Offshore Magazine - August 2013

ENGINEERING, CONSTRUCTION, & INSTALLATION
Safety, feasibility highlight
platform removal plan

Nick Terdre

Contributing Editor

Even high oil prices cannot sustain depleting felds forever, so CNR International (CNRI) has decided to end production on the Murchison feld in the UK northern North Sea. Last oil is cheduled for early 2014. In May, the company submitted a draft decommissioning program for the feld, which extends partly into Norwegian waters, to the UK’s Department of Energy and Climate Change (DECC). This details proposals for disposing of the feld facilities. DECC will liaise in its review with Norway’s Ministry of Petroleum and Energy.

One notable feature of the program is the recommendation to seek derogation (exemption) from the usual requirement to recover the jacket to shore in its entirety. Primarily for safety reasons, CNRI proposes leaving the jacket footings in place. Another point of interest is the availability of new single-lift technology which is under consideration for both the topsides and jacket removal operations.

Assuming DECC approves the request for derogation, this also needs backing from member countries in OSPAR – the Oslo-Paris convention for the protection of the marine environment of the North Atlantic. The OSPAR consultation is expected to be completed by next spring at the latest, opening the way for DECC to give its fnal approval by mid-2014.

In the meantime, CNRI is already preparing the main contract awards.

One for engineering-down and cleaning the topsides is to be let this month (August), and tenders for both jacket and topsides removal will also be issued, according to Mike Corcoran, the Murchison decommissioning strategy consultant. Confrmation of the awards will follow approval from DECC for the decommissioning program.

Later this year CNRI will assign a contract for drilling rig activation and well plug-and-abandonment, and another with a subsea scope covering the pipelines and recovery of equipment on the seabed will be put out to tender.

Ownership history

Murchison was discovered in 1975, approved for development three years later via a single production, drilling and living quarters platform, and brought onstream in 1980. First oil fowed through subsea wells which were shut-in when the frst platform wells came onstream in 1982.

Production at that point was expected to end in 1997.

The feld has changed hands several times during its production lifespan. It was discovered and developed by Conoco, which remained in charge until 1995

when operatorship passed to Oryx, subsequently acquired by Kerr-McGee in 1999. In 2002, CNRI took on Kerr-McGee’s interest and became operator. The current licensees are CNRI and Wintershall; CNRI’s

77.8% stake corresponds to the UK share of the feld;

while the 22.2% held by Wintershall represents the Norwegian share. Two further small felds have been tapped by wells drilled from the Murchison platform: Playfair, 5 km ( 3. 1 mi) to the north, is wholly owned by CNRI, and Delta, a discovery undergoing test production on the Nor wegian side, is owned by Wintershall.

metric tons ( 27,099 tons), comprise 26 modules providing facilities such as drilling, processing, water injection, power generation and other utilities, and accommodation for 198 personnel. There is also a drilling derrick servicing the 33 well slots, a fare boom, and a helideck.

The heaviest structures are the two parts of the module support frame (MSF). One, which is connected to Cellar Deck East, has a dry weight of 2,409 metric tons ( 2,655 tons) while the other, connected to Cellar Deck West, has a dry weight of 2,345 metric tons ( 2,585 tons).

Excluding piles, the eight-leg, 188-m (617-ft) tall jacket – fxed in

156 m (512 ft) of water – weighs 24,640 metric tons ( 27,161 tons). Including piles, grout, marine growth, and water in fooded members, its maximum weight is estimated at 27,584 metric tons ( 30,406 tons).

The jacket is an all-welded, steel tubular construction, in which the legs are stiffened by horizontal and vertical bracings. Each corner leg is secured to the seabed by eight 80-m (262-ft) long piles of 82-in.

diameter, driven through sleeves 40-50 m (131-164 ft) into the seabed. Each cluster of piles is known as a bottle assembly and weighs

3,000 metric tons ( 3,307 tons).

The footings¸ weighing around 12,700 metric tons ( 13,999 tons), constitute the section of the jacket from the seabed to the top of the bottle assemblies some 44 m (144 ft) above the seabed. The jacket may be considered for derogation because it weighs more than 10,000

metric tons ( 11,023 tons) and was installed prior to Feb. 9, 1999. Jackets installed after that date must be designed for eventual removal.

Murchison’s oil heads through a 19.1- km ( 11.8-mi), 16-in. pipeline to the Fair-feld Energy-operated Dunlin platform, and from there via the TAQA-operated Cormorant A complex to the Sullom Voe terminal on the main Shetlands island.

A subsea link into the Northern Leg Gas Pipeline (NLGP), originally built for gas export, is now used to import gas to the Murchison platform for fuel.

As part of the decommissioning planning process, CNRI consulted with stakeholders, holding a workshop in early 2012 at which available decommissioning options were discussed. During a subsequent workshop late last year the recommended options were presented. The draft decommissioning program is now out to consultation, with Xodus appointed as an independent review consultant.

Before deciding to remove and dispose of the platform, the partners considered re-use options for oil and gas production and alternative uses, such as putting a wind energy generator on the topsides. None were found to be commercially viable.