Considering Multiple Initiating Events in a LOPA

Abstract

Layer of Protection Analysis (LOPA) is a risk reduction evaluation methodology well suited to consider hazard scenarios with multiple initiating events. The presence of multiple initiators increases the likelihood that a hazard scenario could occur placing additional demands on the Independent Protection Layers (IPLs) that prevent and mitigate the hazard. This paper will discuss the impact of multiple initiating events on demand frequency, discuss methods to evaluate the effectiveness of IPLs, and determine which may be considered to reduce the demand on a Safety Instrumented Function (SIF) and Safety Integrity Level (SIL) targeting. Finally, the impact of demand frequency and proof test interval on SIF demand mode will be illustrated.