Review of the Prince William Sound, Alaska, Risk Assessment Study (1998) / Chapter Skim
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2 Models Used in the Prince William Sound Study
2 Models Used in the Prince William Sound Study
Pages 10-18
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From page 10... ...
; fault tree analyses, also developed by DNV; and a dynamic simulation model, develope |
From page 11... ...
One of the leading practitioners of this approach, Nonnan Rasmussen, has cautioned that relevant data are necessary for fault trees to be effective: Fault tree analysis is a technique used to predict the expected probability of failure of a system in the absence of actual experience of failure .
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From page 12... ...
· 6 based on actual data · 6 based on estimates or calculations · 3 deemed not applicable to PWS The NRC committee believes that describing this as true fault tree modeling is erroneous and misleading because there was no logical analysis relating basic events to accidents. It might have been better if the PWS Study team had used only the top blocks and labeled the estimates expert judgments instead of implying that a real fault tree analysis had been done.
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From page 13... ...
The traffic mode} also includes weather calculations updated hourly and imposes closure conditions in both the Narrows and the Hinchinbrook Entrance when winds are greater than 45 knots, in which case the inbound tanker drops anchor and the outbound tanker circles. At the heart of the simulation are the probabilities of incidents and the conditional probabilities of accidents, given an incident.
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From page 14... ...
Event trees represent these probabilistic dependencies; fault trees are logical tools that allow computing some of the failure probabilities shown in the event trees. The order of the vanables in event trees is somewhat arbitrary and depends, in practice, on how the information is structured.
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From page 15... ...
, they used a common traffic image and oil outflow model, and they relied on expert judgments made by members of the of} and maritime community, who, it may be presumed, shared many expenences, had similar knowledge of industry literature, had similar cultural biases, and communicated with each other. The analyses were also based on shared knowledge of the worldwide body of scientific and analytic literature.
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From page 16... ...
But the conservatism and accuracy of the modeled events were not documented. Sensitivity Analysis and traceability The PWS Study contains no sensitivity analyses or discussions of uncertainties, with the exception of the uncertainty in the fault tree modeling of powered grounding in the Narrows.
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From page 17... ...
Limitations of the Analytic Approach The NRC review committee found that the fault identification logic was both questionable and redundant. Because event frequency values were generally assigned and used at the highest level in each tree chain, the fault trees do not provide real logical analyses.
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From page 18... ...
By way of explanation, the PWS Study team stated that the modeling of of} outflow probability was adequate because (~) the available residual steerage way in the case of propulsion failure makes evasive maneuvers possible, thus lowenng the probability of grounding, and (2)
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