MULTIPHASE BOOSTER UPS PRODUCTION FROM SUBSEA WELL

The Rogn South subsea well has the world's first commercial subsea multiphase boosting system. The well produces to A/S Norske Shell's Draugen field, in the Norwegian Sea.

The Smubs (Shell multiphase underwater booster station) provides additional energy to transport a mixture of gas and liquids over long distances. This reduces the back pressure on the reservoir to potentially enhance both production and recovery (Fig. 1).

In-house Shell Internationale Petroleum Maatschappij B.V. (SIPM) has studied estimated facility costs and performance for a multiphase boosting system for a typical small (50 million bbl) field between 20-50 km from a host facility in water depths between 1501,000 m. The studies showed that technical costs per barrel of oil produced could be cut by up to 30% compared to conventional technology.

The Smubs development program began in 1987.

CONCEPT FEATURES

The Smubs main features are:

A single retrievable cartridge that houses all active components susceptible to wear
No orientation requirements for the pump cartridge unit
Hydraulically set and tested seals
Vertical installation and retrieval with a single tool, and a remotely operated vehicle (ROV) only for monitoring.

The retrievable cartridge is encapsulated in a receiver barrel arrangement (Fig. 2). The major components are the interface handling and locking mechanism, the suction flow mixer, the pump, and the pump driver.

The system uses an integrated helico-axial pump and hydraulic-drive turbine. A performance envelope was estimated for the range of helico-axial pumps with hydraulic design originating from the Institut Francais du Petrole (IFP). The design is based on tests on several pumps in the Smubs development program and the published results of the Poseidon P300 pump tests.

The helico-axial booster, a dynamic PUMP, was selected after evaluation of various pumps, including positive displacement, twin screw, and piston types. Dynamic pumps were favored because they are small and light, with simple and reliable mechanical design and simple operation.

Water-turbine drive has the advantage of simplicity, compatibility with the environment, ease of control, and mechanical robustness. Also, it eliminates the need for subsea electric power transmission, including a wet mateable connector, lubricating oil systems, and mechanical shaft seals.

Norske Shell decided to incorporate a Smubs system in the Draugen project after bench and loop testing between 1987 and 1990 confirmed the performance and demonstrated the practicality of installation, operation, and retrieval.

PROJECT DESCRIPTION

The Rogn South subsea well, in 270 m (886 ft) of water, is about 10 km (6.2 miles) from the Draugen platform. The Smubs unit is located in the southern water injection template, 4 km from the well and 6 km from the platform (Fig. 3).

The system takes advantage of excess water pressure in the injection system to provide hydraulic power to drive the booster. The booster drive could use the full water injection flow of 12,700 cu m/d (80,000 b/d).

DESIGN BASIS

The principal design details of Draugen Smubs are summarized in Table 1.

The 38-ton Smubs module is installed on a standard well slot within API guideposts (overall module dimensions are 3.5 x 3.5 x 5 m).

The 2.3-ton pump/driver cartridge is 4.8-m long and 0.4-m in diameter. It is installed in the inboard connector as a separate unit and can be retrieved separately for maintenance (under these conditions the well by-passes the unit and flows unboosted to the platform).

The cartridge is designed for the maximum system pressure of the well, 207 bar (3,000 psi). A seal arrangement on the pump/driver cartridge separates inlet and outlet ports on both the water and production side, as well as creating a double seal barrier to the environment.

IMPLEMENTATION

Norske Shell let the contract to Framo Engineering on Aug. 30, 1990, for the design, manufacture, and factory acceptance testing of a turbine-driven Smubs unit. Design began shortly thereafter, with manufacturing taking about 12 months through to the first quarter 1992. Factory acceptance testing and an integration test with the complete water injection template continued into early 1993. The unit was delivered to Norske Shell in May 1993 (Fig. 4).

The unit was installed in August 1993 by the Seaway Condor diving support vessel in about 2 days including vessel mobilization/demobilization. Start-up awaited the completion of other platform operations.

START-UP

First started on Oct. 5, 1994, the Smubs initially ran for about 30 min under stable conditions before being shut down after the control system logic was checked out. It was restarted when a test separator became available and performance conditions, both natural and boosted, could be established.

During the restart on Nov. 8, 1994, the Smubs initial water injection (turbine) rate was 355 cu m/hr (2,233 bbl/hr). This was increased in 6-hr intervals to the planned operating water injection rate of 490 cu m/hr. The unit has operated at this rate since Nov. 9, 1994.

Preliminary data show that total production from the Rogn South well increased by about 60%. Natural flow was about 1,900 cu m/day (11,950 b/d) compared to a boosted level of 3,100 cu m/day (19,500 b/d). The increase in net oil production was 40%, or 600 cu m/day (3,800 b/d).

The unit has operated continuously apart from two minor nonsystem related shutdowns.

ONGOING DEVELOPMENT

Over longer distances, hydraulic losses become more significant, as does the cost of the line for providing the high-pressure water drive. Electric power is thought to be more practical for extended satellite applications. An electric version of Smubs (known as Elsmubs) is now being developed.