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2 Journal of Polish Safety and Reliability Association

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

Abrahamsen E. B., Røed W.

Journal of Polish Safety and Reliability Association

2010

Vol. 1, No. 1

1--6

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.

Safety oriented bubble diagrams in project risk management

Abrahamsen E. B., Aven T.

Journal of Polish Safety and Reliability Association

2008

Vol. 1

1--5

In project risk management many firms use bubble diagrams to get a graphical presentation of a project’s most uncertain attributes. The bubble diagrams and procedures used to put attributes into these diagrams are seen to provide a rational framework for managing risks. In this paper we review and discuss the use of these diagrams and procedures. Special attention is given to the way safety is treated. We show that the standard use of bubble diagrams is not adequate for identification and follow up critical activities that affect safety. The main problem is that the present structure means that the uncertainty is not properly taken into account. In this paper a reformulated bubble diagram is suggested that better reflects safety related uncertainties. The offshore oil and gas industry is the starting point, but the discussion is to large extent general.