Risk-Based Approach – Risk Evaluation

The risk evaluation of a hazardous facility entails the calculation and quantification of the risk based on the Loss of Containment (LOC) scenarios identified in the Hazard Identification step as a function of the likelihood of occurrence (i.e., Frequency Analysis) and the associated impacts (i.e., Consequence Analysis). When conducting a risk-based quantitative assessment, several pieces of data need to be accounted for in order to ensure the completeness of the study: (1) infrastructures and associated population; (2) process equipment; and (3) ignition sources.

Once the required data is correctly defined, the risk evaluation allows the development of several tools: (1) Individual Risk Contours (IRC), (2) Hazard Risk Contours determined at several thresholds due to explosions, fires and hazardous dispersions, (3) Individual and Societal Risk Indices; (4) FN Curves and (5) Exceedance Curves (EC) Approach.


A key step on a risk-based assessment is the quantification of the risk as a function of the likelihood of occurrence (i.e., Frequency Analysis) and the associated impacts (i.e., Consequence Analysis) based on the Loss of Containment (LOC) scenarios identified via systematic Process Hazard Analysis (PHA) (i.e., Hazard Identification) [1], [2]. A Quantitative Risk Analysis (QRA) considers generic and non-generic LOC scenarios; i.e., generic frequencies of occurrence based on historical data and non-generic frequencies of occurrence estimated via the Fault Tree Analysis (FTA) [3]. In addition, it accounts for the consequences of all outcomes from given LOCs that could lead to explosions, fires and flammable and toxic dispersions in a facility that handles hazardous materials [4]. Once the consequences and likelihoods of occurrence are determined, the risk is calculated as function of the population present at a location and at a specific time (i.e., Societal Risk) and as a function of the acceptability of a particular level of risk to an exposed individual (i.e., Individual Risk). The primary purpose of this manuscript is to explain in detail the different tools that can be developed during a risk-based assessment. Figure 01 illustrates a simplified risk management program flowchart and the risk evaluation step is highlighted.


Risk evaluation is the fifth step of a complete risk-based assessment. In this step, both the individual and societal risks are calculated.

On one hand, the individual risk is characterized by the Individual Risk Contours (IRC), a graphical tool that allows the user to view which are the areas that have major risk within a site that handles hazardous materials. The IRC usually considers the effects of all the hazards present in a typical facility: toxicity, thermal radiation and overpressure. Additionally, contours at specific thresholds for explosions, fires and dispersions can be calculated.

On the other hand, the societal risk is characterized by the FN Curves (FN), a discrete curve in which the X axis is the number of N fatalities or more and the Y axis is the cumulative frequency of having N or more fatalities. Furthermore, other risk indices that help the decision-makers to conduct valuable statistical and sensitivity analyses are discussed as these parameters offer insight and can help tracking the progress of a facility in terms of process safety (e.g., conducting risk-based assessments before and after risk reduction measures are implemented, or between different project phases, etc.). These graphical results need to be compared to risk tolerability criteria in order to decide if additional risk reduction measures are required or not (i.e., inherent safer operation, prevention and mitigation).

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