3. JONSWAP Wave Spectra Family 



The JONSWAP wave spectra family consisting of five members for a 

 specified significant wave height was derived for evaluating reponses of 

 a marine system operating in fetch-limited seas (Ochi 1979 b) . It is based 

 on the following JONSWAP wave spectral presentation: 



5("i)=c( 











iCO~^y») 



1 



-1 



A^i 



e 



2 6%^ 



e 





r 





(6) 



where, a =0.076 (X)~°*^^ 



= 2t:(3.5)^ (X) 



-0.33 



X = fetch length 



U = wind speed 



Y = peak shape parameter 



0.07 for 0) < w 

 — m 



C: 



09 for 0) > 0) 



From a statistical analysis of the measured data, five values of the 

 peak shape parameter, y> ai^d the associated weighting factors are determined 

 as given in Table 3. It is noted that the JONSWAP spectral formulation is 

 given as a function of wind speed. Hence, for the design of mooring system, 

 it is necessary to estimate the extreme value of wind speed expected in the 

 area where the system is located. However, for a location where the effect 

 of fetch length as well as the effect of bottom topography on wind-generated 

 sea may exist, statistical information of both wind and significant wave 

 height are required. In this case, the extreme value of significant height 

 should be estimated first, then evaluate the equivalent wind speeds for deep 

 water by the following formulation: 



-0,61 S /,o§ 

 U = ^^ X H, (7) 



where, k = constant given in Table 4 

 H = significant wave height 



98 



