TABLE OF CONTENTS The best-laid plans o' mice an' men
Gang aft a-gley,
An' lea 'e us nought but grief an 'pain,
For promised joy.
Robert Burns
In this chapter, we shall discuss the estimation of risk. The concept of risk has been described in Chapter 1, and some ways of expressing risk outlined in Chapter 2. Generally, risk is a vague and ambiguous term and writings on risk tend to be correspondingly oracular in nature. We shall follow the definition in Chapter 1, relating to the process industries.
Risk is the likelihood of a specified undesirable event occurring within a specified period or in specified circumstances (Jones 1992).
Generally risk is expressed as a frequency, e.g. number of specified events per unit time. We find the following formula often quoted in the literature
Sometimes risk is also expressed as a probability, e.g. probability of a specified event for a certain condition.
The results of the consequence and frequency analysis are combined for each outcome of each individual event, to obtain a measure of risk associated with each outcome. These risk contribution from each of the events may be summed to provide the total risk for the installation.
An estimate of risk can be qualitative or quantitative. Further, risk assessment component of overall process safety assessment takes its input from hazard identification (hazard register), consequence analysis results and frequency estimations, and provides a risk estimate as the output. Tixier et al. (2002) have reviewed 62...
