TECHNOLOGY U.K. offshore project employs new technologies

Dec. 18, 1995
Guntis Moritis Production Editor The Liverpool Bay project, to produce oil and gas from four offshore fields in the British sector of the Irish Sea, includes a number recently developed technologies. GE Power Systems and GE Marine & Industrial Engines says that this project is the first to use its GE LM6000 aeroderivative gas turbine for power generation. On the Lennox platform, Cooper Cameron Corp.s first platform spool tree was installed. Marine fouling protection of the platforms is provided

Guntis Moritis
Production Editor

The Liverpool Bay project, to produce oil and gas from four offshore fields in the British sector of the Irish Sea, includes a number recently developed technologies.

GE Power Systems and GE Marine & Industrial Engines says that this project is the first to use its GE LM6000 aeroderivative gas turbine for power generation. On the Lennox platform, Cooper Cameron Corp.s first platform spool tree was installed. Marine fouling protection of the platforms is provided by LEV Groups proprietary ocean-powered marine growth preventers.

Liverpool Bay project

The blocks in the bay were awarded to Hamilton Oil Great Britain plc and partners in 1987 and 1991. Hydrocarbons were discovered between 1990 and 1992 in the:

  • Douglas oil field

  • Hamilton gas field

  • Hamilton North gas field

  • Lennox oil and gas field.

Together these fields are known as the Liverpool Bay project. BHP Petroleum Ltd. now owns Hamilton Oil.

The estimated reserves in the project are about 1.2 tcf and 150 million bbl of oil (OGJ, Aug. 28, p. 60). Production is expected to peak at 300 MMcfd and 70,000 bo/d.

The Hamilton, Hamilton North, and Lennox fields are developed with three similar wellhead structures, normally unmanned platforms without processing equipment. Production is processed at a three-platform complex on the Douglas field (Fig. 1).

From Douglas, gas will flow through a 21-mile pipeline to Point of Ayr. Oil is exported through a 10-mile subsea pipeline to an offshore loading unit about 20 miles north of the Welsh coast.

The gas will be processed onshore at Point of Ayr before entering a 17-mile pipeline to a 1,400-megawatt, combined-cycle PowerGen (North Sea) Ltd. power station at Connahs Quay, North Wales, that is scheduled to be completed in 1996.

Platform power

According to GE, the 39.0 megawatt electric load required by the project is broken down as follows:

  • Export compressor14.5

  • Injection compressor12.0

  • Stripping gas compressor3.5

  • Seawater system5.0

  • Oil processing4.0.

The project has large variable-speed electric motors to drive all process compressors. GE says that by using these electric motors instead of small gas turbines for compressor drives, the project will realize overall savings in weight, size, maintenance, and cost.

Natural gas and condensate in a dual-firing combination is the primary fuel for the turbines. In addition, diesel engines provide power generation for essential services for the complex. The diesels will start the main generation system and provide minimum services during generator shutdown. A battery-based uninterruptible power system will sustain the main safety system during a power loss.

GE describes the LM6000 gas turbine as being 90% similar to its CF6-80C2 aircraft engine. In 60-hz applications, GE says that one unique feature is that the turbine can be directly coupled to an electric generator load without using a separate power turbine or load gear, thus improving cycle efficiency and reducing the turbine size and maintenance costs.

Two LM6000s will be installed on the processing platform. One unit will be in continuous operation while the other will act as a standby unit.

The LM6000s are described as being two-shaft gas turbines. The low-pressure rotor consists of a five-stage, low-pressure compressor and a five-stage, low-pressure turbine connected by means of a mid-shaft through the center of the engine. The high-pressure rotor consists of a 14-stage, high-pressure compressor and a direct-coupled, two-stage, high-pressure turbine.

Designed shaft speed is 3,600 rpm. Three systems control airflow.

Christmas trees

The four well platforms on the development include 29 wells. BHP selected the Cooper spool trees because they met the design requirements for the predrilled and platform-drilled oil production, water injection, condensate injection, gas production, oil/gas production, and gas injection wells.

Electric submersible pumps can be used with the trees.

Cooper describes the trees as having a compact size and valve arrangement that requires only a minimum space on the platform. A platform spool tree is said to be only half the height of a conventional tree and wellhead.

Previously, spool trees have been used only in subsea applications.

Marine growth

BHP had marine growth preventers placed on the submerged structures from the splash zone to the second bracing level of the Lennox, Douglas production, Douglas wellhead, Hamilton, and Hamilton North platforms. Areas with protectors include inside sacrificial anodes, boat landing supports, and between guide frames (Fig. 3).

The IEV group, from Petaling Jaya, Malaysia, supplied the ocean powered marine growth protectors. The strong currents and high tidal fluctuation in Liverpool Bay are expected to provide a continuous brushing action against steel surfaces. Changes in current speeds and flow directions also affect the movement of the preventers and cause them to rotate and move longitudinally along the protected members.

The brushing action disrupts the settlement of micro-organisms and consequently prevents formation of colonies.

IEV says that these protectors eliminate the costly and hazardous need to periodically remove marine growth from the structure and offer significant benefits for protecting the structures.

A total of 643 preventers were installed on the platforms during the final construction phase. Remote-operated-vehicles (ROVs) will be used to install preventers on the well conductors. n

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