Turrets / Mooring Systems

In the early 1980s, SBM Offshore pioneered the turret concept to moor vessel-based Floating (Production) Storage and Offloading systems (FPSOs/FSOs). In essence, a turret mooring system is composed of a fixed turret column supported by an internal or external vessel structure via a bearing arrangement. The vessel-bound components can therefore freely weather vane around the turret, which is fixed via a number or anchor lines to the seabed.

This turret arrangement allows the FSO/FPSO to adopt the direction of the least resistance against waves, wind and currents. Toroidal swivels allows for the transfer of fluids across the rotary interface while the FSO/FPSO is weather vaning.  Above deck level, located between the fixed turret column and the swivel stack, is a structure that supports the piping manifolds. These manifolds comingle the fluids (oil, gas, water, etc.) and thus reduce the number of fluid paths in the swivel stack.

In general, a turret mooring system consists of 5 main components:

1. A number of anchor lines, to keep the vessel on station;
2. A turret column, to connect the anchor lines to the vessel;
3. A bearing arrangement, to allow the vessel to weather vane;
4. A vessel structure to support the bearing arrangement;
5. The fluid transfer system consisting of an underwater fluid transfer system, a manifold, a toroidal swivel stack, and vessel piping arrangement.

In a turret mooring system, the turret column is secured to the seabed by catenary mooring legs to maintain the vessel within a design excursion limit. This excursion limit ensures the safe working of the underwater fluid transfer or riser system from seafloor to turret. The turret concepts come in a number of different design layouts, but all use a number of catenary or, in deep waters, (semi) taut mooring lines.