Estimating the Conditional Probability of Release (CPR) of Tank Cars in Freight Train Derailments
Advisor: Professor Paolo Gardoni
Abstract
Freight trains are a common mode of transporting goods and products over extensive distances worldwide. Tank cars, in particular, are used to carry hazardous material (hazmat), such as ethanol, crude oil, anhydrous ammonia, and sulfuric acid. Despite their low likelihood, train derailments resulting in hazmat releases can have catastrophic human, environmental, and economic consequences. As such, there is a need to adequately assess the safety performance of the hazmat tank cars in a freight train derailment. The conditional probability of release (CPR) is one of the most insightful safety metrics used to assess the reliability of these derailed tank cars. However, given the chaotic and complex nature of train derailments, quantifying the CPR of derailed cars is quite challenging as it requires a thorough understanding of the dynamic behavior of derailed railcars and information on the derailment impact environment, which is typically unavailable.
This dissertation develops a probabilistic formulation to determine the CPR of tank cars, which captures the underlying physics of a derailment and impact event. The formulation proposes probabilistic models to quantify the resistance and demand for tank cars and their key components (i.e., tank heads, tank shells, top fittings, and bottom fittings). These models characterize the structural behavior of each tank car component in an impact scenario. Moreover, these models incorporate key elements, such as the derailment-caused impact forces, impact types, impact event characteristics, and tank car properties, to determine the quantities of interest at the level of an impact type affecting a component of a tank car. As a demonstration of how the formulation works, the CPR values for three simulated train derailment scenarios, based on the real-world derailment of Graettinger, Iowa, are determined.
Focusing on the impact environment in a derailment, this dissertation supports the refinement of a high-fidelity three-dimensional (3D) finite-element (FE) derailment model to accurately model a complete train derailment, including the interaction of the derailed railcars with each other and the surrounding environment. The model is able to incorporate a variety of accident and train characteristics, as well as sub-models of different train and site features, such as car and equipment, train braking, and ground resistance.
To leverage the output from the FE derailment model, this dissertation develops a comprehensive validation procedure to assess the performance of train derailment models in replicating the kinematics of a full train derailment. By validating the modeled real-world derailment kinematics, the FE derailment model can subsequently be used to simulate the impact environment affecting the railcars. To that end, the validation procedure introduces a series of primary and derived metrics to quantify characteristic dynamic behaviors of the train and derailed railcars, such as the extent of the derailment, the spatial dispositions of derailed railcars, their longitudinal and lateral spread, and their misalignment with the track. The procedure also employs standard comparison measures to objectively evaluate the alignment of a modeled derailment with a real-world accident. With that, the kinematic performance of standalone models and that of multiple model iterations, which feature varying model parameters, can be assessed using the developed validation procedure. This procedure is applied to three real-case freight train derailments that occurred in Aliceville, Alabama; Graettinger, Iowa; and Crosby, Texas.
Overall, this dissertation provides tank car safety experts with a rigorous approach for assessing the reliability of hazmat tank cars, including current and novel tank car materials and configurations, across a wide range of derailment conditions.