The tanker “Prestige” was on its way from the refinery in Ventspills in Latvia carrying 77 000 tonnes heavy fuel oil. Off the coast of Spain the “Prestige” suffered hull damage and started leaking oil on the 13 November 2002. The boat was towed out from the coast, broke in two and sank at a depth of 3500 metres on the 19 November 2002.

In connection with the Norwegian Clean Seas Assosiation for Operating Companies (NOFO) and Norwegian Coastal Administration (Kystverket) participation in the oil recovery operation off Spain and France, samples of emulsions from the sea were collected regularily. SINTEF has performed a characterization of a selection of these samples. An unweathered sample of the oil released was also received from CEDRE and used as reference. The oil recovery operation continued for several months, and it is important to take advantage of such incidents to obtain good data and knowledge on the long term weathering properties at sea for these heavy fuel oils, and to verify model predictions. After the oil spill recovery operation it is also important to characterise the collected emulsions to gain experience and map limitations with the oil spill contingency equipment used. Physical-chemical documentation of what is actually collected is essential to be able to plan later actions with the same or improved equipment.

The oil onboard the “Prestige” was a very heavy fuel oil (burner oil) with a viscosity of approximately 650 cSt at 50°C and a density of 0.995 kg/L, indicating that it was an IFO-650. The results from the study of the weathered samples taken during a period of approximately three months showed :

• The evaporation was very low (<5%) because the oil / emulsion layer was so thick that the evaporation was limited.
• The Prestige oil emulsified with a water content at approximately 60%.
• The Prestige oil reached very high viscosities with time at sea. After one month at sea the viscosity at 15°C was approx. 100 000cP at 10 s-1, after two months approx. 200 000 cP at 10 s-1, and after three months approx. 300 000 cP at 10 s-1as shown in the figure below.
• The viscosity would have been 2-3 times higher if happened at typical Norwegian winter sea temperature (5°C).
• The emulsions changed appearance and consistency over time at sea from viscous but still flowing emulsions after few days to more rigid, rubber-like emulsions after months at sea.
• Changes in consistency over time shows that these emulsions were more difficult to handle after months at sea, compared to a few weeks.
  

The work with the weathered Prestige oil has shown the extent to which the emulsions may change with time at sea. Although the water content and the evaporation were nearly constant over months, the emulsion’s properties changed over time. The change in viscosity, consistency and appearance for the emulsions over time may be due to changes in water droplet size and the droplet size distribution together with changes in the physical and chemical properties of the oil. E.g. precipitation of wax and asphalthenes in the emulsion may with time stabilize the “skin” formation around the water droplets, and result in increased elasticity.

The influence and extent of these processes when oil weathers at sea during long time (months) is not well documented and understood, and should be further investigated.

Contact person: Merete Øverli Moldestad and Per Snorre Daling