- Heavy fuel to blame -
The Kerch Strait between Russia and the Moscow-annexed peninsula of Crimea was polluted after two Russian oil tankers sank during a storm on December 15, and the resulting spill is still spreading.
The two tankers were carrying some 9,200 tonnes of fuel oil and slightly more than a quarter of it is believed to have leaked into the sea, according to the latest estimates by Russian authorities.
Russia's maritime rescue service Morspassluzhba said the accident was the first in the world involving M100 grade "heavy" fuel oil.
This type of fuel "does not float on the surface" and "no proven technology exists in the world to eliminate it in water", the agency said. "That's why the main method is collection on the coast."
"Mazut (M100) is a trade name for a particular type of Heavy Fuel Oil (HFO) that is produced in Russia and Kazakhstan," said Rob James, senior vice-president Europe and Asia at Lamor, which specialises in oil and chemical pollution control.
HFOs are low-cost residual fuels produced by the distillation of crude oil in refineries and are typically used as fuel for large ships and industrial boilers, he told AFP.
According to the International Maritime Organization, approximately 62 percent of the 211 million tonnes of fuel used by ships in 2023 was classed as HFO.
- Heavy or light oil. What's the difference? -
There are two types of oil: heavy fuel oils with almost tar-like consistencies and so-called "light" oils including diesel, petrol and kerosene.
The first are very polluting and are characterised by high viscosity and density. They can also be as heavy or heavier than water, sink, float or stay longer in water.
The second, which have low viscosity and density, degrade quickly.
"A heavy fuel oil can remain in the environment for years while a lighter one, such as diesel fuel, may completely evaporate and disperse in days or weeks," said Thomas Azwell, a researcher at the University of Berkeley.
"If it is spilled, unlike crude oil, HFO does not evaporate, and dispersants have little effect on it", said James.
"With over 20 years in the oil spill response industry, I can tell you that any HFO generally presents more challenges to clean up than a spill of crude oil or of other refined petroleum products.
"HFO clean-up is a mechanical/manual process which takes time and requires labour resources -- a good example would be the 'Erika' spill in December 1999 that impacted the shoreline of Brittany."
- How does an oil spill develop? -
The type of oil as well as weather and sea conditions are determining factors.
Once spilled, petroleum products can undergo chemical and physical changes such as: spreading on the surface; evaporation, which is enhanced by high winds and temperatures; dispersion into droplets by waves; emulsification, which increases the volume of oil as it absorbs water; and sedimentation, which causes oil to interact with sediment and sink.
"Spreading, evaporation, dispersion and emulsification are important in the early stages of the spill stages of the spill whereas photo-oxidation, sedimentation and biodegradation are long-term processes that determine the ultimate fate of the oil," the International Tanker Owners Pollution Federation (ITOPF) said in a technical note.
"A small number of residual oils are sufficiently dense to sink when spilled. However, most oils will float and may sink only if mixed with denser sediment," it added.
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