LOPA or QRA

[octane]

A HAZOP identified a scenario with multiple simultaneous failures.
How would you determine whether it should proceed to LOPA or QRA?

[octane]

Any pointers here?

[opo21]

A HAZOP identifies credible process deviations and their consequences, but if it identifies a scenario involving multiple simultaneous failures, I would first determine whether the combination of failures is credible, independent, and within the assumptions of LOPA.
LOPA is suitable only if the scenario can be represented by:
A single initiating event
Independent Protection Layers (IPLs) that are truly independent
Well-defined initiating event frequencies
Consequence severity that is reasonably understood
Failure events that are largely independent
If the multiple failures are simply an initiating event followed by independent IPL failures, LOPA is appropriate.
For example:
Cooling water failure (initiating event)
High-pressure trip fails
PSV provides protection
This can usually be handled by LOPA because frequencies and IPLs can be quantified.

[luca]

I would first verify whether the scenario fits LOPA assumptions—a single initiating event with independent protection layers and independent failures. If the multiple failures involve dependencies, common-cause failures, cascading events, or complex consequence interactions, I would recommend a Quantitative Risk Assessment. In practice, LOPA is used to verify whether existing safeguards achieve the required risk reduction, while QRA is used when the accident sequence or consequences are too complex for LOPA's simplified approach.