Fate of Oil Spills

Some of these processes, like natural dispersion of the oil into the water, lead to the removal of the oil from the sea surface, and facilitate its natural breakdown in the marine environment. Others, particularly the formation of water-in-oil emulsions, cause the oil to become more persistent, and remain at sea or on the shoreline for prolonged periods of time.

The speed and relative importance of the processes depend on factors such as the quantity spilled, the oil’s initial physical and chemical characteristics, weather and sea conditions and whether the oil remains at sea or is washed ashore.

Ultimately, the marine environment usually eliminates spilled oil through the long-term process of biodegradation.

A number of models are available for predicting the trajectory and weathering of oil spills at sea.  These can serve as a useful guide to understanding how a particular oil is likely to behave and help in assessing the scale of the impact  which a spill might cause.

Oil and its Properties

Crude oil is made up of a wide range of hydrocarbons ranging from very volatile, light materials such as propane and benzene to more complex heavy compounds such as bitumens, asphaltenes, resins and waxes.

Refined products such as gasoline or diesel are composed of smaller and more specific ranges of these hydrocarbons.

The main properties which affect the fate of spilled oil at sea are:

  • specific gravity (its density relative to pure water - often expressed as ° API* or API gravity. Oil can be classified into four groups according to their API.
  • distillation characteristics (its volatility)
  • viscosity (its resistance to flow)
  • pour point (the temperature below which it will not flow).

In addition, the wax and asphaltene content influence the likelihood that the oil will mix with water to form a water-in-oil emulsion. Oils which form stable water-in-oil emulsions persist longer on the water surface.

Persistence of Oil

When considering the fate of spilled oil at sea, a distinction is frequently made between persistent oils and non-persistent oils.

As a rule, persistent oils break up and dissipate more slowly in the marine environment and usually require a clean-up operation. Persistent oils typically include crude oils, fuel oils, lubricating oils and heavier grades of marine diesel oil.  These oils pose a potential threat to natural resources when released, in terms of impacts to wildlife, smothering of habitats and oiling of amenity beaches. 

In contrast, non-persistent oils will dissipate rapidly through evaporation. As a result, spills of these oils rarely require an active response. Non-persistent oils include gasoline, light diesel oil and kerosene. 

Impacts from non-persistent oils may include effects on paint coatings in marinas and harbours and, at high concentrations, acute toxicity to marine organisms.

Persistence is also important when it comes to the international compensation regimes and the International Oil Pollution Compensation Funds (IOPC Funds) have developed guidelines, widely accepted, which define the term "persistent oil". Under these guidelines an oil is considered non-persistent if at the time of shipment at least 50% of the hydrocarbon fractions, by volume, distil at a temperature of 340°C (645°F) and at least 95% of the hydrocarbon fractions, by volume, distil at a temperature of 370°C (700°F) when tested in accordance with the American Society for Testing and Materials (ASTM) Method D86/78 or any subsequent revision thereof.

Explore Documents on Fate of Oil Spills

01. Observación aérea de derrames de hidrocarburos en el mar

01. Observación aérea de derrames de hidrocarburos en el mar

Este Documento de Información Técnica ofrece asesoramiento y directrices para lograr un reconocimiento aéreo eficaz.

Categories: Fate of Oil Spills, Response Techniques, Planning & operations, Spill Response, Technical Information Paper (TIPS)

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02. Destino de los derrames de hidrocarburos en el medio marino

02. Destino de los derrames de hidrocarburos en el medio marino

Este documento describe los efectos combinados de los diversos procesos naturales que

actúan sobre los hidrocarburos derramados, conocidos en conjunto como “meteorización”. Los

factores que determinan las posibilidades de persistencia de los hidrocarburos en el entorno

marino se analizan junto con las consecuencias para las operaciones de respuesta. El destino

de los hidrocarburos derramados en el entorno marino presenta repercusiones importantes

para todos los aspectos de una respuesta y, en consecuencia, este documento debe leerse

junto con otros documentos incluidos en esta serie de Documentos de Información Técnica.

Categories: Fate of Oil Spills, Technical Information Paper (TIPS)

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