Analyses of Multiple-Float-Supported Platforms in Waves 



A set of normal modes and natural frequencies for a particular 

 assumed island structure and float -attenuator size have been calculat- 

 ed by J.Rice of Goodyear Aerospace [67] . The first eight elastic 

 "free-end" modes of oscillation were found to have natural frequencies 

 corresponding within 0.23% of the pure heave natural frequency '. 



While these results are applicable to the particular large is- 

 land which Rice considered, it seems probable that the articulated 

 model, with its essentially negligible elastic interconnections, will 

 also have normal modes whose frequencies correspond to the free 

 heaving frequency of the float elements. Thus the dynamic load factors 

 (DLF's) for all modes, symmetric and asymmetric, will be essential- 

 ly the same. Then, according to the definition of the amplitude func- 

 tion, A n , , the motion should correspond closely to a weighted sum 

 of the distributed load. A detailed evaluation of the response would 

 require significant numerical work but, intuitively, it does not seem 

 reasonable to expect the modest asymmetry of the wave induced heave 

 force (Figure 51) to produce the pronounced asymmetry of the heave 

 motion response (Figure 48). 



Surge Force Interaction 



Results given in the carpet plot (Figure 52) indicated virtually 

 no influence of position in the array on surge force due to waves at 

 low frequencies, but as much as 43% increase (monotonic with di- 

 stance from the bow) at f = 1.4 Hz. 



Scale Effects 



Force measurements shown in Figures 49 and 50 include the 

 small scale model results. They are seen to be somewhat lower, in 

 general, than the larger model results, but the trends of the results 

 are quite similar. Differences may be partly attributable to experi- 

 mental error. The magnitudes of the oscillatory forces being measur- 

 ed on the small models are of the order of 0.001 lbs : such small 

 measurements are not routinely executed in hydrodynamic laboratories 

 such as Davidson Laboratory. The scale effect exhibited may be due 

 to either viscous effect (Reynolds Number) or surface tension effects 

 (Weber Number). 



Wave Measurements 



A few wave elevation measurements were made at locations 

 within the array of the rigidly-held large models. The results show 

 that the wave amplitudes are significantly higher (about 10 to 20 per 



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