This paper shares observations and lessons learned from two originators of LOPA and provides further guidance on how to and how Not to use LOPA. LOPA is only one option for judging risk. A common method for judging the risk of most scenarios is the process hazard analysis (PHA) team; their judgment is qualitative, but the “fuzzy” math of the individual team members frequently coalesces into excellent judgment of risk for most accident scenarios.

The introduction of the streamlined semi-quantitative risk assessment method, LOPA, has had a tremendous impact on the chemical and related industries. 99% of the quantitative risk assessments that may be necessary can now be performed in 1/10th the time of a QRA (quantitative risk assessment). Many benefits have been reaped, including a continual improvement on the identification and control of critical features and actions (IEs and IPLs). However, the initial rollout of LOPA has led to a few problems, including repetition of over-reliance on theoretical calculations, overly aggressive risk tolerance criteria which encourages misuse of EEs and CMs, and of course failure to maintain IEs and IPLs properly. The problems are easily remedied by: • Increased (renewed) focus on the qualitative analyses (PHAs/HAZOPs) • Judicious use of LOPA • Carefully adhering to the rules of LOPA, especially validation of the maintenance of the IPLs and IEs at each site • Avoiding use of EEs and CMs • More focus on maintaining the failure rates of IEs and IPLs • Not believing the numbers but believing the comparison of alternative risk reduction alternatives