Fault Tree Analysis

Define system

Analyse Consequences

Hazardous Events

Continuing Hazards

Identify Hazards

Estimate Risks

Is Risk Tolerable?

RiskControlStrategy

Monitor

Fault Tree Analysis

A Fault Tree is a graphical display of

how the failures of the components in a

system can lead to a pre-defined

system failure state (Top Event).

Failure Tracing Methods

4 © Integra Training and Consulting

F A

Fault Tree Analysis – starts with

the top event (eg accident), asks

“Why”, work down until root

causes are revealed

Fault Tree Analysis (FTA)

• Based on deductive logic.

• Identify combinations of events (faults) resulting in a given outcome (loss).

• Systematic basis for qualitative and quantitative risk assessment.

• Combines hardware failures and human error in the same study.

• Useful technique for accident investigation and analysis.

Disadvantages

• Time consuming and complex.

• Need an experienced team.

• Some top events might be missed.

Examples:• Fire.• Steam flow less than X.• Chlorine release greater than Y.• Explosion.

“Top

Event”

After top event

• Ask “why” or “how” did this

happen

• Use “And” / “or”

• Keep going until the root causes

or basic faults are found

AND Gate

AND Gate: Output exists only if all input events have

occurred.

OR Gate

OR Gate: Output exists if any (or any combination) of the inputs are present.

Basic Fault

Basic fault or event:Tree Branch no longer

developed.

Undeveloped Event

Undeveloped event to due lack of information

or importance.

Tree Transfer

Tree to tree connection

Example Fault Tree

Top event

Example Fault Tree

Failure Probability

• Probability fails = 0:

– The event will not happen.

• Probability fails = 1:

– The event will always happen.

• 0.01 = 1 in 100 chance of failure.

Quantification OR Gate

Probability P1Probability P2

P=P1+P2 • Add probabilities

OR

Quantification AND Gate

• Multiply probabilities

Probability P1Probability P2

AND

P =P1xP2

Example

• For a pressure vessel to explode there must be a

source of excess pressure and a failure in the protective

systems.

• For a particular pressure vessel it has been established

that the excess pressure could arise from either a failure

in the control mechanism which switches off the pump

when the vessel reached the designated pressure or as

a result of a fire under the vessel.

• The protective systems comprise a pressure release

valve and a bursting disc.

Example

Draw a fault tree to represent the events leading the explosion of the pressure vessel.

Pressure vessel explosion

Explodes

And

Excess

pressureFailure in

protective systems

Or

Pump switch fails

Fire RV fail BD fail

And

Probability Data

Fire under the vessel 1x 10-4 ie 0.0001

Failure in pump control mechanism

2x 10-3 ie 0.002

Failure in pressure release valve

5x 10-4 ie 0.0005

Failure of bursting disc 1x 10-5 ie 0.00001

Calculate the probability of the pressure vessel exploding.

Pressure vessel explosion

Explodes

And

Excess

pressureFailure in

protective systems

Or

Pump switch fails

FireRV fail BD fail

And

0.00010.002

0.0001 + 0.002 = 0.0021

0.0005 * 0.00001 = 0.000000005

0.0005 0.00001

0.0021 * 0.000000005 = 1.05 × 10-11

Fault tree example