Analyses of Multiple-Float-Supported Platforms in Waves 



Pitch response for the models of Group A is given in Figure 8. 

 The auxiliary wave-length-displacement chart is also shown beneath 

 this figure. An additional scale of wave length/ yV^= X/y 1/3 is added 

 to this Figure, which related to angular motions of the vehicle. This 

 is incorporated for these cases because the wave exciting moment 

 must be expected to be crucially dependent on the ratio of wave length 

 to float spacing. If the wave forces which act on the floats were purely 

 vertical, for instance, as is considered to be the case in the strip 

 theory of ship motions, it would be found that when the wave length 

 equalled the hull float spacing, the exciting moment would vanish since 

 wave crests would occur at each of the floats simultaneously, etc. 

 Table 1 shows the hull float spacing/ V ' to be around 4, while the 

 response curves of Figure 8 indicate practically null motion for A/vV3 

 in the range of 5.5 to 7. The apparent discrepancy is, of course, ex- 

 plained by noting that horizontal wave exciting forces on the floats 

 contribute greatly to the moment about the center-of-gravity, which is 

 quite high above the still waterline. 



Short-term Statistics 



While the transfer functions are interesting for making qualitat- 

 ive comparisons, especially because they are familiar, quantitative 

 judgments of performance in realistic irregular waves require consi- 

 deration of the spectra of the irregular response. For purposes of 

 design and analysis, particular waves must be considered. 



The 12th International Towing Tank Conference in Rome [l6] 

 adopted the following recommendation relative to wave spectra. 



11 The Conference recommends that wherever possible, use 

 shall be made of information on wave conditions for the ship's service 

 in presenting predicted ship behavior in waves. When information on 

 typical sea spectra is not available, it is recommended that the follow- 

 ing sea spectral formulation shall be used as an interim standard : 



T,/ 4 



Sj.(«) - — — e 



f a, 5 



2 

 S (w) = spectral density, ft -sec 



o) = circular frequency, rad/sec (2) 



"If the only information available is significant wave height, 



H l/3 ft 



' -3 2 2-4 



A = 8.10x10 g = 8.38 ft -sec 



B = 33.56/(H l/3 ) 2 

 801 



