LOPA Explained: From HAZOP to Safety Integrity Level

Where LOPA fits

Layers of Protection Analysis (LOPA) is a semi-quantitative risk assessment that sits neatly between two methods you may already use. A HAZOP is excellent at identifying what can go wrong, but it is qualitative. Full quantitative risk assessment is rigorous but expensive. LOPA is the practical middle ground: it puts approximate numbers on a specific scenario to answer one question — “do we have enough independent protection?”

How LOPA works

For a single cause-and-consequence scenario, LOPA estimates:

• The frequency of the initiating event (how often the cause occurs).
• The probability of failure on demand (PFD) of each independent protection layer.
• The mitigated frequency that results when you multiply them together.

You then compare that mitigated frequency against your tolerable risk target. If it is not low enough, you need another layer of protection — or a stronger one.

What counts as an Independent Protection Layer?

An IPL must be independent of the initiating cause and of the other layers, effective at preventing the consequence, and auditable. Typical layers include basic process control, alarms with operator action, relief valves, mechanical trips and — where required — a Safety Instrumented Function.

From LOPA to SIL

When the required protection lands on a Safety Instrumented Function, LOPA effectively defines the Safety Integrity Level (SIL) that function must achieve. That is the direct handoff into functional safety and IEC 61508/61511: LOPA sets the target, and SIL verification proves the design meets it.

How iSquare helps

Our process safety engineers facilitate HAZOP and LOPA studies and carry them through to SIL determination and verification — see our safety studies services. To scope a study for your facility, contact our team.