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Oil Discharges to the Marine Environment |
Oil discharge to the marine environment has a high public profile, particularly when it is in the context of a spillage yet latest figures available show that it amounts to 0.006% of the total oil produced from the UK sector of the North Sea. Oil enters the marine environment from a variety of sources, many of which are not directly connected with the offshore oil industry. In recent years the major spills have been due to shipping accidents rather than being due to the oil and gas production industry.
Estimates of the total quantity of oil discharged into the North Sea vary. Data for 1985 - 1990 from the 1993 OSPARCOM North Sea Quality Status Report (Fig. 1) give an indication of the relative size of the discharges from different sources. By this estimate, now some years old, around 20% of the oil discharged to the marine environment is reported to come from offshore oil and gas operations. No new data is readily available and it seems likely that this figure will have declined substantially in recent years.
When forming a view on oil industry discharges, three separate sources must be considered: oil spills, discharges from drill cuttings, and discharges in association with produced water. Unlike accidental oil spillage, oil in produced water is permitted, subject to strict regulatory control at a national and at a European level. While oil on drill cuttings is accounted for in the above table it should be noted that discharge of oil based mud cuttings ceased from 1 January 1997. As such, they are subject to strict regulatory control both at a national and at a European level. Each of these sources of oil discharge to the marine environment will be discussed in detail in this report.
When these discharges are combined with the volumes of oil spilled, as is shown in Fig. 2, the amount of oil discharged to the marine environment can be seen to have reduced steadily in recent years. When the information is presented as a percentage of the total amount of oil being produced, both the reducing trend and the relatively small amount involved when compared to the amount produced becomes apparent.
Accidental Oil spills
Accidental oil spillage can occur from production platforms, drilling rigs, and pipelines. Spills can also occur as a result of associated operations such as the boats supplying offshore facilities. The 'oil' involved may cover a range of types, from crude oil itself through to lubricants and refined products.
 Any spillage, however small must be immediately reported to a range of government bodies (DTI, Coastguard, etc). Government policing of spill reporting includes the use of airborne scanning equipment to detect even relatively minor spills. Although some critics question the accuracy of the industry's reporting of spills, information from DTI surveillance flights indicates that less than 5% of spills observed went unreported and when investigated virtually all of these were found to be from sources outside the offshore oil and gas industry.
Although any spillage of whatever size is to be avoided, recovery and restoration processes, including containment and subsequent cleanup, are also important. There is a broad range of environmental issues relating to the effects of oil spills on marine life, and on the aesthetic and commercial values of an impacted site. These issues include - the effects of direct exposure to oil in the water on fish, marine mammals, and birds; the effects of oil on bottom-dwelling and inter-tidal species; the toxicity of dispersants and oil/dispersant mixtures to marine life and the decision as to when it is appropriate to use dispersants; and the effects of marine oil spills on commercial and/or recreational resources and facilities.
More than any industry, oil companies have led the way in understanding how to develop proactive action plans which take these factors into account. The UK government requires all oil companies to put such Oil Spill Contingency Plans in place before drilling or production operations take place and new legislation has recently reinforced this.
A key stage in the development of an Oil Spill Contingency Plan is gaining an understanding of the local environment and potential impacts of a spill. In some cases, where the environment is particularly sensitive or complex, a large amount of data is compiled which may be of interest to local communities. In these situations, some oil companies have published the information and made consultation with the community part of the planning process.
Fig. 3 shows the UK offshore oil industry spill performance over the last 5 years. The volume of oil spilled was showing a progressive reduction. Unfortunately one large spill of 685 Tonnes in 1997 contributed to a recent deterioration in this trend. UKOOA members are conscious these figures can be improved upon and there is no room for complacency.
In terms of numbers of spills occurring, the data suggests that spill frequency has been on the increase. However, the steady reduction in average size of spill over the same period, is clear evidence that a determination to record and report even the smallest spill has been successful.
Oil in produced water
In the North Sea, produced water is derived predominantly from two sources. Firstly, it consists of water that has been trapped for millennia in the subterranean oil bearing rock formations and which is brought to surface along with oil during routine production operations. The second source of produced water is injected seawater. Seawater is injected through a well into the oil-bearing formations in order to increase the reservoir pressure and thereby maximise the recovery of oil and gas. Over time, more and more of this injected seawater is produced with the oil in production wells.
The mixture of oil, gas, and produced water that comes to the surface through the production wells is separated in complex processing systems onboard the oil platform. Provided that it meets a specification set by regulators, the produced water is discharged to the sea.
