Fault and Event Tree Analysis Flashcards
How do major accidents result?
From combinations of events. We want to identify all combinations of minor events which produce a major unwanted failure.
How do we calculate the frequency of major events?
From the probabilities/ frequencies of minor events
What is the top event of a fault tree?
The major accident
Where do we put events that lead to the major event?
Below that for the major accident.
What is the procedure for fault trees?
- Identify the top event i.e. the major fault
- Work down the tree linking fault events with logic gates
- Once all branches are resolved down to basic faults, minimum cut sets can be identified
- frequencies of basic events are combined to calculate frequencies of cut sets
- Cut set frequencies are combined to give top event frequency
What are minimum cut sets?
The combination of events that can produce the top event
What are the two types of logic gates?
- AND gates
- OR gates
How do we progress through the tree via an AND gate?
All the faults leading to the gate must have to happen.
How do we progress through the tree via an OR gate?
At least one of the faults leading into the gate must happen
Which gate means that the accident is more likely to happen?
An OR gate. If there are three events with an AND gate, all 3 must happen to get the main event. If you have an OR gate it would be enough to have 1 to happen for the accident to happen.
What does a rectangular symbol represent on a fault tree?
An intermediate event, resulting from a combination of previous events
What does a circle symbol represent on a fault tree?
A basic fault event that requires no further development (spontaneous event). It may appear multiple times on a fault tree.
What does a diamond symbol represent on a fault tree?
The failure event.
What does a triangle symbol represent on a fault tree?
A transfer from one point to another in the tree indicated by a number. The triangle means that it is replicated.
What do letters indicate and what do numbers indicate on a fault tree?
Letters are usually used for gates and then a number for basic events.
In which direction should the table be continued with an and gate?
Horizontal
In which direction should the table be continued with an or gate?
Vertical
What is the aim of analysis of fault trees?
To replace each gate in turn with possible combinations of gates and basic events which make the gates “true”.
What is the analysis of an OR gate?
Replace the gate’s letter with the first input to the gate then make a new row for each of the remaining inputs, copying the other entries on the original row.
What is the analysis of an AND gate?
Replace the gate’s letter with the first input to the gate and then add all remaining inputs at the end of the original row.
What is a superset?
If a set already contains a set, we take them out.
What is the difference between fault and event trees?
Fault trees are constructed on a “top-down” basis starting with the major unwanted event and resolving this down to sets of basic events and faults.
Event trees are constructed from a “bottom-up” perspective, starting with a major failure with wide effects and working forwards from that event to all possible outcomes (not all of which will be bad)
In which directions are success and failure?
Success is vertically upwards and failure is vertically downwards.
Do event trees have logic gates?
No
What does one branch represent in an event tree?
A single action.
Event trees involve identifying the safety functions. What is a safety function?
Safety functions can act as barriers. These include systems such as trips, safety valves, alarms which alert the operator, operator actions,.
What does the event tree begin with?
The initiating event
What is the initiating event?
This should be a system or equipment failure, human error or process upset which will result in a variety of effects depending on the response of the system.
In what order is an event tree constructed?
Chronological order
What is the end of an event tree?
The outcome. Outcomes are described in terms of the state of the process at that point.
How can outcomes be grouped?
- Hazardous
- Non-hazardous
