Abstract
Safety instrumented systems (SIS) are to be followed up during operation at a facility. An important part of SIS follow-up is to ensure that all relevant SIS requirements from the design phase are fulfilled during the entire lifetime of the facility. The safety integrity level (SIL) and the corresponding probability of failure on demand (PFD) requirements should be verified regularly. The PFD of a component is a function of the dangerous undetected failure rate and the proof test interval of the component. For follow-up purposes it is relevant to group comparable components within the same equipment group, such as shutdown valves or level transmitters. For analysis purpose, the method focuses on equipment groups where all components within the same group are assumed to have a comparable (common) failure rate and PFD budget. This paper presents an approach for estimating updated failure rates and assessing the proof test intervals based on operational experience. The method for updating failure rates is based on a Bayesian failure rate estimation with additional periodisation. For proof test interval assessment two methods are given: One method based on comparison of the “experienced” failure rate and the failure rate assumed in design, and one method based on comparison of the “experienced” PFD and the PFD budget requirement.
