After more than three decades’ production, the mighty Murchison platform was finally down-manned this year. Work to dismantle the 25,000-tonne topsides is ongoing. Elaine Maslin reports.
 |
|
CNR International’s Murchison platform with Heerema Marine Contractor’s vessels Thialf and Hermod, at work earlier this year. Photos from Jan Berghuis/Flickr. |
On 31 March, the last scheduled flight departed from the Murchison platform, two years after production ended at the huge North Sea facility, and more than 10 years after production was expected to cease when the field was acquired by present operator Canadian Natural Resources (CNR) International.
The platform is one of the North Sea’s largest, standing 254m from the seabed to the tip of the flare stack – and described as one of the friendliest. It has been subjected to the harsh environment of the North Sea, with waves of up to 28m-high and wind speeds in excess of 50m/s (110mph) over its 36-year life.
At peak production, in 1982, it produced 150,383 boe/d and overall it has produced about 400 MMboe, or about 50% of the oil in place.
Now, however, the facility is the subject of an ongoing decommissioning campaign, with about 40,000-tonne of topsides and jacket to be removed.
Origins
The field was discovered in 1975 by Conoco (UK), at the height of the early North Sea boom when Texans were a regular sight in Aberdeen.
The topsides, designed for a 30-year life, were installed through 26 modular lifts and, in 1980, oil production started, initially from three subsea wells tied back to the main platform, before the main platform wells came onstream. The field itself extends into the Norwegian sector, but oil was exported to Shetland via the Dunlin and Cormorant platforms.
In 2002, CNR International took over Murchison, undertaking a major program of asset integrity management, reservoir management, well repair and infill drilling, resulting in a 10-year life extension and delivering an extra 22 MMboe. In 2005, under CNR International, production peaked at 18,000, boe/d. However, by 2013, the facility was producing just 2200 boe/d, and the time had come to shut it in.
A 25,000-tonne challenge
The size of the challenge is not to be underestimated. Murchison comprises a 25,000-tonne topside facilities consisting of some 26 modules, including accommodation for nearly 200 people. The eight-legged, 24,500-tonne jacket – excluding marine growth built up over the platform’s lifetime –with 32 piles, is one of the largest in the North Sea, with main legs measuring 6m in diameter.
In addition, there were 33 platform wells and one subsea well that had to be plugged and abandoned (P&A), as well as a 16in export line; a 6in gas spur; three, 12.75in disused bundles; and a subsea isolation valve umbilical. Subsea wellhead protection frames also have to be removed. There is also a 22,500cu m drill cuttings pile.
In 2009, decommissioning studies were started, sparking three years of extensive surveying, data collection and studies before the decommissioning program could be submitted to the UK’s Department of Energy & Climate Change (DECC – now part of a new department, Business, Energy & Industrial Strategy, or BEIS) in 2013, and approved in 2014, with well P&A starting the same year.
Because the platform had changed hands several times over the years, CNR had the job of tracking down all the information needed by the regulator for the decommissioning scope. This included carrying out its own surveys to fill in knowledge gaps. In total, 70 studies from 30 different companies, from marine contractors to environmental experts, were carried out. CNR International also had to bring the platform’s drilling facilities out of warm stacked condition, with the help of PD&MS and KCA Deutag, to enable the P&A operations, which were completed on 31 March this year.
Aker Solutions was picked to lead the engineering down and conversion of the platform to NUI (normally unmanned installation) status in 2014/2015, going on to support, as required, the preparation for removal scheduled for 2016.
Sureclean provided cleaning and waste management during the engineering-down phase, including descaling pipework and vessels to remove hydrocarbon deposits.
Bilfinger Salamis was used for support and access solutions in the early phase, including alternative system scaffold, rope access teams and fabric maintenance services.
To support decommissioning activities, Interpower supplied five, 1MW offshore containerized generators, positioned on Murchison’s cellar deck over three years starting Q1 2015.
In March this year, one of the cranes on the facility was damaged during a lifting operation, but no one was hurt. The incident happened while the crane was lifting supplies from an offshore vessel.
 |
Removal
One of the most visible work scopes is the topsides and jacket removal. AF Gruppen’s subsidiary AF Decom Offshore UK and Heerema Marine Contractors (HMC) were awarded a consortium contract for Murchison platform and jacket removal and disposal.
The original removal method was going to be a hybrid approach, combining short sailing times for the piece-small tonnage (approximately 10,000-tonne) to a UK disposal yard and direct offloading of the remainder from HMC’s heavy lift vessel at AF Environmental Base Vats, Norway.
However, when the consortium found it had two heavy lift vessels available in 2016, Hermod and Thialf, they took the opportunity to revise the method to a full reverse installation, using both vessels to remove the complete topsides, and shaving a full year off the project schedule. Mobilization was on 18 June and by 27 August, after just 80 days, the topsides scope was completed, via 26 heavy lifts and five vessels transits from the field to Vats.
Of the installation material to be removed, some 82% is estimated to be steel. Of the pipeline infrastructure to be removed, about 48% is estimated to be steel with some 50% concrete.
CNR International was given an exemption to allow the jacket’s footings – including 32 piles, up to 44m above the seabed, or down to 112m below lowest astronomical tide – to be left in place under an Ospar derogation ruling, due to their weight and installation date. Under Ospar rules, which cover the entire North Sea, under the Oslo Paris Convention agreement, facilities otherwise have to be completely removed.
CNR International is also being allowed to leave the 22,500cu m drill cuttings pile in place in line with the Ospar recommendation on drill cuttings piles. The view is leaving it to degrade naturally will be better for the environment than to disturb it by attempted excavation (OE: June 2016, Mud Glorious Mud).
 |
Pipelines
The main oil export pipeline will also be left in situ, with remedial rock placement over exposed sections. This involved some 75,000-tonne of rock placement in the summer of 2016, comparable to the already placed 63,000-tonne during the 1985 and 1987 operations in the field.
To remove exposed sections of the main export pipeline, totaling 17 sections, would have involved 746 cuts to lift and handle 729 x 12m-long sections, according to the decommissioning program submitted to DECC.
The program says the potential loss of life risk of the cut and lift operations of exposed sections was considered more than five times that of remedial rock placement. The cost of cut and lift would also be increased by a factor of 10. Stiff boulder clays also made trench and burying a difficult option.
The main pipeline tie in spools, at either end, will be removed, as will short early production pipeline bundles.
As OE went to press, the project was on schedule and below budget, with the work 80% completed. The remaining activities are to remove what needs to be removed of the jacket in 2017, to plug and abandon one suspended subsea well, and to remove the redundant subsea flowlines and wellhead protection structures.
Once this is completed, together with the clean-up of identified oilfield debris, a seabed overtrawl and as-left survey will complete the program.
Subscribe
