WHAT IS LOPA?
- Layer of Protection Analysis (LOPA) is a risk management technique to identify the scenarios that present the most significant risk and determine if the consequences could be reduced by the application of inherently safer design principles.
- LOPA is used to understand how a process deviation can lead to a hazardous consequence if not interrupted by the successful operation of a safeguard called an independent protection layer (IPL).
- An IPL is a safeguard that can prevent a scenario from propagating to a consequence of concern without being adversely affected by either the initiating event or by the action (or inaction) of any other protection layer in the same scenario.
LAYERS OF PROTECTION
Layer 1: Process Design
Layer 2: Basic controls, process alarms, and operator supervision
Layer 3: Critical alarms, operator supervision, and manual intervention
Layer 4: Automatic action
Layer 5: Physical protection
Layer 6: Plant emergency response; and not shown
Layer 7: Community emergency response
LOPA can be represented mathematically using the following computational equation, which multiplies the frequency of an initiating event by the probabilities that each independent protection layer will fail to perform its intended function.
fIC = IEFi × PFDi1 × PFDi2 × …× PFDij
where,
fi C = Frequency of the consequence occurring for scenario i., Typical units are per year (Low Demand) or per hour (High Demand).
IEFi = Frequency of the IE for scenario i., Typical units are per year
PFDij = Probability of Failure on Demand of Independent Protection Layer J for scenario.
METHODOLOGY
The LOPA steps involved are:
(1)Identify impacts events, determine the type of impact (people, environment, property), and classify for severity.
(2)List of causes for each impact event.
(3)Estimate the frequency of each initiating cause.
(4)List independent protection layers for each cause – consequence pair.
(5)Determine the probability of failure on demand (PFD) for each IPL.
(6)Calculate the mitigated event frequency for each cause consequence pair by multiplying the initiating event frequency by the PFD for each applicable IPLs.
(7) Compare the mitigated event frequency to the criteria for tolerable risk.
RISK MATRIX
CONCLUSION
The LOPA method allows safety engineers to understand the risks of their processes, the independent layers of protection that are in place, and where additional risk reduction is needed to achieve tolerable risk. It allows for relative comparisons of the risks of different plants and processes. The LOPA methodology also points out the significance of the initiating event frequency and illustrates the benefits of basic process designs that apply principles of inherent safety.
