INTRODUCTION
This paper gives an overview of the response to the 72.000 t (19 million gallons) of Forties Blend and 480 t of Heavy Fuel Oil (HFO) spilt during the SEA EMPRESS incident. Quantification of the effectiveness of the response operations have been obtained through a monitoring programme and carried out by the National Environmental Technology Centre (NETCEN) of AEA Technology at the request of the Marine Pollution Control Unit (MPCU) of the Coastguard Agency, UK Department of Transport. In the past, researchers and responders have suggested that a monitoring programme could not be mobilised for the initial stages of a response. MPCU's early mobilisationof the NETCEN monitoring team has set a precedent in demonstrating the use of scientific measurements made in real time at an incident to guide a successful response to an oil spill.
The decision making at the incident was also aided considerably by the fact that the crude oil spilt, Forties Blend, is one which NETCEN has used extensively in field trials in the North Sea. Therefore, we were able to place a great deal of confidence in our predictions of oil fate and the potential for different response techniques. We have shown in field trials that Forties Blend forms emulsions readily and that in the absence of treatment that these emulsions can be relatively persistent (Walker et al.1993).On the other hand, in field trials before the SEA EMPRESS spill, Forties Blend has been shown to be amenable to treatment both by dispersants and demulsifiers (Lunel & Lewis 1993; Walker and Lunel 1995; Lunel & Davies 1996) giving confidence that a successful dis-persant operation could be mounted.
OVERVIEW OF THE FATE OF THE CRUDE OIL
Around 59.000 t Forties crude oil cargo was transferred to the Texaco refinery once the SEA EMPRESS had been brought alongside a jetty in Milford Haven. The oil budget considered here, therefore, refers to the 72,000 t of Forties crude which was spilt at sea. The oil budget was clearly not fixed for the entire duration of the spill, it is a time dependant variable. The oil budget on the 29 February is described in this section At this point, 2 weeks after the SEA EMPRESS went aground, only sheens remained at sea and the majority of the bulk oil had been removed from the beaches.
Evaporation - 40% (range 35-45%):
The evaporation of the lighter components of crude oil is one of the most important processes that removes oil from the water surface. Hydrocarbons of a composition containing 15 carbon atoms or less will have evaporated within the first 2-5 hours after being spilt reducing the acute (short-timescale) toxicity of the remaining surface oil to the marine environment since themost toxic components have been lost to the atmosphere.
Samples collected during the first 2 weeks of the incident and modelling predictions showed that between 35% and 45% of the volume of the oil had evaporated.
Table 1. Showing the volume percent that had been lost through evaporation in the samples collected during the SEA EMPRESS incident.
Oil recovered at sea - 2% (range 1.5-2.5%):
Approximately 4.000 t of water-in-oil emulsion, with an average water content of 50-70% was removed at sea by skimming operations. This emulsion was then kept in separating tanks and lagoons to settle out into oil and
