Blackbird Oil and Gas Field Decommissioning, North Sea, UK

Blackbird oil and gas field is located in Block 20/02a, in the UK section of the central North Sea. It is situated approximately 120km north-east of Aberdeen, Scotland, at a water depth of about 115m.

The field was brought into production in November 2011, about one and a half months ahead of schedule and within budget.

Production from the field ceased in June 2016, after the operator had considered various options to extend the life of the field, including a tie-back to Buzzard oil, which is operated by China-based oil and gas company CNOOC. All of the options were deemed uneconomical, and the operator proposed the decommissioning of the field.

The decommissioning programme for the Blackbird field, as well as the Ettrick field, was approved by the UK Department for Business, Energy and Industrial Strategy (BEIS) in April 2017.

Ownership details

The Blackbird project was jointly owned by Canada-based energy company Nexen’s subsidiaries, Nexen Petroleum UK and Nexen Ettrick UK, and Atlantic Petroleum UK, a subsidiary of exploration and production company Atlantic Petroleum.

Together the two Nexan subsidiaries held 90.6% of the interest in the project. Atlantic Petroleum UK held a working interest of 9.4% in the Blackbird field.

Dana Petroleum previously owned a 12% stake in the project but decided not to participate in the development of the field in April 2011. The company’s equity in the field was taken up by Nexen and Atlantic Petroleum.

CNOOC acquired Nexen in 2013, and its subsidiary CNOOC International became the operator of the field.

Blackbird oil and gas field decommissioning details

The Aoka Mizu floating production, storage and offloading (FPSO) was separated from the field and the flowline in 2016, when umbilical flushing activities were also completed. The FPSO was sent to Gdansk, Poland, for future use. The Xmas trees at the field were disconnected in July of the same year.

A section of the gas export flowline was disconnected from the gas export pipeline end manifold (PLEM) and recovered. The gas export PLEM was retained for further gas exports or imports for the Golden Eagle Platform, which is also operated by CNOOC International.

The decommissioning activities for the field were carried out by the COSL Pioneer semi-submersible rig, which can operate in water depths of up to 750m and withstand harsh environments.

Blackbird field discovery and appraisal well

The Blackbird discovery was made by Nexen in August 2008. The discovery well 20/2a-8, drilled by the GSF Arctic IV semisubmersible rig, encountered 33.83m of net pay in Upper Jurassic sandstones. It flowed at an average rate of 3,800 barrels of oil a day (bopd) during the drill-stem test.

The discovery was followed by the drilling of an appraisal well (20/2a-9) in the second half of 2010. The appraisal well encountered a net pay of 27.4m and flowed at a rate of 7,000bopd.

Oil and gas field development

The investment plan was sanctioned by the project partners in December 2010, after assessing the outcomes of the appraisal well and examining the feasibility and economics of developing the field.

The field development plan (FDP) for the Blackbird project was approved by the UK Department of Energy and Climate Control (DECC) in July 2011. The sanctioned scheme called for the drilling of a single production well and single water injection well.

Development drilling for the first production well was started in April 2010 and completed in August of the same year. Drilling of the water injection well was commenced in July 2012.

The Blackbird offshore oil and gas field was developed as a subsea tie-back to the Aoka Mizu FPSO. The hydrocarbons produced were processed on the vessel.

From there, the oil was transported to shore through shuttle tankers. Part of the gas was re-injected into the sea while the remainder was transmitted to the St Fergus Gas Terminal using the Scottish Area Gas Evacuation (SAGE) system.

Subsea infrastructure at the North Sea field

The Blackbird field was tied-back to the Ettrick oil field through a 7in flexible production flow line and a 3in flexible gas lift flow line, both measuring 6km in length.

An electro-hydraulic control umbilical with chemical injection was installed between the production well and Blackbird dynamic umbilical riser base.

The electro-hydraulic control and chemical umbilical facilitated the supply of hydraulic pressure as well as the transmission of power and communications from the Ettrick field’s FPSO to the Blackbird subsea facilities.

To avail this facility, a new umbilical riser was installed at the FPSO. The umbilical was successfully connected to the J-tube on the vessel in February 2012 through the help of UK-based connector supplier First Subsea’s diverless bend stiffener connector.

Contracts involved

Energy consultancy Xodus Group was contracted to provide front-end engineering and design services for the decommissioning of the Blackbird oilfield.

COSL Drilling Europe, a drilling contractor based in Norway, was contracted to perform plugging and abandonment activities for the field in 2017.

John Lawrie Group, a metal re-processor and decommissioning specialist, was contracted for midwater arch degassing and downsizing of the field. The pipeline and control system structure was removed from the seabed and sent to the company’s Nigg Energy park for deconstruction and recycling.