Root cause logic trees

This chapter addresses methods and tools used successfully to identify multiple root causes. Process safety incidents are usually the result of more than one root cause. This chapter provides a structured approach for determining root causes. It details some powerful, widely used tools and techniques available to incident investigation teams including timelines, logic trees, predefined trees, checklists, and fact/hypothesis. Examples are included to demonstrate how they apply to the types of incidents readers are likely to encounter. 

The disciplines of engineering and quality control have long recognized the principles of root cause analysis. Some process safety tools for root cause analysis have been borrowed from these disciplines. For example, fault tree analysis was developed as an engineering tool, but its logic tree structure has been adapted to meet process safety requirements. 

In general, the companies surveyed use one of two main methodologies to determine root causes. The first involves timeline construction followed by logic tree development. The second involves timeline construction, identification of causal factors, followed by the use of predefined trees or checklists. These two approaches are discussed in detail in Chapter 9. 

Logic trees are committee-based investigation toois that use a muitipie cause, system-oriented approach to determine root causes integrated with process safety management program. Exampies fauit tree, event tree, causai tree, and why tree. 

Checklists may also be used to supplement other tools for example, checklists on human factors may be used in conjunction with logic trees. Similarly, checklists may be used in combination with structured brainstorming tools such as What If/Checklist and Hazard and Operability (HAZOP) Analysis.(P It is also a good practice to apply a tool like the 5-Whys to the root causes identified from the checklist to verify whether they are truly root causes. 

Another type of logic tree, the event tree, is an inductive technique. Event Tree Analysis (ETA) also provides a structured method to aid in understanding and determining the causes of an incident.(i) While the fault tree starts at the undesired event and works backward to identify root causes, the event tree looks forward to display the progression of various combinations of equipment failures and human errors that result in the incident graphically. 

Figure 9-1, the two flowcharts describing root cause determinations using Methods A and B, presents general frameworks for root cause determination. Method A focuses on the logic tree method using a simplified fault tree approach. Method B focuses on the predefined tree method. 

Root Cause Determination Using Logic Trees—Method A... 

FfGURE 9-9. Flowchart for root cause determination using logic trees. 

Unlike the procedure followed in developing logic trees, the investigation team does not construct the tree. Rather they apply each causal factor to each branch of the predefined tree in turn, and those branches that are not relevant to the incident are discarded. This prescriptive approach offers consistency and repeatability by presenting different investigators with the same standard set of possible root causes for each incident. 

Predefined trees are relatively easy to use and generally require less training and effort to conduct root cause analysis tban logic trees. 

The use of checklists to supplement another root cause analysis method can be a very powerful technique, for example, human factors checklist(s) may be used in conjunction with logic trees. The checklist may be used as a guide during development of a logic tree, or as a check after the tree has been developed. The checklist essentially acts as a memory jogger to direct the investigation team. This is especially helpful if the team lacks previous experience in the subject matter. 

The following case study describes the investigation work process for a hypothetical occurrence using a logic tree based multiple root-cause systems approach. An example incident investigation report follows the work process description. The example is intended for instructive purposes only descriptions of process equipment and conditions are not intended to reflect actual operating conditions. 

---