WAVES FOR MOORING SYSTEM DESIGN 

 Michel K. Ochi 



INTRODUCTION 



The statistical prediction of responses of ships and ocean structures 

 in a seaway has become common practice in design following the technology 

 developed on the modern probabilistic approach. The prediction of motions 

 and associated forces of a mooring system is not an exception. 



The probabilistic prediction of responses of a mooring system can be 

 carried out in either the time domain or the frequency domain. In either 

 case, application of the linear superposition principle which is often used 

 in predicting responses of ships and ocean structures in a seaway may not 

 be applicable for the mooring system because of strong nonlinear behavior 

 of the system in a seaway. Apart from the nonlinear characteristics of 

 the system, wave information is required as an input (excitation) in 

 applying the probabilistic approach to estimate the responses for design 

 consideration. 



In most design methodologies of a marine system, in general, fairly 

 heavy emphasis is placed on the prediction of extreme wave heights and the 

 corresponding responses that are expected to occur during the system' s 

 lifetime as a result of these waves. Since the response of a system in a 

 seaway is frequency dependent , both wave height and wave frequency (or 

 period) should be considered. This may be of particular importance for 

 the design of a mooring system. More specifically, it is necessary to 

 estimate extreme wave heights for periods that are critical for the moor- 

 ing system. These estimates can be obtained from the joint probability 

 function of wave height and period. However, before this methodology is 

 applied, we have to consider the moored system to encounter various sea 

 severities which are most commonly expressed in terms of significant wave 

 height. Furthermore, in a given sea condition, the mooring system will 

 encounter an infinite variety of wave conditions which are represented by 

 wave spectral shapes. Hence the variability of wave conditions for various 

 sea severities and the frequency of occurrence of these wave conditions has 

 to be reflected in the prediction. 



