Rooting out Causes in Failure Analysis a report by Graeme Keith Special Projects Manager, Lloyd’s Register ODS


The Deepwater Horizon disaster was a terrible reminder of the consequences of equipment failure on facilities operating in challenging environments. Thankfully, catastrophes on the scale of the Deepwater Horizon are rare, but equipment failure is a daily occurrence on installations around the globe. The consequences range from short, unexpected downtime to a total stop on production; from a brief burst of flaring to lasting environmental damage; and from the momentary discomfiture of a worker to incapacity or death. Whenever equipment fails to meet expectations or fails altogether, we must understand what went wrong so that we can safeguard against it ever happening again. A good explanation not only helps to prevent a failure from reoccurring; it can help identify systematic weaknesses that might result in other failures.


To give a good explanation is to give a full account of the relevant causes of a failure. Sometimes these causes are obvious and clear to all; most of the time, however, they are not, even when they seem to be. This article discusses some of the methods investigators use when trying to identify and understand the causes of a failure and some of the issues they face when they use them.


There are a wide variety of methods and procedures for analysing the causes of failure, including the widely used fishbone or Ishikawa diagram, the appealingly simple ‘five whys’, the versatile fault tree analysis and its close cousin the causal map. Each method has its particular advantages and drawbacks. Many were developed for some particular sector or application and while they work very well on their home territory, they are not all as universally applicable as their advocates sometimes hope. All of these methods are essentially about mapping causes: identifying the immediate causes of a failure as well as the causes of those causes and so on. To borrow an example from the philosopher David Lewis: we have the bald tyre, the drunk driver, the blind corner, the approaching car. Each of these is a cause of the crash. But each of these causes has its own causes and these too are causes of the crash.


The Fishbone Diagram


The different methods emphasise different aspects of causal mapping. The fishbone diagram provides a useful categorisation, allowing investigators to focus on one category of possible causes at a time. The


Graeme Keith is Special Projects Manager at Lloyd’s Register ODS. Graeme joined ODS in 2001 from a research position at the University of Cambridge where earlier he wrote his PhD in applied mathematics. He is responsible for the development and teaching of ODS mechanical failure analysis tools and teaches on causality and explanation at the University of Copenhagen.


categories vary according to the application, but a typical list is: equipment, process, people, materials, environment and management.


Figure 1 shows the beginning of a fishbone diagram for David Lewis’ car accident example. The categories are drawn into a thick horizontal line, leading to the problem we are trying to explain. Drawn into these lines are the various causes identified in each category and into these lines may be drawn secondary causes, i.e. causes of causes.


The greatest attraction of the fishbone diagram is also its greatest weakness. While the categorisation provides clarity for the discovery of causes, it imposes unnatural restrictions on mapping the relationships between these causes. For example, we have identified the driver’s drunkenness as a cause in the category ‘people’. Clearly the poor man’s


Whenever equipment fails to meet expectations or fails altogether, we must understand what went wrong so that we can safeguard against it ever happening again.


depression may have contributed to his drunkenness, but there is no natural way to cross categories while passing down a chain of causes in the fishbone diagram. What is more, were we to enquire as to the reason for his depression, we should very quickly run out of space. Some more recent models of the fishbone diagram facilitate longer chains, but the fishbone diagram is severely limited for all but the simplest of problems.


Causal Mapping


Causal mapping dispenses with the categorisation and liberates the connections so that the relationships between causes can be made clearer and more instructive. Lewis’s example is shown in Figure 2, where the causal chain has also been extended beyond the crash to include the true cost of the failure in terms of safety, asset or business performance and environment.


Starting with these ultimate consequences, we work backwards through a series of ‘why’ questions, most of which will have several answers. Why did he crash? He skidded off the road. Why did he skid off the road? The road was icy. The tyre was worn. There was a car coming in the opposite direction, which he swerved to avoid. Why did he swerve? He saw the car too late and was in the middle of the road. Why did he see the car too late? Why was he in the middle of


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