Safety Moment #6: Qualitative Fault Trees

Qualitative Fault Trees in the Process and Energy Industries

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. For example, 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 all that 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 three 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. A second difficulty is that it generally difficult to find trustworthy failure rate data for equipment, instrumentation and human actions.
  3. Finally, the development of a fault tree is often time-consuming and requires the services of costly experts and consultants.

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). It uses fault tree logic as a means of understanding risk and failure mechanisms, but it is simpler than a full, formal analysis. The logic need not be as precise and/or accurate as it would be in a full analysis, and there is little or no quantification.

Not only is the technique useful by itself, it can be used in coordination with other types of analysis. For example, a Qualitative Fault Tree resembles the left-hand side of a bow-tie analysis.

Specific benefits of QFTA are:

  1. It allows the process hazards team to visualize the logic as to how system failures can occur. In doing so, it gets around the “I think/You think” problem that can be a challenge when using techniques such as HAZOP.
  2. It helps identify common cause effects (see the article Two Too Many Common Causes and Safety Moment #9: Let’s Not Make Common Cause for a discussion of this issue). 
  3. It helps the team understand the power of safeguards, and what happens if those safeguards are removed (discussed in Ford’s Missing AND Gate).

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