A report by Oil & Gas consultant Mark MacArthur of EC Harris Built Asset Consultancy detailed the difficulties of decommissioning in the North Sea. In it he talks about the risks and the cost burdens on operators. The strains of balancing the intricate engineering

He also predicted that of the 600 plus offshore platforms in the North Sea, approximately 50 of these would cease production by 2016, with a further 250 also scheduled to be decommissioned in the coming years.

However, an article in The Engineer yesterday moves things on a step announcing that almost a thousand North Sea oil wells will be decommissioned over the next decade. This is going to be one of the biggest technical challenges in recent years involving large scale engineering.

There have already been several decommissioning projects in the North Sea and industry body Oil & Gas UK is pointing out the dangers of decommissioning too early based on falling oil prices. However, the questions about what happens when the hydrocarbons run out are already being asked and it’s clear we’re closer to the end than the beginning. But how do we manage the end of this era?

Decommissioning platforms is not as simple as making a decision and then breaking them down. It can take years of planning to get it right and is a long, complex process that has to start while the platforms are still in use. This is evidenced by Shell who announced this year that they will decommission their Brent oilfield. Of the four platforms in the oilfield only one (Charlie) is still producing but the decommissioning notice is not a reaction to oil prices. It’s evident that this is a decision taken based on the long term understanding of the field and what it can produce. While that one remaining platform is still producing the various stages are beginning to form to decommission the first of the 3 that aren’t. So what steps will they be taking? Here are the typical steps taken in the decommissioning process.

Project Management
It’s likely this started some time ago as it would normally start a few years before the well runs dry. A full review needs to be carried out.
• Operators contractual obligations
• Engineering analysis of the field
• Operators facilities
• Operational planning & pre-planning

Permits and Compliance
When it comes to permits and regulatory permission the process can be very lengthy with many different regulators are involved.

Platform preparation
Until the platform is fully prepared it has to be classed as live because of the hydrocarbons present. That means power, utilities and safety systems all have to be in place and maintained. Preparing a platform involves
• Flushing and cleaning of tanks
• Flushing and cleaning of processing equipment
• Disposal of all hydrocarbons
• Removal of platform equipment
• Removal of any marine growth around any underwater structures

Plugging and Abandonment
This is one of the most costly parts of decommissioning. It involves removing the linings of the borehole and plugging points where hydrocarbons flow into it. You can plug permanently or temporarily if you intend to reactivate the well, but this is a decision that needs to be confirmed well in advance as you can’t change your mind. A permanently plugged well cannot be reactivated.

Mobilisation / demobilisation
Decisions need to made whether to remove the platform in one piece or dismantle in units or pieces.

Pipeline and power cable decommissioning
This can be carried out in place as long as there is no interference with fishing or navigation. If there is then they must be removed to below sea bed level.

Materials disposal and site clearance
This involves the dismantling and treatment of topsides and locating debris and any other materials before and after removal processes. In addition, divers and remotely operated underwater vehicles will be needed to remove any remaining debris and do test trawls to check for obstructions.

As you can see, a very lengthy process involving may regulatory bodies and requiring years of planning, so it’s highly likely that we’ll be hearing a lot more about the decommissioning of platforms if the predicted numbers of closures are correct.

If you’re interested in more specific facts and figures relating to previous decommissions, The Engineer article is a good read.