Beginning of the end

Putting the Brent field online in 1976 was a feat. Decommissioning the field’s four massive platforms is equally challenging. Elaine Maslin sets out Shell’s latest plans and how it came to them.

End of an era at Brent. Photo from Shell.

When Shell’s 24,200-tonne Brent Delta topsides is lifted, now scheduled for 2017, it will be the heaviest lift ever conducted offshore using the world’s largest vessel by displacement.

It will be an immense feat, not unlike the original feat to install the four Brent mega-structures, three of which are ca.300,000-tonne gravity base structures (GBS), some 186km northeast of Shetland, in the harsh northern North Sea back in the 1970s.

While the record lift will hit the headlines and leave many in awe, there have been a string of other feats on this project, including exploration into inner space, with the help of NASA, and creating a moon pool on a 40-year-old platform, not to mention the 400+ wellbore plug and abandonment (P&A) campaign.

There has also been intense work to assess how to deal with the three massive GBSs and the 31,500-tonne steel jacket. Shell’s conclusion is that they would be best left in place, at least in part.

“If you think back to 1971, when the field was discovered, with first oil in 1976, which was really fast, they needed brilliant engineering,” says Alistair Hope, Shell project director. “These are engineering marvels. Now we have to decommission them, we need that same ingenuity and great engineers to allow us to do this.”

Business opportunity manager Duncan Manning, a former Lieutenant Colonel in the Royal Marines, with three tours in Afghanistan under his belt, says the Brent project is akin to the Olympic Games, on which he worked in 2014, in scale, complexity, difficult environment and the need to work closely with a large number of stakeholders.

Discovery to COP

Brent was discovered in 1971, in 140m cold rough water depth. First production was achieved just five years later, with one of the largest sets of infrastructure in the North Sea. Peak production from the four Brent platforms, of three different designs (Bravo and Delta – Condeep, Charlie – Sea Tank, and Alpha – steel jacket), was 500,000 bbl in 1982. Production at Brent Delta stopped in 2011, followed by Alpha and Bravo in 2014. Charlie is still producing and will reach its 40th anniversary this November, despite the field having initially been expected to produce for just 25 years.

The Brent decommissioning campaign started in 2006, some 10 years ago. A large focus – “the big ticket item” – has been on P&A work, with some 154 wells and around 400 wellbores in total to abandon. P&A work on Delta is complete, with work now focused on down-manning. P&A work is ongoing on Bravo, with work on Alpha also ramping up – the Saipem 7000 recently replaced a crane on Alpha to enable P&A work. Meanwhile, on Charlie, an integrity program is being performed, with the support of Prosafe’s flotel Regalia, ahead of P&A work there.

A CGI illustraton of the Brent field. Image from Shell.

Heavy decisions

However, what’s remarkable about this project is the immense amount of background work involved: some 300 reports have been produced, all of which have been scrutinized by an independent review group.

Much of this work has been to inform the decision on how to deal with the three GBS foundations, on Charlie, Delta and Bravo, and the 31,500-tonne (including marine growth) Alpha steel jacket. This has meant understanding the impact on the environment and other users of the sea, the impact onshore, technical feasibility, and cost, of all the various options, Manning says. Alternative uses, from offshore wind to carbon capture and storage, have also been assessed – in detail.

To remove the footings of the steel jacket would mean either boring down through the tops of the footings or excavating down through the seabed and doing a subsurface cut, followed by a lifting operation, Manning says. “On balance, it makes sense to leave them in place,” he says. The footings will stand 55m above the seabed, 85.5m below sea level.

Removing the GBSs would be even harder. In the 1970s, they were floated out then tonnes of ballast was loaded into the cells, to pull them down to the seabed, followed by the installation of a skirt, to pin them in position. They weigh between 297,000-tonne (Charlie) and 341,000-tonne (Bravo), excluding water ballast. These facilities were not designed with decommissioning in mind at the time, Manning says. They’re also relatively unique. Of the 470 facilities in the UK North Sea, there are only nine operational GBSs.

