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Abstract: Environmental Management of Deep-Water Exploration, Rockall Trough

Marathon Oil U.K., Ltd., on behalf of co-venturers Phillips Petroleum Company United Kingdom Ltd., Pan Canadian Petroleum North Sea Ltd., and AGIP United Kingdom Limited, operates a group of four offshore blocks awarded in the UK 17th Licensing Round in 1997. The blocks are located on the eastern flank of the Rockall trough approximately 100km north-west of the Western Isles, in water depths up to 1,500m. A single exploration well, 153/5-A is currently planned.

The environmental sensitivities of this relatively undeveloped area were recognised at the commencement of Marathon's interest in the Rockall Trough. Early consultations with statutory Government agencies confirmed that environmental management would form a key element of the proposed exploration work programme.

In recognition of a lack of existing information at that time, concerning various environmental sensitivities of the Atlantic margin, environmental data has been acquired through a combination of collaborative projects conducted by the Atlantic Frontier Environmental Network (AFEN) since 1995. AFEN now comprises a consortium of 21 Operators and several UK Government departments, and is funded through the Atlantic Margin Joint Industry Group (AMJIG). AFEN also has close ties to the Atlantic Frontier Environmental Forum (AFEF), which provides a mechanism for dialogue with a wide range of interested parties including Local Government and Non-Governmental Organisations.

AFEN has funded and managed a number of major environmental projects including: (1) transect sightings surveys of seabirds and cetaceans (carried out by the Joint Nature Conservation Committee), (2) counts of major seabird breeding colonies, (3) acoustic detection of large whales, (4) sonar and seabed sampling surveys at a regional scale, (5) coastal oil spill protection strategy development, and (6) fingerprinting of beached oil and tar.

These projects are designed to facilitate an understanding of environmental processes and habitat distribution at a regional scale, underpinning the assessment of potential environmental effects resulting from local operations. AFEN data will also be used in, and integrated with, academic science at several research institutes and universities.

In addition to the broadscale approach implemented by AFEN, Marathon and other Operators have also obtained site-specific environmental data, including: (1) current meter deployments, (2) seabed features and hazards, (3) detailed analysis of baseline contaminants and ecological community structure, and (4) survey for specific benthic species, such as the cold water coral Lophelia pertusa.

Prior to acquisition of 3D seismic data in its operated blocks, Marathon conducted an assessment of seismic noise propagation and associated effects. This concluded that risks of significant environmental effects, particularly auditory damage and behavioural disturbance of cetaceans, were relatively low. However, monitoring of cetacean distribution and responses to seismic operations is ongoing through AFEN.

Marathon policy is that Environmental Assessment (EA) of the potential effects of all exploration and production activities should be carried out. The EA process is objective, scientific and transparent. UK legislation introduced during 1998 also requires publication of an Environmental Statement for projects likely to cause significant effects on the environment, providing comprehensive details of the EA. Marathon therefore conducted an EA and published an Environmental Statement for exploration well 153/5-A. This well was planned in a water depth of 1,231m approximately 93km from the coast of Lewis.

To support the EA, quantitative studies were commissioned to model dispersion of discharged drilling wastes. Deterministic and stochastic oil spill trajectory modelling was also conducted.

Review of environmental sensitivities of the location concluded that physical and ecological characteristics were typical of the Rockall Trough area. In particular, high resolution sidescan sonar revealed no evidence for the presence of Lophelia. Sidescan also indicated that the location has been subject to significant physical disturbance by demersal fishing activity. Seabird and cetacean distribution, and vulnerability to surface pollutants and noise, are seasonal and at a minimum during the proposed drilling period.

Potential environmental effects of drilling activities were evaluated through a systematic review of discharges, emissions and disturbance against defined severity criteria. Potentially significant environmental effects of routine drilling operations were identified as noise disturbance to cetaceans, physical disturbance from anchoring, and drilling discharges. These effects were evaluated in detail and all environmental effects of routine operations were judged to be either of minor or negligible significance. Alternatives to discharge of mud and cuttings, including containment and onshore disposal and reinjection were considered, and rejected on environmental, technical and operational grounds.

In addition to routine operations, unplanned events were also evaluated. The most significant unplanned event would be an oil or chemical spill. A major spill could potentially result in large scale fatalities of offshore seabirds and pollution of coastlines, aquaculture and fisheries stocks and the seabed. These consequences can range in significance from negligible to severe depending on the nature of the event, weather and sea conditions, and the effectiveness of response. However, the probability of such an event, and therefore the overall risk, is relatively low.

A systematic approach to the management of oil spill risks was therefore implemented for drilling operations. The approach involved (1) risk assessment, i.e. identification of the potential causes, probability and consequences of spills; (2) prevention and control of spills at source; and (3) contingency planning for containment and mitigation of the effects of spills. The oil spill risk assessment considered six scenarios ranging from a severe loss of well control to a small diesel spill resulting from bunkering. Consultation with JNCC identified a potential risk to seabirds associated with liquid hydrocarbon fall-out from flaring during well-testing, and detailed operation procedures were developed to mitigate this possibility. Sub-sea releases in water depths of >1,000m were also considered. Contingency planning focused on the appropriate use of dispersants, primarily through aircraft application. In conclusion, a major spill capable of severe or widespread effects was considered to be extremely unlikely, given the controls and procedures in place to prevent this occurrence.

Throughout the EA and contingency planning process, Marathon recognised the value of consultation and a range of interested parties were provided with information and invited to respond. Local consultations were also carried out in the Western Isles. After submission of the 153/5-A Environmental Statement to Government, the statutory process of national advertisement and public consultation was followed. After completion of the statutory consultation period in November 1998, and the recommendation of the statutory environmental agencies that a well consent should be granted, Greenpeace made a detailed and critical response to the Department of Trade and Industry. Greenpeace alleged that the ES contained a large number of errors and misleading statements. Marathon responded on a scientific basis to each of the criticisms made by Greenpeace, and following lengthy consideration, DTI has accepted the conclusions of the ES. 

AAPG Search and Discovery Article #[email protected] International Conference and Exhibition, Birmingham, England