It can be seen that the CI-M-AVI will not respond to waves of 

 periods of about 6 seconds, while it will move with waves of periods 

 greater than 14 seconds. The average amplitude for the ship is 1.1 feet in 

 the wave. If it were in phase with an average wave, the total heave would be 

 5+1.1 =6.1 feet with respect to the ocean floor. Figure B-1 5 illustrates the 

 relationship between dynamic axial force and the frequency of oscillation. 

 The values are for average amplitudes and could increase significantly if 

 conditions changed. 



It appears that the ship used for heavy lift will need a longer period 

 in heave than the C1-M-A Vl or if that is found to be difficult to achieve, 

 some type of heave compensating mechanism is needed to reduce the effects 

 of vertical motion. The latter approach is being used on the Alcoa Seaprobe, 

 where a heave compensating crown block will be installed. As far as is 

 known, this is the first ship using such a device; much success is predicted by 

 the designers. 



Motion of FORDS^^ 



Probably the ultimate in stable platforms so far proposed is the Naval 

 Research Laboratory's Floating Ocean Research and Development Station 

 (FORDS). While the prototype was never constructed, models of the 

 platform were tested in the David Taylor Model Basin. The results of the 

 investigation illustrate the response characteristics of the platform in some 

 detail. 



Two important test conditions were studied in the experiments: 



Condition 



Draft 

 (ft) 



30 



265 



Weight 

 (long tons) 



13,510 

 19,220 



Nature 



il Period in Heave 

 (sec) 



1 

 2 





5.3 

 123.3 



The light draft motions (test condition no. 1 ) are comparable in 

 magnitude to those of a ship of the same displacement. 



Figure B-1 6 illustrates response of the platform in heave for regular 

 waves of small amplitude. A summary of the irregular wave data is given in 

 Table B-7. 



Assuming a wave period of 9 seconds for both test conditions, the 

 approximate dynamic axial forces are as shown in Table B-8. 



132 



