The Tri-Catenary mooring system (TCMS) is based on three mooring legs radiating at more or less equal angles from a connecting node below the sea surface, with a single length of chafe chain rising from the node to a chain stopper positioned on the bow of the moored vessel. A key feature of many TCMS systems deployed to date is the existence of only one chain mooring line in any one direction, with the adjacent one being 120º away. The seabed end of each leg is typically held in place by a high holding power, drag anchor, firmly embedded by cross tensioning as an integral part of the standard deployment procedure. The mooring system allows the vessel to weathervane around the node in response to wave and wind movements.
TCMS systems are suitable for marginal fields in
For ultra-shallow water projects at 55m, the TCMS traditional 3-leg mooring system can be used; each leg is typically made up of 500m of Grade 3 chain connected to the mooring system node. This type of mooring project will be close to the shore and so the wave movements will be a good deal less that those found further offshore. In such shallow water vessel rotation is restricted to ±90 degrees movement either side of the mean heading. The TCMS production tower is also shorter and less complex and supports a maximum of 4 risers / umbilical orientated in a lazy wave profile from either pipeline or subsea trees.
As we have seen, the flexibility of the Tri-Catenary mooring system offers independent operators a reliable mooring and production solution for offshore marginal fields. The opportunity to deploy the TCMS mooring system quickly and cost effectively compared with traditional mooring systems presents a compelling case for operators wishing to maximise field production and profitability.
By their very nature, deepwater fields in water depths greater than 1,000m can be expensive to develop and so marginal fields lacking existing pipeline infrastructure are often neglected. Conventional deepwater mooring systems are complex and expensive to deploy at water depths greater than 1,000m. In addition, they often require substantial modifications to the FPSO such as the addition of a mooring turret.
In a deepwater marginal field, a turret capable of handling a small number of risers is inappropriate, whereas a deepwater TCMS, designed for four or five risers, is more practical. And with a life time of around 7 years compared with 25 -30 years for more substantial deepwater fields, the TCMS mooring system provides a less complex mooring at half the cost of a traditional deepwater mooring system.
The differences between the shallow and deepwater TCMS are longer mooring lines based on synthetic fibre rope rather than chain, together with the addition of discrete buoyancy units that give the mooring system a wider profile and prevent clashing between the mooring lines and risers.
The risers too are lengthened and hang off loads from the TCMS production tower maintained by changing the riser profile and enhancing the buoyancy. Subsea the production risers can be supported by buoyancy tanks and connect to the FPSO via flexible jumpers, reducing tension on the production risers.
The major difference between the shallow water and deepwater TCMS is the ability to disconnect the bow-mounted riser production tower in the event of adverse weather conditions. The production system tower is designed to be fully disconnectable within 8 hours and abandoned to 70m water depth while the FPSO sails to a safe port. When the FPSO returns, and is on station, the tower is retrieved and reconnected to the bow of the vessel.