The composition of the discharged produced water varies considerably with location and the length of time that the oil field has been under production. These differences are mostly in respect of the salinity of the produced water and residual levels of both oil and treatment chemicals.
Oil companies are required by the UK government to provide reports on both produced water discharge volumes, and the oil content of the produced water. Moreover, the UK, along with other European countries, sets a stringent discharge limit for the oil content of produced water of not more than four thousandths of one percent (40 Parts per Million). Research by both Government and industry has demonstrated that at this very low concentration, there is no scientific evidence of significant environmental effects. Despite this, the industry has voluntarily accepted a target of 30ppm on a company annual average basis from January 1999.
It is a characteristic of maturing oilfields that the fraction of produced water contained within the oil, gas, and water mixture that comes to surface progressively increases as oilfields age. As North Sea oilfields have matured, oil companies strive to keep produced water volumes to a minimum, and to keep the oil content of the produced water within the regulatory limit. Measures taken by industry have ranged from shutting in wells with high produced water content, through improving platform separation technology, to re-injection of the produced water where circumstances permit.
Continuing efforts in these areas aim to improve the average oil content of produced water discharged into the North Sea. However, based on experience in more mature oil producing regions of the world, the overall volumes of produced water and consequently the net amount of associated oil discharge, is likely to increase for a few more years before the older fields start to close down.
Fig. 4 shows the UK industry produced water performance over the last 5 years. Technological improvements and financial expenditure have resulted in recognisable improvements in produced water quality by steadily reducing the average level of oil content. However, the volumes of both the produced water and the oil discharged with it has shown a slow but steady rise as North Sea oilfields continue to mature.
Oil on drill cuttings
Drill cuttings are the main solid waste resulting from well drilling operations. As the drill bit cuts its way towards the underground reservoir, rock cuttings are generated and carried back up to the drilling rig. In order to lift these cuttings to the surface and keep the drill bit lubricated, drilling muds are continually circulated down the drill pipe and back up the well-bore.
When the mixture of drilling mud and rock cuttings arrives back at the drilling rig, the two must be separated before the drilling mud can be pumped back down to the drill bit again. Unfortunately, the very characteristics that make the drilling mud an effective carrying agent for the rock cuttings, makes the subsequent separation of the two a difficult and complex operation. Inevitably some drilling mud remains adhering to the cuttings.
The name 'drilling mud' belies the sophistication of these fluids. Because they have to achieve very specific characteristics (e.g. viscosity, density) they are very carefully formulated. Very often, the required properties of the mud can best be achieved if it has a significant mineral oil content. The drilling mud is then termed an OBM as opposed to a 'Water-Based-Mud' (WBM). In this situation the residual mud adhering to the cuttings will contain some of this oil.
The limits set for the amount and type of residual oil on drill cuttings that is allowed before the rock cuttings can be discharged to the sea has changed progressively over the last 10 years or so. These changes have influenced both the direction of research into the fate and effects of drill cuttings in the marine environment, and the nature of technological improvements in the design and treatment of the drilling muds themselves. These technological and research developments have themselves influenced the direction of subsequent regulatory change.
Technological developments have occurred in a number of areas: Improved techniques are now available for separating mud from cuttings to allow more of the mud to be recycled. Equipment has been developed which enables cuttings to be re-injected into sub-sea formations. Cuttings can now be reduced to a fine slurry which more easily disperses in the water column. Shore-based cuttings treatment and disposal is now becoming more available and opportunities for recycling and re-use of the end products are being explored. If these are not available, the resulting end products will compete with other wastes for space at land-fill sites.
The most recent phase of regulatory development in respect to drilling mud occurred on 1 January 1997 when regulations came into force which specified that the amount of oil discharged on drill cuttings must not exceed 10gm of oil per kg of cuttings. This is a level of residual oil on drill cuttings which is technically not achievable at present resulting in the discharge of OBM ceasing.
The consequence of the regulatory change can be seen in Fig. 5 which shows the total amount of mineral oil discharged on drill cuttings over the last 5 years. Progressive improvement over the years culminated in mineral oil discharge on cuttings ceasing in 1997.
Further reductions in discharges from cuttings will results from plans which are being drawn up between the UK government and oil companies in respect of the use of synthetic mud systems which have now been found not to give the environmental improvement over OBM which was first envisaged.
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Environmental Report 1998 Index
