The original concepts that led to today's Layers of Protection Analysis (LOPA) were developed in the US in the late ‘80s and early ‘90s. In 2001 CCPS developed the LOPA method as a systematic approach to ensure sufficient protective barriers are available to prevent or mitigate specific hazardous scenarios.

Different LOPA methodologies have been used: some are based on simple frequency-consequences matrices, others are based on risk targets; some follow the order of magnitude approach, others follow a simplified quantitative risk assessment (QRA) approach.

LOPA is not supposed to be a fancy HAZOP, nor a simplified QRA. This paper clarifies and explains some often misunderstood LOPA concepts. The paper also presents a generic case study that compares the order of magnitude approach with the risk-based approach and identifies advantages and disadvantages of each LOPA approach.