methods of calculating these forces have been inaccurate in some cases, so in the WaveNet project, scientists at SINTEF Fisheries and Aquaculture carried out basic research on the hydrodynamic forces exerted on netting-based structures. The researchers conducted computer computations as well as tested physical models. The WaveNet project is presented in the HAVBRUK programme’s latest newsletter (“Nytt fra HAVBRUK” no. 2/2010, available in Norwegian only).
A knotty problem
A net consists of lines and knots – and the knots play a surprisingly major role in the overall strength of a cage net. “When we isolated the forces exerted on the centre of these knots, we found that those forces can be substantially more powerful than previously presumed,” says project manager Pål Lader, Senior Scientist at SINTEF.
A net that tears will allow escapes and lead to further equipment damage. In addition to keeping the fish inside, a net must also be open enough for large volumes of fresh seawater to flow through it.
“Basic research like this enhances our understanding of the various forces exerted on the geometric shape formed by the knot and its lines, and how the current flows around that shape. We’ve produced some interesting findings. And although it may appear that our findings have no immediate utility value for the industry,” says Dr Lader, “this kind of basic research is critical for constructing better nets in the future.”
One component of the project involved simulating various waves.
“It’s important to chart the effects of different wave types on large aquaculture facilities. Some of these sites spread out several hundred meters,” explains Dr Lader, “so there are substantial correlation effects between wave forces at different points within a large site. We must understand these in order to grasp a wave’s full, cumulative effect.”
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