Knowledge Centre
Technical Perspective
Size and scale of the challenge
There are over 600 offshore oil and gas installations in the North Sea, 470 of which are in UK waters. These include sub-sea equipment fixed to the ocean floor as well as platforms ranging from the smaller structures in the southern North Sea to the enormous installations of the northern North Sea built to withstand very harsh weather conditions in deeper water. Many were built in the 1970s and were hailed as technological feats of engineering when they were installed. The industry now faces the equally challenging task of decommissioning them.
Fig: Deep water structures can be several hundred metres high
One of the main challenges is the many different types and designs of structure which mean that there is no single tried and tested method for removal. Most of the structures were designed to suit particular development and field conditions. They range from small steel lattice-work structures, through to large heavy concrete or steel structures, and include floating production, storage and off-loading installations (FPSOs), and under-water (sub-sea) extraction systems. The following diagram gives some idea of the diversity of shape and scale.
Fig: A wide variety of structures exist in terms of overall size and design
The smaller, shallow water installations are themselves large structures by any standards, often towering to heights of around a hundred metres, comparable in size to Big Ben. The enormous concrete or steel structures which are prolific in deep waters such as those of the northern North Sea can be as big as the Eiffel Tower and much, much heavier.
Offshore, as well as such installations, there are more than 10,000km of pipelines, ca. 5000 wells and accumulations of drill cuttings. Associated with these operations are also 15 onshore terminals.
Of the 470 offshore installations present on the UKCS, it is estimated that 10% are floating structures, 30% are sub-sea, 50% are small steel and 10% are large steel or concrete. Under current regulatory requirements, over 90% of offshore structures will be completely removed from their marine sites and brought to shore for re-use, recycling or other disposal means. The rest, which comprises the very large and heavy steel or concrete installations (approximately 10 %), will be looked at on an individual basis to assess whether it is technically feasible (and safe) to remove them, bearing in mind that there is a general presumption for complete removal. If they are too difficult or dangerous to be removed to shore, an exceptional case for 'derogation' can be made (see Legislative Framework)
Decommissioning timescale
It is inherently difficult to predict the exact date of decommissioning for each structure. There are a number of reasons for this including:
- long term trends in oil and gas prices which determine whether it remains economic to keep a field in operation;
- long-term certainty on both fiscal and regulatory regimes which will influence the future investment environment;
- improved production and reservoir recovery methods;
- extending the use of the infrastructure, e.g. for smaller satellite fields tied-back into existing export systems;
- alternative use of the structures e.g. for gas storage or carbon sequestration.
The industry is actively pursuing ways to delay decommissioning and extend the productive life of existing infrastructure, therefore allowing it to be used to maximise recovery of the UK’s remaining hydrocarbon resources. This, along with the other timescale uncertainties, raises another challenge: that of an uncertain market place for decommissioning in which contractors have a difficulty in gearing up technology and the workforce requirements for an ever changing activity plan. To help rectify this, Oil & Gas UK has taken several steps since 2008 to bring together operators and the suppliers of decommissioning-related goods and services; its forthcoming Offshore Decommissioning conference is one such example.
Decommissioning cost estimates
As with timing estimates, projections of the overall cost of decommissioning for the UKCS also vary widely, from around £15 billion to £30 billion. Oil & Gas UK regularly re-issues its estimate of this cost which is published in the annual Economic Report. The variables responsible for this range of estimates include:
- the inclusion/exclusion of wells and pipelines;
- the level of removal i.e. success in achieving derogations for installations (and final status for pipelines);
- cost estimation methods.
A figurative breakdown of overall UKCS costs is noted in the figure below:
Fig: Schematic showing an estimated decommissioning costs breakdown - such estimates are dependent upon several variables (Source: DECC)
Different companies have differing perspectives and methods of estimating the costs involved in decommissioning projects. The introduction of cost uncertainty has a negative impact on, for example, the processes surrounding asset transfer deals, as well as the market place for the award of decommissioning contracts.
To help standardise decommissioning cost estimation, industry guidelines on the methodology behind cost assessment have been published by Oil & Gas UK. The aim is to provide an advisory (as opposed to a prescriptive) service to ensure an industry wide, comprehensive and uniform approach to decommissioning cost estimation, as well as improved estimates for market assessment.
