Knowledge Centre
Oil Spills
Source of emissions
Accidental discharges of oil can arise from equipment failure or from human errors during a range of offshore operations, e.g. the transfer of diesel from supply vessels, overfilling of tanks, well operations and incorrect operation of produced water treatment equipment. Recently there have been concerns that the ageing of infrastructure may increase the risk of incidents. Oil spill types range from crude oil through to lubricants and refined products. However, spills associated with offshore operations rarely exceed one tonne compared to large oil spills from tanker accidents which can result in thousand of tonnes of oil spilled at sea.
Potential environmental impacts
The environmental impact of an offshore oil spill will depend on a number of factors including:
- the quantity and nature of the oil spilled and its behaviour at sea;
- the weather conditions and season;
- the type and effectiveness of the clean-up response;
- the ecological nature of the area and its sensitivity to oil pollution.
Oil spilled at sea will break up and will disperse into the marine environment over time. This dissipation is the result of several weathering processes, including evaporation, dispersion, dissolution, emulsification, sedimentation and sinking and biodegradation.
Experience in the North Sea has shown that, because of these processes, there is very little risk that oil spilled from an offshore location will reach and impact a shoreline.
Environmental effects that have been described in literature (principally from tanker incidents) range from acute toxicity caused by low molecular weight hydrocarbons to smothering effects due to heavier oils and weathered products which find their way to the seabed. The presence of toxic components may induce temporary effects such as narcosis and tainting of tissues, which usually subside over time. Marine bird populations can be particularly vulnerable since they tend to congregate on the sea surface. Oil exposure may cause drowning, starvation and loss of body heat following fouling of plumage. However, there is little firm evidence that seabirds suffer long-term effects from offshore oil spills [Kingston 2002].
Statistics
The Department of Energy and Climate Change (DECC) publishes information on its website on the volume of oil spilled each year from 1991-2004 and 2005-2011.
Key control and mitigation measures
For safety and environmental reasons the offshore industry invests heavily to prevent the release of hydrocarbons. Provision of physical barriers, such as downhole safety valves, maintenance to minimise leaks and procedures and training to influence human factors have significantly reduced spills from UKCS operations.
However should these barriers fail and oil is spilled to the sea, operators will respond to minimize the impact of the spillage. The level of response is predetermined from installation-specific risk assessments which take into account the type of oil, the environmental sensitivities of the location and the likely movement of the oil determined by computer modelling. Depending on the level of risk the response will range from surveillance to monitor how a slick is behaving, through to the use of chemical dispersants and then to physical containment and recovery of the oil.
The response procedures are contained in an Oil Pollution Emergency Plan which is mandated by the Offshore Installations (Emergency Pollution Control) Regulations 2002 and the Merchant Shipping Regulations 1998. This plan describes the resources available to an operator to be able to respond to a spill on a tiered basis. Tier 1 is local resources which can be quickly deployed; Tier 2 resources are regional and Tier 3 are national resources for the unlikely event of a major spillage. Tier 2 resources are usually provided by specialist oil spill responders under contract to the operators.
In all cases where there is a non permitted release of oil to the environment, whether or not a response is required, the operator is required to submit a report to the regulator under the provisions of the Offshore Petroleum Activities (Oil Pollution Prevention and Control) Regulations 2005.
References
- Paul F. Kingston (2002) Long-term Environmental Impact of Oil Spills Spill Science & Technology Bulletin Vol.7 No 1-2 pp53-61
- OLF (2005) Barriers against spills – fact sheet
Updated: November 2009
