When the Brent Field was discovered in 1971, the U.K. North Sea oil and gas industry was in its relative infancy. Brent was to become the heartbeat of the region and the country’s most famous offshore field, with its blend becoming a global benchmark for the price of a barrel of crude.

One of the most significant oil and gas finds ever made in the U.K. sector, at the time of its discovery in U.K. license Block 211/29 in the East Shetland Basin its expected lifespan was put at 25 years at the most.

But continuous investment and a $2 billion redevelopment in the 1990s by the field partners Shell and Esso to unlock large quantities of low-pressure gas extended its life well beyond original expectations.

Brent, located 180 km (112 miles) northeast of Lerwick in the Shetland Islands, lies in a water depth of 141 m (463 ft) and had four platforms installed.

1971 discovery
The discovery well drilled in 1971 hit an oil column of 60 m (197 ft), while the second found 53 m (174 ft) of gas overlying 114 m (374 ft) of net oil sand.

Brent is typical of many of the fields in the area, consisting of a tilted fault block exposing the Brent Formation next to bounding faults. This allowed migration from deeper adjacent “kitchen” areas where the Kimmeridge Clay Formation becomes fully mature and releases hydrocarbons.

Production started up on Nov. 11, 1976, and on Dec. 13, 1976 the first tanker was loaded. The Brent Field consists of Alpha (a steel jacket structure), Bravo (a gravity base structure with 16 storage tanks), Charlie (another gravity base structure with 32
storage tanks) and Delta (a third gravity base structure with 16 storage tanks).

A fifth installation, the now-infamous floating Brent Spar, served as a storage and tanker-loading buoy after being installed early in the field’s construction. Since production began, two-thirds of the revenue generated from the field has been paid to the U.K. government as tax—amounting to more than $31 billion.

Four Bboe produced so far
To date the field has produced around 4 Bboe, which is almost 10% of all the oil and gas produced from the U.K. sector of the North Sea. In its pomp, the field helped the U.K. to become an oil exporter.

At its peak in 1982 the field was producing more than 500,000 bbl/d—its production that year would have met the annual energy needs of around half of all the homes in Britain.

The field, which has mainly produced gas since the mid-1990s, has now almost exhausted its recoverable reserves and is in the process of being decommissioned.

Brent Delta reached cessation of production (CoP) on Dec. 31, 2011, while Alpha and Bravo reached CoP on Nov. 1, 2014. Charlie is still producing with no designated shut-down date at this time.

Alpha and Bravo had not produced since May 2014 when they were shut down for inspection and maintenance work, at which point it became clear that the platforms were no longer economical.

‘Iconic platforms’
A Shell spokeswoman told E&P, “We recognize the major contribution of Alpha and Bravo and the efforts of the many people associated with the platforms over the last 40 years. From a technically innovative installation phase through to a long period of operation and production, these iconic platforms have helped to sustain vital North Sea oil and gas supplies. We will now focus on safely decommissioning these assets.”

The decommissioning process will be a complex and major engineering project, with Shell saying it will take more than 10 years to complete.

The operator said that when the Brent platforms and infrastructure were built in the 1970s during a period of global energy shortages, decommissioning did not feature highly in the design considerations. “The technology, expertise and environmental
standards we rely on today were only in their infancy in the 1970s. Since then, society’s expectations, legislation and technology have moved on, and all offshore installations in the northeast Atlantic built after 1999 are designed to be completely removed [at the end of their useful life.]”

Decommissioning challenges

Elements of the Brent Field infrastructure present particular decommissioning challenges and have been the focus of detailed research.

Three of the platforms (Bravo, Charlie and Delta) have giant concrete “legs,” which support the topsides and have clusters of large concrete oil storage tanks or “cells” at their base.

The storage cells contain large quantities of gravel ballast, used to anchor the structure to the seabed. Many cells were originally used for oil storage and contain some oily sediment.

Accessing the cells to remove this sediment presents a significant technological challenge. This is because of their location deep beneath the ocean’s surface, their size and the thickness of the cell walls.

The Brent Delta topside will be individually removed and transported from the Brent Field to the Tees onboard the newly-constructed Allseas Pioneering Spirit vessel, which has been specially designed for single-lift installation and removal works.

The Pioneering Spirit is the world’s largest vessel in terms of gross tonnage and has a 2,000-tonne pipelay tension capacity, 25,000t jacket lift capacity and a 48,000-tonne topside lift capacity. The vessel cost an estimated $2 billion.

Heaviest offshore lift
At 382 m (1,253 ft) long, Pioneering Spirit will first remove Delta’s 24,000-tonne topsides—one of the heaviest offshore lifts yet attempted.

According to a Shell inventory, the 44-m (144-ft) tall topsides includes 10 tonnes of asbestos (used for insulation and gaskets), 899 tonnes of paint, 31 tonnes of batteries, 6 tonnes of cotton for bedding and 3,446 fluorescent lighting tubes.

The lifting process is expected to take only a matter of seconds, with the topsides to be moved to Able Seaton Port in Hartlepool in northeast England, where Able U.K. has been awarded the contract for the disposal of the four offshore structures. One of Europe’s heaviest load-bearing quays is being built at the port to receive the structures.

About 100 jobs will be created during the 18-month construction of the new quay, with the six-year recycling contract itself generating a further 100 new jobs. It is anticipated that more than 97% of the structures will be reused or recycled.

It has not yet been decided how to deal with the approximate 300,000 tonnes of subsea structure that make up the concrete gravity base structure. Shell is quick to point out that, while this may be the end of an era for Brent, it is far from the end of the line for the U.K. North Sea as a producing region.

Oil and Gas UK estimates that one-third of the U.K. Continental Shelf’s total reserves still remain—some 15 Bboe to 24 Bboe. But the days of finding supergiant fields in the U.K. North Sea such as Brent appear to now be just a happy but distant memory.

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