We utilized Fault Tree Analysis to consider the rationality about the agreement between achievement level of functions before the trouble and 3 behaviour patterns after the trouble.

　

5.1 Construct the Fault Tree

　

 Shown as Fig. 5, we constructed the Fault Tree about the extraordinary situation in Fig. 4.

 The top phenomenon is "unsafe navigation in extraordinary situation". Under this phenomenon, 3 kinds of phenomena follow with OR gate. If one, of these phenomena happened, the situation would grow up to "unsafe navigation in extraordinary situation".

 "Insufficient Extraordinary Reaction" under the top phenomenon would come,, if "Occurrence of trouble" and "Insufficient time for reaction" came at the same time. On this Fault Tree, there is plural "Occurrence of trouble" at different places. Other phenomena are the same. Thus Fault Tree can be constructed.

 "Insufficient Positioning" under the top phenomenon would come, if both "Insufficient Positioning after trouble" and "Insufficient Positioning before trouble" happened. Then again, "Insufficient Positioning after trouble" would come, if either "No Positioning after trouble" or "Positioning after trouble is impossible" happened. Still more, "Positioning after trouble is impossible" would come if both "No time for Positioning because of Extraordinary Reaction" and "No time / No speed reduction" happened.

 "Insufficient recognition on other ships" under the top phenomenon can be constructed m the same way as "Insufficient Positioning". Under this phenomena, phenomena concerned Lookout follows.

　

　

5.2 Analysis on the Fault Tree

　

 We analyzed unsafe navigation in extraordinary situation based on the Fault Tree.

　

5.2.1 Fundamental phenomenon

　

 This Fault Tree has 8 fundamental phenomena. These are bottom phenomena on this tree. These fundamental phenomena are defined as following from x1 to x8.

　

　

5.2.2 Minimal Cut Set and Structure Function

　

 Minimal cut set is the set of fundamental phenomenon, which is necessary and sufficient for occurrence of top phenomenon.

 The minimal cut sets of this Fault Tree are 5 kinds as following.

　

　

 From these minimal cut set, we could obtain the structure function, Ö(x) formula, to calculate the fault tree structure importance.

　

　

 We can obtain each fault tree importance of these fundamental phenomena using formula (1).

　

5.2.3 Occurrence Rate of Top Phenomenon

　

 If we defined each occurrence rate of phenomena as following, we could obtain the g function, g formula, to calculate the occurrence rate of top phenomenon.

　

　

 Therefore, if we could obtain the concrete value for these, rates, we can calculate the occurrence rate of top phenomenon.

　

5.2.4 Fault Tree Structure Importance

　

 The Fault Tree structure importance indicates how much each phenomenon influences on the top phenomenon. This Fault Tree has 8 fundamental phenomena. Table 2 shows the results for each fundamental phenomenon.

　

　

 The higher the value of Fault Tree structure importance is, the more the fundamental phenomenon influences on the top phenomenon occur.

　

 Shown as table 2, there is a large difference between 8 fundamental phenomena from 0.039 to 0.367.

　

 The highest importance is "occurrence of trouble", it is a matter of course.

　

 The following high importance, O.304, was indicated at two phenomena, "Insufficient Positioning before trouble" and "Insufficient Lookout before trouble". These are the highest importance except "occurrence of trouble". It means that these phenomena influence on "Unsafe navigation in extraordinary situation" largely. There calculation results of Fault Tree structure importance shows that Positioning before trouble and Lookout before trouble are important. Positioning and Lookout should be always kept doing in ordinary situation. A decline of these functions causes unsafe navigation in extraordinary situation.

 Therefore this result can become a reason for the agreement between achievement level of Positioning and Lookout functions before the trouble and 3 behaviour patterns after the trouble.

　

 "No Positioning after trouble" and "No lookout after trouble" follow with 0.226. In consideration of the importance of "Insufficient Positioning before trouble" and "Insufficient Lookout before trouble", the influence of both Positioning and Lookout after trouble does not change. But then again, it is thought that this is no more than adding fuel for the acceleration up to more unsuitable situation, because of the phenomenon after trouble. It is also thought that both "Insufficient Positioning before trouble" and "Insufficient Lookout before trouble" cause unsuitable situation after trouble.

　

 phenomena concerned with Positioning and Lookout.

 Compared with above-mentioned phenomena, "No time by speed reduction" and "Insufficient recognition because of no communication" brings nothing but a little influence.

　

5.3 A Sign of Fault Tree Structure Importance on Behaviour

　

 We considered these influences of fundamental phenomena on 6 cases of simulator experiments.

 Fig.6 shows the comparison about frequency in use of 6 categories among 3 behaviour patterns. Case A is from behaviour pattern 1, case C is from behaviour pattern 2, case E is from behaviour pattern 3.