5.4 Structural importance

　

 Some of the basic phenomenon have deeper concerning to the occurrence of the top phenomenon, compare with the other basic phenomenon. Structural importance is not depends on the rate of incidences for each basic phenomenon, but only the structure of the tree. That means structural importance can be used for analysis of accident without actual rate of incidence for each basic phenomenon. Structural importance of each basic phenomenon is shown in table 3.

　

 According to table 3, it is found that the importance of captain's function is much higher than the importance of 2nd and 3rd officer's function in ship handling technique . And the importance of communication and cooperation is as same as ship handling technique in each process as collecting information, deciding the way of avoiding, action for avoiding. In addition, the importance of function of cooperation in the process of making decision is higher than that in the process of collecting information.

　

5.5 Rate of incidences in basic and top phenomenon

　

 In the previous chapter, importance of each basic phenomenon in numerical value is analyzed from the viewpoint of the structure of the tree. In this chapter, quantitative prediction for an accident of collision is carried out. To predict the incidence of collision, it is necessary to obtain the actual rate of incidences for each basic phenomenon.

　

 Author analyzed 30 records of maneuver in the same scenario of avoiding other vessel, performed by experienced captain, navigator and quartermaster. In the scenarios, each member of the Team is assessed based on the guideline for Bridge Team. An example of guideline for Bridge Team is shown in table 4. As a result, actual rate of incidence for each phenomenon is obtained. Table 5 shows the rate of incidence for each phenomenon.

　

　

　

　

 Based on the obtained rates of incidences for each phenomenon, incidence of collision is predicted as 5.168E-04, with assumed incidence for target ship (x1) 1.000E-01. Meanwhile, incidence of collision without function of communication and cooperation is predicted as 1.770E-02.

　

　

 From the result of above-mentioned predictions, it can be said that function of communication and function of cooperation act on reducing the incidence of collision. In the same way, some marine casualties are analyzed utilizing Fault Tree and the results show the effect of Bridge Team that reducing the incidence of top phenomenon.

　

　

 According to above mentioned analysis, the effectiveness of team play is confirmed in the viewpoint of safety navigation. In the next chapter, author discus about the way to control the incidence of top phenomenon efficiently. In other words, to prevent the top phenomenon, which incidence should be reduced by priority, is discussed.

　

5.6 Probability importance and criticality importance

　

 Structural importance depends on only the structure of Fault Tree. Probability importance takes not only the structure but also the incidence of the phenomenon into consideration. So probability importance indicates the effect for incidence of top phenomenon caused by increase or decrease in incidence of basic phenomenon. In other word, it is possible to reduce the incidence of top phenomenon efficiently, if incidence of basic phenomenon that has high probability importance can be reduced.

　

 Furthermore, it is more efficient for the large incidence of basic phenomenon to decrease, than small incidence of basic phenomenon, Criticality importance is the ratio between percentage change in incidence of basic and top phenomenon. Table 6 shows probability importance and criticality importance of each basic phenomenon.

　

 According to table 6, probability importance of x2,x4,x5 and criticality importance of x4 and x5 is relatively larger than the others. The criticality importance of x2 is not large though its probability importance is high, because of reflecting the small incidence of x2.

　

 From the view point of these importance, it can be said that to compensate the mistake of captain in the process of making decision for avoiding, is one of the most efficiently points to prevent occurrence of the top phenomenon of collision.

　

 As a result of Fault Tree Analysis, the effectiveness of organizing Bridge Team is confirmed. Also, essential process for Bridge Team in accomplishing task is grasped.

　

 In above-mentioned study, the following results were obtained ;

　

1) The function of Bridge Team is composed of three functions as the function of ship handling technique, the function of cooperation and the function of cooperation.

　

2) Captain shares a large part in function of ship handling technique. On the other hand, function of communication and cooperation is assigned to all member of the team equally. And the functions of communication and cooperation have same structural importance as the function of ship handling technique.

　

3) To prevent collision efficiently, it is effective to check mistakes in the process of making decision and taking action.

　

4) According to the results of Fault Tree Analysis, the incidence of top phenomenon can be reduced by organizing Bridge Team.

　

[1] KOBAYASHI HIROAKI, et al, (1997) Development of Ship handling Techniques into Elemental techniques, in The Journal of Japan Institute of Navigation, Vol 96, pp 119 - 125.

　

[2] KOBAYASHI HIROAKI, et al, (2000) New Standards of MET using Ship Handling Simulator, Proceedings of MARSIM 2000, Orland, Florida, USA, pp. 159-171.

　

[3] ENDO MAKOTO, et al (2001) New MET System Based on Essential Training Concept, Proceedings of ODRA 2001, Szczecin, Poland.

　

[4] KOBAYASHI HIROAKI (2002) The Condition on Occurrence of Maritime Accidents and Bridge Team Management. Proceedings of Japan and Korea Workshop on Marine Simulator and Simulation Research, pp 1- 8.

　

[5] NIKKAN KOGYO SHINBUN-SHA (1979) FTA safety engineering, pp 69 - 100.

　

 Graduated from the faculty of merchant ship. Kobe University of mercantile Marine in 1990.

　

 Third, second and chief mates of training ships of National Institute for Sea Training from 1990 to 2000.

　

 Master Mariner and Associate professor of National Institute for Sea Training, at present time.