Safety Moment #6: Qualitative Fault Trees

Qualitative Fault Trees in the Process and Energy Industries


  • Overview
  • Further Information


Safety Moment #31: the 26-Year Old HAZOP discusses the need for new methods for hazards analysis that can help generate fresh insights as to how incidents may occur. Techniques such as Layers of Protection Analysis (LOPA) and Bow-Tie method that have been introduced in recent years provide different and fresh ways of understanding risk.

Another technique that is well established, but not used much in the process industries, is Fault Tree Analysis (FTA). It's strictly logical approach to risk analysis provides a useful complement to the more commonly used methods such as HAZOP (Hazard and Operability Study). A description of the FTA approach is provided in the article Fault Tree Analysis, the ebook Frequency Analysis and in the book Process Risk and Reliability Management.)

The Fault Tree method was developed in the aerospace and nuclear power industries. One of the key documents do with FTA was the Reactor Safety Study, published in the year 1975 (Rasmussen 1975). The report, often referred to by it index title, WASH 1400, had its limitations, as became apparent following the Three Mile Island incident. Nevertheless, the method was instrumental in introducing risk management techniques such as Probabilistic Risk Analysis, not only to the nuclear power industry, but to industry in general.

In spite of its ability to generate useful insights into the nature of risk, the fault tree approach does have four limitations that tend to restrict its use in the process and energy industries.

  1. It is difficult for those not thoroughly trained in the technique to work out the logic as to how failures may occur, particularly when common cause effects or the failure of safeguards is considered.
  2. It is generally difficult to find trustworthy failure rate data for equipment, instrumentation and human actions.
  3. The development of a fault tree is often time-consuming and requires the services of costly experts and consultants.
  4. 4. The methodology, and the results that it generates, can be hard for non-experts to understand.

It is, however, possible to use the logical approach of fault tree analysis without needing to spend as much time and money as a formal analysis requires. The simplified approach is known as Qualitative Fault Tree Analysis (QFTA).

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Further information to do with the topic of this Safety Moment is available in the ebook 52 Process Safety Moments.

You are welcome to use this Safety Moment in your workplace. But there are restrictions — please read Use of Safety Moments.

Copyright © Ian Sutton. 2018. All Rights Reserved.