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Domino Effect Evaluation Software Tool
A software tool was developed, implementing the above described risk assessment procedure. The tool is intended to provide a rapid criticality ranking of process units and can act as a decision support system for plant safety managers. The tool was developed in MS Visual Basic language, and is supported by a relational database developed in an MS SQL Server environment. The model has a graphical user interface (GUI) where process units are shown superimposed on a plant area map. The GUI is also used to input equipment data in an interactive manner (see figure 4) and can show the damage areas of selected units (Figure 5). The sequences of all possible domino chains of accidents starting from a selected source unit can also be shown on the map.
Figure 4 — Data Input Window.
Figure 5 — Example of Damage Areas Computation.
In a separate window, the program shows the computation results (Figure 6). In particular, the program shows, for each unit, the absolute values of the DSI and DTI indices. A relative value with respect to the highest-ranking process unit within the plant is also shown. Results are color coded according to the risk level for a rapid guess of the most critical units. Finally, the DEP value is also shown.
For example, Figure 6 shows a case where four units have a DSI value of medium criticality (yellow), while as far as the DTI index is concerned, only two of them have the same criticality range, with only one unit having a high criticality (orange). The overall DEP value is 142. Figure 7 shows a risk simulation after some mitigation measures (including vessel cooling, inert gas supply, remote control valves, drainage system, leak detectors and sprinkler system) were applied to the most critical units. The effectiveness of the adopted measures is demonstrated by the DEP value being reduced to 84, and no unit having a DSI or DTI value above the moderate risk level.
Figure 6 — Ranking Results Window Before Mitigation.
Figure 7 — Ranking Results Window After Mitigation.
Conclusion
In this paper, an improved method for the rapid rating of the domino effect potential in industrial plants has been presented. The method is based on three kinds of rating indices. One index describes the potential of each process unit of being the initiator of a chain. Another index expresses the potential of each process unit to be involved in an accident initiated by other units, thus generating a secondary accident and triggering the domino effect. An overall index expresses the likelihood that a domino effect will occur in the examined plant. A software tool implementing the proposed model was also developed and briefly described.
This domino effect rating assessment method adopts a simplified analytical procedure for ease of use and enables a "rough cut" assessment of the domino scenario to assess whether a more detailed analysis is required and to perform a preliminary screening of process units criticality. Furthermore, the method may help plant managers to correctly select risk mitigation measures for the most critical process units. However, the method is by no means intended as a substitute for a more detailed traditional quantitative risk assessment.
Finally, it should be noted that this new method does not aim to assess the actual probability of a domino effect, but rather to rate the susceptibility of a plant or specific pieces of equipment to a chain of accidents, provided that a primary accident has occurred.
About the Author
Antonio C. Caputo, Ph.D., received his Master's degree in mechanical engineering from the University of Roma, La Sapienza, in 1991. In 1995, he earned a Ph.D. in mechanical engineering and joined the engineering faculty at the University of L'Aquila, Italy, where he is currently a full professor in industrial and plant engineering. In 1991-92, he served as a volunteer in the Italian National Corps of Firefighters. His primary research interests include industrial safety, design of manufacturing systems, and production planning and control.
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