Consequently, Event 5 can be excluded, leaving only cut sets 6–
10. Cut set 6 is considered unrealistic because it requires the failure
of the fans (Event 7) and the failure of the relevant control system
(Event 8) in addition to the operator’s inadvertent omission in
monitoring these failures. The remaining cut sets to be evaluated
include numbers 7–10. As far as Events 9 and 10 are concerned,
they are equivalent, and the distinction between the two situations
is irrelevant to the present case. In any case, Event 10 is considered
more believable as it is in greater agreement with the fact that mud
was present in the plant even in the 2 days preceding the accident.
At this point, the events that are yet to be analyzed include
Event 12 (failure of the operation block if the plant was in automatic
operation) and Event 13 (human error in blocking the feedback
if the plant was in manual operation). The situation is now
represented by Fig. 2, which is obtained from Fig. 1, using the information
obtained by the investigation. It is easy to check that the FT
of Fig. 2 yields the MCS nos. 9 and 10 only (note that most of the
gates in Fig. 2 are not necessary, but they have been left in order
to make easier the connection with Fig. 1).
Consequently, Event 5 can be excluded, leaving only cut sets 6–10. Cut set 6 is considered unrealistic because it requires the failureof the fans (Event 7) and the failure of the relevant control system(Event 8) in addition to the operator’s inadvertent omission inmonitoring these failures. The remaining cut sets to be evaluatedinclude numbers 7–10. As far as Events 9 and 10 are concerned,they are equivalent, and the distinction between the two situationsis irrelevant to the present case. In any case, Event 10 is consideredmore believable as it is in greater agreement with the fact that mudwas present in the plant even in the 2 days preceding the accident.At this point, the events that are yet to be analyzed includeEvent 12 (failure of the operation block if the plant was in automaticoperation) and Event 13 (human error in blocking the feedbackif the plant was in manual operation). The situation is nowrepresented by Fig. 2, which is obtained from Fig. 1, using the informationobtained by the investigation. It is easy to check that the FTof Fig. 2 yields the MCS nos. 9 and 10 only (note that most of thegates in Fig. 2 are not necessary, but they have been left in orderto make easier the connection with Fig. 1).
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