Decommissioning—a view from the Southern North Sea conference

May 20, 2013

Removing platforms in the UK North Sea is a sensitive, costly process that can be made cost effective by developing alternative methods.Innovating and being willing to do things differently could help save in excess of £100million of the cost of decommissioning its southern North Sea assets, says Anglo-French independent Perenco.

But there is still plenty of scope for improvement, says Keith Tucker, decommissioning manager for the firm. In these early years the industry is probably over-engineering decommissioning, he says.

"It looks at this stage as if the sector is over engineering decommissioning," said Tucker. "Operations people working in decommissioning use the same processes of project appraisal. But decommissioning structures is largely salvage."

Perenco's portfolio of assetsit has more platforms than anyone else in the UK North Seawill all need to be decommissioned and, like many others in the basin, it is beginning  the process, with many fields  continuing for at least another 15-20 years.

However, the scale of the task means that Perenco has been keen to try out alternatives to make the process, a sensitive, costly exercise with no returns, as lean as possible.

Its first project was the 10-month-long decommissioning of the minimal production Welland platform, in winter 2011 (OE April 2011). The facility's 1000-ton topsides were taken by transport barge to Vlissingen (Flushing), Zeeland province, where they were refurbished before being reinstalled off Cameroon with a self-installing jacket. The legs were dismantled and recycled.

A key saving on the project was the use of a sheerleg lift barge (Scaldis Salvage & Marine Contractors'  Rambiz). This lift barge is usually used during wind farm installations off Northern Europe - for the heavy lifts of both the topsides and jacket, instead of a more expensive, larger, heavy lift vessel. Despite early concerns by the Health & Safety Executive (UK regulator), the firm proved its case and transported the jacket on the crane hooks instead of transferring to a transport barge, which reduced the spread required.

Perenco also pre-cut the legs for the topside lift using 15,000 psi waterjet, creating a beveled finished so the separated pieces were stilled "cupped" in place, and could be held in position with a clamp.

This meant Perenco could "walk away" from the facility and give the crane barge contractor a wide timeframe for completing the project, avoiding potential costly weather delays and enabling Scaldis to fit it into their schedule.

Cutting the piles was carried out internally with sections cut and pinned to be lifted out with the same tool used for the cutting. Before the lift, the cut piles were held in place with suspended sleeves so that it was still safe to have men onboard or walk away and leave the structure in a safe state if the weather turned.

Another area to save costs is to reduce the need for environmental plugs in wells being abandoned by displacing the residual oil (left-over drilling mud) from the c annulus. Setting each cement plug costs about £30,000. In some cases, that is being spent to seal in only a few dozen barrels of oil-based mud.

On the three trial surface wells, Perenco managed to flush out the residual oil from the annuli which meant they did not have to put in a surface environmental plug before cutting the surface tubulars.

"We are cleaning, rather than perforating," said Tucker, a process costing just £2,000 instead of £24,000. This removes the pollutant from the environment rather than sealing it into the abandoned well.

Perenco also dealt with liquids from the well by re-injecting them, where possible, back into the subsurface reservoir, or pumping into a producing pipeline to another platform for disposal within a contained system.

Combined, these initiatives across Perenco's 46-platform portfolio could save over £100million, and that's the start, says Tucker. They're still learning and there are plenty of areas he can see room for improvement, from refining the process for cleaning out the c annulus to completely new processes.

The Welland platform’s 1000-ton topsides were removed from the North Sea, refurbished, and reinstalled off Cameroon, Africa."We cannot help but think there is a better way of doing things," says Tucker. "There is still more we will be able to innovate."

Tucker wants to see alternatives to HP waterjet cutting - an expensive process.

"We know there is ongoing research into laser cutting and a process called EDM [electro discharge machining] or there could be chemical cutting. Or cold fracture potential - we don't know if there is technology at the moment but we know most pipe will start to fracture at -30 degrees Celsius.

“Perhaps there is potential to use a liquid nitrogen wrap to super-cool a section of pipe and then induce a brittle fracture by some sort of automated local impact device. Pipelines are generally on seabed and any cutting technology which reduces the need for extensive dredging would be very interesting. We want to see methods developed that can reduce the spread cost and the environmental impact.”

Tubular removal also needs to be made easier, he said.

The tubulars can be pulled out with a jacking system that also pins, cuts and lifts out the concentric bundles of tubulars in sections. This requires a capability to lift 100 tons for the first lift, even on a small platform. Another option is to use a lift barge and lift out whole, 50-100 ft sections in a single lift.

“However, the individual components of the tubular bundles weigh less than 20 tons and any method of cutting and lifting small weights at a time using a smaller spread, would be very desirable,” said Tucker.

"It becomes difficult to deploy heavy-duty jacking spreads for the smaller platforms – so spread size reductions will open up further opportunities.”

For abandoning subsea wells, he would like to see a process that could perform the cut subsea, through all the well sections, at the same time as sealing in the annulus, which could save millions of pounds.

Tucker would also like to see an alternative to air diving from a drill rig. Divers are needed as part of the subsea tieback well abandonment process, but have very limited work windows‒only 20 minutes per tide (every 6 hours) at the dive site‒and they require a platform or rig to work off.

A two-hour job can take a week or more if weather comes in and meanwhile, the hire charges from the drill-rig continue to accumulate.

Saturation or TUP (transfer under pressure) diving would give more bottom time but so far no one has developed a mobile unit spread small enough to sit on a small/medium-sized drill rig.

There are likely to be more areas for improvement in decommissioning methods in the North Sea in coming years and decades, not least in the harsher Northern North Sea where structures are larger and costs even greater.

The industry, including Perenco, an early starter in the North Sea decommissioning world, is keen to share what it has learned so far. OE

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