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CM 5 – Probability of fatality
146 This conditional modifier is often referred to as ‘vulnerability’.
147 This conditional modifier may only be used if a single value can be specified for the hazardous
scenario – most likely in an Individual Risk calculation. Otherwise it should be incorporated in the
calculation of the consequence. The value to be used will have to be determined on a case-by-
case basis.
CM6 – Probability of the environmental consequence
148 This conditional modifier is included to account for any factors additional to those considered
elsewhere in the LOPA (eg seasonal factors, if not implicitly included in other factors within the
LOPA) that may influence whether the hazardous scenario can cause the defined environmental
consequence.
Completing the study of the scenario
149 The process should be repeated for the other scenarios as shown in Figure 22. It must
be remembered that the resulting predicted unmitigated frequency of the overflow event is
aggregated over all relevant initiating events. This sum, combined with existing control, protection
and mitigation risk reduction factors applicable to each initiating event must be compared with
the target frequency for the specified consequence defined in the risk tolerance criteria (see
paragraphs 36–53).
150 It is important that a sensitivity analysis should be carried out to explore the sensitivity
of the predicted risk levels to the assumptions made. It is important to be able to identify the
key assumptions and to provide justification that the analysis is based on either realistic or
conservative assumptions. Sensitivity of assumptions on initiating events and consequence side of
a risk assessment are also required.
Concluding the LOPA
151 The conclusions of the LOPA should be recorded. The record should include sufficient
information to allow a third-party to understand the analysis and should justify the assumptions
made and the choice of values for parameters such as human reliability, equipment failure
rates and conditional modifiers. Where assumptions are made about the mode of operation of
the facility (such as the proportion of the time tanks are being filled, or the number of tanks on
gasoline duty) these should be documented so that their continuing validity can be checked.
152 The LOPA should provide the basis for the safety requirements specification of the SIS
(where required). This should include:
clear definition of the SIL required for the safety instrumented system in terms of reliability
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level, eg PFD;
it should also provide the basis of the functional specification of the SIS.
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