Risk Measures and Presentation

In general, risk is a measure of economic loss, human injury or environmental damage in terms of both the likelihood and the magnitude of the loss, injury or damage [2]. The study describes risk measures which estimate risk of human fatality caused by the immediate impact of hydrogen accidents—fire and explosion. Two common ways used of combining incident frequency and consequence data to produce risk estimates, i.e. individual risk and societal risk.

Individual risk (IR) measures are single numbers or a set of risk estimates for various individuals or geographic locations. In general, they consider the risk to an individual who may be in the effect zone of an accident or set of accidents. The size of the accident, in terms of the number of people impacted by a single event, does not affect the individual risk.

Societal risk (SR) measures are single number measures, tabular sets of numbers, or graphical summaries which estimate risk to a group of people located in the effect zone of an accident or set of accidents. Societal risk estimates include a measure of accident size (for example, in terms of the number of people impacted by the accident or set of accidents considered). Some societal risk measures are designed to reflect the observation that people tend to be more concerned about the risk of large accidents than small accidents, and may place a greater weight on large incidents.

The large quantity of frequency and consequence information generated by a QRA must be integrated into a presentation that is relatively easy to understand and use. The form of presentation will vary depending on the goal of the QRA and the measure of risk selected. The presentation may be on a relative basis (e.g., comparison of risk reduction benefits from various remedial measures) or an absolute basis (e.g., comparison with a risk target). Published risk studies have used a variety of presentation formats, including both individual and societal risk measures.

Common forms of presentation of individual risk are risk contour plots and individual risk profiles, also known as risk transects. The risk contour ("isorisk" lines) plot shows individual risk estimates at specific points on a map. It connects points of equal risk around the facility. The individual risk profile (risk transect) is a plot of individual risk as a function of distance from the risk source. This plot is two-dimensional (risk vs distance) and is a simplification of the individual risk contour plot (Fig. 4.18). In order to use this format, two conditions must be met: the risk source should be compact (i.e., well approximated by a point source) and the distribution of risk should be equal in all directions. A candidate for this presentation format is a generic risk assessment for a common hazardous item [2].

Individual Risk Transect
Fig. 4.18 Example of an individual risk, i.e. risk profile [2]
Risk Profile Examples

! 10 100 1000 10.000 FATALITIES, N

Fig. 4.19 An example F-N curve for a single liquefied flammable gas facility [2]. 4.1.1.2. Societal Risk (SR)

! 10 100 1000 10.000 FATALITIES, N

Fig. 4.19 An example F-N curve for a single liquefied flammable gas facility [2]. 4.1.1.2. Societal Risk (SR)

A common form of societal risk (SR) is known as an F-N (frequency-number) curve. An F-N curve is a plot of cumulative frequency versus consequences (expressed as number of fatalities). A logarithmic plot is usually used because the frequency and number of fatalities range over several orders of magnitude. It is also common to show contributions of selected incidents to the total F-N curve as this is helpful for identification of major risk contributors. Fig. 4.19 is a sample F-N curve for a single liquefied flammable gas facility [2]. The facility contains two major parts—a shore-based operation and a marine transfer operation. The F-N curves for these two components of the installation are plotted in Fig. 4.19, along with the F-N curve for the total facility. The societal risk F-N curve for the total facility is equal to the sum of the F-N curves for the two facility components.

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