The Fault-Tree Analysis: Still A Relevant Risk Assessment Tool?

First formulated in order to assess the probability of failure of fairly complex systems over thirty years ago is the fault-tree analysis still applicable in current assessment of today’s complex systems?


By: Ringo Bones


Even though there’s a growing perception of the general public’s declining trust in risk management since the September 11, 2001 terror attacks, it is quite ironic to ponder that risk assessment has further matured since that tragic event. As long as they had been around or had been tenured by insurance companies, risk analysts often start by dividing hazards into two parts – namely exposure and effect. Even though they’ve had it down to a science, insurance companies cannot yet predict whether any single driver will be killed or injured in an accident, even though they can estimate the annual number of crash-related deaths and injuries in the United States with considerable precision. With the salient point in the development of risk assessment during the past three decades had been in large part the search of ways to determine the extent of risks that have very little precedent. Like the link between cellular phone / mobile phone use and brain tumor risks and a more objective assessment of catastrophic global warming risks.

Risk assessments of complex systems are more often than not defined by the enumeration of failure modes. A common technique called failure mode and effect analysis where risk analysts try to identify all the events that might lead to a system breakdown. Usually when all the failure modes have been enumerated, the fault-tree analysis has been routinely used since the last 35 years or so, as an aid to estimate the likelihood of failure of any given mode.

First utilized on a large scale by Norman C. Rasmussen of the Massachusetts Institute of Technology back in 1975 to study nuclear reactor safety. Although specific details of his risk assessment estimates were disputed under peer review, fault-tree analyses are now routinely used in the nuclear industry. As a rule, a fault tree graphically represents how the subsystems of a larger system depend on one another and how a failure of one part affects key operations. Once a particular fault tree of a particular system is constructed, one need to only estimate the probability that once individual elements do fail, the same probability governs the set of circumstances that lead to the entire system’s failure to function.

Due to the method’s good track record of formulating more effective risk mitigation while reducing costs in its implementation, the plane-maker Boeing had been for sometime now been applying fault-tree analysis in the design of large aircraft. Company engineers have identified and remedied a number of potential problems in passenger aircraft design. Such as vulnerabilities caused by routing multiple control lines through the same area, which can be a recipe for disaster during a bird-strike incident. Even though it is already too late for their chemical plant in Bhopal, India, Union Carbide had also employed the technique in designing processes for chemical plants. Particularly in deciding where to situate their plants and in evaluating the risks of transporting particular chemicals. But as a risk assessment tool, is fault-tree analysis still relevant today?

Maybe Barbara Ehrenreich was right for lambasting the overly-optimistic and cavalier attitude of Wall Street when it comes to risk assessment because the fat-cats had never discussed using fault-tree analysis to examine the vulnerability of the global financial system’s propensity to failure. As a fairly complex system, fault-tree analysis could have been used to examine the global financial system’s failure modes that could have averted the widespread collapse of banks and other financial institutions deemed to big to fail back in 2008.

In our eternal struggle for the search for an effective carbon neutral energy source, fault-tree analysis could be used to assess the risks of constructing more nuclear fission power plants in comparison to the global warming risks posed by coal-fired power plants. As the only feasible carbon neutral electricity generating power plant that is here right now, a renewed risk assessment of nuclear fission technology deserves reevaluation until we can find something better that truly works.