Production from the Brent Field began in 1976 and since then has reached about 3 billion barrels of oil equivalent—almost 10% of total U.K. production. Shell, the operator, recently submitted a decommissioning plan to the U.K. Department of Business, Energy and Industrial Strategy (BEIS) to begin removing the Brent Alpha, Brent Bravo, Brent Charlie and Brent Delta platforms in 460 ft (140 m) of water in the North Sea.

Three of the platforms are concrete gravity-based structures (GBS) and the fourth platform is steel. The Brent Bravo and Brent Delta are GBS built by Norwegian Contractors. The Condeep platforms were installed in 1975 and 1976. The Brent Charlie is a GBS built by McAlpine/SeaTank and installed in 1978. Each of the concrete structures weighs about 300,000 metric tons without the topsides.

The Brent Delta ended production in 2011, and the Brent Alpha and Brent Bravo stopped producing in November 2014. The Brent Charlie is expected to continue producing for several more years.

Those huge storage cells at the bottom of the GBS required a different solution than conventional steel platforms. Shell, which is a 50% shareholder in the Brent Field with ExxonMobil Corp. subsidiary Esso Exploration and Production UK Ltd. (50%), recommended that the three GBS, the sediment contained and the attic oil within their concrete storage cells, and drill cutting piles remain in place. The plan also called for removing the upper steel jacket of the Brent Alpha platform and leaving the rest in place.

Work is already underway on the topside lift for Brent Delta, which is expected to be removed in a single lift in summer 2017.

Three options for decommissioning the GBS were identified: complete removal by refloating; partial removal by removing some or all of each leg; or leave in place.

The risks of failure for complete removal are about 1,000x higher than would be considered acceptable by the E&P industry at the start of any new project. Therefore, the refloat option was ruled out.

After a comparative analysis of the remaining two options, leave in place was selected. The conductors and casings in the drilling legs and the pipework and steel infrastructure in the utility leg of the two Condeep platforms would be left in place.

On the SeaTank platform (Charlie), the external conductors and conductor guide frames located between legs C3 and C4 would be cut and taken to shore for dismantling and recycling. Conductors on the platform would be undertaken as part of topsides removal or other subsea removals.

The position and new status of each GBS will be formally notified to other users of the sea through the U.K. Hydrographic Office and marked on charts. Aids to navigation will be fitted to the concrete cap on one leg of each GBS. The three platforms also will be incorporated in the FishSAFE system to provide early warning to commercial fishing vessels.

Assessments for the life of the platforms predicted the GBS will deteriorate and collapse over a period of about 1,000 years.

When Brent Charlie is finally decommissioned, that will end the production of the benchmark Brent crude, which is a mix of production from 15 North Sea fields. So perhaps the market will continue to use “Brent” as the marker name even though there will be no Brent production.