The figure below gives a time/cost estimation of the decommissioning process by UKCS region. It can be noted that the bulk of costs are likely to be incurred in the deeper waters of the Northern and Central North Sea.
Fig: Decommissioning costs by region (WoB = West of Britain, SNS/CNS/NNS = Southern/Central/ Northern North Sea).
Source: Oil & Gas UK / Wood Mackenzie
Offshore infrastructure subject to decommissioning
Fixed platforms
Platforms can be designated as small or large (those in water depths of more than 100m, with jackets or sub-structures weighing more that 10,000 tonnes).
For small platforms, entire removal to shore is the only option. The following pictures show a process of decommissioning used in the southern North Sea, where experience to date runs to more than a dozen fixed platforms having already been decommissioned, including all Esmond, Forbes and Gordon field structures and infield platforms in West Sole, Viking, Leman and Camelot.
Fig: The decommissioning of Southern North Sea fixed platforms
For the large platforms, the top-sides need to be removed to shore, but there may be options, under OSPAR 98/3 derogation, for part of the steel jackets and for concrete substructures.
These include:
- Complete removal;
- Partial removal, leaving 55m clear water column for navigational safety. The cut-off point would be so as to remove as much as reasonably practical. In the case of North West Hutton, this was at the top of the “bottle leg” footings (where the thickness of steel meant that cutting safely with current technology was not possible);
- Leave in place, for concrete gravity based structures only, leaving appropriate navigation aids. The cut-off point is determined by the construction.
On the UKCS there are only 40 platforms which qualify to apply for derogation, 9 concrete and 31 steel. However, not all will take the derogation option so the total number of structures left on the sea-bed will be lower; all new structures installed after 1st January 1998 have had to be capable of total removal.
The first 98/3 derogation approval was for the Ekofisk concrete tank in Norwegian waters; the first UKCS derogations were for three Frigg concretes - two on the UK side approved by UK and one on the Norwegian side approved by Norway. North West Hutton was also given approval in 2006 to remove topsides and the steel jacket down to the bottle-leg footings. It took three years to consult on the options and final plans will re-cycle or re-use 97% of all materials. There is also a drill cuttings accumulation around the footings. The figure below illustrates this.
Fig: Schematic showing the North West Hutton substructure
Floating and subsea
Many of the large structures being decommissioned in the UK sector have been unusual floating structures. These include the Brent Spar, Maureen and Hutton TLP installations:
- Brent Spar - a floating storage and off-loading column, originally approved (in 1995) for removal to a deep sea bed disposal site, this was reversed (following direct action by Greenpeace environmental activists) and in 1999 the structure was cut into segments to make the base of a ferry quay in Mekjarik, Norway (see below).
Fig: Brent Spar as it is a) towed to shore, and b) as part of a new dock construction
- Maureen – a unique steel gravity base platform (100,000 tonnes), which was floated free from the sea-bed and towed intact to Norway. Initially, full re-use options were considered, but eventually it too was re-cycled as a quay in Stord, Norway in 2001. As with Brent Spar, the draft required deep inshore waters to effect the subsequent de-construction and disposal (see below).
Fig: The Maureen installation as it is towed to shore
- Hutton TLP – the first ever Tension Leg Platform (TLP) which was removed for re-use outside the UK in 2002.
Fig: The Hutton TLP installation being towed to shore
Recently, there has been an increasing use of subsea satellite developments tied-back to existing fixed platforms, which makes decommissioning fairly straighforward for those fields.
Fields with floating systems, especially Floating Production, Storage and Offloading vessels (FPSOs) tied into sub-sea manifolds, lend themselves to re-use at the end of field life. Examples of re-use following field decommissioning include floating platform Argyll (1992) and FPSOs Angus (1993), Emerald (1996), Donan (1998), Blenheim/Bladon and Durward/Dauntless (both 2000).
