Response to the Sea Empress Incident, 1996
(Application of Dispersant and Bioremediation Technologies)
Dr Peter Wood
Principal Consultant
National Environmental Technology Centre
AEA Technology Environment
Introduction
The tanker Sea Empress ran aground near Milford Haven, South Wales, on the 15
February 1996. Over the next few days 72 000 tonnes of Forties Blend crude oil and 480
tonnes of heavy fuel oil were released into the sea. A rapid at sea response wasundertaken but, nevertheless, oil came ashore along 200 km of coastline. Over the next
few weeks a massive shoreline clean-up operation was undertaken.
This paper outlines some of the new approaches to oil spill response and monitoringthat were undertaken during this incident. These are:・ monitoring of dispersant effectiveness at sea
・ bioremediation
・ Surf washing of oiled cobbles
・ in-situ pit washing of oiled cobbles
・ shingle washing operations.
Each is discussed in turn.
Monitoring Dispersant Effectiveness at Sea
The main approach to the clean up of oil at sea in the UK is to use approved
chemicaldispersants. These are normally sprayed from aircraft onto relatively fresh oil.
However, it is of critical importance to determine whether or not the dispersant ishaving an influence on the oil. (Not all oils are dispersible, especially once a degree
ofweathering and emulsification has occurred.) Whereas dispersed oil can sometimes beseen in the sea from vessels or aircraft this is not always the case, especially with moreviscous and weathered oils where the dispersant may act only slowly. Consequently, forthe first time in a major oil spill, concentrations of dispersed oil were measured during
dispersant application operations.
Monitoring of the dispersion process using flow-through-flourometry indicated that thedispersant operation enhanced the rate of natural dispersion. Scientists from theNational
Environmental Technology Centre monitored oil concentrations at different depths
under and around the oil on the sea surface. They were able to show that, at 4 m depth,
oil concentrations were around <1500 μg/l in areas treated with dispersant compared
to around <500 μg/l in areas that had not been treated. This information confirmedthat the dispersants were having an effect and was fed back to the responsemanagement centre. The centre was then able to continue dispersant spraying
operations in the knowledge that the dispersants were being effective.
As expected the dispersants were found to be most effective on the fresh oil emerging
from the grounded tanker. Therefore, the strategy for dispersant application was, inthe first instance, to target any significant fresh releases of oil from the tanker. Oncethese had been successfuly treated then a secondary target was the large patches ofmore weathered oil.
When emulsions were being treated with dispersants it was found than an initial
application tended to break the emulsion while subsequent additions increased theconcentrations of dispersed oil.
Part of a successful dispersion operation is the judgement of when to stop treating aparticular patch of oil. In the case of fresh oil emerging from the Sea Empress, while
the oil tended to remain as a coherent slick, dispersant operations reduced it thickness
until only sheens remained. In the case of weathered oil, as the successful dispersant
operation progressed, the main problem became the low surface coverage of emulsion
on the water surface. Once the coverage became around 30 % of the water surface itwas not possible to achieve efficient application of the dispersant as so operations wouldterminate.
During the operations some 445 tonnes of dispersant were applied from aircraft. Anotable feature of the spray response was the highly effective targeting achieved by theuse of remote sensing aircraft positioned above the spray aircraft to direct the spraypattern. This operation is well established in the UK and allows the DC3 spray aircraft

