The function of the mooring system is to keep the farm in a fixed position and to avoid transfer of excessive forces to the cages, especially vertical forces. Different methods are used for mooring depending on the type of cage, how exposed the sites are to the weather, and the requirement for position exactness. Two major systems are used for mooring: pre-stressed and slack (Fig. 15.15). Slack mooring is used to moor ships which drift around one anchorage point. Such mooring systems are well adapted for stiff constructions such as ships. Few cage farming systems are stiff and therefore pre-stressed mooring systems are most often used, but slack mooring has also been tried.24 Pre-stressed systems are well adapted for use in flexible constructions, and in correctly designed systems the forces will be equally spread over the entire farm. Pre-stressing of the mooring system is performed at high tide and forces can be up to several tens of kilonewtons.
A special type of mooring is needed for tension leg cages in which the forces are taken up in the tension legs which, regardless of the weather conditions, are always under tension. In this way the dynamic loads resulting from the weather, that affect traditionally moored cages and create slack and tension in the lines, are avoided. The challenge when constructing tension leg moorings is to find technical solutions where the legs will always be tensioned.
A pre-stressed mooring system
contains three major parts (Fig. 15.15):
· Mooring lines which include the point of attachment to the cages