Environmental Impacts of the Decommissioning of Oil and Gas Installations in the North Sea

Many of the oil and gas installations in the northern North Sea are reaching the end of their economic production life, and proposals for decommissioning them are being prepared by the operators.  In 1995, proposals by Shell to dispose of the Brent Spar oil storage facility provoked an extensive campaign of protest.  The result was a change of plan, with the facility being towed inshore to be dismantled. The material has been recycled as a harbour at Mekjarvik, near Stavanger, Norway.

On 22nd October 1999, Phillips Petroleum Norway announced their plans to decommission 15 installations in the Ekofisk field, an operation on a much bigger scale than the Brent Spar.  If these proposals are accepted by the Norwegian government (full parliamentary approval was given October 2002), and The Oslo and Paris Commissions (OSPAR), 14 steel structures will bereturned onshore for recycling and a large concrete storage tank will beleft in situ.  Perhaps most controversially, Phillips plan to leave the drill cuttings piles in situ.  Drill cuttings consist of the fragments of rock that are removed as each oil or gas well is drilled, mixed with so called "drilling muds" which are used to lubricate the drill bit, carry rock fragments back to the surface and maintain pressure in the well as it is drilled. The drill cuttings are usually discharged into the sea adjacent to the platforms and although some of the drilling muds are recovered and re-used, some adhere to the cuttings and are also discharged.

In the shallow waters of the Southern North Sea, the strong tidal currents disperse the drill cuttings and any environmental impacts from their discharge rapidly dissipate. In the deeper waters of the Northern North Sea, tidal currents are much weaker and the drill cuttings remain in distinct accumulations around the platforms from which they have been discharged, forming so called "cuttings piles", which may contain as much as 40 000 tonnes of contaminated sediment. In the early days of the exploration of the North Sea, drilling muds were based on diesel oil which has a relatively high content of toxic aromatic compounds. As shown by the work of Professor John Gray of Oslo University, adverse effects of these cuttings discharges on the ecology of the adjacent sea bed may extend out to more than 5km from the point of discharge. Early concern about the environmental effects of these oil based drilling muds has led to replacement of oil based cuttings with less toxic alternatives wherever possible. In difficult drilling conditions, oil based muds are still used, but current legislation in the North Sea prevents discharge of cuttings containing more than 1% oil. Current platform based technologies cannot remove sufficient oil to meet this limit, so cuttings containing oil based muds are either re-injected down the well or removed to shore for treatment.

These piles of contaminated sediment can remain toxic to marine life up for 20 or more years after discharge, but there is no proven technology that could cleanly remove the large amounts of heterogeneous sediments from the deep water of the northern North Sea.  In the UK, the seriousness of this problem was recognised by the UK Offshore Operators Association (UKOOA),who collaborated on a large R&D programme to examine the environmental impacts of drill cuttings and the options for their management.  The reports from the first phase of this UKOOA programme were made available on the internet in January 2000.  Areas covered include detailed summaries of cuttings pile volume, contents and toxicity and discussion of management options and their potential environmental impacts. A second phase included field trials of methods to lift material from cuttings piles to the surface, using BP-Amoco's North West Hutton pile as a case study.

Results from the second phase have now been produced, with an overall summary report and executive summaries of individual project reports being available on the web. The full reports are available on CD from UKOOA and have been passed to OSPAR for consultation.

Some other internet resources on this issue are given below.

Our contribution to the discussion

In conjunction with Dr. Simon Gerrard, Roy Marsh and Clare London, I have been involved in a detailed review of the environmental impacts of drill cuttings piles and the options for their management during and after the decommissioning of offshore platforms in the North Sea. This work was funded by Amoco Exploration (UK) Ltd, now part of BP Amoco. It pre-dates the UKOOA initiative, and the results are already in the publicdomain.  Printed copies of the final report:

Gerrard, S., Grant, A., London, C., Marsh, R., 1999. Drill Cuttings Piles in the North Sea: Management Options during Platform Decomissioning .UEA Centre for Environmental Risk, Research Report 31, 224 pages.

Can be obtained at a cost of £50 by writing to Dr. Alastair Grant, School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK.  Please make cheques payable to The University of East Anglia. It can also be freely downloaded from this site as a pdf file.

