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Is it necessary to install a downhole safety valve in a subsea oil/gas well?
5.3 Fault Tree Analysis
The construction of the fault trees is based on the theory given in [8].
Fault tree analysis is a deductive technique that focuses on a particular unwanted system event
and provides a method for determining causes for that event. A risk analysis often includes the
fault tree analysis technique for evaluation of the individual component failure modes and their
impact on the system reliability.
The fault tree is constructed with the use of different logic gates and displays the
interrelationships between a potential critical event in a system and the reasons for this event.
The main logic elements are the ‘TOP’-event, the ‘AND’ and ‘OR’ gates, and the basic events.
The combination of the basic events and the system structure determines whether or not the
‘TOP’-event will occur.
The fault tree provides a static picture of the combinations of failures and events that may cause a
‘TOP’-event to occur. Fault tree analysis, as barrier diagrams, is thus not a suitable technique for
analysing dynamic systems.
5.3.1 Case example
In the case example the ‘TOP’-event is “leakage to the surroundings”. The ‘TOP’-event occurs
either if there is a “leakage to the surroundings from the x-mas tree” or “leakage to the
surroundings from the wellhead”. A more concise definition of the ‘TOP’ event is:
“Sustainable leakage to the surroundings through either the x-mas tree or the wellhead during
normal shut-in conditions.” This ‘TOP' event covers situations where the barrier combination
in one or more cut sets have failed.
The different components of the production well presented in subsection 5.1.2, are represent
basic events in the fault trees. The barrier diagrams, of subsection 5.2.1, provide the basis for
the fault tree construction. In order to simplify the construction of the fault tree, external stress
and common cause failures are not included.
The fault trees constructed for the oil/gas production well with and without a DHSV are
presented in appendix C. The CARA-fault tree program is used in the construction.
Diploma thesis, NTNU 2002
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