Decommissioning Activities in the North Sea Professor W D Dover Emeritus Shell Professor of Mechanical Engineering at UCL
Decommissioning Activities North Sea • Two major decommissioning activities are being considered for the North Sea • BP have completed a study for the NW Hutton platform and are likely to start removal in 2008 • Shell are at feasibility stage in considering the removal of the Brent Field (one steel structure and three concrete platforms).
Decommissioning • Oil companies are not free to remove platforms as they wish • Society has a keen interest in returning the oil and gas exploitation zone to its original state • For this reason the Dept Of Trade and Industry (DTI) in the UK and similar bodies such as OSPAR (arising out of the Oslo and Paris Conventions) have produced directives for platform removal. (Mainly DTI) • It is worthwhile reminding ourselves of some of these requirements
Typical DTI/ OSPAR requirements No accident or harm to people No spills or discharges of oil or chemicals to land or sea No release of ozone depleting substances Maximise efficiency of energy and resource use Restore the site to as close as possible to its original state Balance societal, safety, economic, environmental, and technical considerations
Oil Company response • Oil companies have responded strongly to the DTI requirements • Very wide consultation process • The best contractors and consultants have been asked to consider the wide range of problems • Independent Reviews have been undertaken
Independent Review Group • Both BP and Shell established independent review groups to assess the studies undertaken and give independent advice. • The IRGs although funded by the oil companies retain their independence.
BP NW Hutton • BP have gone through this whole process of consultation for the NW Hutton platform • An Independent Review Group was constituted under the Chairmanship of Professor John Shepherd FRS • The review group has submitted its findings to BP and these are now on the BP website. • Nature and work of the IRG are as follows
IRG work for BP on NW Hutton • Committee • Nature of IRG work • Background to NW Hutton • Main problems for NW Hutton • Details of IRG report
The Independent Review Group (IRG) An independent group of scientists and engineers who could examine and comment, in an independent and objective way, on studies relating to the development of proposals for the decommissioning of the NW Hutton platform in the North Sea.
Important IRG Caveat The Independent Study Review Group, or any member thereof, will have the right to publish the findings of their scientific review including any objection after notifying BP with sufficient notice to enable BP to comment and correct any misunderstandings.
IRG Membership Prof. John Shepherd , MA, PhD, CMath, FIMA, FRS (Chairman) Prof. William Dover , FIMechE, CEng, FINDT (Engineer) Prof. W.B Wilkinson , BScEng, BScGeol, PhD, FICE, CIWEM, FGS, CEng, CGeol, F Russ Acad Nat Sci (Geotechnical) Research Scientist Torgeir Bakke , Cand.real. Marine Biology Prof. Michael Cowling , BEng, PhD, CEng, CMarSci, FIMarEST, FSUT, MIM (Metallurgist) Prof. Dr. Jürgen Rullkötter , Dipl.-Chem., Dr. rer. nat.habil., AAPG, DGMK, DGMS, EAOG, GDCh, GS (Sediments and Geochemistry) Mr Richard Clements , CEng, MIMechE, MIMarEST(Secretary )
IRG Activities for N W Hutton 7 meetings & 2 teleconferences (March 2003 to March 2004) Study and critique of about 40 major reports about 120 man-days of work in total In a few cases, several review/revision cycles
The work of the IRG read and review all the reports commissioned for or produced by BP, provide views/guidance on the above in respect of the scope, clarity, completeness, methodology, relevance and objectivity of conclusions, advise on any further research or actions to address identified gaps, be satisfied that all relevant stakeholder comments have been addressed
N W Hutton Northern North Sea 143m
NW Hutton Topsides consisting of module support frame and modules which house the accommodation and processing equipment. 20,200 tonne. An eight-legged steel jacket that supports the topsides, approximately 17,000 tonnes. Steel footings of approximately 40m height (lower sections of the legs, which are also referred to as ‘bottle legs’).
Bottle Legs NW Hutton
Potential problem areas considered • Drill Cuttings • Structure • Pipelines
NW Hutton (Drill cuttings) During the period up to 1992, drill cuttings were discharged to the sea from the platform. The cuttings comprise mainly rock fragments but they also contain a residue of drilling mud, that adhered to the rock fragments after the cuttings were treated, also some paint and detergent. This discharge accumulated on the seabed directly under the platform, to create a cuttings pile (mound) with an approximate weight of 41,600 tonnes.
Drill cuttings • Leave in situ and monitor • Leave in situ and cover • Move, bury and monitor • Recover to surface and inject • Recover, take to shore and treat
Drill cuttings • In situ cuttings will slowly leak oil into the water column but not enough to give a surface slick • The pile could be disturbed by fishermen and this would give a greater release. • If left undisturbed the release would gradually become negligible
Drill cuttings • Covering the drill cutting pile by rock dumping was one possibility considered. • The result would be a pile with no oil emissions that could be trawled by fisherman without disturbing the pile • Energy and CO2 emission would be high for this solution
Drill cuttings • Recovering to the surface for either re-injection or cleaning and disposal onshore is also an option • Lifting would cause a relatively large release of oil to the water column • Large energy requirements and CO2 emissions would appear to eliminate this option but it is the only way to fully restore the seabed.
NW Hutton Structure: Cutting tools • Both the diamond wire and abrasive water jet cutting techniques are prone to operational difficulties that can lead to incomplete cuts. • For the final structural cuts required to free each section for lifting, failure to complete a cut would represent a major source of risk because the crane would be attached to the section in readiness to lift. • A cutting failure at this stage of the operation could result in the equipment and vessels being exposed to a severe risk of damage due to loss of stability.
Structural deterioration • During the production life of a platform it is likely that a deterioration in strength will occur. • In addition the cutting and lifting process will load the platform in a different manner to service loading • For these reasons great care must be taken to ensure that the condition of the platform is established
1 Corrosion • Corrosion is likely to occur and the level of corrosion must be established. • A simple change in wall thickness is not the most important measure of corrosion • More important is the loss of section at welded joints and stiffener connections. • Weld toe corrosion must therefore be established
2 Weld toe cracking • Offshore structures experience cyclic loading and hence can suffer from fatigue cracking at welded joints. • It is therefore important to inspect and measure the size of any fatigue cracks • Conventional MPI is not suitable for this purpose; instead ACFM type equipment should be used • These techniques will give the crack length and depth and can be deployed from ROVs
3 Structural strength • The strength of a cracked body is a function of the stress applied and the crack size • Fracture mechanics is the method of analysis and stress intensity factors (SIFs) are used to characterise the combined effect of stress and crack size • SIF s have been produced for tubular welded connections and should be used to determine the structural strength
4 Global stresses • The method of manufacture and the difficulties met during installation of a platform can lead to the presence of large built-in stresses . • Cutting and lifting operations can release these stresses possibly resulting in large deflections during final separation of parts of the platform
5 Measurement of stress • Attempts should be made to measure the built in stresses so that springback can be minimised • Non contacting measurement of stress is possible with equipment such as the StressProbe. This is a device based on magnetostriction. • StressProbe has been used subsea and can be deployed from an ROV in a pick and place mode of operation.
Diver activities • It would be preferable, from a safety point of view, to deploy ROVs to carry out all the work. • Significant advances in ROV use have been made, but the studies indicate it is probable that divers would be required for some of the removal operations. • Use of divers is most likely during the removal of the lower-most parts of the jacket where operations would be complex. These operations would pose a high safety risk to divers.
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