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External Turret Mooring systems
The external turret mooring system includes a turret system that is located at the extreme end of an outrigger structure attached to the bow of the vessel. This turret contains one large-diameter 3-race roller main bearing that transfers both the vertical and the horizontal loads from the riser and mooring system to the outrigger structure.
In addition, it transfers the own weight of the turret and the turntables above the main bearing to the outrigger. The turntables include the process facilities for commingling fluids from the risers using manifolds, as well as pigging facilities and/or chemical injection facilities to assure the flow in the risers. The swivel stack is located at the highest deck of these turntables and transfers fluid and electrical power to the turret access structure located on the outrigger. This access structure also provides support access to the different turntable decks.
The external turret including (a part of) the outrigger can be installed as one integrated structure on the vessel bow, minimising the scope of work for the integration of the turret into the vessel. Furthermore this integration work can be executed while the vessel is afloat. Typically the cantilever type outrigger structure is integrated above the main deck of the vessel. However, depending on the departure angle of the mooring lines and on the size of the outrigger structure, part of the bow might need to be removed to avoid clashes between the vessel and the mooring lines.
Optionally, risers are available that allow for fast disconnection from the turret. This disconnectable turret uses a buoy to keep the risers afloat when disconnected. Typically the disconnectable option is required when the vessel operates on dynamically positioned (DP) station keeping. Disconnection within minutes is required to avoid damage to the risers during failures in the DP system causing drift-off or drive-off conditions.