Thus, for rational design of a mooring system, it is necessary to 

 consider (a) various sea severities (significant wave heights) expected 

 in the area where the system is located, and (b) various wave conditions 

 (spectra) in a given sea severity. Then the responses such as motions 

 and wave-induced forces of the system can be evaluated. The design 

 values can be determined by applying extreme value statistics to the 

 responses. Another way to evaluate the design value is to obtain the 

 responses to extreme wave heights for wave periods that are critical for 

 the system. For either approach, it is highly desireable to estimate the 

 extreme value responses expected to occur over a period of time sufficiently 

 long to cover the desired lifetime of the system. 



In general, there are two approaches for estimating the extreme 

 responses expected to occur in the lifetime of the mooring system, i.e., 

 the long-term prediction approach and the short-term prediction approach. 

 The former approach considers responses to all wave heights associated 

 with all sea conditions expected to be encountered by the system irre- 

 spective of their magnitude, while the latter approach considers responses 

 to wave heights within a specified sea condition (significant wave height) 

 taking into account the total exposure time during the system's lifetime 

 in that short-term sea condition. 



It may appear that estimation through the long-term prediction 

 approach is superior to that through the short-term prediction approach, 

 since it deals with the accumulation of responses to all waves. However, 

 in reality, the method of estimating the extreme value through the long- 

 term approach appears to be insensitive to the frequency of occurrence 

 of mild sea conditions, but is sensitive to the frequency of severe sea. 

 The magnitude of responses will not reach the level critical for the 

 system irrespective of how long time the system is operated in relatively 

 mild seas. While, the magnitude reaches the critical level within a few 

 hours when the system is in severe seas. Hence, the short-term appraoch, 

 which involves only the more severe sea conditions, is appropriate for 

 estimating the extreme values. The approaches for evaluating the extreme 

 responses of the mooring system are summarized in Figure 1. 



94 



