A Benefit /Cost Approach for Prioritizing Plant Modifications to Reduce Risk below Acceptance Limit

Abstract

The paper describes practical experience with the application of a benefit/cost approach for prioritization of plant improvement expenditure to reduce both Health Safety and Environmental (HSE) risk and production loss risk below acceptable level. The method has been developed during a three-year SIL allocation programme implemented in more than 10 petrochemical plants where risk associated to safety functions failing upon request has been determined in order to establish safety function required availability to meet risk level requirements. Conducting a SIL analysis using a combination of technical documentation (P&I Diagrams, Cause and Effect Matrix), safety documentation (HazOp, Fire Fighting System data) and economical parameters (equipment cost, product prices, raw material cost) leads to the identification of initial risk level of petrochemical plant units with regards to three categories, i.e. Safety, Environment and Economic. Resulting SIL level of each safety function corresponds to the highest reduction rate required to lower initial risk within tolerable range. Plants built before the issue of IEC 61508/61511 typically result in high SIL level associated to safety functions, being that safety system architecture was developed without considering reliability requirements for safety loops. Criteria to achieve adequate risk reduction minimizing SIL level of safety functions, e.g. ensuring full independence of safety functions final elements, minimizing frequency of causes, etc. have been evaluated considering the benefit in less severe maintenance requirements and lower cost for modifications.