Layer of Protection Analysis (LOPA) Basics

Layer of Protection Analysis (LOPA) is a semi-quantitative risk evaluation tool. The methodology is widely used in setting Safety Integrity Level (SIL) targets for safety instrumented systems (SIS). The methodology is often used to evaluate the adequacy of existing protective systems against corporate risk targets. The LOPA method is based on the use of conservative but representative estimates of the failure probability of protective systems.

A LOPA study begins with the definition of a set of undesired events (accidents, chemical releases, fires, explosions, etc.) and the estimate of the consequences of these undesired events. Independent layers of protection are identified and evaluated in a LOPA, based upon assigning protective credits for various management control systems (procedures, inspections, etc.), basic process controls, and defined safety systems. The need for additional protective layers is based on the organization’s risk tolerance. The LOPA methodology used is explained in the Center for Chemical Process Safety’s (CCPS’s) book “Layer of Protection Analysis, Simplified Process Risk Assessment”.

Typically a team of knowledgeable personnel is gathered and a systematic evaluation of the hazards of the process is completed. The LOPA team typically includes personnel who are knowledgeable in the process, control system, maintenance, design, and operations of the facility. The participants in a LOPA Team are essentially the same personnel as for a PHA study team. The difference between a LOPA and a typical PHA is the depth of analysis of a particular issue and the breath of the scope of the study. This talk reviews the basic principles of a LOPA study and presents the authors views the determination of Independent Protection Layers.