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284 Chupfer 16

Fault Tree Analysis
Event Tree Analysis
0 Synthesis Models
0 Monte Carlo Simulations
0 Equipment Failure Rate Databases

16.43 Fault Tree Analysis
The fault tree is a graphical diagram of logical connections between events and conditions,
which must be present if an initiating event should occur. A fault tree for a system can be
regarded as a model showing how the system may fail or a model showing the system in an
unwanted situation. The qualitative analysis maps systematically all possible combinations of
causes for a defined unwanted event in the system. If available data can be supplied for the
frequencies of the different failure causes, quantitative analysis may be performed. The
quantitative analysis may give numerical estimates of the time between each time the
unwanted event occurs, the probability of the event etc.

The Fault Tree Analysis (FTA) has three major phases:

1. Construction of the Fault Tree: this is the identification of combinations of failures and
circumstances that may cause failures or accidents to occur.

2. Evaluation of the Fault Tree: this is the identification of particular sets of causes that
separately will cause system failure or accident.

3. Quantification of the Fault Tree: this is overall failure probability assessment from the sets
of causes as defined above.
16.43 Event Tree Analysis
An event tree is a visual model for description of possible event chains, which may develop
from a hazardous situation. Top events are defined and associated probabilities of occurrence
are estimated. Possible outcomes from the event are determined by a list of questions where
each question is answered yes or no. The questions will often correspond to safety barriers in
a system such as “isolation failed?” and the method reflects the designers’ way of thinking.
The events are partitioned for each question, and a probability is given for each branching
point. The end events (terminal events) can be gathered in groups according to their
consequence to give a risk picture.

16.5 Probability of Initiating Events

16.5.1 General
The methods stated above gives a methodology which can be applied to any scenario such
that it is possible to determine the conditions which will result in an initiating event. However,