A survey of inputs to the North Sea resulting from oil and gas developments

Abstract

The annual input of petroleum hydrocarbons to the North Sea has recently been estimated to be between 100 and 170 kt and is derived from a variety of sources. Although there is uncertainty about the size of inputs from some sources, there is general agreement that the atmosphere, rivers and land run-off (including coastal sewage), and coastal oil industry activities combined with shipping, remain sources of major inputs. However, the size of annual inputs from the offshore oil and gas exploration and exploitation activities has recently increased to about 20 kt and these activities now form one of the major sources of petroleum hydrocarbons to the North Sea. This increase is almost entirely due to the use of oil-based drill-muds and the consequent discharge of drill cuttings contaminated with residual mud. At present, experience in the United Kingdom has shown that this input of fresh, unweathered oil rapidly enters otherwise uncontaminated offshore sediments, producing strictly local effects around the point-source discharges. The nature and composition of this input differs from the majority of the inputs to coastal waters and sediments, and from the diffuse atmospheric input to offshore waters. Of the 140 kt of materials other than oil discharged annually to the North Sea from oil and gas developments in the United Kingdom, 98-99% arise from drilling operations, but the vast majority of inputs from this source are biologically inert or derivatives of natural products. Surveys indicate that, of the remaining materials, less than 50 t of the more toxic products (i.e. those with a 96 h LC 50 to Crangon crangon of less than 1 part/10 6 ) are discharged into United Kingdom waters annually. The largely uncontaminated offshore North Sea waters and sediments remain little affected by offshore oil and gas developments, but if these activities enter already contaminated estuarine and coastal waters, the contamination and effects from this source will be harder to distinguish.