Modifying LOPA for Improved Performance
Glenn G. Young, CSP
Glenn Young & Associates, LLC PSM Consulting
Baton Rouge, Louisiana
Glenn S. Crowe, CSP
Safety Manager
PCS Nitrogen
Geismar, Louisiana
Introduction
Layers of Protection Analysis (LOPA) is a semi-quantitative risk analysis method that has been popularized by the 2001 book, Layers of Protection Analysis – Simplified Process Risk Assessment. This book is offered by the American Institute of Chemical Engineers (AIChE).
Finding wide support in the chemical, refining, and pipeline industries, the LOPA process has become a popular tool for assessing risk.
The LOPA process is built upon the principle that process experts in a given industry are
cognizant of and accurate in assessing the severity of possible process events, but do a poor job of assessing the likelihood of these possible process events. Because risk is composed both of severity and likelihood, a divergence in either of the factors gives a skewed assessment of true risk.
LOPA attempts to overcome the inherent “human-nature problem” of misdiagnosing likelihood by taking likelihoods from insurance industry data. Using historical data, the likelihood is more likely to be accurate.
LOPA suffers from a number of shortcomings. Among these is the fact that each LOPA analysis is restricted to a single cause-consequence pair. When multiple causes can instigate the same consequence, multiple LOPA analyses must sometimes be done. When a single cause can instigate multiple consequences, multiple LOPA analyses must sometimes be done.
An additional shortcoming is that LOPA is a coarse tool. LOPA likelihoods are broken down  into orders of magnitude change, and exact probabilities are impossible to calculate unless a large amount of field data is available. The Independent Protection Layers (IPLs) that protect against a specific scenario are also given probabilities-of-failure-on-demand (PFDs) that are broken down into orders of magnitude change.
Because LOPA has been so widely adopted over the past five years, and because its penetration of the process industries has been so deep, a broad and deep knowledge base has been developed.