Statistical Character of the Demand on Longitudinal Strength (Third Report)

Abstract

In the theoretical estimation of the long term distribution of the ship's wave induced variables, it is well known that the estimated extreme values are significantly affected by the supposed wave conditions and the ship operating conditions. However, there are few reports which investigate the statistical characteristics of the wave conditions. Furthermore, there is no report which concern with the varying of ship operating conditions of every each ship, which may appear as so-called the captain's spectra.Authors studied the statistical characteristics of the wave conditions which are observed by 35 voluntary ships in the North Pacific Ocean during the period of 6 years (1976-1981). From this study, it was clarified that meteorological phenomena such as winds and waves in the ocean had wider scatter band than which was estimate by the usual simple population with constant parameter. Authors proposed the probabilistic model of the population of such phenomena which had a complex distribution with random variable population parameters.In this report, authors study the statistical characteristics of the extreme values of wave induced variable of the ship with the supposed wave condition having the population of the complex distribution.The general formulation of the extreme distribution taking number of N samples from the population of the complex distribution is shown, and the asymptotic distribution is proposed. From the analysis of the distribution of the maximum wave height observed by 35 voluntary ships, it is found that the mean value of the distribution is proportional to the logarithmics of the sample size and the standard deviation is nearly constant. The properties of the statis-tical characteristics of the theoretical maximum distribution agree with the observed distribu-tion. On the other hand, the distribution of the usual simple population with constant parameter is found to be inaccurate.The maximum wave height and its probability of each wave period interval have direct effects upon the estimation of the extreme values of the wave induced variables. Authors' study on the distribution of the maximum wave height shows that the theoretical maximum distribution of the population with the complex distribution well agree with the observed maximum distribution of each wave period. From above result, it is clarified that the coefficient of variation of the maximum wave load is 14% to 18% and its value increase proportional to the ship length. This value is larger than the value from the usual supposed wave conditions of the population with constant parameters.