Understanding ethanol tank wagon transportation risks using Societal Risk maps

The transportation of ethanol is a vital component of its supply chain, often necessitating travel from production sites—typically located near agricultural areas—to other locations for further processing, storage, or distribution to users. This logistical necessity, however, is not without its challenges. Previous accidents in ethanol transport¹ have shown the risks of harm to nearby communities, underscoring the urgent need for thorough risk assessments to mitigate possible undesired scenarios effectively.

A valuable tool for assessing risks associated with transporting, storing, and handling hazardous materials is the Societal Risk map. This method provides a clear geographic representation of risk and allows the possibility to incorporate varying transport intensities or changes to transport routes. Moreover, the clear visualisation aids significantly in facilitating communication between risk assessment professionals and urban planners.

This blog post explores the calculation of Societal Risk maps through a case study of an ethanol tank wagon accident using Gexcon’s RISKCURVES QRA software.

Ethanol transport using tank wagons

The transportation of ethanol utilises various methods, including road tankers, tank wagons, barges, and pipelines.

Figure 1. Transportation of ethanol from the production site using tank wagons.

Tank wagon transport, in particular, emerges as the dominant force in this logistics chain, handling 95% of ethanol shipments in 2022 in the United States,² the world’s leading ethanol producer, because of their large capacity.

However, this mode of transport is associated with significant risks, including the potential for accidents such as derailments and collisions. When accidents occur, the large volumes of ethanol involved can lead to serious outcomes due to its flammability, corrosive nature, and toxicity, posing dangers to infrastructure, the environment, and public health. Therefore, complying with safety and regulatory guidelines is essential to mitigate these risks and prevent unwanted accidents. ^3,4 

Read: Ethanol: Associated hazards and safety measures

A brief introduction into Societal Risk maps in RISKCURVES

Societal Risk is the yearly cumulative probability that more than 1 person will be killed due to their exposure to an accidental chemical release. The risk is typically represented by an f-N curve where “f” stands for frequency (per year) and “N” for the number of fatalities.

However, this representation is a simple 2D graph that merely illustrates risks as “acceptable” or “not acceptable”. This is where the geographical presentation of Societal Risk, known as Societal Risk map, proves beneficial by providing more detailed insights, such as: 

• Locations with highest population being at risk from an accident. 
• Populations that contribute most to Societal Risk. 
• Areas where Societal risk is considered not acceptable. 
• Potential areas for urban development. 

Societal Risk map is not a new criterion for assessing risk, but a visualisation method developed by Gexcon, exclusively available in RISKCURVES QRA software, that helps identify areas of concern on a map.​ 

Read: Societal Risk maps as a clear geographical representation of risk

Calculating Societal Risk of ethanol tank wagon transport accident in RISKCURVES 

Approaches and considerations 

Train derailment serves as a common example of accidents involving ethanol transport by rail. In such events, collisions often lead to multiple tank wagons being damaged.  

The scenario we analyse draws from an actual derailment of a train loaded with the denatured ethanol which comprised 83 tank wagons, 20 of which sustained structural damage, leading to the release and subsequent ignition of ethanol spill.⁵ 

The train derailment results in extensive shell damage to 12 tank wagons, causing in the instantaneous loss of their full containment and leakage from the valves and connectors of 8 tank wagons.

The spill subsequently ignites, leading to a fire simulated utilising RISKCURVES’s “Pool Fire” model. It is presumed that the entirety of the released ethanol was consumed by the fire.⁵

Since the scenario is set in an urban area, we explore an alternative scenario alongside this actual incident, representing an average derailment accident consequences based on speed limit regulation for an urban area extend assuming a damage of 4 tank wagons.⁴

Input parameters

To calculate Societal Risk, the information on the population in the analysed area is required, which can be imported into RISKCURVES. Since derailments can occur anywhere along the transportation route, the exact route is defined as “Transport Equipment” in RISKCURVES by marking coordinates as route points with simple clicks on the map. Next, we select meteorological data for the location, in this case Utrecht, which is predefined in the software or the latest data can be imported. 

Figure 2. Defining the transportation route coordinate named “Transportation Equipment” in RISKCURVES.

Each scenario requires specifying the chemical involved in the accident. For this purpose, we create a mixture of ethanol denatured with natural gasoline, consisting of ethanol (95 wt.%), natural gasoline (5 wt.%), and benzene (0.25 wt.%), using chemicals available in the DIPPR chemical database within RISKCURVES. This database automatically calculates the properties of the mixture, eliminating the need for users to manually define the mixture’s properties.

