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FORMAL SAFETY ASSESSMENT OF A FISHING VESSEL 41

Fault Tree Analysis is a logic diagram showing the causal relationship between events. It is
used to determine the probability of a top event which may be a type of accident or unintended
hazardous outcome. Fault trees can include failure events or causes related to human. Therefore,
they are useful for undestanding logically how an accident occurred and the probability of a top
event can be calculated taking into account the failure probabilities of system components.
The development of a fault tree is by a top-down approach considering the causes or events at
levels below the top level systematically. To carry out an analysis, a description of how the
output states of each component are influenced by the input states and of how the components
are interconnected. Therefore, a functional layout diagram of the system must show all
functional interconnections.
If two or more lower events cause the next higher event, this is shown by a logic .and. gate. If
any one of two or more lower events causes the next higher event, this is shown by a logic .or.
gate. The logic gates determine the addition or multiplication of probabilities (assuming
independence) to obtain the values for the top event (IMO Circ. 1023). Construction, usually,
starts with the top event which would normally be the major hazard such as ship loss, and works
down towards the basic events. The event symbols are rectangle, circle, diamond and triangle.
The rectangle resresents a fault output event which results from a combination of basic faults
and/or intermediate events acting through the logic gates. The circle is used to designate a
primary or basic fault event. The diamond describes fault inputs that are not basic events but
considered basic fault input since the cause of the fault has not been further developed due to
lack of information. The triangle is not strictly an event symbol but is traditionally classified as
such to indicate a transfer from one part of a fault tree to another.
The fault tree does not offer an exact solution to the problem. The main purposes of this
approach are to provide the systematically derived information that makes a rational decision
and to design safe fishing vessels.
As a preliminary model of the fault tree shown in Fig. 3 considering the accident category
„collision‟. The fault tree analysis for a fishing vessel is given in details as an example in Köse,
E. at al (1998).

Sayı 1, 2015 GiDB|DERGi