LOPA onions: Peeling back the outer layers

Abstract

Layer of protection analysis (LOPA) has quickly gained acceptance in the chemical processing industries and has risen to be one of the leading risk assessment techniques used for process safety studies. LOPA generally uses more rigor and science than what is encountered with qualitative risk assessments, while still not becoming overly onerous when compared with detailed quantitative risk assessments. In the interest of balancing time and resources against science and accuracy, certain tradeoffs and assumptions are made within the LOPA assessment. In turn, these tradeoffs and assumptions can lead to inaccurate conclusions. For example, one issue that arises is with the treatment of protection layers associated with mitigation of consequences. LOPA teams have a choice to account for mitigation layers in the consequence assignment or alternatively treat these layers as independent protection layers (IPLs). Although this may appear to be an inconsequential decision, it can in fact result in very different conclusions. In the course of treating mitigation layers as IPLs, organizations must ensure the necessary inspection, testing, and preventive maintenance practices are in place for these layers. Furthermore, recognizing this dichotomy in treatment, one can also show that these mitigation layers should be designed so as to achieve a balance between consequence reduction and desired reliability. This article discusses alternative treatments of risk mitigation layers that are commonly applied by LOPA teams and demonstrates their impacts through case studies. © 2011 American Institute of Chemical Engineers Process Saf Prog, 2011

Risk criteria, protection layers, and conditional modifiers

“…According to Wasileski and Henselwood [10], there are two approaches in handling these layers within the LOPA framework. Their conclusions can be applied to risk assessments in general.…”