The base frequencies for each scenario are established by incorporating instantaneous release frequencies for tank wagons and probability of direct ignition available in the Purple Book.⁶ 

Pool fire model

In addition to the parameters above, the input parameters of the “Pool Fire” model in RISKCURVES include multiple categories. The selected calculation method is the Two-zone model, specifically the Rew & Hulbert model, which distinguishes between a clear and sooty part of the flame.

To determine the total mass released and maximum Pool Fire surface area, the consequence modelling tool EFFECTS can be utilised by employing “Liquid Release” and “Pool Evaporation” models.

The total mass released and pool surface area combine the predominant event of full loss of containment of 12 tank wagons and a leak of 50% of their released content from 8 tank wagons. Similarly, the principle applies to the scenario with reduced speed limit resulting in a total loss of containment of one tank wagon and a leak from 3 additional tank wagons.

The temperature of the pool is set to the ambient temperature at which the ethanol is transported. Furthermore, the model allows for defining the height of the confined pool above the ground level and including shielding at the bottom side flame, which is not considered in this scenario. 

Finally, a reporting distance of 25 m was set to all scenarios, reflecting the half track width and reaching the infrastructure around the railroad. 

Calculation results

The Societal Risk in RISKCURVES is represented through f-N curves and Societal Risk maps, offering a multifaceted view of potential risks along the ethanol tank wagon transportation route.

RISKCURVES graphs (Figure 3) enable us to examine the f-N curve in specific route sections, such as the city centre where the maximal expected value is observed, or to assess cumulative Societal Risk and Societal Risk along the entire transport route. This reveals that while the Societal Risk for ethanol transport is deemed unacceptable in areas close to the city centre, it is considered acceptable in less populated areas. 

Figure 3. f-N curve in section with the highest expected Societal Risk value along the transportation route.

The Societal Risk area map (Figure 4) illustrates the geographical distribution of Societal Risk along the transportation route through colour coding, offering comprehensive and immediate insights into areas where Societal Risk is deemed unacceptable. In contrast to the f-N curve which is only able to present a Societal Risk per section of transport route, the Societal Risk area map identifies the exact locations on a map which are exceeding risk acceptance criteria indicated by the red colour in Figure 4. Notably, the Societal Risk decreases for an average derailment accident, particularly along the transportation route across the city. It highlights areas with potential issues or growth opportunities and is valuable for urban planning purposes. Additionally, the results of the Societal Risk area map emphasise the necessity for strict speed limits regulations for ethanol transport in urban areas, more so than in rural areas.

Figure 4. Societal Risk area map based on an actual ethanol tank wagon derailment accident.

On the other hand, Societal Risk contribution map (Figure 5) identifies areas where ethanol transport contributes most significantly to the total Societal Risk, essentially indicating where the highest number of fatalities could occur in an accident scenario. In other words, it is representing Expected value, also known as a Potential Loss of Life (PLL). The colour coding reveals several areas inside and outside of the city, often referred to as “hot spots”, which are the most critical and require attention in emergency planning or risk reduction strategies. In this case, both scenarios, regardless of content, highlight the most severe hot spots due to increased population. 

Figure 5. Societal Risk contribution map based on an actual ethanol tank wagon derailment accident.

To understand the meanings of various colours in the Societal Risk maps results, please refer to our blog titled “Societal Risk maps as a clear geographical representation of risk“.

Conclusion

The transportation of ethanol using tank wagon brings challenges, notably the risk to nearby communities. This article has highlighted the benefits of Societal Risk maps in assessing these risks, through a case study using Gexcon’s RISKCURVES QRA software. These maps are valuable for planning safer ethanol transport routes and facilitating communication between risk assessors and urban planners. 

In conclusion, using Societal Risk maps is essential for enhancing the safety of ethanol transportation and protecting communities. They allow for informed decisions and risk mitigation, proving to be a vital tool in the management of hazardous materials transport. We recommend their use in improving safety protocols within the ethanol supply chain. 

Any questions about calculating Societal Risk in RISKCURVES? 

Reach us at riskcurves@gexcon.com.

References

1. University of Illinois (2022) Ethanol and the Fire Service: Ethanol Incidents. Available at: Link

2. U.S. Energy Information Administration (2022) 95% of the fuel ethanol moved in the United States in the first half of 2022 moved by rail. Available at: Link

3. International Labour Organization (no date) Ethanol (Anhydrous). Available at: Link

4. Commonwealth of Massachusetts (2018) Large Volume/High Concentration Ethanol Incident Response Planning Guidance. Available at: Link

5. National Transportation Safety Board (2006) Derailment of Norfolk Southern Railway Company Train 68QB119 with Release of Hazardous Materials and Fire New Brighton, Pennsylvania October 20, 2006. Available at: Link

6. Purple Book: Guidelines for quantitative risk assessment. Available for download: Link

Writers

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