National Academies Press: OpenBook
« Previous: E Uncertainty, Sensitivity, and Bayesian Methods
Page 199
Suggested Citation:"F Event Sequence Diagram Method and Risk ScenarioDevelopment." Transportation Research Board. 2009. Risk of Vessel Accidents and Spills in the Aleutian Islands: Designing a Comprehensive Risk Assessment - Special Report 293. Washington, DC: The National Academies Press. doi: 10.17226/12443.
×
Page 199
Page 200
Suggested Citation:"F Event Sequence Diagram Method and Risk ScenarioDevelopment." Transportation Research Board. 2009. Risk of Vessel Accidents and Spills in the Aleutian Islands: Designing a Comprehensive Risk Assessment - Special Report 293. Washington, DC: The National Academies Press. doi: 10.17226/12443.
×
Page 200
Page 201
Suggested Citation:"F Event Sequence Diagram Method and Risk ScenarioDevelopment." Transportation Research Board. 2009. Risk of Vessel Accidents and Spills in the Aleutian Islands: Designing a Comprehensive Risk Assessment - Special Report 293. Washington, DC: The National Academies Press. doi: 10.17226/12443.
×
Page 201
Page 202
Suggested Citation:"F Event Sequence Diagram Method and Risk ScenarioDevelopment." Transportation Research Board. 2009. Risk of Vessel Accidents and Spills in the Aleutian Islands: Designing a Comprehensive Risk Assessment - Special Report 293. Washington, DC: The National Academies Press. doi: 10.17226/12443.
×
Page 202

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

APPENDIX F Event Sequence Diagram Method and Risk Scenario Development The event sequence diagram method is a simple and powerful mod- eling tool for developing possible risk scenarios. It enables visualiza- tion of the logical and temporal sequence of causal factors leading to various states of the system. Figure F-1 illustrates the event sequence diagram method. The figure depicts the change of state of a vessel initially operating within the “safe functional/physical zone” (shaded area). At Point A, an event (e.g., equipment failure) occurs, causing deviation from the normal operating zone and putting the vessel in an undesired state (Point B). Another event (e.g., crew recovery action) is initiated at that point, and depending on whether it suc- ceeds or fails, the vessel returns to the safe zone (Point C), or an acci- dent occurs (Point F). The sequence of events from A (the initiating event) to the end states (C or F) forms two simple scenarios. These scenarios provide the context within which the events and their causes are evaluated as potential hazards or sources of risk. In event sequence diagramming, a set of graphical symbols is used to describe the various elements of a scenario. Figure F-2 shows a simple event sequence diagram. The diagram starts with a circle sym- bolizing the initiating event or condition. The possible events or con- ditions (rectangles in the diagram) that can follow the initiating event are then listed in the proper temporal or logical order, connected by lines, forming various possible strings or sequences of events that ultimately end with a diamond symbol representing their end states. Pivotal events have a single input line and a (Yes/No) pair 199

200 • Risk of Vessel Accidents and Spills in the Aleutian Islands Undesired state F (failure/accident) Event causing deviation B from normal operation Recovery (initiating event) A Normal C Operation FIGURE F-1 Event sequence diagram method. of output lines, depending on whether the pivotal event occurs (Yes output) or otherwise (No output). The same applies in the case of conditions where Yes means the condition is satisfied and No means the opposite. An event sequence diagram, therefore, is a visual representation of a set of possible risk scenarios originating from an initiating event. Each scenario consists of a unique sequence of occurrences and non- occurrences of pivotal events (Point B or C in Figure F-1). Each sce- nario eventually leads to an end state, which designates the severity of the outcome of that scenario. Yes Yes No No Yes No FIGURE F-2 Event sequence diagram concept.

Event Sequence Diagram Method and Risk Scenario Development • 201 Figure F-3 is an example of a simple event sequence diagram where, given the occurrence of the initiating event, the state of System 1 (a pivotal event) determines whether the sequence leads to success (end state S), when it works, or a human action is required, when it fails. Given the success of human action, another pivotal event (state of System 2) will determine the final outcome: success state (S) if System 2 works or failed state (F) if it fails. The failure of human action also leads to failed state F. Therefore, this simple event sequence diagram depicts four possible risk scenarios, two leading to success and two leading to a failed state (accident). Event sequence diagrams are extremely versatile and can be used to model many situations, ranging from the behavior of purely static systems to that of many types of dynamic systems. Historically, event sequence diagramming has been loosely defined and has been used in a variety of industries for different purposes. It has been used in probabilistic risk analyses by the nuclear power industry to develop and document the basis for risk scenarios, as well as to communicate risk assessment results and models to designers, operators, analysts, and regulators. Event sequence diagrams have also been used in the aviation industry as part of safety and reliability analyses of aircraft systems. The National Aeronautics and Space Administration has used event sequence diagrams to help identify accident scenarios. In all three applications, the diagrams have been used both qualitatively for identification of hazards and risk scenarios and quantitatively to determine probabilities of risk scenarios. SYSTEM 1 S Initiating Human Event SYSTEM 2 S Action F F FIGURE F-3 Simple event sequence diagram.

202 • Risk of Vessel Accidents and Spills in the Aleutian Islands External Safeguard Safeguard Safeguard Influences Failures Failures Failures Hazards Accidents Safeguard Conse- Incidents (marine Effects Failures quences casualties) Causes External External External External Influences Influences Influences Influences FIGURE F-4 Characterization of elements of marine risk scenarios. Developing scenarios that can be analyzed efficiently requires good engineering knowledge, extensive experience in systems operation, and familiarity with modeling. Figure F-4 characterizes the main aspects of marine scenarios. The scenario begins with an initiating cause, for example, a fire, flooding, or adverse weather. What happens next is a sequence of events that represents the response of the “system” (the ship, its hardware and software, its crew) to the cause and the safeguards in place (barriers, operational controls, and risk control options). The cause can be controlled by the first safeguard; if not, failures may occur (hardware failures, human and organiza- tional failures, or failures caused by environmental stressors). This sequence of events either is arrested or leads to an accident that can have immediate consequences, such as loss of life, physical damage to the ship, or a spill of hazardous materials. If a spill is involved, the scenario continues through transport and deposition of the material in the environment. Mitigation measures (additional safeguards) can limit the damage before environmental and subsequent eco- nomic and social consequences accrue. Remediation measures can limit harm to life in the area by cleaning up the contamination.

Next: G Sample List of Salvage Incidents near Dutch Harbor »
Risk of Vessel Accidents and Spills in the Aleutian Islands: Designing a Comprehensive Risk Assessment - Special Report 293 Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

TRB Special Report 293, Risk of Vessel Accidents and Spills in the Aleutian Islands: Designing a Comprehensive Risk Assessment, provides guidance for a comprehensive risk assessment of vessel accidents and spills in the Aleutian Islands. The report examines data related to the risk of oil, chemical, and other hazardous cargo spills from vessel traffic through the Aleutian Islands and identifies key information needed to conduct a comprehensive risk assessment.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!