Analytical trees

Analytical trees can be used in a variety of ways in the system safety effort. The most common application of analytical trees in current system safety programs is probably the use of fault trees for fault tree analysis (FTA). However, analytical trees can also be used as planning tools, project description documents, status charts, and feeder documents for several hazard analysis techniques (including fault tree analysis). Analytical trees can be multipurpose, life cycle documents and represent one of the most useful tools available to managers, engineers, and safety professionals. 

Analytical trees are nothing more than graphic representations (Fig. 10-1). They are literally pictures of a project. Analytical trees use deductive reasoning that is, they start with a general top event or output event and develop down through the branches to specific input events that must occur in order for the output to be generated. 

Trees are called trees because, basically, they have a structure that resembles a tree, that is, narrow at the top with a single event symbol and then branching out as the tree is developed. 

There are two basic types of analytical tree. The positive tree, or objective tree, which is developed to ensure that a system works properly, and the negative tree, or fault tree, which is generally used for troubleshooting and to investigate system failures (Fig. 10-2). 

Positive or objective trees are extremely useful planning tools. In the early stages of a project, they can be used to outline project requirements and list alternatives. As decisions are made, they evolve into graphic checklists and also make excellent status charts and project description documents. 

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Buys, R. J. Standardization Guide for Construction and Use ofMORT-Type Analytical Trees. Idaho Falls, ID System Safety Development Center. Idaho National Engineering Laboratory 1977. (ERDA 76-45/8)... 

Fault tree analysis is many times requested because of complex hazards identified by a HAZOP review. Engineering has some people trained in analytical tree development and analysis and they are responsible for conducting these type studies. 

Input data for preparation of the PHL include project description documents (narrative descriptions, sketches, preliminary drawings, artists concepts, drawings or photographs of existing similar products, and analytical trees), historical data (safety, reliability, and loss data and accident reports from similar end products and/or components), and any other relevant studies or research. 

Figure 10-2 Analytical trees. (Derived from the student workbook from the Management Oversight and Risk Tree Workshop presented for the Department of Energy by the System Safety Development Center.)...

Analytical trees display clear thinking. By graphically putting a project on paper, the project engineer ensures that he or she has looked at all the alternatives and understands project requirements. 

Analytical trees force individuals to use the deductive analysis process and to think about the events that must occur at lower levels for output events to be generated. 

Analytical trees show how relationships and interfaces occur during a project. 

Root causes are sought and identified when analytical trees are used for accident analysis. 

Critical paths, that is, the key events or key elements within a particular project, can be determined by the use of analytical trees. 

Analytical trees make excellent systems status boards. Because the tree is a graphic representation of the project, the current status is readily depicted by color coding the tree or simple checking off completed events or items. 

Analytical trees also provide a basis for rational decision making, especially early in the concept stage when they characteristically have a number of alternatives and or gates. 

There are seven basic steps in constructing an analytical tree. 

To construct an analytical tree, the first thing to be done is to define the top event. For an objective or positive tree, define what you want to design, build. 

The second requirement in preparing an analytical tree is to know the system. In and of itself, this requirement is an excellent reason for using analytical trees because it provides reviewers and other parties the assurance that the individual project engineers or others involved with the project do, in fact, know the system well enough put the project on paper logically and systematically in the form of an analytical tree. At times, particularly for consultants, this step may be very time-consuming because it requires considerable research. 

Sequence from the left. Many times analytical trees are not specifically concerned about sequencing, but a natural sequence or a sequence that must be followed should be indicated with a constraint symbol, and the events should be sequenced from the left to right. 

Analytical tree construction can be accomplished with only seven different symbols, even though some programs like MORT include a few other special purpose symbols. 

TTiree event symbols are used in preparing analytical trees (Fig. 10-4). The general event symbol is a rectangle. The rectangle is the main building block for analytical trees. It is used throughout analytical trees as the event symbol, except in two cases. 

A simple example may be helpful in understanding the role of analytical trees as important life cycle, multipurpose documents. The top event or goal, for the example, is to obtain a home computer system (Fig. 10-11). 

Analytical trees make excellent project description documents because they are relatively easy to construct and easy to understand. Many end users and some safety professionals, engineers, and managers are not skilled in reading blueprints, electrical schematics, or other technical documents. The analytical too is an excellent means for communicating project information within a system safety working group. 

Analytical trees are also very useful as feeder documents for several hazard analysis techniques, for example, failure mode and effects analysis (Chapter 14), fault tree analysis (Chapter 15), energy trace and barrier analysis (Chapter 13), and project evaluation tree analysis (Chapter 16), the primary hazard analysis tools for many projects. Virtually any analytical technique or any type of analysis can be simplified by starting with the analytical tree as a base document. 

Construct an analytical tree. The tree can be on any subject and either a positive tree or a negative tree. Include at least four tiers in its longest branch, not including the top event as a tier. 

Preliminary drawings or sketches may be adequate to prepare a preliminary hazard list. More detailed drawings are required for a preliminary hazard analysis, and even more detail is required for subsystem and system hazard analyses. Analytical trees, copies of maintenance and operating procedures (if available), and site maps may also be helpful. 

The functional FMEA requires less detail and can be performed with the upper branches of an analytical tree. The requirement is to identify subsystems and their functions, not details of subsystem configuration or design. 

First, collect the input documents and reference resources. The primary input documents include an analytical tree and/or block diagram, drawings, and narrative descriptions of the system. The most important resources are a data base, lessons learned file, historical data, and/or other source(s) of failure rates or other reliability statistics. The preliminary hazard list (PHL), preliminary hazard analysis (PHA) and any other hazard analyses should also be available. 

The scope is easily defined if an analytical tree is used as a feeder document (Chapter 10). The indenture levels can be identified by specifying the number of tiers of the tree to be included in the analysis (limits of resolution). A functional FMEA includes upper and/or middle tiers only a hardware FMEA includes the entire tree. 

If an analytical tree is not used, in addition to defining the system and clarifying the scope in terms of breadth (limits of resolution) and depth (indenture levels), a block diagram or other project description document may be required. 

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