We have also carried out research assessing the toxicity of material from drill cuttings piles to marine invertebrates and have an ongoing research programme seeking to determine which components of the sediments are responsible for the very high toxicity that we have observed.  A preliminary account was presented at a decommissioning conference in January 2000 (Grant 2000) and a fuller account was published in the journal Marine Environmental Research (Grant and Briggs, 2002).

Grant, A., 2000.  Toxicity and environmental risk assessment of drill cuttings piles.  In: Decomissioning of Offshore Oil and Gas Installations, IBC Global Conferences, London.

Grant, A. and A.D. Briggs, 2002, Toxicity of sediments from around a North Sea oil platform: Are metals or hydrocarbons responsible for ecological impacts? Marine Environmental Research, 53, 95-116. (read abstract) request a reprint via email

This work forms part of a larger research programme on the ecological effects of pollutants in the marine environment. There are opportunities in the School of Environmental Sciences to study for a PhD in ecotoxicology and in a number of other areas of environmental pollution and environmental management. A list of available PhD projects is available on our School Studentship pages, but we are always happy to discuss ideas for research topics not on this list. Please email me for more details.

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If you are interested in undergraduate study in ecology, environmental sciences or related fields, please look at the web pages describing the University of East Anglia's degree programmes in Ecology and Environmental Sciences.

Other internet resources on the decommissioning of offshore installations

Decomissioning of offshore installations came to international prominence as a result of Shell's proposals to dispose of the Brent Spar oil storage tank by dumping it in deep water beyond the edge of the continental shelf.  The Brent Spar was occupied by protesters from the pressure group Greenpeace and in response to large scale protests the disposal plans were altered.  A full record of these events is given by Shell's site on the Brent Spar.  A Greenpeace perspective on the issues can be obtained from their  website. An independent assessment of the options for disposal of the Brent Spar is also available on their web site and some comments on the implications of the events for including stakeholders in environmental decision making are on the Sustainability web site.

The United Kingdom Offshore Operators Association (UKOOA) is funding a collaborative R&D project on the environmental effects and management options. Details of contracts issued in the second phase of this initiative were posted on the UKOOA web site in December 2001.

Regulation of disposal at sea, including decommissioning of offshore installations is governed by the Oslo and Paris Commissions (OSPAR).  In July 1998, the OSPAR meeting at Sintra in Portugal resolved that the great majority of offshore installations should be returned onshore for disposal. A document giving a summary of main results of Sintra meeting, July 1998 and guidance notes on decommissioning produced by the UK Department of Trade and Industry, including material on the implementation of the Sintra agreement, are both available.

The largest decommissioning project proposed so far is the abandonment and removal by Phillips Petroleum Norway of 15 installations in the Ekofisk field, collectively referred to as "Ekofisk I".  These proposals were submitted to the Norwegian government on 22nd October 1999 and approved on 21st December 2001.  Details are on the following web pages:

Phillips Petroleum Norway, Proposals for Ekofisk decommissioning
 Greenpeace press release on Phillips proposals for decommissioning Ekofisk
 Comments on Ekofisk plans by Alexander's Gas and Oil Connections, March 1999

Other relevant sites

UK Department of Trade and Industry decommissioning home page

Document produced by the London Convention giving guidelines on whether platform decommissioning would constitute dumping at sea

List of approved and pending decommissioning programmes in the UK sector

Article in Offshore Environment about the impacts of drillcuttings

Muddied Waters a report by Jonathan Wills on the environmental impact of drilling wastes

Proposals by Phillips petroleum to decomission the Maureen facility in the North Sea

Proposals by Total Fina Elf Norway to decomission the Frigg field

General comments by Greenpeace on the decommissioning issue

Det Norsk Veritas decommissioning page
Rogaland Research decomissioning page and  report by Rogaland on the disposal of oil based cuttings
Index of a number of documents mostly on platform decommissioning

International Association of Oil and Gas Producers 

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Page created by:

Dr. Alastair Grant,
Reader in Environmental Sciences,
University of East Anglia,
NR4 7TJ,


Last modified, 9th April 2003