LOPA (Layer of Protection Analysis) is a Level of Safety and Hazard Protection in addition to PHA (Process Hazard Analysis)
Industrial process safety methods continue to advance from the traditional Process Hazard Analysis (PHA) to Layer of Protection Analysis (LOPA).
Process Hazard Analysis (PHA) is a critical component of the modern Process Safety Management (PSM) and is required by federal law under OSHA 1910. Fundamentally, PHA is a team-based qualitative method of safety review that identifies hazards and solutions to mitigate them. OSHA 1910 prescribes that a PHA must include at least one of the following analytical safety review processes:
- Hazard and Operability Study (HAZOP)
- Failure Mode and Effects Analysis (FMEA)
- Fault Tree Analysis
PSM and PHA analysis techniques have substantially advanced safety in industrial facilities; however, tragic accidents have still occurred, and countless ways remain where the process safety measures may again fail or overlook a potential situation of endangerment. At times, PHAs can suffer from "tunnel vision," where a specific process hazard is overanalyzed to the lessened attention of other risks, thereby making the safety review less effective while overlooking potentially critical measures. PHAs are qualitative in nature and evaluate a specific failure's consequences while not quantifying the probability of a failure. This is where the LOPA method can provide a higher level of review and recognition for increased safety measures effectiveness.
A Layer of Protection Analysis (LOPA) is a semi-quantitative method that analyzes a single one-cause one consequence pair, thereby allowing a facility to apply uniform risk across entire operations quantitatively and measured against risk tolerance acceptance criteria. Where PHA can be somewhat general (i.e., good, better, best), LOPA can provide numeric values (probabilities) to more effectively quantify safety options such as having a 5%, 0.5%, 0.1%, or 0.01% chance of occurrence.
LOPA establishes Independent Protection Layers (IPLs) in overall safety systems to achieve the same quantitative results across various causes and consequences. LOPA's critical component is the IPLs, with each fully independent layer capable of preventing the hazard from progressing further, maintaining process safety for all involved. The following diagram illustrates IPL's principle, where each separate layer helps protect the public from the process. LOPA combines event severity, frequency, and the likelihood of failure to calculate a level of risk. LOPA also allows the analysis of significant incidents and expected outcomes, such as a power interruption or loss to an entire process unit, what happens if the process loses all power, the physical protection for a control system is compromised, human activity does not occur, etc.? LOPA recognizes more comprehensive decision-making during the development and design of safety systems through quantitative considerations, thereby enabling safety engineers to reduce risks with corresponding increases in plant safety.
LOPA is a safety tool that became more prominent due to the Deepwater Horizon incident, where critical equipment performance should be redefined based on the likelihood of failure.
(Layers of Protection (AICHE-CCPS) Adapted from Layer of Protection Analysis: Simplified Process Risk Assessment, 2001)
The semi-quantitative aspect associated with LOPA is depicted in the following Figure. This diagram illustrates the independent protection layers and the probability of a batch reactor system's failures to achieve an end risk assessment. If the likelihood is considered unacceptably high, additional layers of protection are needed.
Summary of LOPA mapping for a batch reactor with several layers of protection (Adapted from Layer of Protection Analysis, Willey, R. J, 2014 International Symposium on Safety Science and Technology)
LOPA's semi-quantitative method has a distinct advantage relative to the qualitative approach employed with PHA methodology. Additionally, LOPA allows for analysis of one-cause one-consequence pairs, while PHAs can be all-encompassing, at times limiting their effectiveness. LOPA is further protection through process safety and LOPA allows semi-quantitative methods to reduce the overall risk to the process.
Implementing Layer of Protection Analysis (LOPA) enables advancement towards the prevention and elimination of industrial accidents, injuries, fatalities, and environmental releases, from the traditional Process Hazard Analysis (PHA).