Environment Report 2014

3.3.3 Potential Environmental Impacts Chemicals discharged from offshore operations are immediately diluted into the sea. The level of dilution depends on the water depth and currents. The dilution is at least 1,000 times at a distance of 500 metres from the discharge point, reducing the discharge to levels that are not acutely toxic to marine organisms. 3.4 Discharge of Drill Cuttings Drill cuttings are rock fragments generated during subsurface well drilling offshore. To prevent the well becoming clogged, the cuttings are carried back to the surface in the drilling fluid known as ‘mud’ due to its colour and consistency. There are two main forms of drilling fluids: water-based and non-aqueous-based (synthetic or oil). It is common practice to use both when drilling various sections of the same well. Water-based fluids are generally applied in the upper sections while non-aqueous fluids are used in the more technically demanding sections. The choice and composition of the drilling fluid depends on reservoir characteristics and consideration of the safety and environmental risks. On the drilling rig, the cuttings are separated from the fluids and disposed of, while the drilling fluid is recycled. The disposal option selected depends on the type of drilling fluids used. Cuttings are either thermally treated onboard to reduce the oil content and then discharged to sea, returned to shore for treatment and disposal, or injected back into a suitable rock strata.

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Since 2001, following OSPAR decision 2000/3, cuttings contaminated with non-aqueous drilling fluids cannot be discharged to sea without treatment to reduce the oil content to below one per cent of the total volume.

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Prior to any discharge, operators must conduct a risk assessment to investigate the potential environmental effects as part of their permit application to DECC.

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