06: SIL Study

Process hazard and risk assessment

Owners of major accident hazard sites (duty holders), need to identify hazards and make some grading of different risks posed by their process to workers, visitors, the surrounding public and the environment. For process hazards, this usually involves looking for events that result in the “loss of containment” (LOC) of hazardous materials.

There are many different ways of approaching process hazard analysis (PHA), often using a combination of study methods like “What-If” or HAZOP - Hazard and Operability studies. Whichever methods are adopted, the key is to use a systematic procedure that covers all process areas using a multi-discipline team approach.

PHA / HAZOP must use multi-discipline teams to be effective.

If approached methodically, a PHA/HAZOP should provide solid information on possible causes of hazards, estimated consequences of hazardous events, and lists of safeguards that can prevent or mitigate escalation.

Tolerable Risk Criteria

After hazards and safeguards have been identified, there often needs to be some further analysis to determine how effective the safeguards are at risk reduction. There are no mandatory techniques for this step in IEC 61511-1, so the duty holder needs to decide their own approach.

Tolerable risk is usually summarized in the form of a risk matrix, which should be calibrated with a set of numerical targets and specified frequencies for different consequences.

Risk Graph

Risk graphs come in many different forms. There is no such thing as a "standard" risk graph, because they need calibrating. The best guidance on calibration is provided in IEC 61511 part 3.

When they are calibrated and agreed by the duty holder, risk graph's get used by teams to produce estimations fairly quickly. This is often useful alongside a PHA/HAZOP to provide a "first pass" risk/SIL assessment that filters lower risks from those that are potentially high-risk.


Another effective technique for SIL target assessment is a layer of protection analysis (LOPA) study. A LOPA study can consider each major hazard scenario and determine whether the safeguards identified during PHA/HAZOP are effective as independent protection layers (IPL).

When IPL have been considered, it may be the case that one or more Safety Instrumented Functions (SIF) are needed. LOPA studies usually result in the selection of appropriate IPL's and the required Safety Integrity Level (SIL) of SIF to prevent or mitigate each hazard.

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