System Safety for the 21st Century, 2nd Edition

CHAPTER 19Management Oversight and Risk Tree

Bill Johnson developed MORT or the management oversight and risk tree in the 1970s as part of his overall systems safety approach and systems safety effort for the Department of Energy, at that time the Atomic Energy Commission. The MORT chart contains approximately 1,500 base items arranged into a large and rather complex fault tree that is primarily used for accident investigation.

The symbols used on the MORT chart are basically those used for other analytical trees (Chapter 11) and fault tree analysis (Chapter 16). They include the rectangle as the general event symbol, the circle as the base event symbol, the diamond as an undeveloped terminal event, the and gate, the or gate, and the ellipse as a constraint symbol (Figs. 19‐1 and 19‐2).

In addition, Bill Johnson used a scroll as a “normally expected event” and an oval as a “satisfactory” event. The normally expected event distinguishes events that are typically a part of any system, such as change and normal variability. The satisfactory event describes events that may be accident causal factors but are a necessary part of the operation, like “functional” (part of the system) people or objects in the energy channel. Also, in addition to using the traditional transfer symbol (a triangle), the MORT chart includes capital letters as drafting breaks and small ellipses as risk transfers (Fig. 19‐3).

Risk transfers are found in two places. They are found next to undeveloped terminal ...