This paper not to be cited without prior reference to the author. IMPACT OF OFFSHORE OlL PRODUCTION INSTALLATIONS ON THE BENTHOS OF THE NORTH SEA

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1 \ This paper not to be cited without prior reference to the author International Council for the Exploration of the Sea CM1990/MINI:7 Biological Oceanography Committee IMPACT OF OFFSHORE OlL PRODUCTION INSTALLATIONS ON THE BENTHOS OF THE NORTH SEA by Paul F Kingston Institute of Offshore Engineering HeriotWatt University. Edinburgh INTRODUCTION Biological monitoring around the offshore oil production platforms of the North Sea has now been going on for over a decade and a half. During that time there have been many changes in the approach to monitoring the impact of oilfield development. This has resulted in a change from predominantly exploration and production weil drilling to oil production and from the change in government legislation relating to drilling activity and monitoring. Over this period there has been an unbroken increase in overall oil industry related activity which looks like continuing weil into the future. INPUTS OF OlL TO THE ENVIRONMENT Figure 1 shows the quantities. of oil on cuttings discharged since By 1985 the use of diesel oil based drilling muds had ceased being replaced by low aromatic oil based muds which are theoretically less harmful to the environment.."1":="1". _._._.. The discharged cuttings after the use of oil based drilling muds is by far the greatest source of oil finding its way to the North Sea seabed. Other sources include the discharge of produced water and accidental spillage. Figure 1. Total oil in cuttings discharged from oil and gas platforms in the North Sea Produced water is made up of formation water (the water associated with oil in the reservoir) and possibly from breakthrough of water injected into the reservoir to maintain pressure and oil production. I

2 J ~.' At present the quantity of oil discharged into the North Sea with produced water is small compared to other sources and is estimated at about 4,500 tonnes per annum. Oil spills from platforms contribute relatively small quantities of oil to the overall total discharged to the North Sea. Although spills are frequent (250 were recorded by the UK Department of Energy in 1987 alone) they are usually less than one tonne. The spills generally result in a sheen in the water around platforms and can be detected by aerial surveillance. ENVIRONMENTAL MONITOR1NG APPROACH 1t was originally thought that the impact on the benthos would be much more widely spread than has been subsequently shown, so that early monitoring studies featured wide ranging sampling grids. After the first exploration phase, when the production platforms became operational, it was found that gross impact was, for the most part. confined to the immediate vicinity of the installations and so the sampling approach was modified to concentrate more on the inner zones. The most common approach currently used is to sample the seabed by 0.1 m 2 grab sampler on a transect along the prevailing current direction. Sampies are usually taken at 200, 500, , 2500 and 5000 metres from the installation. The sediment is sieved through either 0.5 or 1.0 mm mesh aperture sieves and the retained material analysed back at the laboratory.samples for physical and chemical analysis of the sediments are taken at the same time. BIOLOGICAL IMPACTS The effect of discharging a large quantity of solids on to the seabed will have an immediate effect on the benthos directly underneath. Most monitoring surveys have been concerned with measuring the intensity of this impact and the spread of its effect. Typically, species richness falls the nearer a production platform is approached. (Figure 2) This is to be expected since it reflects on increasing gradient of disturbance towards the point of cuttings discharge.,, 'N, M STATFUORO AI.PHA PLATl'ORM 1884 No. 8pectea,:. L''_'"""'_''_L'l U u, u u.~.m Figure 2. Number of species along transect from a North Sea oil platform Diversity, which is greatly influenced by species richness, follows a similar pattern. Preoperation levels of the Shannon Wiener Index (H s )' one of the most commonly used indices, varies between 4.0 and 6.0 in the northern and central North Sea (Kingston 1987). M a 2

3 , DIVERSlTY Relatlonehip between He and Dl8tance ex'y lttel 1,,,.a o.a ~ :'. 0.'... : I E.: t.i 1 _OCIU LI Figure 3. Relationship between diversity (Hs), shown as a proportion of background diversity and distance from production platform (after Davies et a1 1988). 4 4,' Figure 3 shows the relationship between macrobenthic diversity and distance from a produetion platform. To equate the diversity values from a range of monitoring studies using different sampling and analytical techniques, the diversity has been expressed as a ratio to background levels in the area. For the most part "background levels" are reached for all installations within 1000 m suggesting that the extent to which the benthic community structure is disrupted, as defined by H s is confined to a relatively small area around each platform. Within this zone there is a rapid drop in H s suggesting that there is gross disturbance close to the installation. The graph gives combined data for surveys carried out between 1982 and 1988 and shows that the pattern of effect has differed little over the years. Whilst the response of species richness and diversity is similar around all installations, the number of individuals shows two types of response (Figure 4). The first is indicative of a predominant organic enrichment effect with huge numbers of individuals representing. relatively few species. This suggests that the material being discharged is relatively nontoxic and is being rapidly biodegraded by the sediment organisms. Similar responses have been described when there has been a high input of biodegradable organic material (Pearson and Rosenberg, 1978). Figure 4. No. of individuals with respeet to distance from four North Sea oil platforms expressed as % of background level. Statfjord, Brent, Murchison and Beatrice. (after Kingston 1987) The second suggests a toxie effeet with low numbers of species and individuals. The discharged material appears to be preventing the growth of macrofauna, the gradient of effect indicating that the drop in numbers is unlikely to be attributable to simple smothering close to the site of dumping. 3