in particular to target effectively ribbons of oil as narrow as 10-20 m wide.
It is difficult to estimate accurately the split between the volume of oil that would havedispersed naturally and the volume that was dispersed chemically. In total it isestimated that some 47 % (33 800 tonnes) was dispersed of which some 7200 tonnes was
naturally dispersed. This means that some 26 600 tonnes was chemically dispersed.
Given that 445 tonnes of dispersant were used then this results in a ratio of 1 tonne ofdispersant to 60 tonnes of oil chemically dispersed. Although this ratio is greater thanthat normally assumed (1 tonne of dispersant disperses 20 tonnes of oil), it is considereda reasonable estimate. High dispersion would be expected as the dispersant was applied
to fresh crude oil and the monitoring did show that successful dispersion was readily
occurring.
Bioremediation
Bioremediation was not used as a standard clean-up procedure for oiled beaches during
the Sea Empress incident. However, the response management centre did take theopportunity to undertake some experimental studies.
AEA Technology scientists carried out a survey of a number of beaches to evaluate thepotential for bioremediation. A preliminary survey suggested that six beaches should
be considered once the manual clean-up had been completed on them. Factors
considered in this selection process are the energy levels of the beach, thebiodegradability of the oil, the natural nutrient levels of the shoreline and the type of
substrate that is oiled.
The survey indicated that nutrient levels were adequate in the winter and early spring,but that reduced temperature was a limiting factor. However, a later survey indicatedthat, in the summer when temperatures had increased, nutrient deficiency became thelimiting factor.
However, it would be possible to overcome this problem by the addition of artificialnutrients.
An experimental plot was established at Bullwell Bay. This is a low energy beachoccurring within Milford Haven. The beach consists of pebble, gravel and stones
overlying clay and is highly permeable to water and air. Both Forties Blend and heavy

fuel oil were present and sediment samples indicated that an appropriate bacterial
population existed. Following relevant regulatory approval, additions of nutrients suchas nitrogen and phosphorus were undertaken to stimulate the growth of thesenaturally occurring hydrocarbon degrading bacteria. Nine plots were established on thebeach; three receiving weekly additions of nutrients, three receiving monthly additionsof a slow release nutrients, and three receiving no additional nutrients. TheEnvironment Agency also undertook monitoring during the experiment to ensure thatthe nutrient additions had no impact to the near-shore environment.
The results indicated that both nutrient types increased the rate of biodegradation of
the heavy fuel oil and the Forties Blend with equal effectiveness The treatment had nomeasurable effect on the nutrient content of the near-shore seawater and there were nodetectable toxic effects. It was concluded that the slow release method may prove to bea cost effective technique for enhancing the natural recovery of low energy shorelines.
Surf Washing of Oiled Cobbles
A technique known as surf washing was used on two beaches during the Sea Empress
incident. The technique is essentially a matter of using tracked excavators while thewater is at low tide to relocate material from the oiled zone at the high water mark (theberm) towards the middle of the intertidal zone. As the tide rises the energy impartedin the surfzone is then sufficient to remove significant amounts of the Forties Blendemulsion from the oiled cobbles. The natural mineral fines found within the beachand/or in the waters of the surf zone interact with oil to form mineral-oil flocs and so
prevents the dropletes from coalescing. The flocs then become dispersed in the watercolumn.
Two locations, Amroth and Marros, had extensively oiled cobble beaches and wereconsidered appropriate sites for surf washing operations.
At Amroth stranded emulsion on the cobble storm beach was found to be associatedwith fine minerals including mica, chlorite, kaolinite, quartz, calcite, feldspar. When
this emulsion was introduced into the sea clay-oil flocs formed which again had thesame suite of minerals but with less of the non-clay mineral (quartz, calcite andfeldspar). The flocs that formed were generally 30μm or more in size. This provided anopportunity for the responders to undertake a surf washing strategy to clean the cobble

