given in Equation (3) is included in Figure 2. As can be seen in the 

 figure, the probability distribution thus derived represents well the 

 observed data over the entire range of the cumulative distribution function. 

 The magnitudes of significant wave height most likely to occur in 10 years 

 and 50 years are estimated from the distribution function and are shown in 

 Figure 3. 



WAVE SPECTRA 



The sea condition in a specified sea severity (significant wave height) 

 varies considerably depending on the geographical location, duration and 

 fetch of wind, stage of growth and decay of a storm, and existence of swell. 

 Hence, the question always remains as to how realistic the predicted marine 

 system's responses are if we use commonly available simple spectral formu- 

 lations which have been developed for some idealized conditions. Since a 

 mooring system encounters a variety of wave conditions even though the 

 significant wave heights are the same, the variability of wave conditions 

 (wave spectra) has to be reflected in the prediction. 



One way to cover a variety of spectral shapes which the mooring 

 system may encounter in a sea is to use a series (family) of wave spectra 

 consisting of several members for any specified sea severity. 



A significant benefit obtained by using a family of wave spectra for 

 predicting responses of a marine system in a seaway is that, for each sea 

 severity, one of the family members yields the largest response, while 

 another yields the smallest response with a statistical confidence 

 coefficient of 0.95, for example. Hence, by connecting the largest and 

 smallest values obtained in each sea severity, we have the upper and lower 

 bounds of responses. The results of computation made on wave-Induced 

 forces of an offshore structure have Indicated that the bounds reasonably 

 cover the variation of responses computed by using spectra measured at 

 various oceanographlc locations in the world (see Figure 4) (Ochl 1978) . 



The following are the families of wave spectra developed for open and 

 fetch-limited seas: 



1. Two-Parameter Wave Spectra Family 



The two-parameter wave spectra family consists of nine members for an 

 arbitrarily specified significant wave height. It is based on the two- 

 parameter spectral formulation given by (Bretschneider 1959) , 



96 



