The data on oil discharges are presented in the Department of Trade and Industry's annual "Brown Book" on the UK's oil and gas resources.1The UK has been by far the largest contributor to oil discharges from offshore oil and gas exploration and production activities in the North Sea since the 1970s. In 1990, it accounted for 86% of the 19,000 tonnes of oil which entered the North Sea from these sources. The UK's lead is not due wholly to its dominance in the offshore production league, but is attributable in part to specific practices, such as the continuing large-scale use of oil-based drilling muds, which have been largely phased out by other countries (ENDS Report 213, pp 8-10 ). However, the latest statistics, summarised in the table below, show an encouraging trend.
Dumping of oil-based drilling muds has dominated oil releases from UK offshore platforms, contributing 89% to their total discharges in 1985. However, in 1992 oil-based muds were used with only 55% of the 298 wells drilled, compared to 66% of the 331 wells drilled in 1991.
Releases from this source also declined following a cut in the level of oil permitted to be discharged with drill cuttings from 15% to 10% from 1 January 1992. The limit will be reduced further to 1% from 1 January 1994 for exploration and appraisal wells and for new developments.
The second largest oil source is the discharge of oil-contaminated production water from oil wells. In 1992, the total quantity discharged was 4,850 tonnes, a little below the peak figure in 1991. Larger amounts of production water are being generated as oil fields in the North Sea age, but improved treatment facilities are being installed to curb the upward trend in oil discharges.
The number of reported oil spills from North Sea platforms began an upward trend in 1986, when aerial surveillance flights were initiated by the Government. The figure peaked at 345 spills in 1990, but by 1992 the total had been halved to 168.