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Environmental Impact of the Offshore Oil and Gas Industry by Stanislav Patin - a unique summary of world-wide studies on the environmental issues associated with offshore oil and gas exploration and development

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Muddied Waters

A Survey of Offshore Oilfield Drilling Wastes and Disposal Techniques to Reduce the Ecological Impact of Sea Dumping

by Jonathan Wills, M.A., Ph.D., M.Inst.Pet., for Ekologicheskaya Vahkta Sakhalina (Sakhalin Environment Watch); 25th May 2000

Environmental Effects of Drilling Waste Discharges (continued)

The Effects of Discharges of Drill Cuttings

If oil and gas exploration rigs and production installations are allowed to dump drilling wastes unchecked, the effects on marine life can be extensive and biologically significant. Over the past 40 years in the UK and Norwegian sectors of the North Sea, for example, about 1.3 million cubic metres of drill cuttings and associated wastes have built up on the seabed in 102 individual "cuttings piles" with an estimated mass of from 2 to 2.5 million tonnes. The largest pile contains over 66,000m3 of material and weighs about 100,000 tonnes ( For fuller details, see: Det Norske Veritas. 2000. op. cit.; and Grant, A. 2000. Toxicity and Environmental Risk Assessment of Drill Cuttings Piles. University of East Anglia, Norwich). The ecological effects extend for several kilometres from some platforms and can be detected up to 10km from discharge points. These cuttings piles smother seabed life and remain toxic for many years, mainly because of the hydrocarbons they contain.

Although the discharge of cuttings contaminated with OBMs and SBMs is now effectively banned, or about to be banned, in Western Europe, WBMs may still be discharged with cuttings and additive residues here and in most jurisdictions, including North America, provided they are treated to prevent the formation of surface slicks from crude oil entrained in the wastes. According to the US Department of Energy, "WBMs produce short-term, minor impacts on the seabed, whereas OBM cuttings introduce long-term, more severe impacts" (Veil, J.A. et al. 1999. op. cit.). According to UKOOA (UKOOA spokesman, April 2000, pers. comm.), WBM is relatively harmless because it "contains water as its primary lubricant/coolant and no oil". In some situations "additives may have organic properties but these are only allowed in very small quantities". As a result, UKOOA believes there is no residual oil on the cuttings. Any salts or minerals coming from the mud are "not biologically available as they are in non organic forms coming generally from the barite used as the cutting medium". A review of the literature suggests that WBMs may not be as benign as this suggests.

While the areas of affected seabed are much smaller where only water-based muds have been used (Grant, A. 2000. ibid.; and Olsgard, F., and Gray, J.S. 1995. A comprehensive analysis of the effects of offshore oil and gas exploration and production on the benthic communities of the Norwegian continental shelf. Marine Ecology Progress Series 122: 277-306.), the ecological effects are still significant because, as discussed above, WBM drilling wastes may contain free oil, dissolved aromatic hydrocarbons, heavy metals, radionucleides (Minerals such as barite and bentonite, and some drilling chemicals, may contain minute amounts of radium. For example, in 1999 a cargo of 800 tonnes of calcium chloride powder, destined for use in North Sea drilling muds, triggered a radioactivity detector at Shell's Aberdeen supply base. The material contained 1 becquerel per gram of radium226, which brought it under the control of the 1993 Radioactive Substances Act. The contamination proved to have come from Swedish phosphate ore used in the production process. The cargo was returned to Sweden. See website: Information for Industry (IFI). 2000. Environment Business News Briefing. 13/1/00. London.), biocides and other additives. Thus, WBM wastes can poison marine life as well as smothering it with artificial sediments or suffocating it with plumes of superfine suspended particles (Such a suspended plume is a plausible contributory cause of mass deaths of pelagic fish, such as the June 1999 Piltun Bay incident in Sakhalin, but more research is needed before this can be established. JWGW ). It is therefore vitally important, even when no OBMs or SBMs are used, to minimise the discharge of drilling wastes if at all possible. If this is not done, offshore oilfields will eventually face the same problems as the countries around the North Sea: how can the production platforms be removed without disturbing the cuttings piles and releasing buried toxins to the sea; how can the cuttings themselves be removed, treated and disposed of without causing further ecological disruption; and would it be better, in some circumstances, to leave them where they are and let nature take its course?

