Knowledge Centre
Project Management, Engineering and Safety
One of the challenges concerning decommissioning is that most North Sea structures were custom built for the particular field conditions, so decommissioning solutions will be equally as varied, with no readily tried and tested techniques. The work is non-productive so a key objective will be to find ways to reduce costs. The possibilities of aggregating work over several structures or scheduling it to coincide with slack time, with flexible deadlines, have been mooted as possible strategies. Developing new techniques or vessels to improve capabilities beyond straight de-construction reverse-engineering processes may also raise possibilities.
However, with the continual change in field cessation of production (COP) and decommissioning timings, it is difficult for contractors/suppliers to understand and react to market signals. A key part of the way forward for industry therefore is to be more lucid on its future operational plans so the supply chain can respond more effectively.
Issues which should be considered in the decommissioning programme include:
- Well killing, plugging, abandoning and surface structure removal operations: this, can take place before the formal process decommissioning programme is complete;
- Clean-up, preparation and removal of top-sides: hydrocarbons have to be removed and systems isolated, purged and cleaned before removal;
- Steel jacket removal: many deep water structures were floated in place, not lifted with a crane-barge, and so may have to be removed in sections;
- In-field flow-line removal: especially difficult if adjacent fields are still producing;
- Clearance of all debris on the sea-bed.
These processes will involve a great many hazards which will need to be safely managed during the decommissioning programme implementation. These may include:
- Use of explosives, for under-water cutting: giving consideration to possible collateral damage to structures and their stability, divers, marine mammals or floating vessels;
- Ageing and corroding structures: suitability for decommissioning operations, un-planned eventualities arising from old/poor/incomplete records etc;
- Dealing with contaminants: this needs careful consideration, especially for low specific activity (LSA) scale, poly-chlorinated biphenyls, PCBs, asbestos etc.;
- Lifting operations: especially heavy-lift or through water/air interface;
- Marine vessel interactions and sea-state/weather conditions;
- Personnel travel (helicopters) and accommodation/ maintenance of life-support on ‘dead’ platforms.
Criteria to be considered in determining a solution
The DECC guidelines state that the criteria used to determine the most appropriate decommissioning solutions needs to take into account:
- Local and international regulations;
- Impacts on the marine environment and other users of that environment;
- Re-use and re-cycling opportunities;
- Cost, safety and the practical availability of technology.
Environment: when considering the environmental impacts of a given option it is necessary to assess the wider effects on the land, sea and air of the energy used, the emissions, discharges and waste streams generated and the effects they have on the marine flora, fauna and other users of the sea.
Re-use, recycling and disposal: Apart from FPSOs, the re-use of platforms or even process equipment has been non-existent to date. The reasons why this does not make economic sense in the North Sea include the lack of operational confidence in second-hand equipment in a high cost environment, re-certification issues, tax and/or duty anomalies and customisation mismatch. A more likely possibility exists of refurbishment and re-use in other areas of the world ,e.g. Hutton TLP in Russia, or for some other purpose, e.g. Brent Spar and Maureen which were cut up to build quays. Failing that, most are brought to shore and re-cycled as scrap. There is always a proportion that will end as landfill, but this should be reduced to as low as practical and is usually less than 10% by weight.
Cost and safety: it is the operator’s responsibility to find the most cost-effective option which doesn’t compromise the safety of workers or the environment. At present, the cost for platforms is in the order of £30million for a small platform in the southern North Sea, and £100-200million for medium to large sized platforms in the northern sector.
Public opinion: the views and concerns of people outside the industry are important, not just overall for setting the legislative climate but also individually, through statutory and other consultees influencing regulatory approvals.
Technology: New offshore technologies are continually being developed, evaluated and tested in order to ensure that the best technology is available to provide safe and efficient procedures. To date most decommissioning programmes have relied upon heavy-lift vessels which are designed to take the structure apart offshore piece by piece.
Research and development
Research and development (R&D) is carried out across the industry by operators, major contractors, specialist suppliers and universities sometimes through programmes supported by government. R&D areas to date include the following:
- Alternative removal methods, looking to develop new vessels attempting to remove top-sides in one go or in the floating removal of jackets;
- Underwater cutting. There have been a lot of advances here through the use of abrasive water-jets, diamond wire cutters and shaped explosive charges;
- Drill cuttings removal;
- Lifting, back-loading and sea-fastening methods.
Clearly different technologies will be needed dependent upon the type and location of structure to be decommissioned. Technological research and development is also being conducted to assist with the re-commissioning of some structures for potential re-use, the recycling of unwanted scrap materials, and the orderly and efficient disposal and treatment of wastes onshore.
