Oil major Shell has set out its plans for decommissioning the Brent field facilities in the UK North Sea, including leaving its foundations in place.
The firm, which has already spent 10 years working on decommissioning the four platforms, installed in the 1970s some 186km east of Shetland, said is it planning to apply for a derogation order to leave the foundations in place.
It also said the removal of the first topsides, on Brent Delta, will now be delayed to next year, due to the commissioning of the lifting beams on Allseas' Pioneering Spirit mega lifting vessel taking “longer than anyone anticipated.” The vessel was due to lift out the Brent Delta topsides in one piece this year, with the other topsides in following years.
Duncan Manning, business opportunity manager on the Brent Decommissioning project, said it was a “very large and complex project.”
He said commissioning the lifting arms have taken longer than Allseas' or anyone anticipated. “We need 16 of these [the lifting arms] to be in place to lift Brent Delta. The impact isn’t that great for us because our focus is on preparing the platform for lift and going into lighthouse mode, getting staff off the platform and leaving it in a safe manner,” he said.
The Brent field is one of the largest in the North Sea with four platforms, Alpha, Bravo, Charlie and Delta. At peak production in 1982, the field was producing more than 500,000 b/d. Delta ceased production in 2011, followed by Bravo and Alpha in 2014. Plugging and abandonment (P&A) work on Delta is complete. In total more than 150 wells, including more than 400 well bores, will need to be P&A'd. P&A work is nearing completion on Bravo at the moment, with plans to start on Alpha and Charlie next.
Deciding how to deal with the three gravity base foundations and the 31,000-tonne Brent Alpha steel jacket has involved a lot of work around understanding impact on the environment, other users of the sea, the impact on shore, technical feasibility and cost, said Manning. “On balance, it looks like the best option is to leave it in place -85m from top of sea,” he says.
The gravity based structures are also to be left in place, subject to approval by the Oil and Gas Authority, which will apply to Ospar (named after the Oslo Paris conventions which agreed terms for anti-dumping in the North-East Atlantic) for a derogation order to leave them in place. Under Ospar rules, structures weighing under 10,000-tonne have to be removed. Those above that weight can apply for a derogation order.
Gravity based structures are relatively unique, says Manning. Of 470 facilities in the North Sea, just nine are gravity based. They're 70m-tall, made of 1m-thick reinforced concrete and each weigh more than 300,000 – or the same weight as the Empire State Building, he said.
They were “perhaps put in place perhaps without decommissioning in mind,” says Manning. "That’s given us a real challenge, understand how feasible it would be to refloat them or to cut the legs and leave them in place.” The conclusion is “it’s extremely challenging to refloat” the facilities, technically and from a point of view of safety, he says.
Discussions with fishermen suggested leaving some of the structure standing out of the water with beacons on, so that they can be seen and avoided, he said.
As part of working out if the structures can be removed or not, Shell had to investigate the contents of the cells, including sediment at the bottom and so-called attic oil. Each cell is 60m-tall and 20m in diameter – or the same height as Nelson’s Column in Trafalgar Square, London. Key to this work was being able to collect samples from inside the cells, which has been a multi-year, technologically challenging project in itself. The result was a tool developed to clamp on to the cells, bore through and siphon out some sediment for testing.
Results showed the samples closely mirrored assumptions made during modelling, with no normally occurring radioactive material and trace levels of heavy metal. “On balance the environmental impact [of this leaking to the environment] would be small but negligible,” leading to the conclusion it could be left, says Manning.
Shell also plans to leave drill cuttings in place, which means they will not be disturbed. Treatment of the 28 pipelines around the facility, varying in size from 3-30in diameter, will be on a case by case basis, with some flushed and left and some removed.
The Brent decommissioning program started in 2006. Much of the work to date has been plugging and abandonment operations, with about half of the field’s 140 wells now plugged and abandoned. An independent review group led by Prof John Shepherd has scrutinized over 300 documents, with comments having to be addressed during the program.
More recently, Shell has been working to install hundreds of tonnes of steel structural reinforcement to enable the Brent Delta topsides to be lifted. At the moment, Saipem's 7000 is at the Brent Alpha ready to complete the lift out and replacement of the Brent Alpha’s crane and to remove platform modules.
On Brent Charlie, an integrity program is being performed, to make the facility ready to go into next phase for reservoir isolation, said Manning.
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