Environment Report 2014

3.4.2 Composition of Discharged Cuttings Drill cuttings vary in size and texture, ranging from fine sand to gravel, depending on the type of rock being drilled and the type of drill being used. Drill cuttings may contain residuals of the drilling fluid used and are disposed of accordingly as previously outlined. 3.4.3 Potential Environmental Impacts The Oil and Gas Industry Marine Monitoring and Assessment Committee commissioned (through Oil & Gas UK) a series of wide-area monitoring studies between 2005 and 2009. The work used a wide-area stratified random sampling strategy in the Fladen ground, east Shetland basin, east Irish Sea and CNS. Each of the four surveys involved collecting over 100 sediment samples that were analysed for grain size, carbonates, organic matter, metals and radio-chemicals. The samples revealed that all contaminants were at background levels and that far-field contaminants from offshore activities, including drilling operations and their associated drill cuttings, were not detectable. From 2005 to 2006, surveys were also undertaken around five producing platforms on the UKCS to investigate long-term trends in contaminant persistence and biological recovery. Following the cessation of oil-based mud discharges in 2001, the surveys confirmed significant reduction over time of sediment hydrocarbon concentrations and a return to background or near background levels. This year, Oil & Gas UK commissioned Heriot-Watt University to undertake a spatial and temporal assessment of North Sea benthic species using the UK Benthos database. A statistical framework was developed to test the far-field effects of drilling operations, accounting for the physical factors relevant to benthic distribution such as water depth. Nineteen platforms were considered, 90 per cent of which did not reveal far-field effects on the benthic data. The spatial extent of the impact footprint was greatest within the first one to two years after drilling, but with time the footprint shrank, in some cases to about 50 metres.

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The statistical framework was also able to detect environmental effects and signs of recovery within the benthic dataset; the benthos’ capacity for recovery was notable.

The PROOFNY study on the NCS (see section 3.2.3) also found that the effect on benthic organisms fromdrill cuttings contaminated with water-based muds was confined to a distance of 100 to 500 metres from the installation.

Overall, recent studies indicate that chemicals and cuttings discharges fromoffshore installations are not increasing contaminate concentration within seabed sediments beyond the installation’s immediate vicinity. Over time, the area of impact of cuttings shrinks to the immediate vicinity of the installation and the benthic organisms are able to recover.

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