The Eastern Trough Area Project (ETAP) is located in the Central North Sea. (Credit: BP)
Oil is exported to Kinneil terminal at Grangemouth through the Forties Pipeline. (Credit: Richard Webb/Wikimedia Commons)
All the fields are located within a 35km area. (Credit: Koichi Hayakawa/ Flickr)

The Eastern Trough Area Project (ETAP) is a complex integrated development of multiple oil and gas reservoirs located in the Central North Sea, around 240km east of Aberdeen, the UK.

The project was sanctioned in December 1995 and it came on stream in July 1998.

As standalone development of the reservoirs were not commercially viable, the ETAP was conceived to develop all the fields as a single project using a common central processing facility (CPF).

Initially, the development included seven fields- BP-operated Machar, Marnock, Mungo and Monan fields, and Shell-operated and Egret, Heron and Skua assets.

Two additional BP-operated fields, Mirren and Madoes began production in 2002, taking the total number of producing assets through the CPF to nine.

Two Shell-operated fields have since ceased production.

The ETAP hub had an estimated production life of 20-25 years, with decommissioning planned to begin in 2023. However, a $1bn investment programme in 2015 extended the life to the 2030s.

In November 2023, BP announced the start of production from the Seagull field, making it the first tieback to the ETAP hub in 20 years.

The Murlach project, which received government and regulatory approval in September 2023, is expected to commence production in 2025.

ETAP Fields

All the fields are located within a 35km area.

Marnock reservoir is a high-pressure/high-temperature (HPHT) field at initial conditions of 9000 psi and 150°C hosts a gas condensate accumulation. It is expected to contain around 600 billion cubic feet of gas and 50 million barrels of condensate.

Mungo reservoir is a large oilfield with a small gas cap. The development of the field involved combined water and gas injection on a NUI located above and then tied back to the CPF.

The fluids from the Mungo reservoir are sent to the CPF through two separate pipelines. One pipeline delivers liquids to the CPF, while the other transports wet gas and Monan fluids through a subsea manifold.

Monan is a small oil and gas field with hydrocarbons present in a turbidite reservoir in a diapir flank setting. It produces via natural depletion with a subsea manifold.

Machar is an oil accumulation which has a subsea tie-back to the ETAP CPF via a 36km 12-inch pipeline.

Mirren Field is developed subsea where two wells are tied back to the CPF, while Madoes is a three-well subsea cluster connected to the CPF via a 10-inch pipeline. The two fields are located 13km east and 19km southwest of the CPF, respectively.

The Seagull field is a four-well asset developed as a subsea tieback to the CPF. The field is linked to the ETAP via a new 10-mile umbilical providing control, power and communications services between the surface and seafloor.

At peak, Seagull is estimated to produce around 50,000 barrels of oil equivalent gross per day.

Reserves and Production

Initially, ETAP had reserves of approximately 450 million barrels of oil, 35 million barrels of natural gas liquids and 1.1 trillion cubic feet of sales gas.

The addition of Mirren and Madoes increased reserves by another 40 million barrels of oil and 80 billion cubic feet of sales gas.

According to a 2018 report, the BP-operated ETAP fields produced more than 550 million barrels of oil equivalent (gross) after two decades of operations.

ETAP Project Infrastructure

ETAP development includes a Central Processing Facility (CPF) at Marnock field.

It comprises a Processing, drilling and Riser (PdR) platform bridge linked to a Quarters and Utilities (QU) platform along with a normally unmanned installation (NUI) on the Mungo field.

All other ETAP fields are connected to the CPF facility via subsea infrastructure.

The PdR and QU platforms of the CPF,  situated approximately 60m apart, are linked by two bridges. The QU platform can accommodate around 156 people.

A single control room on the CPF is used to manage all process systems.

The facility can manage 160 thousand barrels per day of oil and produce around 1,100 tonnes/day of natural gas liquids.

Additionally, gas production is exported at a rate of around 320 million standard cubic feet per day.

Oil and Gas Export

Oil is exported to Kinneil terminal at Grangemouth through the Forties Pipeline System via a 76km 24-inch carbon steel pipeline.

The peak export capacity is estimated at approximately 250 thousand barrels per day.

The produced gas is transported via a 14km, 16-inch carbon steel spur line that connects to a 36-inch Central Area Transmission System (CATS) pipeline and subsequently to Teesside.

The capacity is estimated around 350 million standard cubic feet per day.

Key Contractor Involved

In September 2015, British firm Amec Foster Wheeler secured an Engineering, Procurement and Construction (EPC) contract from BP to provide additional living quarters needed for ETAP life extension.