To illustrate the scale of the problem in the North Sea (an area about half the size of the Sea of Okhotsk), it has been estimated that removing all the cuttings could impact the ecology over an area of some 4,000 km2, if pollutants from the piles were released into the water column at a concentration of 3%. At 5ppm dilution the area of impact could be 246,000km2. The costs of removal, treatment and disposal are estimated at a minimum of UK£384m [US$614]. Industry working groups are studying more than a dozen detailed proposals for dealing with the problem and minimising its impact (For details, see website: United Kingdom Offshore Operators' Association. 1999a. Drill Cuttings Joint Industry Project. UKOOA, London).

On top of these ecological, technical and economic challenges, the industry faces the potentially enormous financial cost of unquantified legal liabilities (for example: what price might the fishing industry's lawyers place on the semi-permanent loss of formerly valuable fishing grounds or the contamination of remaining fish stocks by pollutants from the cuttings piles?) Knowing what we know now, clearly it is in no-one's interests - not the local people's, not their government's and not the oil companies' - to allow a North Sea scenario to develop in newly developing offshore oil and gas fields such as the Sakhalin Shelf.

After reviewing the literature on drill cuttings, Patin (1999. op. cit.) wrote:

...cuttings discharges from offshore oil activities can cause effects that are more hazardous due to cutting contamination by oil and toxic components of drilling muds. Even after separation and cleaning in special units, drilling cuttings still contain a wide array of organic and inorganic traces especially when oil-based fluids are used. Drilling cuttings usually go overboard the offshore oil platforms in thousands and tens of thousands of tons. Hundreds of tons of oil and dozens of tons of chemical for each drilled well can enter the marine environment with these discharges. This raises serious concerns about the possible eco-toxicological disturbances in areas of offshore production...

...obviously, the composition of cuttings will vary a lot even when drilling a single well, which might explain the absence of any generalized information about this issue...

Most researchers believe that the main toxic agents in drilling cuttings are oil and oil products. These accumulate in the solid phase of drilling cuttings when crude oil and drilling fluids contact cuttings during oil extraction. According to some national and international standards [GESAMP, 1993], the permissible content of oil in discharged drilling cuttings should not exceed 100g/kg. Even if this requirement were observed during actual industrial operations, this concentration is much higher (100-1,000 times) than the thresholds of acute and sublethal toxic effects of oil-polluted bottom sediments...

Toxicological data on produced drilling cuttings (before their discharge) are not available except some mention of low toxicity of the particles of these cuttings, suspended in the water, in concentrations of about 500mg/l... The attention of researches has been concentrated on assessing the ecological effects of oil-containing drilling cuttings after they are discharged and distributed in bottom sediments around the offshore oil platforms. Here, the levels of oil pollution are hundreds and thousands of times higher than any background characteristics. These levels can cause obvious disturbances in the structure and functions of benthic communities up to 10km away from the place of discharge...

