Eastern Trough Area Project (ETAP), North Sea Central

The Eastern Trough Area Project (Etap) is an integrated development of nine different reservoirs, located 240km east of


240km east of Aberdeen, UK, in Central North Sea

Production started


Number of reservoirs



BP (six reservoirs) and Shell (three reservoirs)


The Eastern Trough Area Project (Etap) is an integrated development of nine different reservoirs, located 240km east of Aberdeen, UK, in the Central North Sea.

Six separate fields are operated by BP: Marnock, Mungo, Monan, Machar, Mirren and Madoes (the M fields); and three by Shell: Heron, Egret and Skua fields (the Heron cluster). All fields are geographically located within a 35km area.

Etap reserves

Each field is not commercially viable for standalone developments because of their small scale. Their combined reserves are estimated to be around 490 million barrels of oil, 35 million barrels of natural gas condensate and around 2 billion cubic feet of natural gas. The Etap project began production in 1998 with seven reservoirs. The additional two reservoirs, Mirren and Madoes, started production in 2002.

The facility can handle up to 120,000bpd. The gas produced is exported at approximately 250 million standard cubic feet per day. The project generates around 1,100t per day of natural gas liquid products such as condensate.

Etap reservoirs

Marnock, a gas-condensate accumulation, is a high pressure / high temperature field, at about 9,000psi and 300°F. The field is notable due to the variations in gas richness, with liquid yields ranging from 30 billion barrels / mft³ in the east to 1.7 billion barrels / mft³ in the west. The development plan involves six near-horizontal wells. The accumulation has been developed by natural depletion.

“The Etap facility can handle up to 120,000bpd.”

Mungo is a large oil field with a small gas cap. Development drilling was undertaken in several stages.

Five production wells and a gas injector were pre-drilled, along with the high-angle well used in the Mungo Extended Well test. In all, there were 13 development wells.

Monan is a small oil and gas field, developed under natural depletion with a subsea manifold and a spurline, linking into the wet gas pipeline 15km away.

Machar is an oil accumulation developed in three stages. Phase 1 comprised an extended production test, which was completed in May 1995. This demonstrated the viability of natural depletion and during the test, with 7.7 million barrels were produced.

Phase 2, which started in August 1995, incorporated a pilot waterflood, which entailed oil production from two wells, with water injected into a third; 6.8 million barrels of oil were produced.

“The oil produced is exported to the Kinneil terminal through the Forties Pipeline System.”

The Mirren well is located 13km east of the central processing facility (CPF) and has two production wells.

Mirren is Palaeocene diapir structured and is developed as a subsea with two wells tied back to CPF by a 10in pipe.

Madoes is a subsea cluster tied back to the CPF through a 10in pipeline. It is a pressurised Eocene reservoir and is located around 19km south west of the CPF.

Etap production facilities

The Etap development is based on a CPF over Marnock, which is shared between the fields. This consists of a processing, drilling and riser (PDR) platform and a quarters and utilities (QU) platform, sited approximately 60m apart and linked by two bridges.

This design separates the production risers and Marnock wellheads on the PDR platform from the accommodation on the QU platform. The PDR design also separates the riser area and the Marnock wellheads from a jack-up drilling rig if one is present.

The CPF is designed to handle:

  • Gas injection: 113mft³ per day
  • Gas export: 439mft³ per day
  • Water injection: 115,000bpd
  • Produced water: 104,000bpd
  • Produced water-disposal injection: 120,000bpd

Additionally, a platform that is not normally manned is placed over Mungo to separate liquids from the gas. The liquids flow directly to the CPF in a 12in pipeline.

“Six separate fields are operated by BP: Marnock, Mungo, Monan, Machar, Mirren and Madoes.”

Chemicals and electrical power for Mungo is supplied via an umbilical from the CPF. A 16in wet gas line from the Mungo NNMI is routed to the CPF via Monan. Any separated produced water is cleaned in a hydrocyclone and degasser before being returned to the sea, or fed back into the CPF for re-injection.

For Machar, a subsea pump has been installed along with a 12in water-injection flowline through which injection water is used to power the pump to return the produced fluid. A total of seven wells are required for this – two injectors and five producers.


The oil produced is exported via a 76km, 24in carbon steel pipeline to the Kinneil terminal through the Forties Pipeline System. The peak export capacity is around 250,000bpd.

A 16in diameter spur line carries gas to BP’s 36in-diameter Central Area Transmission System pipeline through which the gas is finally exported to Teesside.

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