Cumulative Lopa Method

Abstract

LOPA (Layer of Protection Analysis) is a simplified risk assessment method that is uniquely useful for determining how “strong” SIF (Safety Instrumented Function – “interlock”) should be designed. LOPA is a semi-quantitative tool which is readily applied after the Process Hazard Analysis (PHA) – for example, HAZOP – and before Fault Tree Analysis/Quantitative Risk Assessment if needed. In most cases, the SIF’s Safety Integrity Level requirements can be determined by LOPA without using the more time-consuming tool of Fault Tree Analysis or Quantitative Risk Assessment. The problem of classical LOPA approach is that it takes into consideration only one hazard scenario at a time. However a SIF may exist in several hazard scenarios, so in practice there is a need for a cumulative LOPA method where we can take into account all hazard scenarios in LOPA calculation which have identical SIF as a Safety Instrumented Independent Protection Layer. We lay down the mathematics of cumulative LOPA, and developed software called Tool4S which uses this mathematics. The article shows some example of the SW application.