Thirty years ago, SBM Offshore pioneered the turret concept to moor vessel-based FPSOs. Since then we’ve installed nearly 50 turrets around the world.
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 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 FPSO is weathervaning. 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.
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.
SBM offers a family of turrets designed to permanently moor vessels in “survival” conditions. These include internal turrets for harsh environments, internal turrets for a large number of flow lines and risers, and external turrets for mild to medium environments.
The internal turret mooring system is integrated into the forward end of a vessel and is supported on a large roller arrangement, located either inside a moonpool towards the keel of the vessel, or at deck level.
Internal Mooring Systems
In 1994, SBM Offshore delivered the first turnkey FPSO project in the North Sea. The Anasuria FPSO consisted of a newly built turret-moored monohull vessel with a storage capacity of 850,000 barrels, capable of a continuous operation for the design service life of twenty years in the unforgiving North Sea environment. The special tailor-made turret mooring system was designed to moor the FPSO vessel in all weather conditions up to and including the survival sea state of 12.8m significant wave height.
An internal turret consists of a chaintable structure anchored to the seabed via an array of mooring anchor lines. The chaintable structure is suspended from a heavy-duty roller bearing, which is located several meters above the keel of the FPSO. A radial assembly is used at the vessel main deck level to absorb the radial loads caused by the accelerations of the upper turret structural mass. The turret cylinder houses the riser i-tubes and supports the turret main deck, the manifold platform, the swivel stack, riser winches, and all of the fixed portions of the rigid piping. The Anasuria-type turret was designed to accommodate up to 14 risers of various diameters.
Another breakthrough in turret technology came in the late 1990s when SBM Offshore was called to moor two large, deepwater FPSOs for Petrobas, Brazil. The challenge was to accommodate 35 and 47 risers respectively in two weathervaning FPSOs for various functions. The swivel stacks used were among the largest in the industry, with up to 12 individual passes for hydrocarbons. This same technology was later applied to the Espadarte FPSO (285,000 DWT). The Espadarte‘s turret system accommodates 45 risers, servicing up to 15 production and 9 water injection wells.
The concept of an External Turret Mooring System became a reality when the stern of the 140,000 DWT Floating Storage and Offloading (FSO) vessel, the Alba Marina, was fitted with an external turret in 1985.
External Mooring Systems
An external turret mooring system comprises of a steel box-type structure that can be either close to or extended some distance from the bow or stern of the vessel, providing a foundation for a rotating bearing arrangement and a turret located above water. The bearing accommodates a fixed chaintable to which mooring chains and fluid transfer hoses are attached. The chain legs are fastened to the seabed either by anchors or piles.
Product and utility connections are made between the facilities on the vessel and the seabed via a swivel stack in the turret, allowing the vessel to weather vane around the fixed part whilst continuing production. An alternative design is the clamped bow turret where the rotating bearing arrangement is split between an above and below water section.
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