The analysis dashes theories that the crude may have been contaminated by hydrogen sulfide or fracking fluids, theories that could have led to focused technical solutions to the crisis of exploding oil trains.
No hydrogen sulfide was detected in the vapor space above the liquid contents of the surviving tank cars, says the TSB. Nor was there any evidence that chemicals used in hydraulic fracturing played any part in the cargo’s explosion.
“The occurrence crude oil’s properties were consistent with those of a light sweet crude oil, with volatility comparable to that of a condensate or gasoline product,” the report says.
The test results establish that the cargo was indeed typical “light, sweet” crude from North Dakota and had not been contaminated by the souring that has affected some wells in the mid-continent Bakken shale formation.
The tests also confirmed the TSB’s earlier emergency finding that the oil had been misrepresented at the point of loading as Class 3, Packing Group III when it should have been the more hazardous Packing Group II. But there was nothing to make the doomed consist chemically exceptional among other shipments of Bakken crude.
“The large quantities of spilled crude oil, the rapid rate of release, and the oil’s high volatility and low viscosity were likely the major contributors to the large post-derailment fireball and pool fire,” TSB said.
The oil’s high vapor pressure, a measure of evaporation rate for volatile compounds, contributed to the ferocity of the explosions at Lac-Mégantic. Combined with poor containment, the conditions were right for the creation of the fireball explosions observed as the runaway DOT-111 tank cars piled up at high speed.
The analysis warns that higher-speed derailments mean bigger initial fireballs and subsequent “pool fires” from accumulations of leaking oil:
“In the present case, a large number of tank cars sustained large ruptures during the derailment and released their content very rapidly. The spilled crude oil had high vapor pressure and a low flash point (less than -35 °C) that was much lower than the temperature at the time of the occurrence (21 °C), indicating it was readily ignitable.
“Multiple sources of ignition were present at the derailment site such as damaged power lines, derailed equipment, etc. Therefore, all of the conditions required for ignition to occur were present.
“When the release is a large spill accompanied by immediate ignition, the result is usually a fireball. The size of this fireball will depend strongly on the amount of flash vaporization and liquid entrainment that occur during the release. This suggests that more volatile materials (with higher vapor pressure) and high speed derailments (with more energetic impacts and release of lading) will result in larger fireballs. Spilled material that does not ignite immediately will spread and accumulate into a pool. The size of this pool will continue to increase until a physical boundary is reached or the material is ignited and burns, resulting in a pool fire.”
The report’s technical conclusions are presented here in full:
The flash point obtained for the occurrence crude oil samples was significantly less than 23 °C and the IBP determined using the ASTM D86 method ranged from 43.9 to 50.0 °C. Consequently, the crude oil samples clearly met the federal regulatory criteria for being classified as a flammable liquid of Class 3, Packing Group II.
The occurrence crude oil samples gave low density (815.9 to 821.9 kg/m3), low total sulphur (0.096 to 0.117 mass %), low viscosity (2.882 to 3.259 cSt at 20 °C), low pour point (less than -65 °C), low flash point (less than -35 °C) and high Reid vapor pressure (62.3 to 66.1kPa) results.
The occurrence crude oil’s properties were consistent with those of a light sweet crude oil, with volatility comparable to that of a condensate or gasoline product.
There was no indication that the occurrence crude oil’s properties had been affected by contamination from fracturing process fluid additives.
The occurrence crude oil samples were taken at atmospheric pressure. This could lead to an underestimation of the crude oil’s volatility due to evaporation loss of very light constituents.
The large quantities of spilled crude oil, the rapid rate of release, and the oil’s high volatility and low viscosity were likely the major contributors to the large post-derailment fireball and pool fire.
The occurrence crude oil contained concentrations of BTEX that were comparable to typical values reported for crude oils. This explains why concentrations of benzene and other VOCs well above exposure limits were detected at the derailment site.
TSB added, “If at any stage during the remainder of the investigation the TSB identifies additional safety deficiencies, it will communicate directly with regulators and the industry, and inform the public.”
The full report can be accessed here.