Operators mull new deepwater concepts

Expected subsea wells worldwide [72,053 bytes]

Expected deepwater fields worldwide [127,272 bytes

Deepwater discoveries around the world have combined estimated reserves of more than 40 billion bbl of oil, and operators are developing ways to produce hydrocarbons in up to 2,000 m of water.

Tim Warren, director of research and technical services at Shell International Exploration & Production BV, gave this estimate at the Deep Offshore Technology conference in The Hague, Nov. 3-5.

Warren broke down the total into major deepwater plays: West Africa 18 billion bbl of oil, U.S. 10 billion bbl, Brazil 8 billion bbl, West of Shetland 3.5 billion bbl, and Norway 2.4 billion bbl.

Yet this estimate is conservative, he said, because a number of deepwater areas were not included, while others are currently inaccessible but likely to open in the future.

Shell is involved in most of these areas already, said Warren: "For Shell, deep water offers the most significant conventional petroleum industry opportunity for expansion of the company."

He said Shell has just formed a multidisciplinary deepwater division. One of its first new projects will be development of giant Bongo discovery off Nigeria.

Although Shell has not fixed estimated reserves for Bongo, some sources have said the figure is more than 1 billion bbl of oil. Warren said Shell hopes to bring the field into production in 5-6 years.

Deepwater market

John Westwood, director of Douglas-Westwood Associates, Canterbury, U.K., told the conference the number of field developments in less than 50 m of water is declining, while the number beyond 200 m is rocketing.

Westwood said identified future deepwater developments have reserves amounting to 65 billion bbl of oil. Deepwater projects are expected to make up 11% of expected developments.

"This is a small proportion of the total," said Westwood, "but a growing one that represents a large future market."

Westwood said 254 field developments are under consideration in more than 200 m of water, while spending on identified deepwater developments is expected to amount to more than $100 billion.

North America will account for 44% of deepwater developments, said Westwood, while Europe will have 28%, South America 11%, Southeast Asia/ Australasia 11%, and West Africa 6%.

"The majority of future deepwater fields will use subsea wells," said Westwood. "In water depths to 200 m we are aware of plans for 421 wells, but beyond 200 m there are plans for 1,350 (see chart, p. 36).

Of 216 expected deepwater developments, Westwood said development schemes had been announced for 139. Floating production systems are involved in 68% of these (see chart, this page).

Helland-Hansen

Helge Skjaeveland, manager of deepwater prospect evaluation at Norske Shell, revealed company thinking on a large Norwegian Sea prospect.

Skjaeveland said a combined tension leg platform and floating production, storage, and offloading development is already being considered for Helland-Hansen prospect in the Voering basin off central Norway.

Water depth in Helland-Hansen is 600-900 m, and initial production is assumed to be 20,000 b/d/well, with a total of 20 horizontal wells to be used for development.

Although the prospect has not even been drilled yet, Skjaeveland said a TLP/FPSO solution can be identified as most suitable because of high predicted production rates of up to 400,000 b/d and the need for large, high-pressure risers for gas reinjection.

Ideas about Helland-Hansen performance are derived through analogy with Schiehallion field in U.K.'s West of Shetland area. And lack of gas export infrastructure and Norway's long queue of developments awaiting allocation of gas transport contracts, mean initial reinjection of gas is inevitable.

"Currently," said Skjaeveland, "a TLP is considered the most appropriate facility for steel catenary risers (SCRs), although it may be possible to use SCRs with an FPSO via some type of stationary-buoyed SCR support structure.

"It is deemed preferable to have the TLP available both to perform the final compression of the gas to be reinjected and to support the SCR necessary for import of gas and export of oil to the FPSO."

Petrobras FPSOs

While Shell and other operators see deepwater as an attractive option, Mauricio de Aratanha of Petroleos Brasileiro SA (Petrobras) told delegates his company sees deepwater technology as a necessity.

Brazil has total estimated oil reserves of more than 15 billion bbl, he said, of which 35% are in water more than 1,000 m deep. Petrobras has long used production semisubmersibles but is now looking to use FPSOs.

The company has five FPSOs under construction, all conversions of obsolete products tankers in the Petrobras fleet. Three will be installed in Marlim field, one in Albacora field, and one in Barracuda field's pilot production scheme.

"The giant Marlim, Albacora, and Barracuda fields," said de Aratanha, "with large volumes of oil at a great distance from the shore, posed a big challenge, inducing Petrobras to change from its traditional approach of using semisubmersible-based floating production systems and pipelines.

"This approach would entail a price tag of more than $1 billion for the offshore and onshore pipelines alone. New construction of large semisubmersibles would also be necessary, since the world market could not possibly supply all the units required for conversion."

De Aratanha said Petrobras also had a number of old tankers, which were coming to the end of their useful life as products carriers because of tighter transportation regulation and requirements for double-hull construction.

"The conversion of these ships into FPSOs was then the best solution company-wide," said de Aratanha. "The FPSO is able to offload produced oil through a dedicated fleet of shuttle tankers. The company's cash flow is optimized, without heavy investment in pipelines, which bring no additional oil revenues."

Petrobras subsea

Denilo Oliveira of Petrobras told delegates of a new concept for subsea systems that eliminates the need for crane barges and pipelay vessels as conventionally used during installation.

Instead, Petrobras has developed a system that enables drilling rigs and supply ships to be used to install manifolds, jumpers, flowlines, and risers, and control umbilicals.

Oliveira explained that two versions of the system have been designed: a subsea development linked through a steel catenary riser to a production semisubmersible and a subsea development linked through a flexible riser to a FPSO (see schematics on p. 37).

With either option, christmas trees would be connected either in piggyback or daisy-chain formations to a shared actuated manifold (SAM), in which one actuator typically enables a number of operations simultaneously.

Oliveira said some of the equipment normally placed on a subsea manifold is installed on the christmas trees in this system. This way, the SAM is light enough to be installed by a supply ship.

The SAM concept has been patented by Petrobras. A SAM typically weighs less than 25 metric tons. The SAM is 5.9 m long by 4.8 m wide by 5.5 m high.

For both options, a drilling rig/supply ship combination is used to install import and export jumpers, while pipelines can be installed with a drilling rig fitted with J-lay equipment.

For the production semi option, risers can be installed by a drilling rig, but for the FPSO version, a flexible flowline laying vessel would still be required.

This new concept will be first used next year for installation of P-18 production semisubmersible in Marlim field. Because specialist vessels are not needed, said Oliveira, installation costs can be reduced by 10-25%.