LOPA: Quantify Risk Reduction from Independent Protection Layers
What is LOPA?
Layer of Protection Analysis (LOPA) is a semi-quantitative method that builds on HAZOP: it estimates the frequency of an initiating event and counts how much risk each independent protection layer (IPL) removes, then judges whether the residual risk is tolerable. It is the risk analysis tool recommended by IEC 61511 and the most common method for SIL rating. The evaluation should be documented and kept in the safety file as evidence for audits and continual improvement.
When to Use It
Use it for SIL determination of safety instrumented functions, quantitative verification of high-risk scenarios identified in HAZOP, comparing the cost-effectiveness of safety measures, and preparing functional safety reports for regulators. Scenarios that remain intolerable after reduction need additional independent layers or a lower initiating frequency.
How to Use It (Step by Step)
Identify the scenario and its initiating event frequency, list the independent protection layers such as BPCS, SIS, relief valves, physical barriers and operator response, and enter the probability of failure on demand (PFD) for each layer. The tool computes the total risk reduction factor and residual risk, which you compare with the tolerable risk target. If a layer is counted, verify its independence, effectiveness and auditability.
Key Formulas / Example
The risk reduction factor is RRF = 1/∏PFD_i over all independent layers. Example: BPCS PFD 1×10⁻¹ × SIS PFD 1×10⁻² × relief valve PFD 1×10⁻² gives RRF = 1/10⁻⁵ = 10⁵. Do not double-count layers that share sensors, actuators or power supplies, and source PFD data from reliable failure databases.