Layers of Protection Analysis (LOPA) is a semi-quantitative method for analsying the likelihood of a hazardous event, considering initiating event (IE) frequency and the mitigating effect of various independent protection layers (IPL).
Initiating events are defined in the HAZOP, such as:
- Piping leak or rupture
- BPCS failure
- Equipment failure (loss of containment)
- Human error (depending on frequency and criticality of task performed)
- Loss of utility (e.g. power supply, instrument air)
- Mechanical overpressure protection (e.g. PRV, burst disk)
- BPCS interlock (where control loop is not initiating event)
- Independent safety systems (e.g. BMS for a fired heater)
- Operator response to alarm (sufficient time to adequately respond must be available)
Additional mitigating factors (e.g. F&G systems, spill containment) and conditional modifiers (e.g. ignition probability, presence of personnel) are considered.
After consideration of all IPLs, the gap between residual risk and tolerable risk gives the required SIL of instrumented safety functions (SIF).