Why is a Heavy Fuel Oil Spill Substantially Worse than a Marine Diesel Spill?

Although a spill of any type of oil can cause irreversible damage to the environment, the ultimate consequences and hazards of an oil spill largely depend on the properties of the specific oil. For example, distillate fuels, such as diesel, tend to evaporate and dissolve faster than HFO and do not emulsify on the ocean surface.[1]   Conversely, HFO demonstrates a strong tendency to solidify rapidly and form tar lumps in marine waters. This not only results in a significant increase in the volume of waste to be handled in the event of a spill, but also makes HFO more persistent in the environment.[2] For example, a study commissioned by the Arctic Council has established that while 90 percent of HFO remains in the ocean after 20 days, marine diesel can take as few as three days to disappear from the surface.[3]

Most liquid hydrocarbons, such as distillate fuel, tend to spread into a slick over the water surface.[4] This quality allows oil recovery teams to detect and track a spill through aerial observations. On the other hand, due to a deficiency of “lighter” compounds in HFO, there may be no sea surface sheen to aid in the detection of an HFO spill.[5]

Furthermore, HFO is not typically as buoyant as distillate fuel and may not float on the sea surface. The long-term consequences of sunken oil are highly complex, but can include the incorporation of oil in ocean and coastal sediments. Although the oil can remain buried for years, it does not always remain submerged. A good example of the challenges associated with buoyancy of an HFO spill is the Swedish vessel THUNTANK. In December of 1986, the THUNTANK ran aground in heavy weather and spilled HFO in the Baltic Sea. When the HFO was spilled, it was denser than the surrounding water and sank to the ocean floor. However, when the water temperature increased during the summer months, the oil warmed, became more fluid and buoyant, and ultimately resurfaced. The resurfaced HFO was repeatedly washed ashore during the summers of 1987, 1989, 1990 and 1991 causing the shoreline to be repeatedly coated in thick oil.

Overall, the THUNTANK spill demonstrates that an HFO spill can be incredibly difficult to clean-up, given the viscous nature of HFO as well as its persistence in the ocean and coastal environments.

[1] Det Norske Veritas, Heavy fuel in the Arctic (Phase 1),Report No./DNV Reg No.: 2011-0053/ 12RJ7IW-4 Rev 00, 2011-01-18, at 38 (2011).

[2] Deere-Jones, T., Ecological, Economic and Social Impacts of Marine/Coastal Spills of Fuel Oils (Refinery Residuals), at 7 (2016).

[3] Det Norske Veritas, Heavy fuel in the Arctic (Phase 1),Report No./DNV Reg No.: 2011-0053/ 12RJ7IW-4 Rev 00, 2011-01-18, at 38-39 (2011).

[4] Deere-Jones, T., Ecological, Economic and Social Impacts of Marine/Coastal Spills of Fuel Oils (Refinery Residuals), at 7 (2016).

[5] Deere-Jones, T., Ecological, Economic and Social Impacts of Marine/Coastal Spills of Fuel Oils (Refinery Residuals), at 7 (2016).