Exhaustive comparative assessments were carried out on ways to remove these structures. Shell even looked at the impact of exploding them to collapse them. Ultimately, the firm feels, on balance, leaving them in place is the best option, following precedents such as Total’s Frigg GBS, which has navigation aids installed on its stub top. The three stubs will also be marked on navigation charts.

Subject to approval of its plans by the Department for Business, Energy and Industrial Strategy (DBEIS), formerly DECC (the Department of Energy and Climate), an application will be made to OSPAR (named after the Oslo and Paris conventions, which agreed terms for anti-dumping in the North-East Atlantic) for a derogation order to leave the steel jacket footings and GBSs in place. This will happen after Shell has officially submitted its decommissioning program, sometime before the end of this year. Under OSPAR rules, structures weighing under 10,000-tonnes have to be fully removed. Those above that weight, which were installed prior to 1999, can apply for a derogation order.

Cells

Going into inner space, with the help of NASA. Photos from Shell.

But, before Shell could decide the best solution for the three GBSs, it had to investigate what was in their 64 concrete cells (there are 16 cells each on Bravo and Delta and 32 cells on Charlie). While the firm was able to do modeling based on production data, etc., it needed to verify these models with real data, i.e. with actual samples sourced from the cells, a task which proved to be a multi-year challenge.

Each cell is cavernous, measuring 60m-tall and almost 20m-diameter (think St. Paul’s Cathedral), with nearly 1m-thick concrete reinforced walls. Of the 64 cells, 42 were used for oil storage and separation. Oil remains in the cells, trapped at the top (“attic oil”), as well as a waxy interface between the oil and ballast water, sediment (containing sand flushed in during a gas blow down in the 1990s) and ballast remain in them. But, access is limited, so new technologies had to be developed to reach the contents.

One of the first attempts to access the sediment was with tractor technology. But, onshore trials found the tractor would not be able to navigate the 90° corners of the filler lines into the cells. Shell then started a program to access the cells from the outside, which would mean boring through the 1m-thick reinforced concrete cell shells.

A number of contractors were engaged to deliver the program, and in 2014 their combined efforts resulted in the first cell sampling. Divers connected a base plate to the top of three Delta cells, some 80m below the waves but accessible from the platform crane. A lubricator tool was then attached to the base plate, through which a sampling tool could be sent.

Shell then hired Enpro, which built upon the original concept, creating a smaller, lighter, simpler and entirely ROV-operable system. To make operations for the removal of the attic oil easier, as Shell didn’t just want to sample the sediment and attic oil, it also wanted to remove the latter, a big, innovative move was made; Shell cut a hole right through the center of the platform, to create a moon pool, to ease access to tops of the cells.

“To cut a moon pool from the top deck, through the cellar decks, allows us to effectively deploy the attic oil recovery tools far more effectively than doing it over the side,” Manning says. Over the summer, the tool has been removing the attic oil from the Delta cells, helped by ROV operator ROVOP, with a platform-based ROV.

North Sea calling NASA

Pre-deployment testing.

But, while the external access tooling was being developed, Shell was also working with NASA on a probe that could go down the filler lines. “We have a technology partnership with NASA and we reached out to them,” Manning says. NASA came up with a tool, a 9in-diameter sphere containing miniaturized sonar equipment, which could be flushed down the 10in-diameter filler lines into the cells to map their insides. The spheres are attached to a tether and pulled out once they’ve completed their mapping. “It is technology they adapted from the technology they use on the space shuttle to understand the internal pipework condition,” Manning says.

It took a series of attempts to get it to work, the first on Delta, then successfully on Bravo earlier this year. The 2D sonar image was converted to a 3D model, which then allowed Shell to model the quantity of sediment.

The external sampling tools and the sphere confirmed some 4m deep of sediment. The sediment was made up of 25% sand, 25% oil and 50% water.

Now the attic oil is being successfully siphoned out. To make sure it is all removed, it is measured using a conductivity tool, to see when the attic oil stops coming through and it turns to the ballast water, as well as a UV light. While the work is being done through the platform moon pool, when the topsides are removed, it could also be done from a support vessel.

