A semi-quantitative approach for verification of Safety Integrity Level requirements

A Safety Integrity Level (SIL) is a measure of performance required for a safety instrumented function. The IEC 61508/61511 standards define four safety integrity levels, SIL1 to SIL4, where SIL4 is the level with the most stringent requirements. For each safety integrity level there are many design requirements, including requirements for the probability of failure on demand (PFD). Verification of the required failure probabilisty is usually based on a quantitative analysis. In this paper we argue that such an approach is better replaced by a semi-quantitative approach. The approach acknowledges that the PFD requirement for a safety function cannot be adequately verified only by reference to an assigned probability number. There is a need for seeing beyond the probability number. The key aspect to include is related to uncertainty. Such an aspect is of ten ignored in verification of a safety integrity level. The offshore oil and gas industry is the starting point, but the discussion is to large extent general.