Product characteristics and behaviour

The oil transported by the Erika was a heavy fuel oil used for two types of applications: industrial combustion (power plants, furnaces, cement factories...) and power supply to ships propelled by slow powerful diesel engines.

The product was a mixture of 10 % light fluxing oil, 30 % heavy fluxing oil and 60 % products of vacuum distillation. Its density was very close to that of seawater. Transported heated, it had a high viscosity at ambient temperature (20,000 cSt at 10 °C). It was therefore not chemically dispersible and, as a consequence, dispersant application either by planes or by ships was not a viable response option.

On the day of the incident, Cedre received from the TotalFina refinery in Dunkirk 100 kg of the fuel n°6 loaded on the Erika. Tests were immediately carried out in Cedre's flume tank or Polludrome®. On 13 December, it appeared that the product had no tendency to disperse naturally in water. It floated and hardly fragmented at all. Its viscosity increased within a day to 70,000 cSt while it formed an emulsion reaching 30 % water. The emulsion reached 50 % water (viscosity: 350,000 cSt) in two days and then stabilised. At sea, the fuel also formed an emulsion. However the water content remained around 30 %. Analysis carried out on samples collected on the shore showed water content nearing 50 %.
 
Emulsification increased the volume of pollutant drifting at sea and made it stickier. Based on these observations, the French Navy confirmed that the heavy fuel oil spilled by the Erika would be difficult to recover and could only be pumped provided water was added to it. This recommendation was followed and over 1,200 m³  of emulsified fuel oil was recovered.

 
A full analysis of the chemical composition of the product was subsequently carried out by the French Petroleum Institute (IFP) and other laboratories: the LASEM (French Navy Laboratory for Analysis Monitoring and Expertise), Cedre, the MNHN (National Museum of Natural History), IFREMER (French Institute for research and exploitation of the Sea) and the University of Bordeaux.

The composition results depended on the laboratory protocols. According to the nature and the polarity of the solvents used, the breakdown between the families can be noticeably different, which modifies their relative proportions. We can conclude that the fuel transported by the Erika contained about 25 % heavy products (resins and asphaltenes) and nearly 50 % aromatic hydrocarbons. Aromatic hydrocarbons included oil compounds with a toxic potential, either in the form of direct toxicity (intoxication), or by mutagenic or carcinogenic effect.

The impact on the ecosystem depended on the conditions of exposure (solubility of the compounds, duration) and on the possibilities of transfer to the food chain.

Evolution of the product

Evaporation depends initially on the nature of the product spilled, but also on the prevailing conditions. It can be significant for some light crude oils (30 to 40 % for the pollution caused by the Amoco Cadiz). Heavy products like the Erika's are much less influenced by the evaporation process, which affects less than 10 % of their mass.
 
Heavy fuel oils are classified as insoluble in water. However, the heavy fuel oil transported by the Erika contained mono-aromatic (6.6%) and di-aromatic (5.9%) fractions that showed some solubility in water. Seawater contamination can also result from a fragmentation of the product due to wave action and water turbulence, causing the production of oil droplets and tarballs of various sizes, able to drift according to movements in the water column. To support this hypothesis, it should be stressed that many small oil fragments were regularly found stranded on the beaches. The fact that this oil product is characterised as insoluble does not exclude the possibility of contamination of the water column.