Kaplan 



II. SCOPE OF INVESTIGATION 



It is easily seen that there are a host of problems associated 

 with the subjects considered in this paper. As a result, some limi- 

 tations are Imposed so that only certain aspects are considered in 

 detail. The region of application of the results in this paper is in 

 deep water, so that no shallow water effects are considered. This 

 limitation thereby excludes problems of ship oscillation when 

 moored at docks in harbors, which is an important problem that can 

 be treated in a similar fashion to those herein by proper inclusion 

 of shallow water effects. The main emphasis within this paper is 

 on the seaway and its effects, and in some cases the influence of a 

 current will not be considered. However it is known that currents 

 are often present together with sea waves , and their combined effect 

 is often very important. In addition the presence of a current is 

 often necessary to establish certain static equilibrium conditions 

 for a vehicle about which the seaway disturbances are imposed, 

 and in that case certain assumptions are made as to the existence 

 of such initial conditions for purposes of simplifying the analysis. 

 Similarly, the presence of any wind effects is also not considered 

 in detail within this paper. 



When considering the problem of the motions of moored 

 systems, it is known that the effects of drift forces are also present 

 and that they produce an important influence on the resulting motions 

 and cable forces. However, in an effort to obtain tractable solutions 

 and to provide information on the characteristics due to different 

 force mechanisms, these effects will be considered separately. 

 Illustrations of the different influences that act on vehicles and 

 systems in a seaway will be presented separately, with some dis- 

 cussion given to the expectations with combined effects In a realistic 

 situation when more than one mechanism Is acting on a system. 

 The discussions of results are devoted to the more Innportant 

 phenomena Influencing performance of a system In the sea, and they 

 will be given throughout the paper for each case treated. 



III. TECHNIQUES USED FOR MOORED SHIP ANALYSIS 



In order to determilne the motions of a moored ship In Irregular 

 waves. It Is necessary to determine the response In regular sinu- 

 soidal waves. The aim Is to predict these motions, and the technique 

 to be utilized Is that of spectral analysis [ l] wherein the statistical 

 definition of the seaway in the form of Its energy spectrum Is used 

 as the Initial data. The energy spectrum of the time history of each 

 motion of the vessel In response to Irregular waves is evaluated for 

 the corresponding degrees of freedom to the energy spectrum of the 

 seaway. These operators are obtained from the solutions for the 

 motions in sinusoidal waves, and In accordance with the basic 

 premise of this technique of analysis, a linear theory of ship motions 

 Is a prerequisite. 



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