BP selected SBM Inc.'s large-diameter internal turret to moor the Schiehallion FPSO in the harsh environments off-shore the Shetlands in the Atlantic Frontier area. This internal turret allows the FPSO to freely weathervane while trans-ferring fluids and services to the subsea fac-ilities.
The diameter required for a turret, which can accommodate up to 24 large-diameter risers and umbilicals, precluded the use of a conventional slewing ring arrangement as used in many previously built SBM turrets.
Two designs were studied and compared – a split roller bearing of 12 m mounted on the vessel main deck level and a bogie bearing arrangement derived from previously used systems in the large offshore cranes delivered by SBM's sister company, Gusto Engineering. A bogie system with a diameter of 16.25 m was chosen for the application.
The Schiehallion turret is based on an entirely passive mooring, that is, no dynamic positioning assistance is required to keep the vessel on station. The natural weathervaning tendency of the system ensures minimum environmental loading on the catenary mooring arrangement. Analytical methods were used to conceive a mooring system, which was confirmed by a series of model tests representing the 395 m of water depth.
The turret mooring consists of a fixed and a rotating/vessel part. The fixed part of the turret is composed of a 14-m diameter cylinder, inserted through the vessel hull and supported at the vessel deck level on the turret collar by a bogie bearing arrangement. This allows the vessel to weathervane around the turret, and comprises a series of 20 vertical bogie assemblies and 18 radial wheels. The vertical bogies and the radial wheels run on bolted and sectored rails. All components can be replaced in situ if required.
The main design features of the bogie bearing turret as supplied for Schiehallion are:
- The number of vertical bogies and horizontal wheels can be adapted easily to meet different survival and fatigue load requirements
- The turret diameter can be adapted to suit the required number of flow lines and risers; designs catering up to 36 risers were considered in early Schiehallion work
- In addition, the large diameter turret provides extra space during hook-up for riser and mooring installation and inspection requirements.
Bearing in mind the turret flexibility mentioned above, SBM designed a large diameter bogie bearing system for an FPSO to be located offshore Brazil in a water depth greater than 1,000 m. The vessel used during the design was based on a converted very large crude carrier but a new-built vessel could easily be used instead. The proposed turret design was based on the Schiehallion bogie bearing design and allowed for the tie-in of up to 75 risers, including umbilicals.
A bogie system with a diameter of 16.25 m was chosen for the application. The turret mooring consists of a fixed (green colored) and a (blue colored) rotating/vessel part.
Due to the large turret diameter, only a small number of decks are required to house the piping headers, emergency shutdown and shutdown valves, the chain and offshore installation winches, and the electric equipment room, above the turret.
The large diameter decks are straddled by a four-legged gantry structure. This structure carries all the piping from the manifold room, via the swivel stack, to the vessel main deck. An additional advantage of the large manifold deck is that it can accommodate an upside-down swivel stack at its center. This upside down or "reverse" swivel stack leads to the several significant advantages:
- Decreases overall turret height
- Decreases gantry height
- Reduces deflections between manifold and swivel inlet piping
- Reduces pipe lengths
- Provides easy swivel access for inspection and maintenance.
This type of turret mooring construction would make the turret capable of resisting harsh hurricane-type environments like the Gulf of Mexico.
SBM has gone one step further by integrating the drilling function on this type of FPSO turret for a floating production, drilling, storage, and offloading application. Drilling will take place through the center of the geostationary large diameter turret. This capability is achieved by placing the drilling support structure over the turret-like gantry, creating a drill floor above the turret. Up to 75 risers can be tied to the turret with enough space available for all material handling/inspection requirements between the FPSO and subsea.
The Schiehallion FPSO has now been on location for almost five years and has proven to be a good solution for this harsh environment. This proven turret design can be directly applied to large deepwater field developments with a great number of flow lines and risers, and high production rates.
The authors would like to thank BP and IHC Caland. Companies in the IHC Caland Group that have been involved in the design and supply of the Schiehallion turret are SBM and Gusto Engineering.
References available from Leen Poldervaart at email: Leen.Poldervaart@singlebuoy.com.