Fault and Event Tree Analysis

Question 1

How do major accidents result?

Answer 1

From combinations of events. We want to identify all combinations of minor events which produce a major unwanted failure.

Question 2

How do we calculate the frequency of major events?

Answer 2

From the probabilities/ frequencies of minor events

Question 3

What is the top event of a fault tree?

Answer 3

The major accident

Question 4

Where do we put events that lead to the major event?

Answer 4

Below that for the major accident.

Question 5

What is the procedure for fault trees?

Answer 5

1. Identify the top event i.e. the major fault
2. Work down the tree linking fault events with logic gates
3. Once all branches are resolved down to basic faults, minimum cut sets can be identified
4. frequencies of basic events are combined to calculate frequencies of cut sets
5. Cut set frequencies are combined to give top event frequency

Question 6

What are minimum cut sets?

Answer 6

The combination of events that can produce the top event

Question 7

What are the two types of logic gates?

Answer 7

• AND gates

- OR gates

Question 8

How do we progress through the tree via an AND gate?

Answer 8

All the faults leading to the gate must have to happen.

Question 9

How do we progress through the tree via an OR gate?

Answer 9

At least one of the faults leading into the gate must happen

Question 10

Which gate means that the accident is more likely to happen?

Answer 10

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.

Question 11

What does a rectangular symbol represent on a fault tree?

Answer 11

An intermediate event, resulting from a combination of previous events

Question 12

What does a circle symbol represent on a fault tree?

Answer 12

A basic fault event that requires no further development (spontaneous event). It may appear multiple times on a fault tree.

Question 13

What does a diamond symbol represent on a fault tree?

Answer 13

The failure event.

Question 14

What does a triangle symbol represent on a fault tree?

Answer 14

A transfer from one point to another in the tree indicated by a number. The triangle means that it is replicated.

Question 15

What do letters indicate and what do numbers indicate on a fault tree?

Answer 15

Letters are usually used for gates and then a number for basic events.

Question 16

In which direction should the table be continued with an and gate?

Answer 16

Horizontal

Question 17

In which direction should the table be continued with an or gate?

Answer 17

Vertical

Question 18

What is the aim of analysis of fault trees?

Answer 18

To replace each gate in turn with possible combinations of gates and basic events which make the gates “true”.

Question 19

What is the analysis of an OR gate?

Answer 19

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.

Question 20

What is the analysis of an AND gate?

Answer 20

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.

Question 21

What is a superset?

Answer 21

If a set already contains a set, we take them out.

Question 22

What is the difference between fault and event trees?

Answer 22

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)

Question 23

In which directions are success and failure?

Answer 23

Success is vertically upwards and failure is vertically downwards.

Question 24

Do event trees have logic gates?

Answer 24

No

Question 25

What does one branch represent in an event tree?

Answer 25

A single action.

Question 26

Event trees involve identifying the safety functions. What is a safety function?

Answer 26

Safety functions can act as barriers. These include systems such as trips, safety valves, alarms which alert the operator, operator actions,.

Question 27

What does the event tree begin with?

Answer 27

The initiating event

Question 28

What is the initiating event?

Answer 28

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.

Question 29

In what order is an event tree constructed?

Answer 29

Chronological order

Question 30

What is the end of an event tree?

Answer 30

The outcome. Outcomes are described in terms of the state of the process at that point.

Question 31

How can outcomes be grouped?

Answer 31

• Hazardous

- Non-hazardous