Guidance for Performing an Effective LOPA – Part One

Posted on 31/01/2019

By Dr Esteban Bernechea, PhD FS Eng (TÜV Rheinland)

Dr Esteban Bernechea

Introduction

Layers Of Protection Analysis (LOPA) is a technique that can be used to determine or allocate the Safety Integrity Level (SIL) of a Safety Instrumented Function (SIF) according to IEC-61511 [1] Part 3.

Guidance for Performing an Effective LOPA

The technique is based on the principle that all of the Independent Protection Layers (IPLs) designed to prevent a scenario must fail at the same time for it to occur. In order for the hazardous consequences of the scenario to manifest, several Conditional Modifiers (CMs) might also have to coincide with the failure of all IPLs. This is the principle applied to calculate the demand rate on the SIF (before CMs) and the Intermediate Event Frequency (considering CMs). This can then be divided by the risk target selected for the scenario in order to determine the Probability of Failure on Demand (PFD) or Probability of Failure per Hour (PFH) and the SIL requirement of the function.

LOPA is normally performed after completion of a Process Hazard Assessment (PHA) or process Hazard & Risk Assessment (H&RA), as a brainstorming exercise by a team of professionals that have expertise on the plant (or system) under analysis. Due to its semi-quantitative, brainstorming nature, LOPA can suffer from several pitfalls that can negatively impact its results. The consequences of a poorly performed LOPA can be significant over the life of a process plant, whether due to overestimation or underestimation of the requirements of the SIFs. In the worst case, it can cause the plant to operate in an unsafe state, while in the best case, it can lead to over expenditure during the lifecycle of the plant.

This paper presents and explains simple tips that users can apply at the moment of performing LOPA in order to avoid potential pitfalls that can lead to poor results and a myriad of issues during the lifecycle of the Safety Instrumented Systems (SISs).

The tips are related to the various stages of a LOPA study as presented in Figure 1, from the preparation to the completion of sessions and management of results:

• Preparation is key
• Calibrate and define risk targets
• Select a competent team
• Always keep traceability in mind
• Time it right
• Specify hazardous consequences
• Justify the initiating events, as well as their frequencies
• Be aware of potential dependency issues between IPLs
• LOPA is a live study

In the opening part of this work, the first tip is expanded; other tips will be further developed in the next parts.

1. Preparation is key

LOPA is no different to any other study or project in that preparation prior to the sessions is paramount for success.

One of the key aspects when preparing for a LOPA is gathering the required process safety documentation. This information relates to PHA study(es) that should be performed in advance of LOPA, as well as the engineering documentation that is necessary to carry out the study workshop in an appropriate manner. A Terms of Reference (ToR) document or company procedure should also be prepared prior to the LOPA workshop, in order to define the rules and assumptions that will be applied during the study, as well as clarifying the roles and responsibilities of each of the parties involved in the study.

A common misconception when of performing SIL determination is to use the representative Risk Graphs / models presented in IEC 61508 [3] and IEC 61511 [1], which are not specific to any process or organization and should not be used in real applications. Definition and calibration of methodologies, including graphical methods (risk graph / risk matrix) should be undertaken by competent people, with an understanding of the hazards associated to processes that will be assessed.

Table 1 presents a list of typical required documentation, emphasizing which documents are required for the preparation of the LOPA workshop and which ones will be required during the study.

Once the ToR document has been issued, and the rest of the required documentation is available, the preparation centres in the LOPA team studying the scenarios and SIFs to be analysed during the workshop.

In case the minimum required documentation is not available, it is advisable to postpone the workshop sessions, in order to avoid performing an incomplete study, which could result in the requirement for further meetings and project delays.

Part two will be available next week.

If you have any questions or would like to leave us feedback please use the comments box below.

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