As part of the design process, a HAZID and HAZOP are typically performed to identify and address the design and process hazards, Once the hazards have been identified and assessed, mitigation is proposed to control and reduce the likelihood of the hazard occurring. The mitigation proposed is typically based on engineering judgement with no formal quantification of how effective the proposed mitigation is, or whether the mitigation has reduced the risk to an acceptable level.
To determine whether proposed design and mitigation is acceptable, a safety integrity level (SIL) assessment is required to quantify the risks and to determine the requirement of safety instrumented functions (SIF) to ensure the AGI meets the proposed safety integrity level. Techniques and methods are contained within standards IEC 61508 and IEC 61511 to determine the Safety Integrity Level (SIL) of non-safety instrumented systems.
AFAA are familiar with the quantitative (fault-tree analysis), risk graph, and layer of protection analysis (LOPA) contained within the standards. The most commonly used method is the Layer of Protection Analysis (LOPA) to quantify the risks.
The LOPA method is a quantitative risk analysis that basically consists of four steps;
Identifying the scenario/consequence and initiating event probability
Evaluating the unmitigated risk
Evaluating the Independent Protection Layer probability
Evaluating the risk
In addition to the SIL rating methods contained within the codes, AFAA are able to combine the results of such analysis with probabilistic assessments of system failure conditions and scenarios, to provide robust justification for the selection SILs.
AFAA have developed very sophisticated in-house software for undertaking SIL Assessment studies, it has been subjected to extensive validation and verification processes.