LOPA

LOPA, or Layers of Protection Analysis, is a semi-quantitative risk assessment method used in process safety engineering to assess whether a hazardous event is adequately protected by existing or proposed safeguards. The approach focuses on identifying a specific hazard scenario, estimating the frequency of the initiating event, and evaluating the effectiveness of independent protection layers that would prevent escalation or reduce consequences. The result is typically a calculated risk reduction factor or a residual risk level that is compared with a risk target to determine if additional safeguards are required.

In practice, LOPA is often performed after an initial hazard analysis such as a HAZOP. The process involves selecting a scenario, estimating initiating event frequency, listing independent protection layers such as safety instrumented systems, relief devices, mechanical integrity, alarms, procedural controls, and human actions, and assigning a Probability of Failure on Demand (PFD) to each layer. The safeguards are treated as independent; the combined protection is assessed to determine whether the residual risk meets the site's risk criteria. If not, additional layers or improvements are recommended.

LOPA is widely used in chemical processing, oil and gas, pharmaceuticals, and other industries with significant process safety concerns. It is valued for its balance between rigor and practicality, offering more detail than qualitative checklists while avoiding full probabilistic risk assessment. Limitations include reliance on accurate input data and independence assumptions; results can be sensitive to PFD values and scenario selection. It does not model dynamic interactions or common-cause failures comprehensively.

See also: CCPS, HAZOP, PHA.