New fuzzy uncertainty assessment approach of target SIL evaluation by risk graph optimization

Abstract

The requirements on the design of safety instrumented systems based on safety integrity level (SIL) have been developed continuously in the oil and gas industries. IEC 61508 and IEC 61511 explain various methods to determine the target SIL for specified safety functions, such as risk graph, risk matrix and layer of protection analysis. These methods could achieve different target SIL for the same safety function, principally due to uncertainty in the models. Additionally, uncertainties in the input parameters donate uncertainty in the output (target SIL). Based on conventional practice in oil and gas industry, engineer usually use different methods such as risk graph, hazardous event severity matrix and LOPA to evaluate target SILs for the same function and accept the most conservative value as the target SIL. In this paper, the uncertainty assessment in target SIL determination evaluated by the risk graph method using new fuzzy set approach has been done and finally a new framework based on this method is proposed.