4 OlL CONTAMINATION AND DIVERSITY There is a strong inverse relationship between Diversity (H s ) and concentrations of oil in the sediments. Figure 5 shows this for two North Sea oil fields (Matheson et a1,1986; Mair et a1, 1987). Reductions in diversity appear to begin around ppm. Other authors suggest a wider range, between 10 and 100ppm (Reiersen et al, 1988). There is also evidence that certain sensitive species may be excluded by levels of sediment oil below 10ppm as for example the polychaete worm Aonides paucibranchiata (see Figure 6), which shows a dramatic drop in numbers at levels of sediment hydrocarbon as low as 5ppm. DIVERSfTY (Hs) AND SEDIMENT Oll (ppm) DIwnlty (Hel 11.._. ~. _.. _ _._.. :: n.:. }. I._._,, ~ 4.. a _.. 2 _.. _. 1.. _ o LlJ''I.uLLLlJ'''."'u.JL"'LLüLI _ 1 Conc:ent~tlon oll (ppm) NIO.8. Nos Aonides paucibrsnchists,', _... _...._. _. _.. _..._ _. _..+ _,~ _._. _..._.. _..; :i=:.+ _._.. _... _ + _.. o.. _. _.._. + x _.._.. _ ' ""''" Sectiment Hydrocartlon loild (ppm) x+ 0 la +.0 L.'"JU.UI.'''.JlJUoL_''LL&. Figure 5. The relationship between sediment hydrocarbon concentration and diversity (after Davies et a1, 1988) Figure 6. Relationship between the numbers of the polychaete Aonides paucibranchiata and sediment oil concentration at four North Sea oilfields OlL CONTAMINATION OF THE SEA BED As the oil contamination spreads outwards from the cuttings piles the concentration of hydrocarbon in the sediments decreases. Levels of oil that may have a detectable effect on the benthos (50 60ppm) are reached usually in the range of 750 to 1000 metres from the point of input (Figure 7). Very high concentrations, usually between 1000 and times "background" are apparent elose to the p1atform with a steep downward gradient between 500 and 1000 metres from the installation. This pattern is typical for most North Sea oil developments. SEDIMENT Oll CONC. WITH DlSTANCE._r:e.==: ::;==,,., +.,.,..' Figure 7. Relationship between sediment hydrocarbon concentration and distance from production platforms after 1985 (adapted from Davies et a1, 1988) '+ upper and lower limits of SD i 4

5 , I Although gro s contamination is confined to a relatively small area around the production platforms, there is increasing evidence of a more widespread dispersion of oil in sediments at low levels. Figure 8 shows the concentration of sediment hydrocarbons for areas outside a distance of 5000 metres from installations in the Shetland basin (north 'rn North Sea). This data, collected by DAFS between 1980 and 1988 shows a marked rise in hydrocarbon contamination of sediments (as UVf diesel equivalents) over this period (see Davies and Kingston, 1990). Gradient analysis points to this oil as emanating from production platforms. However, chromatic evidence suggests that this oil may not be primarily from CONCLUSIONS Total oll (UV)tn Hdlment. lb c,. 1000,.'',, '00 '0, +.,,rrr.,.,' n _ ~ u u ~ M U M YEAIl Figure 8. Total oil (UVf diesel equivalents) vs Time for sites greater than 5KIn from North Sea platforms (data from DAFS). drilling cutting (Matheson, pers comm). The impact of North Sea oil developments now has weil defined limits of gross effect which can be summarized as follows: 1. Oil levels of 1000 times "background" may be expected up to 750 metres from a production platform with a highly disrupted benthic community of low diversity within this zone. 2. There is a transition from areas of high disturbance (as measured by diversity) to "background" diversity between 750 and 1500 metres. Hydrocarbon levels in this region may vary from 20 to 100 times "background" level. 3. Although diversity measures appear normal between 1500 and 3000 metres, effects of contaminants on sensitive benthic indicator species may be detected. Hydrocarbon levels may vary from 5 to 20 times "background" levels. 4. There is no confirmed evidence of any effect on the benthos beyond 3000 metres although levels of hydrocarbons above preoperational "background" levels show a significant elevation at distances of 10 kilometres and beyond. AcknowledgemEmts: My thanks to Hamish Mair and Ian Matheson for their helpful discussion. I am grateful to Philippa Ambrose for help in preparation of the manuscript. 5

6 REFERENCES Davies, J., Bedborough, D. Blackman, R.A.A. Addy. J., Appelbee, J., Grogan, W., Parker. J.G. and Whitehead, A The environmental effects of oil based mud drilling in the North Sea. in: Proc. Calgary Conf. Drilling Wastes. April 5 8. Calgary, Alberta. Davies, J. and Kingston, P.F Sources of environmental disturbance associated with offshore oil and gas developments. ICOE, Chapter 16 (in press) Kingstfln, P.F Field effects of platform discharges on benthic macrofauna. Phil. Trans. R. Soc. Lond. B 316 pp Mair, J.McD., Matheson, 1. and Appelbee, J.F Offshore macrobenthic recovery in the Murchison field following termination of drill cuttings discharge. Mar. Pollut. Bull. 18 (l2) pp Matheson, 1 Kingston, P.F., Johnston, C.S. and Gibson, M.J Statfjord Environmental Study. in: Proc. Conf. Oil Based Drilling Fluids Cleaning and Environmental Effects of Oil Contaminated Drill Cuttings. Trondheim, Norway. Feb pp Pearson, T.H. and Rosenburg, R Macrobenthic succession in relation to organic enrichment and pollution of the marine environment. Oceanogr. Mar. Biol. anno Rev. 16 pp Reiersen, L 0., Gray, J.S., Palmork, K.H. and Lange, R Monitoring in the vicinity of oil and gas platforms: results from the Norwegian sector of the North Sea and recommended methods for forthcoming surveillance. Proc. Calgary Conf. Drilling Wastes. April 5 8, Calgary, Alberta. 6