MSCI231 Risk Assessment and Control
Seminar Tasks
Attempt both scenarios.
Scenario 1
A fault tree analysis of a hazardous operation shows that there are two basic ways in which the top event can occur. One involves a hazard plus the failure of a series of protective measures. The other is similar, except that each protective measure itself can fail in two possible ways. The basic events are shown as Bn.
Question 1
Which set(s) of basic events are sufficient on their own to cause intermediate event G1?
Question 2
Which set(s) of basic events are sufficient on their own are sufficient to cause intermediate event G6?
Question 3
How many set(s) of basic events, on their own, are sufficient to cause intermediate event G2?
You are told that the probability of each of the two hazard events B1 and B5 is 0.001, and that the probabilities of all other events are 0.1.
Question 4
What is the probability of the top event T?
Question 5
Suppose you could modify the risk by eliminating B7, B9 and B11. What would the probability of T now be?
A: 2 in 10
B: 2 in 1000
C: 2 in 100 000
D: 2 in 1 000 000
E: None of the above
Question 6
Suppose instead you could improve risk by adding a new event B12 (probability = 0.3) which is added as an extra input under the existing AND gate immediately below G2. What would the probability of T now be?
Question 7
Risk in a system is reduced by (explain):
A: Removing events under OR gates and adding events under AND gates
B: Removing events under AND gates and adding events under OR gates
C: Removing events under both OR gates and AND gates
D: Adding events under OR gates and adding events under AND gates
Scenario 2
A company operating a sports stadium has been asked by a regulatory organisation to estimate the risks of potential accidents involving major loss of life. It has identified crowd surge as the primary initiating event. It has also identified a number of safeguards that should operate in response to early signs of crowd surge:
• Marshalling action by its own gate staff
• Emergency full opening of the gates
• Police action to stop forward movement of the crowd
• Rapid evacuation from inside the stadium to relieve congestion near the gates
• Local emergency action by first aiders to deal with injuries
• Large scale emergency service response to deliver first aid and evacuate dead and seriously injured
These safeguards would be activated in this sequence. Estimates of the number of fatalities in the case of each of the safeguards being needed and succeeding has been estimated as follows (in the same order):
• 0
• 0
• 0
• 5
• 10
The worst-case scenario (where even large scale emergency response fails) would be around 100 deaths based on the worst disaster in the UK to date. Estimates of the probabilities of
each safeguard succeeding are as follows (in the same order):
• 0.5
• 0.7
• 0.4
• 0.5
• 0.7
• 0.8
The probability of the initiating event is estimated at .01 per annum.
Question 1
Construct an event tree and annotate with probabilities.
Question 2
Work out the probabilities of each scenario.
Question 3
If the regulator has stated that the maximum probability of an incident killing 10 or more people should be 1 in 10 000 per annum, are risks in this case acceptable?