Analyses of Multiple-Float-Supported Platforms in Waves 



bending rigidity of the fabric attenuator on float forces and other per- 

 formance features. Using classical vibration theory [6lJ , with an as- 

 sumed bending rigidity E = 15. 268 x 10 6 lb-ft 2 for a six-foot diameter 

 pressurized inflated float tube constructed of 3000 lb/in fabric, the 

 computed lowest lateral bending frequency of the attenuator is 0. 11 Hz, 

 assuming hinged-free boundary conditions (corresponding to tests with 

 the hinge, if the hinge is omitted, the lowest two cantilever natural 

 frequencies are 0.025 Hz and 0. 16 Hz ). Since these frequencies are 

 within the range of significant wave energy, it appeared to be impor- 

 tant to study flexible model performance. The scale -equivalent flex- 

 ural rigidity of the 1/5 7. 6 -scale model was provided by a central 

 0.45" x 0. 06" plexiglas flat strip. The external shape of the float was 

 simulated by cylindrical segments secured to the flexure strip by ple- 

 xiglas bulkheads. A system of baffles and flow passages were used to 

 inhibit flow passing from outside the model to inside it but which per- 

 mits flow communication within the float. For shorter floats, the e- 

 lastic laterla natural frequencies are higher and consequently, the in- 

 fluence of rigidity is lessened. 



The slender float of Figure 3 6c was used in one set of inter- 

 action tests, while the full float of Figure 36d was used in the other set 

 of float interaction tests and for the dynamic island tests. The float of 

 Figure 36e is the same as that of Figure 3 6d except for the rounded 

 bottom. 



Apparatus 



The apparatus used for these tests was an improved version of 

 the equipment described by Mercier in published references [62,63] . 

 Existing force balances and motion transducers were adapted for these 

 tests. 



A new data reduction procedure was applied for these tests. 

 Electronic signals for forces, moments, waves and/or forced motion 

 (heave, surge, pitch, sway, roll ; but not yaw) were recorded on 

 analog magnetic tape as well as on oscillograms. For some of the 

 tests, the signals were immediately processed by an on-line computer- 

 controlled analog-to -digital converter and further evaluated by the 

 computer (PDP-8E) to determine the Fourier coefficients of the signals. 

 Other tests were "played-back" later during off-line processing. 

 Sampling was carried out at the rate of eigher 20 or 50 samples per 

 second, permitting accurate (and easy) interpretation of electronic- 

 wave-forms which were sometimes relatively "noisy". Usually only 

 the first harmonics of the wave forms were determined, but for sever- 

 al tests the second and third harmonics were also computer. Higher 

 harmonics are of interest primarily for large amplitude waves or 



833 



