The Dunlin field was discovered in 1973, in blocks 211/23a and 211/24a, 195 km (121 miles) northeast of Shetland in some 500 feet of water.
It was one of the first generation of deep water oil fields in the Northern North Sea, following hard on the heels of the Brent field, with production facilities built to withstand the harsh conditions of the region.
The platform is a concrete gravity base structure (CGBS) built by ANDOC (Anglo Dutch Offshore Concrete) in The Netherlands and weighing 320,000 mT. It has four concrete legs supporting steel leg transitions, a module support frame and topsides. CGBS completion and topsides installation took place in June 1977 in Norway with float out later that year. Production began in August 1978, with peak production of 120,000 bopd achieved in 1979. Dunlin and Dunlin South West wells were drilled from the platform, using the platform rig. Development of the Dunlin subsea satellites, Osprey and Merlin, followed in 1991 and 1997 respectively.
When Fairfield Energy and JV partner acquired the Greater Dunlin Area assets from Shell in 2008, the economic and design life expectancy had already passed original expectations. Fairfield‘s aim was to further extend productive life through a programme of investment in improvements to bolster water injection and well performance, and enhance field economics. This included restoration of the platform drilling rig, platform and subsea modifications and a 3D seismic acquisition to support reserves development opportunities. It also included a successful campaign to restore production from redundant wells in Dunlin Block 10 through installation of electrical submersible pumps and water injection, resulting in additional recovery of 1.5MMbbls of oil.
However, in 2015 a combination of challenges – the need for high cost subsea infrastructure investment, negotiation of revised fuel gas contracts, growing conductor integrity issues and finally a sub $40 oil price with little prospect of an early recovery – led to the determination that Dunlin had reached maximum economic recovery (MER).
Termination of Production from the Greater Dunlin Area took place in June 2015; this date was later formally approved as Cessation of Production (COP) by the Oil and Gas Authority.
Following this significant change in strategic direction, Fairfield’s imperative was to swiftly transition from a producing operator focused on field life extension to a new kind of company – an ultra-late life decommissioning operator. This would necessitate the development of an organisation with the decommissioning expertise and project mind-set to meet the challenges of the Dunlin platform and subsea infrastructure, including the plug and abandonment (P&A) of the 61 wells that had contributed more than 522 million barrels of oil in the field’s 37 year lifespan.
There were major challenges. Previous planning for decommissioning had been undertaken as required by the regulator, but the decision to shut down production in June 2015 had been sudden. P&A planning had to begin immediately and at an accelerated pace.
Critically, the organisation had to be staffed appropriately to plan and execute an extensive, multi-year year P&A programme, requiring the very best in drilling and subsea wells expertise. Fairfield embarked upon a recruitment campaign to supplement the existing team and fill the knowledge and experience gap, at a time when much of the industry was downsizing.
Fairfield’s Drilling Manager, Ed Pritchard, who joined Fairfield at this time, reflects. ‘It was a difficult time for the industry; companies were letting people go but we were hiring against the trend due to our urgent need for people with the skills to carry out a major P&A programme on 61 platform and subsea wells. And to meet our aggressive target, we had to focus on developing a project based mentality as a cornerstone of our new organisational capability.’
Initially, the plan was to commence the 45 platform wells programme in January 2016, and the 16 Osprey and Merlin subsea wells later that summer. However, it became clear that starting two major campaigns in close succession was not the optimum approach.
‘We realised that this plan would impose too heavy a burden on both the Wells and Subsea Abandonment team and the supporting organisation, and thus expose the project to too much risk,’ Ed explains. ‘There was also the issue that the specialist well intervention equipment needed for the subsea wells required extensive refurbishment and testing. This was a major challenge that required new thinking. These two factors led to the decision to start the subsea wells abandonment programme in spring 2017.’
Meanwhile, the platform wells abandonment began in early 2016 with slickline work whilst the platform rig was re-activated. Restoration work on the rig undertaken early in Fairfield’s tenure meant that this could now be used for the P&A campaign for Dunlin’s 45 platform wells. Ed observes that other operators faced with decommissioning old platform wells may not be so fortunate. ‘Keeping Dunlin’s rig viable, even allowing for the cost of taking it out of cold stack, was a really good decision. Some platform rigs are no longer viable or have been scrapped altogether, adding complexity and a lot of cost to abandoning old platform wells as a rig has to be brought into the field.’
The main challenge of the Dunlin platform wells is their age and the poor condition of the annular cement. Innovative solutions, such as perforate and wash techniques, have been trialled but so far these methods have proven not to be suitable with the Dunlin platform well architectures, and traditional abandonment techniques, such as section milling, has been required to establish the required permanent isolations. Typically, the team aim to engineer three wells ahead of the well they are working on to balance flexibility of the schedule, capturing the lessons from each well for input into the future programmes.
The subsea wells abandonment programme is in the early stages of execution. The decision to commence this in spring 2017 had the added benefit of facilitating monitoring of the rig market and going to tender when conditions were favourable for the project. A key selection criterion was crew competency and experience. When the Transocean 712 came on contract to Fairfield in April 2017, it had the additional advantage of having been in continuous work, with a stable and experienced workforce – factors which have supported the good performance to date.
The decision on timing of the commencement of the subsea wells abandonment programme also allowed the capacity to address the issue of the age and obsolescence of the subsea trees and associated intervention equipment. Dating back to the 1980s and thus over 30 years old, they were designed for diver intervention. This challenge led to the development of new tools enabling ROVs to eliminate the majority of the diving work scope.
The Greater Dunlin Area platform and subsea wells P&A programme is a major component of the decommissioning project. It is a critical path activity in terms of the overall project plan both in terms of schedule and budget and represents a substantial opportunity for the UK supply chain. The P&A challenge has the capacity to create business opportunities for UK suppliers who have the expertise and innovation to develop new techniques to deal with old platform and subsea wells that may benefit the wider industry in the decommissioning challenges ahead.