Heavy lifting

NASA’s survey tool up close.

One of the next steps will be more visible – the Delta topsides removal in 2017. It’s a year late, due to commissioning on the Pioneering Spirit lifting arms taking “longer than anyone anticipated,” (OE: April 2016 and August 2016). Shell’s Hope describes the work on each arm as being similar in complexity to commissioning a small southern North Sea platform, and then all 16 arms work together, but also act independently. However, the delay will not impact Shell’s operations, Manning says. The vessel has already successfully performed a test lift. And, as OE went to press in late August, the vessel completed the Yme topsides removal offshore Norway.

The Delta lift will involve the Pioneering Spirit bow slot ballasting into position beneath the topside, before using 16, 65m-long, lifting arms to lift the platform from its base. It’s an ambitious system, but one in which Shell has confidence, having done its own studies on the design prior to signing a contract for the unit in 2013.

To do the lift, however, reinforcement work had to be carried out on the topsides. Eight cruciform lifting points, weighing 120-tonnes in total, had to be added to the underside of the platform, for the mating with the Pioneering Spirit’s lifting arms. Structural reinforcement was also added to the lower decks.

Three shear restraints, at around 12m-diameter and weighing about 36-tonne each, have also been installed in each leg, to hold the platform in place after leg cutting, by Cut UK. They will also accommodate the shear forces during the lift, Manning says. “They effectively make the cut legs as strong as they were before the cuts,” Hope says.

Based on the Delta lift plan, eight lifting points were added, two more than the six originally thought necessary. The 16 lifting arms will also be acting in pairs (two for each lifting point), Hope says, adding further redundancy. It’s a conservative approach for the first lift. “We are actually looking at how we can make this less conservative in the future, as there is substantial conservatism in this,” he says.

Others have also had to invest to make this project happen. Able UK, in northeast England, had to upgrade its quayside in order to receive the topsides, with some 1200 piles installed then covered in a concrete pad ready to receive Delta’s topsides. Able will also lead recycling efforts, hoped to reach 97%, with a level of re-use. Able already has designs on the galley and that elements such as valves are likely to be reused, more and more so when this market grows, Hope says.

Cleaning up

It’s not just about the platforms. Shell also has some 28 pipelines, including umbilicals and flexibles, to deal with, measuring from 2.5-36in in diameter, on a case-by-case basis, with either removal or flushing and burial. And even then, clean-up work will be needed around the platforms, to clear all debris, such as scaffolding, broken off during severe weather.

Life goes on

Delta will be the first topside to be removed in the campaign, in 2017, with three more to come, as well as completion of the P&A program. Lessons learned in this project will be passed on, Manning says. “Some of this is about driving greater efficiencies in delivery,” he says, “conducting planning and execution and preparation early. Some of it is about ensuring the platform is prepared for end-of-life activity, that cranes are robust, there’s robust power solutions and you’re not reliant on reservoir gas to provide power. Some of it is ensuring there is the right focus on the platform.”

There’s also scope for new technology to come in and impact how decommissioning is done, not least in the P&A space, which is nearly half the total cost of decommissioning. “Being able to P&A more efficiently so you can abandon a well in one run would be the dream ticket,” Manning says. “We are tracking a number of technologies in the P&A space with various levels of technology readiness and expectations.” The issue here, he says, is finding test wells for these technologies. Doing it 186km offshore in the rough northern North Sea isn’t perhaps ideal. More broadly, doing things quicker and more efficiently is an all-round goal, he says.

But, it’s not all about decommissioning. In order that fields, which currently produce over the Brent facilities, i.e. the Penguins development, which produces through Brent Charlie, FLAGS (Far North Liquids and Associated Gas System) pipeline, are not left stranded, a bypass route is due to be installed.

The facilities will also require periodic inspection, something that could be well-suited to autonomous underwater robotics, which could swim out from onshore, Manning suggests. It’s a future technology, but one which could have use here. We look forward to reporting on it, when the time comes.