Definition:
Layer of Protection Analysis (LOPA) is a semi-quantitative method to evaluate risk reduction and assign Safety Integrity Levels (SIL). This has become more common over time. It is often done with the HAZOP in one large/long meeting and with a single tool.
LOPAs are typically done in a big spreadsheet. Sometimes with macros and various functions to auto-colored cells. There are also various software packages that manage HAZOPs and LOPAs.
LOPA strikes a balance between qualitative hazard identification and fully quantitative risk assessment, enabling efficient, defensible risk decision-making without exhaustive data
IEC 61511-3 Annex F is a great breakdown.
Note that where LOPA sits can confuse practitioners as it is at the boundary between two phases of the IEC 61511 life-cycle. The scenario evaluation, IPL crediting, and risk gap calculation are Clause 8 risk H&RA (specifically on the “A” side of that), while the resulting required risk reduction is consumed by Clause 9 when that risk reduction is allocated to a SIF and the SIL is assigned.
Deep dive: For a much more in-depth treatment, see Layer of Protection Analysis (LOPA): The Engineer’s Guide to SIL Selection.
How it fits: For an overview of how LOPA fits alongside the other hazard and risk analysis methods in an IEC 61511 H&RA, see Hazard and Risk Analysis Methods.
Key Points:
- LOPA reviews existing alarms, relief valves, and safety systems to ensure they meet independence and effectiveness criteria before assigning a risk reduction credit.
- Commonly used for SIL determination.
- Often managed via a software package or an Excel spreadsheet.
- Not technically part of the H&RA, but could be done with the HAZOP.
- Each independent protection layer (IPL) must be independent and auditable to be credited for risk reduction.
Example:
A HAZOP determines that a reactor is at risk from over pressurization from various scenarios. A LOPA determines that an independent high-pressure trip is required for the reactor system. The LOPA then examines the protection layers present and decides that a SIF is needed and its target SIL is SIL 2.
See Also: HAZOP, H&RA, semi quantitative, IPL, target SIL
Cited Source:
- IEC 61511-3:2016, Annex F
- University of Michigan Chemical Engineering – Layers of Protection
- Wikipedia article Layer of Protection Analysis
- Sphera – What is the Difference Between PHA, HAZOP, & LOPA
- Mary Kay O’Connor Process Safety Center – On the Use of LOPA and Risk Graphs for SIL Determination