Quantitative evaluation of equipment fragmentation and missile impact hazards in industrial accidents

The projection of fragments is an important scenario for industrial accidents involving loss of integrity in process equipment. It was recorded as the cause of fatalities, injuries and of damage to process equipment in several accidents occurred. Moreover it is one of the primary causes of accident escalation (domino effect). The generation of missiles usually follows the catastrophic rupture of process equipment, either due to internal pressure exceeding allowable values or to mechanical failure of rotating components. The generated fragments may trigger secondary accidents by impacting and damaging equipment at relevant distances from the actual primary accident. As well, they can damage population, either internal or external to the plant, contributing to accident severity. The relevant projection distances that were experienced in past accidents (up to 1 km) hinder the application of safety distance criteria and preventive actions to avoid domino effects. On the other side, physical barriers for consequence mitigation can be expensive and require specific data for an effective design. The risk of damage and of domino scenarios caused by fragment projection shall be evaluated by a quantitative risk analysis (QRA) approach based on both expected frequency and expected consequence assessment. However the quantitative assessment of missile impact damage requires the availability of dedicated models for the assessment of fragment generation, trajectory and impact probabilities up to a given target. In the present study, an approach based on quantitative risk analysis was developed for missile impact studies. It comprises of several models to capture relevant factors in the event chain spanning from missile generation to impact on a target. The different categories of missile sources were identified. The analysis of fracture mechanics fundamentals allowed the exploration of the relations between the fracture characteristics and the final event leading to equipment collapse. Reference fragmentation patterns were defined on the basis of the geometrical characteristics of the categories of equipment that are more frequently involved in fragmentation accidents. Primary scenarios leading to fragment projection were correlated to specific fragmentation patterns. Industrial accidents involving fragment projection were investigated and statistics from more than 140 vessel fragmentation events provided the data needed to support and validate the approach. The available data also allowed the calculation of the expected probability of fragment projection following vessel fragmentation, and the probability of the alternative fragmentation patterns with respect to the different accidental scenarios. Furthermore, probability distributions of expected number of fragments and initial velocities were inferred. These models allowed the assessment of input data for trajectory analysis. The categories of critical targets were identified, considering both damage to personnel and damage of structures that may trigger escalation events. A probabilistic model was used to assess fragment impact probability on a given target, based on the ballistic analysis of possible fragment trajectories. Simplified functions for drag factor calculation were developed for the expected shapes of fragments. The model made possible a probabilistic approach to the estimation of the projection distance and direction. The impact condition was verified taking into account the actual target geometry and its distance from the fragment source. The models in the developed approach were validated using available literature data and data retrieved from past accident analysis.