Limited QRA in SIS systems

This paper discusses the application of applying limited Quantitative Risk Analysis method in Safety Integrity Level (SIL) Verification. In certain cases, the number of critical actions required to achieve SIL target is high, therefore difficult to attain. A limited QRA can be proposed for these types of SIFs to identify the Risk Reduction Factor (RRF) based on the gap between the risk value obtained from QRA and Target Maximum Event Likelihood (TMEL) established by the end user company. QRA can be performed by considering the consequence and frequency analysis on the particular hazard scenario. The paper discusses a compressor release scenario which was assigned a SIL target of 1 by the SIL assignment team. Based on the number of critical outputs, it was difficult to achieve the SIL target even with a minimal test interval. The QRA on the particular SIF identified the required RRF by identifying the consequences of the compressor failure considering the weather conditions and type of release. These consequences were combined with the frequency of failures obtained from industry databases and the occupancy in the unit to obtain the total risk associated with the event. The gap between the total risk calculated with the QRA and the TMEL was the RRF required by the SIF to be successful.