Logic trees example

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. 

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. 

Examples of logic trees—fault, event, causal, and why—are discussed below in order of increasing rigor. Chapter 9 contains detailed information on developing logic trees. 

In addition, the team should also update the in-progress analysis of the occurrence. For example, update the fact list, logic tree, or fact hypothesis matrix. Finally, the information from the interview must he communicated to the remainder of the investigation team. 

The next item in the loop is a decision point for possibly introducing the use of inductive reasoning methods into the deliberations. If the deductive process continues to indicate progress, then additional facts are procured or the logic tree is restructured. For example, one witness stated a particular valve was open, yet the post-incident inspection found it to be closed. The team must be careful to ensure that the valve is closed because of the actions taken prior to the incident, and not as a result of post-event response activities. The position of this particular valve may be a critical item in determining which of two scenarios is the more probable case. The incident investigation team would then initiate a short-term action item to conduct a mini-investigation to resolve this question. 

FIGURE 9-15. Example top of the logic tree, employee slip. 

FIGURE 9-16. Example logic tree branch level, oil spill. 

FIGURE 9-17. Example logic tree, hand-carried containers. 

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. 

An example of a decision logic tree which could be used when evaluating a workplace for use of air-purif3dng respirators is presented in these pages. 

In the example depicted in the logic tree in Figure 5.2, a site had a complete failure of the bottom of a fiberglass product tank and a resulting spill of product. The logic dialogue would have been ... 

Fig. 18 Example of a logic tree model (Adopted from Kramer (1996))...

Fault Tree Analysis. Fault trees represent a deductive approach to determining the causes contributing to a designated failure. The approach begins with the definition of a top or undesired event, and branches backward through intermediate events until the top event is defined in terms of basic events. A basic event is an event for which further development would not be useful for the purpose at hand. For example, for a quantitative fault tree, if a frequency or probabiUty for a failure can be deterrnined without further development of the failure logic, then there is no point to further development, and the event is regarded as basic. 

An example of a faiilt tree logic diagram using AND and OR gate logic is shown in Fig. 26-5. 

A logic model that graphically portrays the range of outcomes from the combinations of events and circumstances in an accident sequence. For example, a flammable vapor release may result in a fire, an explosion, or in no consequence depending on meteorological conditions, the degree of confinement, the presence of ignition sources, etc. These trees are often shown with the probability of each outcome at each branch of the pathway... 

The flat tire example is pictured using a fault tree logic diagram, shown in Figure 11-12. The circles denote basic events and the rectangles denote intermediate events. The fishlike symbol represents the OR logic function. It means that either of the input events will cause the output state to occur. As shown in Figure 11 -12, the flat tire is caused by either debris on the road or tire failure. Similarly, the tire failure is caused by either a defective tire or a worn tire. 

RuleMaker is a knowledge extraction utility for building and testing the decision logic contained within Radial modules. The logic is specified as a table of examples of correct expert decisions for each module. RuleMaker transforms each example set into an equivalent decision tree, and automatically generates the body of the module in the form of Radial code. System builders may also choose to enter Radial code directly, although they usually prefer to work with example tables. 

Fault tree analysis is based on a graphical, logical description of the failure mechanisms of a system. Before construction of a fault tree can begin, a specific definition of the top event is required for example the release of propylene from a refrigeration system. A detailed understanding of the operation of the system, its component parts, and the role of operators and possible human errors is required. Refer to Guidelines for Hazard Evaluation (CCPS, 1992) and Guidelines for Chemical Process Quantitative Risk Assessment (CCPS, 2000). 

Causal Trees were developed in an effort to use the principles of deductive logic found in Fault Tree but make it more user-friendly. Originally, private companies developed the Causal Tree Method (CTM) for safety, process safety, and environmental incident investigations applications. Rhone-Poulenc, for example, was an early user.<20.21) Multiple-Cause Systems Oriented Incident Investigation (MCSOfl) is another name for the CTM. At this time, most companies use simplified versions of fault trees for complex incident investigations. 

---