Inherent Risk is the consequence and likelihood of a hazardous event occurring if nothing is in place to prevent or mitigate it.
In the context of IEC 61511 and Layer of Protection Analysis (LOPA), inherent risk is the starting point of any risk assessment. It is calculated by combining the initiating event frequency with the consequence severity, before crediting any independent protection layers (IPLs) such as a safety instrumented function (SIF), relief devices, or emergency response.
Inherent Risk should not be confused with “inherently safer design,” which is a separate concept referring to the elimination of hazards at the source through process design changes. Inherent risk is a quantified measure — expressed as an event frequency — while inherently safer design is a methodology.
Key Points:
- Calculated as: initiating event frequency x consequence severity (expressed as a frequency, e.g., events/year)
- Represents risk with zero credit given to any protection layer
- The starting point for LOPA and other risk assessment methodologies under IEC 61511
- Must be compared against the tolerable risk target to determine required risk reduction
- A large gap between inherent risk and tolerable risk drives a higher target SIL requirement
- Often referred to as “unmitigated risk” in practice
Example:
A high-pressure vessel has an initiating event frequency of 0.1/year (a control valve failing open). The consequence is a toxic release. Before crediting any IPLs — relief valve, SIS shutoff, or operator response — the inherent risk is 0.1/year. If tolerable risk is 1E-4/year, a total risk reduction factor (RRF) of 1,000 is needed, typically achieved through a combination of IPLs including a SIL-rated SIF.
See Also: LOPA, tolerable risk, residual risk, IPL, RRF, target SIL
Cited Source:
- IEC 61511-1:2016, Clause 8
- CCPS — Layer of Protection Analysis: Simplified Process Risk Assessment
- ISA — IEC 61511 Functional Safety Standard
- CCPS — Layer of Protection Analysis Overview