On the Importance of Using Realistic Data for Safety System Calculations

The use of realistic failure data is an essential part of any quantitative reliability analysis of safety systems. It is also one of the most challenging parts and raises several questions concerning the suitability of the data, the assumptions underlying the data and what uncertainties are related to the data. The IEC 61508 and IEC 61511 standards present requirements to safety instrumented systems (SIS) for all relevant lifecycle phases, and have become leading standards for SIS specification, design, implementation, and operation. The IEC 61511 explicitly states that applied reliability data shall be credible, traceable, documented and justified and shall be based on field feedback from similar devices used in a similar operating environment. The paper discusses challenges that arise when collecting and applying field data from operational experience, including how to identify and treat systematic failures such as repeating failures and bad actors. Guidance is provided on use of failure data for different applications such as design calculations versus operational follow-up. The paper is based on extensive reviews of some thirty thousand SIS maintenance notifications from the Norwegian petroleum industry, documented in the new 2021 revision of the PDS data handbook.