Almost all of the shape parameters for machinery and main diesel engine parts are considered range between 1.0〜1.9, except in the case of crank shaft more than ten years of age, turbochargers more than seven years of age, and shafting equipment more than ten years of age. Although these shape parameters of 1.0〜1.9 indicate that major failures occurred randomly, major failure of crankshafts more than ten years of age, turbochargers more than seven years of age and shafting equipment more than ten years of age can be understood to have occurred due to the influence of constructive defects such as degradation of machinery. It can be further supposed that the chain of mistakes caused by human factors, as shown in Fig.2, could not be broken under conditions where the source of constructive defect could not be found through normal daily or periodically maintenance.
Table.1 Shape parameters of Weibull Distribution of major failures during 1993 - 1998
Fig. 9 Weibull Distribution of each major failure categorized by error mode
Fig.9 shows Weibull Distribution of each error mode for 499 major failures, it is principally understood that Weibull Distribution for human error should have shape parameter of about 1.0 because failure caused by human error may be assumed to occur randomly. Factors depending upon constructive defect may be supposed to be stronger than human factors if the shape parameter has a deferent value from about 1.0.
Table.2 shows value of shape parameters of each Weibull distribution categorized with error mode. It is supposed from Table.2 that errors of commission at more than six years of age and errors of omission at more than ten years of age occurred in large part due to the strong impact of constructive defects. Such defects can be supposed to have gradual increasing failure type caused by degradation of system performance due to such factors as wear and tear or corrosion, or the like.
However, Fig.9 and Table.2 show that major failures caused by errors of commission occurred randomly on ships less than six years of age while major failure caused by errors of omission occurred randomly on ships less than ten years of age. It can be supposed that preventing human error may be more effective in reducing the occurrence of majors failure in such cases.
A list of major human factors affecting failures and accidents onboard ship is shown in Table.3. This list was derived from a study done using a questionnaire to Japanese shipowners. Table.3 shows major human factors from the perspective of preventing failures and accidents in the engine room which had been caused in ninety-six actual cases of failures of machinery in the engine room. It is generally recognized that human factors can be categorized into four types as summarized below [5];
(1) Personal factors: cases where problems arise due to the failings of individual workers;
(2) Management factors: cases where problems exist in management systems;
(3) Work characteristics & environmental factors: cases where problems arise due to special characteristics of the work or environment conditions; and
