Chapter 12 - Quantitative Risk Assessment

Abstract

CCPS (2005) defines chemical process quantitative risk analysis as the quantitative evaluation of expected risk from potential incident scenarios. It examines both consequences and frequencies, and how they combine into an overall measure of risk. In process safety, the quantitative risk analysis is a preliminary step, aimed at quantifying the final risk (QRA), in order to ensure that this is mitigated and controlled, and can be accepted according to some validated criteria. Process safety engineering does not deal only and always with QRA; however, most of the qualitative and semi-quantitative studies, such as HAZID, HAZOP, PHA, FMECA, usually adopt a quantitative parametrisation to risk ranking. Regardless of this, all process safety engineering tasks contribute to ensuring that any quantifiable risks demonstrably fall within the limits. The QRA world encompasses a multifaceted series of exercises, which present a different degree of complexity and difficulty. Regardless, all of these are approached through the combination of hazard scenarios, which are identified according to the techniques of consequence assessment, with the corresponding probabilistic levels of occurrence. Particular importance is given to the risk associated with Major Hazard Installations, which are regulated by very stringent and comprehensive international laws and standards, such as the Seveso Directives/COMAH in Europe and the EPA Risk Management Program Rule in the USA. In addition, a plethora of standards and regulations, both in Europe and in the USA, require undertaking a lot of QRA-based studies as a support for an appropriate and safe design. Among these, hazardous area classification, functional safety, reliability review, and even the registration of new chemicals within the REACH regulation framework are quantitative risk-based processes. A full coverage of the QRA scenario would deserve a whole volume, so the systematic description of the methodology and of the associate techniques is out of the scope of this book. Nevertheless, most parts of the concepts and the theoretical aspects underpinning the execution of a QRA have largely been presented. In this chapter, an overview of the techniques has been provided, referring, when required, to the specific chapters where consequence assessment and probabilistic aspects have been treated. The following chapters focus on the most relevant thematic aspects of the quantitative risk assessment and have been developed to provide the reader with a complete description of the related techniques, along with a significant number of calculation examples.