Optimizing the design of railway tank cars to minimize accident-caused releases

Abstract

The design of vehicles transporting hazardous materials has important public safety and economic implications. Conventional wisdom among industry and government has held that a thicker tank on railroad tank cars and trucks reduces risk. However, a thicker tank increases vehicle weight and thus leads to an increase in the number of shipments required to transport the same amount of product and consequently greater exposure to accidents. In this research we develop a model that analyzes the tradeoff between increased damage resistance and greater exposure to accidents in which the objective function is minimization of the probability of release. The model accounts for the reduction in tank car release probability as a function of tank thickness, and the increased exposure to accidents that occurs due to the increased number of shipments needed for the heavier car. Three variables affecting this optimal thickness are considered in this paper: the volumetric capacity of the tank, the probability of release from other, non-tank sources, and the weight capacity of the car. Sensitivity analyses using the model indicate that for any particular configuration of tank car there is an optimal thickness. This optimal thickness is affected by several factors and there is no single optimum for all tank cars.

Introduction

In many aspects of industrial society there are tradeoffs between risk and economics and such is the case with hazardous materials transportation. Society derives benefit from the use of these materials, but also incurs certain risks due to the need to transport them from their place of production to the point of consumption. To minimize the risk, considerable attention has been paid to the proper handling of hazardous materials in transport, including packaging of the materials, loading and unloading practices, transportation operations, routing of shipments, emergency response practices and hazardous materials shipment information [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21].

One of the most important elements of ensuring hazardous materials transportation safety is the “packaging”. Packaging refers to the design of the container and its ability to transport its intended product and withstand both the ordinary and the extraordinary physical aspects of the environment that may be experienced in transit. Both industry and government agencies have developed extensive specifications, regulations and practices for hazardous materials packages [20], [21], [22], [23] that are intended to minimize the likelihood of a spill. However, it is also important that these do not place undue economic burden on transportation. Although some overland transport of hazardous materials is via pipeline, a substantial majority of the bulk transportation tonnage is by truck or rail. Consequently, the design of tank trucks and railroad tank cars is of considerable importance when considering the safe transport of hazardous materials.

Type of hazard varies widely among the many different materials classified as hazardous in transportation [21]. For example, the principal hazard for some materials is their flammability, whereas for others it is their acute toxicity to humans or their potential to cause damage to the environment. Within each of these groups there is substantial variability in the degree of hazard they pose.

In the design of containers for transportation of hazardous materials, the first consideration is complete containment of the product throughout the variety of conditions that can reasonably be expected to normally occur in transit. This aspect is driven by safety concerns as well as commercial interest in maintaining product quantity, quality and purity. Above and beyond this, however, is the need to prevent spillage in the event of unusual occurrences such as accidents. Although the probability of such events is low, it is a factor affecting the design of nearly all containers transporting hazardous materials. And for certain hazardous materials it may be the most important factor affecting specific aspects of container design.

Not surprisingly the robustness of containers is generally commensurate with the hazard posed by the products they are intended to transport, the more hazardous the product, the more robust the container. The objective is to design an efficient and economical transportation container that is sufficiently resistant to damage that it will not spill its contents in an accident. A common approach to achieving this is to construct containers with stronger walls, either through use of thicker or stronger material, or both. Extensive engineering and statistical analysis has been conducted to document and quantify the benefits of this [7], [24], [25], [26], [27], [28], [29]. However, there are limits to the beneficial effect of this approach. Increasing tank thickness results in higher tank car weight and consequent loss in capacity of the car. This means that more shipments must be made to transport the same amount of product, and there is a resulting increase in exposure to an accident. As will be shown in this paper, if this is not properly integrated into the overall design of the container, thickening the tank will not minimize the probability of release of a hazardous material in transit.

Optimization techniques have been previously used to address hazardous materials safety questions particularly regarding routing, modal choice and emergency response [15], [30], [31], [32], [33], [34], [35], [36]. However, we are unaware of any previous work that considers container design from a formal optimization stand point. We develop a model here that considers the question of optimizing container thickness so as to minimize the probability of an accident-caused release. The model is developed in the context of railroad tank cars, but the concepts and methodology apply to and can be adapted to other transport modes, particularly truck transportation of hazardous materials.