The oil transported by the Prestige was a heavy fuel, used for two kinds of applications: the industrial combustion (oil-fired power stations, furnaces, cement works) and the supplying of ships propelled by powerful slow diesel engines. Heavy fuels are obtained at the end of the chain distilling crude oils. To facilitate the mixing of the heavy residues of distillations, distillation cuts are added sometimes.
The fuel the Prestige transported came up to the M100 Russian classification, with a sulphur content of 2.58%. Its English appellation is “fuel oil n°6”. This is a very viscous product, considered as insoluble and which has an oil characteristic smell. Transported when reheated, it shows at an ambient temperature a high viscosity (30,000 cSt at 15°C) and can’t be dispersed chemically.
Oil response at sea by spreading dispersants, from planes or vessels, was out of question then. The Prestige wreck lying off Galicia by 3,500 metres of depth, IFREMER (French Research Institute for the Exploitation of the Sea) experimented in its hyperbaric testing chamber, the buoyancy of fuel depending on the depth of submersion. It clearly demonstrated that the fuel had a smaller density than the water which surrounds it on the whole water column (fuel density at 3°C and 350 bars: 1.012 against 1.045 for seawater at this depth).
|Prestige (average)||Erika||Baltic Carrier|
|Saturated hydrocarbons (%)||22.9||22.2||40.9|
|Unsaturated hydrocarbons (%)||52.7||55.6||37.9|
|Analysis of the Prestige heavy fuel|
|Density at 15°C||0.993|
|Pour point||+ 6°C|
|Viscosity at 50°C||615 cSt|
|Viscosity at 15°C||30,000 cSt|
On November the 22nd, analyses were carried out at Cedre on an oil sample taken at sea by the Ailette ship on the 18th of November. They revealed information about the global chemical making of the product. The water content of the fuel aged at sea was 45% then. Therefore, it formed an emulsion with the equivalent of its weight of water. With a viscosity of 100,000 cSt at 15°C and a measured density of 1,01, this emulsion remained thick (slicks, plates or oil cakes) at the surface of the water, indeed under the surface in rough sea.
For the record, the viscosity of the oil transported by the Erika contained 50% of water, which was just like a viscosity of 60,000 cSt in the same conditions. It’s the same for the saturated and unsaturated parts. After several days of ageing at Cedre’s polludrome, the water content of the emulsion stabilized in the region of 60%.
On the 28th of November, Cedre got another sample provided by SASEMAR, taken from the cargo on November the 5th, after the loading in Ventspills harbour. This one was considered as the reference sample for the whole identification analyses managed afterwards.