Interrelationship Among the Long-Term Wave Data Bases And Their Unification

                                                             by Yong-Ho CHOI

                                  ABSTRACT

     For the design of ships or offshore structures, a reliable database of long-term waves 
is very important. Nowadays this valuable information is mainly available in three forms: 
such as the visual data and hindcast data from the paper [Winds and Waves of the North Pacific 
Ocean 1974 - 1988] made by SRI (Ship Research Institute) in Japan, and GWS (Global Wave 
Statistics) data compiled and edited by British Maritime Technology Limited in United Kingdom. 

     Since there are many differences between those databases, such as the difference of the 
sea area subdivision, the distribution tendency according to the process of establishment, 
and the estimated results of the response according to the used database, there are many 
difficulties in determining which database should be chosen, and the designer must choose 
the appropriate database for each area. In the specific case of visual data where errors 
are larger, the data cannot be directly used for general purposes. But it can be said that 
observed significant wave height has relatively good reliance without regarding wave period, 
and to throw away the enormous visual data will be a great loss of information. Furthermore 
the visual data measurements are performed directly on the ocean site, and therefore deserve 
more attention.

     In this work, the long-term prediction and mutual transformation among the long-term wave 
databases were considered to give the basic information aiming at the unification of those 
different databases.


     This paper is organized as follows. Firstly after brief overview of characteristics of 
the three long-term wave databases, the frequency response functions of the wave external 
forces are estimated by using the NSM (New Strip Method) and principal dimensions of considered 
ship-types in this work. And then the short-term extreme estimation of the Midship Wave Bending 
Moment for every ship-types at the regular waves and the directional waves is performed. 
Using the long-term wave databases in the North Pacific Ocean, the long-term extreme estimation 
of midship wave bending moment is obtained. To continue, from the long-term prediction of 
extreme point of view, the difference among characteristics of databases is reviewed. 

     After obtaining the frequency response function of the ship motions for every ship-types 
by using the NSM, the short-term estimation of the Mean Added Wave Resistance at the regular 
waves and the directional waves are performed. Considering two sea routes (designated sea routes 
in this work) such as the PN sea route called the great-circle track route from Yokohama to 
St. Francisco in the North Pacific Ocean and Brunei sea route from Yokohama to Brunei in the 
Southeast Asia, the long-term mean added wave resistance and the Most Probable Extreme Value 
are obtained for every ship-types by using the long-term wave databases according to two 
designated sea routes. Using calculated results, from the long-term prediction point of view, 
the difference among characteristics of databases is reviewed. 

     And next, to investigate the conversion tendency among the databases, parameters, those 
represent the characteristics of wave mean period - significant wave height scatter distribution, 
are estimated based on wave period proposed by Kawabe et al. and based on wave height proposed 
in this work individually, and it is tried to estimate the mean-ratios and standard deviation-ratios 
between GWS data and visual data, and between hindcast data and visual data. The investigation 
shows that the possibility of the unification of the three databases exists from the similarity 
of the tendency between GWS data or hindcast data and visual data.

     After that, the interrelationship among the databases is described through the conversion 
of visual data in area subdivisions of the North Pacific Ocean given by SRI to corresponding 
area subdivisions given by GWS. Two conversion methods such as the Transfer Coefficient Method and 
the Transfer Matrix Method are proposed for the purpose of mutual relation among those databases. 

     Finally, comparing differences of long-term extreme value of the midship wave bending moment 
between given databases and calculated databases from visual data obtained by using transfer 
matrix method, a comparative consideration is done. Moreover, comparing differences of the 
most probable extreme value between given databases and calculated databases, the basic information 
of unification of those databases is investigated and such investigation will be the first time 
in this field.