(71ae) Uncertainty Analysis for LOPA in the Offshore Industry

According to IEC 61508 and IEC 61511, a variety of methodologies, e.g., risk graph, hazard matrix, LOPA(Layer of Protection Analysis), are introduced to determine target SILs of SISs(Safety Instrumented Systems). Recently, in the offshore industry, LOPA has become the most popular method since it reflects the effects of existing safeguards on a system.

However, even the LOPA method is powerful, there are some uncertainties mainly due to assigned numeric values. With these uncertainties, the target SILs can be affected in terms of credibility. In order to assure that the determined target SILs are credible, uncertainty analysis for the LOPA method can be utilized to show the level of uncertainty of target SILs. To investigate the uncertainty level of target SILs which are determined by the LOPA method, the appropriate uncertainty analysis method should be selected among the numerous methods developed in many applications.

This paper suggests the proper LOPA uncertainty analysis technique based on the natures of the LOPA method. The LOPA method has both qualitative and quantitative attributes. The qualitative attributes come from the C(Consequence or Severity) value which is determined by experts' consensus. The quantitative attributes, on the other hand, exist due to numerical parameters such as the initiating event frequency, PFD(Probability of Failure on Demand) for IPLs(Independent Protection Layers), and intermediate event likelihood. To reflect these qualitative and quantitative characteristics at the same time, the combination of the fuzzy set approach and the Monte Carlo simulation are proposed. By using the proposed method, the case study has been conducted. The results of the analysis will help establish the guidline when applying the LOPA method.