Large oil and gas discoveries have been made in the deep waters of the Gulf of Mexico, Brazil and West Africa. So far, development plans for these fields in West Africa and Brazil have favored development with floating production, storage and offloading vessels (FPSO) and either subsea wells or a combination of subsea wells and dry tree wellhead platforms. The favored development for these large fields in the ultra-deep Gulf of Mexico has not emerged yet, but there is intensive study going on with respect to the application of FPSOs in the Gulf of Mexico.
The application of subsea wells in the Gulf of Mexico has already been established. The high costs of drilling and producing these ultra-deepwater fields using conventional approaches naturally yields an incentive to consider new methods and paradigms. One approach being promoted by several contractors for West Africa is the "multi-function" vessel, which can do drilling, production, storage and offloading.
These floating drilling, production, storage and offloading vessels (FPDSO) have the advantage of eliminating the need for costly mobile offshore drilling unit (MODU) operations for development drilling and completions, and they allow dry tree operations and workovers. The FDPSO combines the functions of a wellhead platform and an FPSO, reducing the total costs for facilities.
A disadvantage of the FDPSO approach is that the vessel's schedule is driven by the longest lead time component, usually the process facilities. While combining all functions on one vessel might save some capital expenditures, the advantage is loss of early delivery of a drilling/wellhead platform. This means that more development drilling is required from a MODU. Some operators also perceive added risk in "placing all the eggs in one basket" as is the case for an FPDSO.
Most FDPSO designs are based on conventional mono-hull construction. These are suitable for subsea wells and flexible risers in most environments, but these hulls are not ideal for rigid risers and steel catenary risers in harsh environments or those subject to cyclonic events.
Alternative
A new FPDSO design has been developed by Aker Maritime to address these issues - the Aker DPS-2000 (formerly known as the shoebox spar) was originally intended for West Africa. Recently, the configuration has been optimized for improved motions and is capable of operations in Brazil and the Gulf of Mexico as well.
The present DPS-2000 configuration has been developed for 2 million bbl oil storage, 200,000 b/d production, 40 dry tree wells, plus additional steel catenary risers (SCR) slots for subsea wells and/or export lines. The hull, which is similar to a deep draft semisubmersible, consists of a lower hull, four edge columns, and a central upper hull to support the drilling module and centerwells.
The columns provide support for the process module and the quarters/utilities module on either side of the drilling module. The hull arrangement is designed for optimal motions, but perhaps equally important, it allows deck floatover at the drill site. This allows for flexible scheduling of process and hull delivery to accommodate an accelerated drilling program, early production, and deferred process selection. The present design accomodates a total deck operating weight of 40,000 metric tons.
The motions of the Aker design make it suitable for operations in Brazil as well as West Africa. A new variation of is under development which will also allow operations with dry trees and SCRs in the Gulf of Mexico. Oil storage volume is in the lower hull. Oil displaces water at ambient pressure in this tank using the same method employed on the Brent Spar in the North Sea and numerous gravity base structures.
The dry tree risers are situated in centerwells. The risers are located by buoyancy cans contained within the centerwell. Similar riser systems are currently operating on spar platforms in the Gulf of Mexico. The advantages of this design are:
- Wellhead and FPSO functions are combined on one floater with the flexibility provided by two floaters
- Risers use proven spar technology
- Motions are suitable for dry tree and SCR application in West Africa, Brazil, and with some hull optimization, in the Gulf of Mexico
- The design is not depth limited
- Mooring with steel strand is feasible in water depths up to 10,000 ft.
Aker recently received shipyard estimates for construction and has performed economic comparisons with other field development options for a typical West African field. These studies indicate a significant cost and net present value advantage for this approach, over conventional approaches to large West African fields. Development work is currently focusing on the following areas:
- Motion of the oil/water interface in the storage tank
- Hull outfitting definition
- Floatover operations in West Africa
- Optimization of the hull for the Gulf of Mexico, with and without oil storage.
We expect to have these issues resolved in the coming months.
References
Choi, G., Halkyard, J. and Kumpis, H., "FPSO With Drilling and Dry Trees for West Africa," West Africa '99 Conference, Abidjan, Côte d' Ivoire, March, 1999.
