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Chapter: Aquaculture Engineering : Sea Cages

Fixing point - Mooring systems

The mooring line is fixed to the collar by a shackle. If a rope is used this should be spliced and a thimble used to reduce the bending of the rope.

Fixing point

The mooring line is fixed to the collar by a shackle. If a rope is used this should be spliced and a thimble used to reduce the bending of the rope. All bending will weaken ropes to some extent. A bending diameter of three times the rope thickness is necessary to avoid significant weakening. A knot can reduce the strength of the rope by 50%. Rings may also be used in the connections because they tolerate chafing better. It is an advantage to over-dimension the connection, for instance by doubling the size of the maximum transferred forces.

The force through the connection point can be divided into horizontal and vertical components. To avoid breakage of the cage collar by the tide, the vertical component should be transferred so that it is as low as possible, almost negligible. By using several connection points to the collar, the forces transferred at each point will be reduced, although this will increase the mooring costs. Normally at least four points are used. As said previously, it is advantageous to split the single mooring line before attaching it to the collar, so more connection points are achieved; such an arrangement is known as a hen foot mooring from its design.


To secure the fixing point against breakage, a secondary fixing may be used (Fig. 15.17). An extra rope or wire transfers forces directly from the collar to the mooring lines if the main fixing point breaks.

 


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