Layers of protection analysis

A 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 layer (IPL), and the consequences of the event.[2] LOPA aims to identify the counter measures 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 ILP 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]

Layers of Protection

Safety protection systems for process plant typically comprises eight layers.

Layers of Protection for process plant[2]
Layer of protection Protection measure Examples Safeguards
Layer 1 Process design Design to standards, Inherently safer design
Layer 2 Basic controls Process controls, process alarms (yellow), operator supervision
Layer 3 Critical alarms Process alarms (red), operator intervention Preventive safeguards
Layer 4 Automatic actions Shutdown, emergency shutdown (ESD)
Layer 5 Physical protection Relief valves, rupture discs, fire and gas system
Layer 6 Physical protection Fire walls, dike, berm, bund (local containment) Mitigative safeguards
Layer 7 Plant emergency response Emergency response teams, muster, evacuation
Layer 8 Community emergency response Warning, evacuation, emergency services

LOPA is used to determine how a process deviation can lead to a hazardous event if not interrupted by an IPL.[2]

The LOPA procedure

LOPA is a risk assessment undertaken on a ‘one cause - one consequence’ pair. The steps of a LOPA risk assessment are:[4]

  1. Identify the consequences, see Risk Matrix
  2. Define the risk tolerance criteria (RTC), the tolerable/intolerable region on the Risk Matrix
  3. Define the relevant accident scenario, e.g. mechanical or human failure
  4. Determine the initiating event frequency, see 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.

The LOPA technique can be used to determine the safety integrity level (SIL) for an instrumented safety system.[1]

Risk Matrix frequency and consequences[2]
Consequences
Effect on reputation Negligible Marginal Critical Catastrophic
Cost (indicative) $0.1m $0.3m $0.6m $1.2m
Frequency Improbable 1/100/yr Low Medium Medium Serious
Remote 1/50/yr Low Medium Medium Serious
Occasional 1/10/yr Low Medium Serious High
Probable 1/2/yr Medium Serious High High
Frequent 1/½/yr Medium Serious High High


LOPA is complementary to HAZOP and can generate a second in depth analysis of a scenario. This could be used to challenge the HAZOP findings in terms of failure events and safeguards.[3]

Other uses

Although the LOPA methodology started in the process industry the technique can be used in other fields. These include:[4]

  • General design
  • Management of change
  • Facilities siting risk
  • Mechanical integrity programs
  • Incident investigations
  • Screening tool for Quantified Risk Assessment (QRA)

See also

References
  1. CCPS (Center for Chemical Process Safety) (2001). Layer of protection analysis: simplified process risk assessment. CCPS (Center for Chemical Process Safety). ISBN 978-0-816-90811-0.
  2. Willey, Ronald J. (2014). "Layer of Protection Analysis". Procedia Engineering. 84: 12–22.
  3. Mokhatab, Saeid (2019). Handbook of Natural Gas Transmission and Processing, Principles and Practices 2019 (fourth ed.). pp. Chapter 17. ISBN 978-0-12-815817-3.
  4. Kingsley, John (4 July 2017). "LOPA or Layer of Protection Analysis". Retrieved 4 September 2023.
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