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Risk assessment is an effective means of identifying process safety
risks and determining the most cost-effective means to reduce risk.
Many organizations recognize the need for risk assessment, but most do not have the tools, experience and resources to assess risk quantitatively. Therefore, these organizations use qualitative or semiquantitative risk assessment tools, such as risk ranking.
Although risk matrices are easy to use, unless they are designed properly, they can create liability issues and give a false sense of security.
An effective risk ranking matrix should have the following characteristics:
• Be simple to use and understand
• Not require extensive knowledge of quantitative risk analysis
to use
• Have clear guidance on applicability
• Have consistent likelihood ranges that cover the full spectrum
of potential scenarios
• Have detailed descriptions of the consequences of concern for
each consequence range
• Have clearly defined tolerable and intolerable risk levels
• Show how scenarios that are at an intolerable risk level can be
mitigated to a tolerable risk level on the matrix
• Provide clear guidance on what action is necessary to mitigate
scenarios with intolerable risk levels
Risk ranking uses a matrix that has ranges of consequence and likeli-
hood as the axes. The combination of a consequence and likelihood
range gives an estimate of risk or a risk ranking. Although there are
many risk matrices that have been developed and published, the de-
velopment and application of risk matrices present their own chal-
lenges.
Construction of a risk matrix starts by first establishing how the matrix
is intended to be used. Some typical uses for risk ranking are process
hazard analyses, facility siting studies, and safety audits. A key initial
decision that has to be made is to define the risk acceptability or toler-
ability criteria for the organization using the matrix. Without adequate
consideration of risk tolerability, a risk matrix can be developed that
implies a level of risk tolerability much higher than the organization
actually desires. Another key aspect of risk matrix design is having the
capability to evaluate the effectiveness of risk mitigation measures.
The risk matrix should always allow the risk ranking for a scenario to
move to a risk tolerable level after implementation of mitigating measures. Otherwise it may be difficult to determine the effectiveness of
mitigation measures.