beach.
Successive surf washing episodes were subsequently undertaken at Amroth and weresuccessful in reducing the concentrations of oil in the beach material During this surfwashing operation chemical evidence indicates that oil found within the particulatematter of near-shore waters was biodegraded to a greater extent than the oil strandedon the shoreline.
At Marros stranded emulsion on the cobble storm beach was again found to beassociated with fine minerals and again formed flocs when introduced into seawater.
Observations at Marros indicated that high concentrations of emulsion on the surface
cobbles persisted for about 14 days after oiling. However by 50 days after oiling theemulsion had become unstable and more mobile and had penetrated the beach todepths of up to 3 m and had reached a less permeable substrate of superficial
glacial/solifluxion deposits. This resulted in some oil being subjected to tidal
fluctuations in the cobble beach and a subsequent loss of some oil by sheening.
Examination of the oil composition in samples of stranded emulsion at Marros
indicated that no significant biodegradation occurred on the beach between 14 and 51
days following the spill. Consequently during this time it is considered that the major
process contributing to a reduction of the quantity of oil on the beach would be sheening.
It is considered that this sheening process would have continued for many months hada surf washing operation not taken place.
A surf washing operation was undertaken at Marros over a seven day period
commencing 47 days after oiling. Table 1 indicates the statistics of the operation. An
estimated 8150 tonnes of oiled cobbles was moved a distance of between 12 and 18 m
seaward along a length of 850 m. The tonnage of cobbles that it was necessary torelocate was a direct result of the depth of penetration of the emulsion within the beach.
Had the beach been relocated earlier, before the emulsion became unstable, then thetonnage requiring relocation would have been less.
Analysis of samples collected after 2 tides following surf washing show that oilconcentrations did not exceed 22 ppm both at the surface and at depth. This represents
a considerable reduction compared to concentrations prior to relocation, up to > 700
ppm, which is considered to be the direct result of enhanced oil dispersion processes
coupled with the surf washing operations. Further support for this hypothesis isprovided by detailed GC-MS analysis (alkanes:hopane ratio) which confirmed that theloss of stranded beach emulsion was not due to a stimulation in biodegradation activity.
Determination of oil concentrations off-shore during the surf washing process
confirmed that dispersion of the oil into the water column was occurring. Oilconcentrations at 1 m depth were up to 600 μg/l near the shore but dropped to 150 μg/l within a kilometre of the coast.
From the results obtained, it is concluded that, in those situations where clay-oilflocculation can be demonstrated as occurring, surf washing should now be considered
as an effective oil spill countermeasure for the treatment of polluted beaches.
Furthermore, as biodegradation of the oil appears to be enhanced in the sea followinginteraction with fines, surf washing should be considered as a mechanism that
effectively enhances the rate of oil removal from the ecosystem.

Taqble 1:Nominal Statistics of the Relocation Exercise at Marros Beach* determined as volume x 2.6 (assumed cobble density) x 0.7 (assumed packing density)** material originally relocated on 19 April but which was not reworked by the tides and therefore needed to be
moved lower down the beachIn-situ Pit Washing of Oiled Cobbles
Pit washing was used for the first time during the Sea Empress incident to clean
cobbles. Large pits were dug to hold between 50 and 100 tonnes of material and linedwith a heavy duty plastic liner. Cobbles were added and washed under high pressure
water and an approved degreaser (surface cleaner). Oil could then be skimmed off thesurface of the pit and the cleaned cobbles returned to the beach. In some cases sunken
skips were used as the pit. It should be noted that this method of cleaning cobbles
removes only the bulk oil. The remaining stained cobbles were not returned to thesurface of the cobble zone but were buried to prevent re-oiling.
Shingle Washing Operations
Washing stations were set up at several locations. These were established from readily
available equipment such as cement mixes, skips, temporary tanks, conveyors andscaffolding. Oiled material is fed into a lorry-mounted cement mixer and seawater with
an approved degreaser, and occasionally diesel, added. (However, later operations usedneither degreaser or diesel as it was found that these were not necessary.) Operating
the cement mixer agitates the material and loosens the oil. The mixture is then left toseparate. The oily water is then run-off into watertight skips or tanks and the oilremoved by surface skimmers. The cleaned shingle was then returned to the beach.

Treatment rates would depend on the degree of oiling, the capacity of the cement mixer
and the number of cement mixers used. Cement mixers have a nominal capacity of 10
tonnes and, with a treatment cycle of 2 hours, some 50 tonnes per day can be treated.
However, more recent work by AEA Technology suggests that treatment cycles of thislength may not always be necessary.
Conclusion
The clean-up response to the Sea Empress incident can be considered to have been very
successfiul. An opportunity was taken to undertake innovative approaches to oil spill
response and monitoring for both at sea operations and shoreline clean-up. Real time
monitoring has proved to be a very useful to enable decision making during a responseand to enhance protection of the environment.
Monitoring of dispersed oil concentrations during dispersant spraying operations
provide proof of dispersant effectiveness. Surf washing should now be considered as aneffective oil spill response measure. In-situ pit washing and shingle washing operations
in cement mixers have also proved to be successful for the cleaning of otherwise
difficult material. Bioremediation with the use of additions of slow release nutrients
may prove to be a cost effective technique for enhancing the natural recovery of low
energy environments.