Risk assessment includes incident identification and consequence analysis. Incident identification describes how an accident occurs. It frequently includes an analysis of the probabilities. Consequence analysis describes the expected damage. This includes loss of life, damage to the environment or capital equipment, and days outage. The hazards identification procedures presented in chapter 10 include some aspects of risk assessment. The Dow F&EI includes a calculation of the maximum probable property dam- age (MPPD) and the maximum probable days outage (MPDO). This is a form of consequences analysis. However, these numbers are obtained by some rather simple calculations involving published correlations. Hazard and operability (HAZOP) studies provide information on how a particular accident occurs. This is a form of incident identification. No probabilities or num- bers are used with the typical HAZOP study, although the experience of the review committee is used to decide on an appropriate course of action. In this chapter we will
review probability mathematics, including the mathematics of equipment failure, show how the failure probabilities of individual hardware components contribute to the failure of a process, describe two probabilistic methods (event trees and fault trees), describe the concepts of layer of protection analysis (LOPA), and describe the relationship between quantitative risk analysis (QRA) and LOPA.
We focus on determining the frequency of accident scenarios. The last two sections show how the frequencies are used in QRA and LOPA studies; LOPA is a simplified QRA. It should be emphasized that the teachings of this chapter are all easy to use and to apply, and the results