Decommissioning Activities in the North Sea Professor W D Dover - - PowerPoint PPT Presentation

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

• riginal 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

• f 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

• bjection 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

• nly 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

• SIFs 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

• f 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

• f 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

• perations would be complex. These operations

would pose a high safety risk to divers.

IRG Influence

• One potential decommissioning option which it

considered had not been examined in sufficient depth i.e. removing the structure down to the cuttings pile level.

• Several other areas where additional work was

needed were identified (eg the cuttings pile modelling study).

• BP responded by carrying out additional studies

IRG Influence

• IRG comments sent to contractors,

responses were also considered

• Advice given by IRG on long term

problems

• Decommissioning route left to BP and DTI

Permission granted in 2006

Shell Brent Decommissioning Project

INDEPENDENT REVIEW GROUP (IRG)

John Shepherd

(Chairman)

National Oceanography Centre University of Southampton

Membership

• Prof. John Shepherd, FRS (Chairman)
• Prof. W.B Wilkinson (Project Co-ordinator)
• Research Scientist Torgeir Bakke (Mar. Biol.)
• Prof. Dr-Ing. Gunther Clauss (Engineering)
• Prof. William Dover (Engineering)
• Prof. Dr. Jürgen Rullkötter (Sediments &

Geochemistry)

• Professor J Croll (Concrete Structures)
• Mr Richard Clements (Secretary)

Brent field background

• Brent has three concrete and one steel

platform

• Water depth is 140m
• Much larger decommissioning task than

NW Hutton with extra possible problems to consider

Some Brent Statistics

• Bravo 139 metres water depth, substructure

165,664 tonnes

• Charlie 141 metres water depth,

substructure 287,542 tonnes

• Delta 142 metres water depth, substructure

177,809 tonnes

Brent scope of work

• Gravity Based Structures (GBS)

remediation and decommissioning

• Steel jacket removal
• Subsea and pipeline decommissioning

studies.

Concrete storage cells

• Concrete storage cells form part of the

GBS at the seabed

• It is very important to know the amount

and type of content in the storage cells (weight). Sampling will be a big problem.

• This is not easy either through the

existing pipeline system or external hot tapping of the cells.

Drill cuttings

• Cuttings exist on the seabed and on top of the

storage tanks (cells).

• Cuttings could move and/or make the structure

unstable during floatation.

• Assessment of the quantity of the drill cuttings and

the nature of the cuttings will be necessary.

• Removal of the drill cuttings must also be

considered.

GBS removal

• Several options are being considered.
• Refloat and tow onshore
• Partial Removal (remove legs)
• Leave structure
• Initial problem is to determine the weight
• f structure.

Refloating the GBS

• This would consist of deballasting part of

the structure to provide the platform with near neutral buoyancy, prior to providing underbase hydraulic overpressure sufficient to jack the platform (and skirts) from the seabed without creating a “pop-up” effect.

• A quick release from the seabed (pop-up)

could lead to instability

Platform removal

• Cutting large diameter members could need

new technology

• New heavy lift vehicles becoming available.
• Buoyancy tanks could be attached
• Structural analysis could be very important
• Energy CO2 emissions becoming more

important

Summary for Brent

• Heavy Lift Vessels, Sampling, Remote working,

Cutting, Inspection/NDT (cracking, corrosion, residual stress), Modelling, Structural Analysis (Strength and Stability).

• These are some of the areas where advances are

being made (and need to be made) and these will prove to be valuable for future decommissioning work

Final point

• Consumption of energy and the emission of

carbon dioxide has become a bigger issue recently

• These factors will feature more strongly in

consideration of the options and society needs to reconsider the desire to return the oil exploitation area back to its original condition. It may be necessary to live with some pollution.