Sunken and submerged spilled oils



Alun Lewis, Oil Spill Consultant, UK


Despite stringent precautions, accidents can happen at sea, and at times oil may be spilled. Oil has the potential to cause significant environmental damage, especially if spilled near sensitive resources. Counter-pollution response measures are used where possible to minimise any damage that may be caused. Oil spill response techniques may appear straightforward; oil is spilt, oil floats on water, so corral the oil with booms and pick it up with skimmers, or disperse it into the sea if appropriate. In reality, the problems of responding to spilled oil at sea are never simple; spilled oil will spread out over a large area, the properties of the spilled oil change with time and the oil drifts with currents and the wind. Prevailing weather and sea conditions often impose severe limitations on the effectiveness of any response and the time available for response before the oil impacts a sensitive location is often very short.

There can be a further unwelcome complication. What if the oil does not float, but sinks or submerges below the sea surface? This has happened at various locations around the world in the past and will almost certainly happen again.

The Agency with responsibility for dealing with oil pollution at sea in UK waters is the Maritime and Coastguard Agency (MCA). Together with ITOPF (International Tanker Owners Pollution Federation), the MCA has sponsored a scientific study into the processes that can lead to sinking and submergence of oils, how we might predict this and how we can best respond to it. The study is being conducted by BMT Cordah Limited, Oil Spill Response Limited and the author, Alun Lewis.

Why oil does (and does not) float
Mineral oils exhibit a wide range of physical and chemical properties. There are crude oils, fuel oils and refined oil products and many further sub-divisions within these three categories. Crude oils can be ‘light’ or ‘heavy’, there are distillate and residual fuel oils, and refined oil products cover a range from LPG to bitumen. Most, but not quite all, oils float on the sea because the density of the oil is less than that of seawater. Under standard conditions, the density of freshwater is 1.000 gm/ml and full salinity seawater is 1.025 gm/ml.

Oils that sink
Not many oils have a density higher than that of seawater, but there is one notable exception; a refined oil product used to make carbon black. It is a by-product of the ‘cat cracking’ process used to produce more gasoline from a crude oil. This oil has many names, but is often described as ‘slurry oil’ and has a typical density of 1.075 gm/ml. Slurry oil spilled at sea will sink. This happened to 30,000 tonnes of slurry oil off the coast of France in 1979 (from the oil tanker ‘Gino’). In freshwater, on a river or lake for instance, several other oil types will sink if spilled.

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