This suggests that it may be possible to prevent the majority of cases of failures or malfunctions caused by human error by improving the work management system and work characteristics/ environmental conditions of work onboard ships.
4. Summary
On a study using data based on 499 major failures of machinery in the engine room of ships registered in ClassNK which has reported by ClassNK's surveyors during the period 1993 - 1998 as failure after which the subject ship had to be towed by tug or operate at reduced speed, the cause of failure was selected automatically from assembling error, design error, fatigue, insufficient maintenance, fire etc., operation error, vibration and poor workmanship considering failed condition and environmental condition from the viewpoint that the failure had occurred after degradation of system performance as a result that crews does not carry out required tasks properly; for example, "insufficient maintenance" was selected with "omission error" when the failure could be considered to be avoided if enough maintenance and check had been carried out periodically. The result is as follows;
(1) It may be supposed that 53% of the major failures reported occurred as a result of insufficient maintenance while 33% were due to careless operation
(2) Given that human error occurs randomly; the shape parameter m of cumulative failure rate of failures caused by human error may be about 1.0, This may be understood to mean that major failures on ships less than about ten years of age may be due in large part to human error mainly. However, the occurrence of the major failures onboard ships over ten years of age may be recognized as being caused by constructive defects that resulted from degradation of system performance due to wear and tear corrosion or the like. This is because all shape parameters are about 1.0 - 2.0 for ships less than ten years of age but over 2.0 for ships more than ten years of age in Fig.8 and Table.2
(3) From this study on major failure, it may be supposed that errors of both commission and omission occurred with same ratio, however these percentages may be subjected to modification in the event of a more detailed examination of the process of error.
(4) Cumulative failure rate of major failures caused by errors in commission as seen in Weibull Distribution tends to be of a random failure type for ships less than six years of age. Hence, it may be supposed that more attention should be paid to errors in commission in the case of ships less than six years of age. Moreover, attention must be paid to errors of omission for ships less than ten years of age because of the shape parameter of 1.1.
(5) It may be supposed that source of constructive defects caused by degradation of system performance will disturb to break the chain of mistakes caused by human factors for ships over about ten years of age that are subject only to daily and periodical maintenance.
(6) Failures and malfunctions caused by problems in management systems accounted for more than 40% of the total number of cases reported, while work characteristics and environmental factors account for 30%, personal factors account for 20%, and human error related machinery factors represented less than 10% of cases. It can thus be seen from the Table.3 that about 70% of failures and malfunctions in engine rooms occurred either as a result problems with the work management system or with work characteristics and environmental conditions.
5. Conclusion
From a viewpoint of preventing failure on machinery in engine room, a study was carried out to determine the characteristics of human error which are supposed to occur daily. It may be recognized from the result of this study that major failures of machinery onboard ships are mostly attributable to human factors for ships less than about ten years of age. On the other hand, constrictive defects may have a greater impact than human factors on the occurrence of major failures in the case of ships over about ten years of age.
Additional future study will be pursued in order to seek which characteristics of human factors and methods that can have the greatest impact on breaking the chain of mistakes caused by human factors under the existence of source of constructive defects.
Reference
[1] K. SATANI; Journal of Japan Society for Safety Engineering, Vol.38, No.6, 1999, Page380
[2] T. MIYAMURA; Journal of Japan Society for Safety Engineering, Vol.36, No.5, 1997, Page329.
[3] Instruction books for reliability engineering, for example, H. SHIOMI; Guide to Reliability Engineering, Maruzen.
[4] H. FUJIMORI; "Statistical Evaluation of the Reliability of Diesel Engines". Journal of the Japanese Society of Marine Engineer. Vol.30, No.6. P.426.
[5] Isao KURODA, "Approach to Human Factors in the Safety Management", Journal of the Japanese Society of Marine Engineer, Vol.26, No.5. P.262, 1987.
