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Use of Chemical Dispersants in the Sea Empress Oil Spill - Milford Haven, UK

The following article has previously been published in Waves, Vol 4, No 3, Spring 1997 - pp6

The use of chemical dispersants in maritime oil spills has always been a controversial issue and the topic of much debate between responders, scientists, the public, media and environmental action groups.

There is still widespread concern and lack of awareness, and knowledge, on the composition and use of oil spill dispersants (OSDs) as a response measure. These concerns are often highlighted during a spill incident by environmental groups, the public or even regulatory bodies and can paralyse decision making of the combat agency.

As the 'window of opportunity' is limited in the use of oil spill dispersants at sea, due to the weathering of the oil, it is important that decisions are made rapidly, the response is well planned and managed and any environmental concerns can be quickly and effectively addressed. This is often the criticism of the Exxon Valdez oil spill response in Alaska in 1989 that the window of opportunity was missed by responders for use of chemical dispersants, which resulted in significantly more foreshore and wildlife impact by the oil than if dispersants had been used in the first few days of the spill.

In the Sea Empress oil spill in Milford Haven (UK) in February 1996, 72,000 tonnes of Forties Blend crude oil was spilt into the marine environment from the grounding of the vessel along with 370 tonnes of Heavy Fuel Oil. The use of around 445 tonnes of chemical dispersants sprayed by aircraft onto the oil slicks at sea prevented at least 36,000 tonnes of oil, which is approximately 57,000-110,000 tonnes of oil/emulsion, from the Sea Empress coming ashore in this sensitive region of Wales.

The volume of oil that actually came ashore was around 10,000-15,000 tonnes; without dispersant spraying at sea 5 to 10 times this amount of oil/emulsion would have impacted the foreshores. The environmental impact on sea birds, coastal waders, intertidal invertebrates, amenity areas and the cost of clean up would have been much greater that it was. The adjoining coastline of Milford Haven has the only coastal National Park in the UK and nearby islands are bird sanctuaries having internationally important populations of puffins, guillemots, gannets, and Manx shearwaters. Parts of the region are special scientific interest areas and areas designated Special Protected Areas under European Birds Directive.

During the operation scientific personnel monitored oil and oil/dispersant concentrations in the water column and in the spill region and assisted in the direction of spraying aircraft to target oil slicks still amenable to dispersants as well as the area of greatest oil concentration. Oil/dispersant concentrations in the water column were observed rapidly diluting at sea by natural mixing energy.

Although environmental impact assessment is still continuing, and information and data is limited at this time, some impact on lower shore bivalves and urchins was observed due to dispersed oil concentrations. The scientific team have concluded that during dispersant application operations they 'do not believe that the dispersant operation generated acutely toxic dispersed oil concentrations of environmental significance'.

The major conclusion to highlight in the response was the value of having a well planned aerial dispersant capability in place as a first response measure to oil spills at sea, along with close targeting and monitoring of application operations by boat and aerial surveillance craft.

The use of trained scientific observers and field monitoring staff 'on-site' during the various dispersant spraying operations allowed direct and valuable feedback to responders and planners on the success of the response options used and refinement to achieve maximum effectiveness and minimise environmental impact.

As mentioned in a previous Waves article - it must be remembered that dispersants do not eliminate the problem of an oil spill but are intended as a means of reducing the overall environmental impact of an oil spill. In an oil spill event environmental trade-offs of protection and sacrifice will occur. These decisions are not taken lightly by response authorities and will be based on the best available advice and scientific data.

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last updated: 27 April 2000