A recent literature review by Norwegian scientists (Det Norske Veritas. 2000. Technical Report - Drill Cuttings Joint Industry Project. Phase I Summary Report. Revision 2: 20th January 2000. DNV doc. order No. 29003500. Oslo) underlined the dangers of allowing cuttings piles to develop, even if they contain mainly WBM wastes: "Higher concentrations of heavy metals such as chromium, copper, nickel, lead, zinc, barium and hydrocarbons were observed in association with the cuttings than those seen in natural North Sea sediments," they reported. In contrast to Neff's earlier findings (Neff, J. M., Hillman, R. E. and Waugh, J. J. 1989. Bioaccumulation of Trace Metals -from Drilling Mud Barite by Benthic Marine Animals. In Engelhardt, F. R. et al. (eds). 1989. Drilling Wastes. pp. 461-480. Elsevier Science Publishers Ltd, Barking, England) that there was little evidence of bioaccumulation of heavy metals from drilling wastes, the Norwegians described "biogeochemical pathways such as adsorption and desorption, particularly from oxyhydroxides of iron and manganese and adsorption into organic matter of the assimilation into the gut of benthic infauna". This view is backed by other studies, one of which (Grant, A. 2000. Toxicity and Environmental Risk Assessment of Drill Cuttings Piles. University of East Anglia, Norwich) said: "...these [heavy] metals will be rendered largely immobile by the formation of insoluble metal sulfides in the anoxic conditions that prevail within [cuttings] piles ( Di Toro, D. M., Mahony, J. D., Hansen, D. J., Scott, K. J., Hicks, M. B., Mayr, S. M. and Redmond, M. S. 1990. Toxicity of Cadmium in Sediments - the Role of Acid Volatile Sulfide. Environ. Toxicol. Chem 9:1487-1502. and Di Toro, D. M., Mahony, J. D., Hansen, D. J., Scott, K. J., Carlson, A. R. and Ankley, G. T. 1992. Acid Volatile Sulfide Predicts the Acute Toxicity of Cadmium and Nickel in Sediments. Environmental Science Technology 26:96-101)."

While not denying Patin's warnings about hydrocarbon pollution of the seafloor, the Norwegian research did, however, point out that once hydrocarbons are buried in seabed sediments, whether they originate in WBM cuttings or OBM cuttings, not much happens to them if left undisturbed: "Hydrocarbons within the [cuttings] piles remain relatively unchanged with time as a result of depleted oxygen, low ambient temperature, type of drilling fluids used and lack of significant bio-turbation. They do not leach out in any substantial quantities over time but will stay bound to the sediment particles trapped within pore water and degrade slowly."

Another recent study of cuttings piles around the North-West Hutton oilfield between Shetland and Norway (Ibid) found widely varying pollutant thresholds at which ecological effects occurred and concluded:

More fundamentally, there is no agreement on which components of the drill cuttings are responsible for the environmental effects. As in some cases, effects have been observed at very low hydrocarbon levels, some authors have suggested that it is not plausible that effects are due to hydrocarbons. (1. Davies, J.M., and Kingston, P. F. 1992. Sources of Environmental Disturbance Associated with Offshore Oil and Gas Developments. In Cairns, W. J. (ed.). 1992. North Sea Oil and the Environment. Elsevier Applied Science. London. 2. Kingston, P. F. 1992. Field Effects of Platform Discharges on Benthic Macrofauna, Proceedings of the Royal Society of London B316:545-565. London. ) Concentrations of hydrocarbons are usually correlated with concentrations of a number of metals and with sediment grain size and organic carbon content. Measures of environmental effect may be more strongly correlated with these other environmental variables than they are with hydrocarbon concentration (Olsgard, F., and Gray, J.S. 1995. op. cit.... This uncertainty over what is responsible for observed ecological effects clearly causes some fundamental difficulties in determining dose-effect relationships.

The indisputable fact remains, however, that the precautionary principle strongly suggests minimising all discharges from oil and gas production platforms.

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"Muddied Waters":



List of Abbreviations

Summary of Conclusions

Drilling Waste Streams from Offshore Oil and Gas Installations

The Law on Offshore Wastes Discharges in Different Jurisdictions:

The OSPAR Convention

United Kingdom



United States

Inviting Regulation

Environmental Effects of Drilling Waste Discharges

The SBM Controversy

"Non-Water Quality Environmental Impacts"


Drill Cuttings

Produced Water

Minimising Waste Discharges and Their Effects

Reinjection Offshore

Cleaning Produced Waters

List of Main Sources

Selected References



Articles on Offshore
Oil&Gas; and Environment

Impact of Offshore Oil&Gas; Industry


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