i 7, 



(13) 



where, T = total time considered for a specified sea in hours. 



As a numerical example of estimating the extreme height with specified 

 periods, computations are carried out using the six-parameter wave spectra 

 family for a sea of a significant wave height 7.5 m (24.6 ft) with various 

 ranges of period as shown in Table 4. The time T in this sea is estimated 

 from the available data as 78.8 hours in 50 years. As can be seen in the 

 table, the magnitudes of the conditional extreme wave height decreases 

 substantially for the range less than 7 sec. in this particular sea. 

 This implies that if a system is designed such that the ranges of periods 

 critical for the system's behavior is less than 7 sec, then the extreme 

 wave height for design consideration may be reduced in this sea condition. 



REFERENCES 



Bretschneider, C. L. (1959), "Wave Variability and Wave Spectra for Wind- 

 Generated Gravity Waves", Beach Erosion Board, U. S. Army Corps of Engineers, 

 Tech. Memo No. 118. 



Longuet-Higgins, M. S. (1975), "On the Joint Distribution of the Periods 

 and Amplitudes of Sea Waves", Journal of Geophysical Res., Vol. 80, 

 No. 18, pp 2688-2694. 



Ochi, M. K. and Hubble, E. N. (1976), "On Six-Parameter Wave Spectra", 

 Proc. 15th Coastal Eng. Conf. Vol. 1, pp 301-328. 



Ochi, M. K. (1978) , "Wave Statistics for the Design of Ships and Ocean 

 Structures", Trans. Soc. Nav. Arch, and Itor. Eng., Vol. 86, pp 46-76. 



Ochi, M. K. and Whalen, J. E. (1979a), "Estimation of Extreme Waves 

 Critical for the Safety of Offshore Structures", Offshore Tech. Conf. 

 OTC 3596, Vol. 3, pp 2085-2090. 



Ochi, M. K. (1979b), "A Series of JONSWAP Wave Spectra for Offshore 

 Structures Design", Conf. Behavior Offshore Structures, Vol. 1, pp 76-86. 



Ochi, M. K. and Whalen, J. E. (1980), "Prediction of the Severest Signifi- 

 cant Wave Height", will appear in Proc. 16th Coastal Eng. Conference. 



102 



