Much of the US’ new abundant domestic oil and gas supply is being produced from unconventional resources—particularly light sweet crude oil from the Bakken shale in North Dakota, as well as the Eagle Ford and Permian Basins in Texas. This rapid growth has also created challenges in moving crude oil to market. Rail is increasingly relied upon to transport crude oil because production has exceeded the capacity of pipelines to move oil from these areas to refineries in the West, Midwest and Northeast.

In the last five years, oil-by-rail transit has grown from 9,500 carloads (2008) to 407,761 carloads (2013), according to the American Association of Railroads. The great majority of this crude oil transport takes place without incident. However, the massive increase in the quantity of crude being shipped by rail and the significant distances traveled, along with several notable recent train derailments, have raised serious transportation safety concerns. The DOE commissioned the investigation in response to the occurrence of several rail accidents involving crude oil combustion in the US and Canada during 2013-2014, some of which involved loss of life, property damage, and environmental impacts.

Key objectives of the investigation were to characterize and to define tight crude oils based on their chemical and physical properties, and to identify properties that could contribute to increased potential for accidental combustion.

The initial report complies and summarizes publicly available literature and data pertaining to the chemical and physical properties of tight crude oils. Key literature/data sources reviewed include recent reports on Bakken crude properties commissioned by the American Fuel & Petrochemical Manufacturers, North Dakota Petroleum Council, and US Department of Transportation Pipeline and Hazardous Materials Safety Administration, and data from the US Strategic Petroleum Reserve.

The initial investigation identified gaps in important crude oil characterization data; uncertainty regarding how best to sample and analyze crude oil to ensure that its properties are accurately determined; and deficiencies in the understanding of how crude oil properties impact its potential for accidental ignition, combustion, and explosion.

However, the report also confirmed that while crude composition matters, no single chemical or physical variable—flash point, boiling point, ignition temperature, vapor pressure or the circumstances of an accident—has been proven to act as the sole variable to define the probability or severity of a combustion event. All variables matter.

Available analysis of tight crude oil does not provide the necessary data or conclusion to enable meaningful comparison with other crude oil. The report recommends additional research to identify the best way to collect and compare oil samples, while developing correlations between a particular property or set of properties and the likelihood or severity of rail transport-related combustion events.

Among the key findings of the investigation (Literature Survey of Crude Oil Properties Relevant to Handling and Fire Safety in Transport) are:

• Due to significant variability in criteria and procedures utilized in selection, acquisition, and analysis of crude oil samples, the available data are of insufficient quality to enable a meaningful comparison of crude oils—either to each other or against a designated standard.

• In addition to variability due to sampling and analysis methods, variability may also be introduced through crude oil conditioning, storage, and transport. “Conditioning” refers to processing conducted—typically at or near the well site—to remove crude oil impurities prior to transport.

• Currently used methods for assignment of crude oil transportation hazard classification and packing group are often inadequate. As mandated by current federal law, hazard classification and packing group assignment is done on the basis of crude oil initial boiling point (IBP) and flash point; however, there was widespread agreement among the studies reviewed that the methods commonly used for IBP determination are inappropriate for application to crude oils, especially tight crude oils containing significant quantities of dissolved gases. While recommendations for improved methods have been offered, no widespread agreement has been reached regarding the adoption of more appropriate methods.

• Relationships between crude oil properties and probability or severity of combustion events in rail car spill scenarios have not been established. Although it is likely that a combination of crude oil properties—especially those associated with potential for flammable vapor formation—could be used to predict combustibility, no specific, objective data were found that correlated known crude oil properties with the likelihood or severity of rail transport-related combustion events. While industry groups actively working on this problem have been identified, their progress and results have not yet been released to the public.

• General lack of uniformity in methods and QA/QC across industry makes comparison of crude oil vapor pressure difficult.

• Bakken crude is a light, sweet oil that exhibits a statistically higher true vapor pressure than the slightly heavier, blended sweet and sour oils that are stored at the US Strategic Petroleum Reserve (SPR).

• Numerous combustion events can occur from an accident involving hydrocarbons and hydrocarbon mixtures including crude oils, with severity dependent on the amount of fuel involved, surrounding infrastructure, and environment. Possible combustion events include:

  • Pool fire, which results from the burning of a liquid fuel pool.

  • Boiling liquid expanding vapor explosion (BLEVE), an explosion resulting from the failure of a vessel containing a liquid at a temperature significantly above its boiling point at normal atmospheric pressure.

  • Fireball, which refers to partially pre-mixed diffusion flames that rapidly combust due to enhanced turbulent mixing and atomization.

  • Deflagration: Classification of an explosion. Burning of a fuel-air mixture where the flame travels at subsonic velocities.

  • Detonation: Classification of an explosion. Burning of a fuel-air mixture where the flame travels at supersonic velocities.

  • Flash fire, which refers to the burning of a fuel vapor cloud that was ignited at a location away from its release point.

  • Flare, which refers to the burning of fuel vapors at the source of a release.

• No single parameter defines the degree of flammability of a fuel; rather, multiple parameters are relevant. While a fuel with a lower flashpoint, wider range of flammability limits, lower auto-ignition temperature, lower minimum ignition energy, and higher maximum burning velocity is generally considered more flammable, the energy generated from an accident has the potential to greatly exceed the flammability impact of these and any other crude oil property-based criteria.