Layers of protection analysis explained

Layers of protection analysis (LOPA) is a technique for evaluating the hazards, risks and layers of protection associated with a system, such as a chemical process plant. In terms of complexity and rigour LOPA lies between qualitative techniques such as hazard and operability studies (HAZOP) and quantitative techniques such as fault trees and event trees.[1] LOPA is used to identify scenarios that present the greatest risk and assists in considering how that risk could be reduced.

Introduction

LOPA is a risk assessment technique that uses rules to evaluate the frequency of an initiating event, the independent protection layers (IPL), and the consequences of the event. LOPA aims to identify the countermeasures available against the potential consequences of a risk. An IPL is a device, system or action that prevents a scenario from escalating. The effectiveness of an IPL is quantified by its probability of failure on demand (PFD), in the range 0 to 1.[2] An IPL must be independent of the other protective layers and its functionality must be capable of validation.[3]

LOPA was developed in the 1990s in the chemical process industry but has found wider application.[4] In functional safety, LOPA is often used to allocate a safety integrity level to instrumented protective functions. When this occurs in the context of the analysis of process plants, LOPA generally leverages the results of a preceding HAZOP. LOPA is complementary to HAZOP and can generate a second in-depth analysis of a scenario, which can be used to challenge the HAZOP findings in terms of failure events and safeguards.

Layers of protection in process plants

Safety protection systems for process plant typically comprises eight layers:

!Layer of protection!Protection measure!Examples!Safeguards
Layer 1Process designDesign to standards, inherently safer design
Layer 2Basic controlsProcess controls, process alarms (yellow), operator supervision
Layer 3Critical alarmsProcess alarms (red), operator interventionPreventive safeguards
Layer 4Automatic actionsShutdown, emergency shutdown (ESD)
Layer 5Physical protectionRelief valves, rupture discs, fire and gas system
Layer 6Physical protectionFirewalls, dikes, berms, bunds (local containment)Mitigative safeguards
Layer 7Plant emergency responseEmergency response teams, muster, evacuation
Layer 8Community emergency responseWarning, evacuation, emergency services
LOPA is used to determine how a process deviation can lead to a hazardous event if not interrupted by an IPL.

The LOPA procedure

LOPA is a risk assessment undertaken on a 'one cause–one consequence' pair. The steps of a LOPA risk assessment are:

  1. Identify the consequences, using a risk matrix
  2. Define the risk tolerance criteria (RTC), based on the tolerable/intolerable regions on the risk matrix
  3. Define the relevant accident scenario, e.g. mechanical or human failure
  4. Determine the initiating event frequency, again using the risk matrix
  5. Identify the conditions and estimate the probability of failure on demand (PFD)
  6. Estimate the frequency of unmitigated consequences
  7. Identify the IPLs and estimate the PFD for each one
  8. Determine the frequency of mitigated consequences
  9. Evaluate the need for additional IPLs.
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Consequences
Effect on reputationNegligibleMarginalCriticalCatastrophic
Cost (indicative)$0.1m$1.0m$10m≥$50m
FrequencyImprobable1/100/yrLowMediumMediumSerious
Remote1/50/yrLowMediumMediumSerious
Occasional1/10/yrLowMediumSeriousHigh
Probable1/2/yrMediumSeriousHighHigh
Frequent1/½/yrMediumSeriousHighHigh

Other uses

Although the LOPA methodology started in the process industry, the technique can be used in other fields, including:

See also

Notes and References

  1. Book: CCPS . Layer of Protection Analysis: Simplified Process Risk Assessment . . 2001 . 978-0-816-90811-0 . New York, N.Y. . American Institute of Chemical Engineers.
  2. Willey . Ronald J. . 2014 . Layer of Protection Analysis . . 84 . 12–22 . 10.1016/j.proeng.2014.10.405. free .
  3. Book: Mokhatab, Saeid . Handbook of Natural Gas Transmission and Processing, Principles and Practices . Poe . William A. . Mak . John Y. . . 2019 . 978-0-12-815817-3 . 4th . Cambridge, Mass. and Oxford . 517–518.
  4. Web site: Kingsley . John . 2017-07-11 . LOPA or Layer of Protection Analysis . live . https://web.archive.org/web/20231215050550/https://www.linkedin.com/pulse/layers-protection-analysis-lopa-q-john-kingsley . 2023-12-15 . 4 September 2023 . LinkedIn.