Release Risk Model to Evaluate Highway Cargo Tank Component-Specific Safety Performance in Transporting Hazardous Materials

This paper introduces a release metric as the expected value of the quantity lost from a highway bulk package given that it is involved in an accident. The associated release risk model is developed in terms of potential releases from tank head/shell components and tank appurtenances, such as top and bottom fittings, valves and larger openings. The model defines conditional probabilities of release for release size, failure mode, and failure source. The severity measure used in the model is the percentage of packaged lading released. The use of the risk model is illustrated in estimating the severity levels of releases normalized with respect to the conditional probability of release given that an accident has occurred. The normalized release risk model is used to evaluate the performance of tank head/shell components and tank appurtenances in accidents reported to the U.S. Department of Transportation Pipeline and Hazardous Materials Safety Administration over a five year period. The analysis showed that given a release event involving a cargo tank, the expected release severity for the head or shell is higher than for tank appurtenances. Furthermore, “burst or ruptured”, “ripped or torn” and “gouged or cut” failure modes correspond to the greatest severity given a head/shell failure.