Fig: An FPSO vessel (with a tanker at the rear) operating in the Foinaven Field
Wells
Following a field's Cessation of Production (COP), the decommissioning of production and injection wells is often the first phase of the overall programme. On platforms this requires the removal of conductors from the sea-bed to the platform deck, which can also have structural implications. On floating or sub-sea developments, it requires the removal of sub-sea wellheads to around 5 metres below the level of the surrounding sea-bed. After this time it is recommended that fishermen are commissioned to check the sea-bed for any remains that could pose a potential hazard for nets and issue a clearance certificate.
Likewise, the decommissioning of exploration wells is also an important issue. Companies sometimes choose to plug and suspend successful exploration wells for future development. However, there is no record of a well over six years old being reactivated in such a way, while there are many wells older than this that have remained suspended.
Oil & Gas UK's subsidiary, Common Data Access Ltd (CDA), manages the rigorous tracking of well ownership and location, the information from which is regularly communicated to fishing vessels to allow on-board plotters to be updated to help them avoid the hazards. Since 1998, Oil & Gas UK has promoted a campaign for the full abandonment/removal of suspended wells to reduce the sea-bed inventory, lessen the hazard and maintain a good working relationship with the fishing industry.
The standards of safety required before wells can be left are of importance for all these cases. Oil & Gas UK provides a framework of good industry practice and regulatory requirements through its Guidelines for the Suspension and Abandonment of Wells, which is regularly updated to incorporate recent experience and learning. The main issue is to ensure that effective barriers adequately isolate reservoir fluids both down hole and at its surface.
Pipelines
Pipeline decommissioning involves the pigging, flushing, filling and plugging of lines, followed by removal or abandonment in situ. However, the regulations relating to pipelines are currently controlled by national rather than international requirements. This, together with a low level of experience to date, means that the standards required are difficult to predict.
Pipelines can be laid on top of the sea-bed, trenched or buried and this may also influence the decommissioning programme required. In general, if the pipeline is of a small diameter (less than 12”) it is likely that it will need to be removed or fully buried. This would apply to all in-field flow-lines and control bundles. For larger trunk lines, the options depend on other factors and the practicality of removal, disposal etc. There are a number of issues which influence these decisions and will need to be addressed, including overlapping or crossing pipelines, concrete mattresses, spans, sea-bed stability etc.
Currently the Government recognises that during the course of field life, pipelines or parts of pipelines may be taken out of use, at which point operators must give notification of disuse to the DECC. This can be done via a 'disused pipeline notification' pro forma available for download from DECC's oil and gas website. When this happens, the Secretary of State has the option of immediately calling for a full decommissioning programme under the Petroleum Act 1998. However, this is often not considered the most appropriate option and it has therefore been agreed that consideration will be given to handling suitable pipelines under an informal decommissioning regime (the 'Interim Pipeline Regime') thereby deferring the full formal programme until the end of field life.
The Interim Pipeline Regime is intended to ensure that out-of-use lines do not pose a risk to other users of the sea or to the environment and that they are covered by an appropriate surveying and maintenance regime from the point when they are taken out of use until approval of the formal decommissioning programme, which is usually at the end of field life.
Drill cuttings
Fig: Drill cuttings / drilling mud mix
Under some platforms there are large mounds of drill cuttings, deposited when the wells were drilled. In the main these only exist in northern North Sea waters, where sea-bed currents are not strong enough to have dispersed them. These accumulations also contain drilling mud weighting material (barite) and hydrocarbons from when oil-based drilling muds were used. The composition, size and nature vary greatly, but some piles weigh over 10,000 tonnes, are up to ten metres high and can extend hundreds of metres from the platform which they originate from.
Please click here to learn more about the way in which drill cuttings are treated at the cessation of prodution.
Debris
Once the main structures have been removed, there may remain small items of debris on the sea bed which has fallen overboard during the decommissioning process or at some point during the lifetime of the installation. Examples include scaffolding, cables, concrete blocks etc. These remains must also be entirely removed or protected and monitored. A comprehensive survey of the site should be carried out – usually in the presence of an independent organisation, very often the fishermen. Any materials which have been left behind can then be picked up and disposed of onshore, at which point a certificate can be issued to confirm that the sea bed has been fully and comprehensively 'cleaned' of any oil and gas related debris.
The aim of the post-decommissioning activity is to ensure anything left behind does not adversely affect the marine environment or other users of the sea (e.g. fishermen, shipping).
