Tri-Catenary mooring systems (TCMS) provide a secure mooring system, particularly when used with storage tankers and floating production, storage and offtake (FPSO) vessels
All mooring system design codes require there to be a means of safe positioning in the event of a single line failure. The TCMS offers a choice of options to ensure that there are no single point failure modes; this can either be with the use of additional mooring legs and a redundant chafe chain; or, as has been approved by a number of Classification Societies, a package of individual measures which together have been accepted as providing an adequate level of protection. One of these measures is an enhanced strength factor of safety for non-redundant systems, which is recognised in the latest version of the DNV Position Mooring Code OS-E-301. This measure takes advantage of the efficiency of this type of mooring system, where the peak mooring forces can be reduced by “tuning” the mooring system’s response to match the vessel’s response in severe weather.
The Tri-Catenary Mooring System can be tuned by the careful design of such factors as the length of the chafe chain and the mass of the node. Thus, use of larger components to increase the strength, and reduce the risk of equipment failure, is not financially burdensome.
By using three lines compared with other mooring systems including CALM buoys and turrets which have typically 6, 8, or more legs, the TCMS is quicker to deploy and less ‘stiff’ than other tanker moorings. This ensures that the tanker’s natural fore and aft motions, with the right engineering, can be damped by the mooring catenaries and the pendulum effect of the chafe chain.