Analyses of Multiple-Float-Supported Platforms in Waves 



DISCUSSION 



Michel K. Ochi 



Naval Ship Research and development Center 

 Bethesda 3 Maryland, U.S.A. 



My discussion is directed toward the evaluation of under- 

 water float configuration which is one of the most difficult deci- 

 sions to make for the design of a float- supported ocean platform. 



The authors have derived a significant conclusion that a 

 slender float with displacement relatively uniformly distributed 

 along its length appears to be superior in mild seas, while a float 

 with displacement concentrated near the bottom is preferable in 

 relatively severe seas. This writer concurs with this conclusion. It 

 may be well, however, to call the authors 1 attention that this con- 

 clusion is true from the view point of platform motions, but the va- 

 lidity of the conclusion should also be confirmed from the view point 

 of wave -induced forces and moments of the platform. 



To clarify the point of discussion, let us consider the follow- 

 ing example : suppose a platform of proper size is floating without 

 any restrictions, the minimumization of motions is of utmost im- 

 portance. Suppose the platform is moored, on the other hand, a float 

 configuration which yields the minimum wave-induced forces and 

 moments of platform is highly desirable. The same way be true for 

 a platform of relatively large size consisting of several element plat- 

 forms which have been connected into a single unit. This is because 

 the minimumization of the hydrodynamic forces and moments is ne- 

 cessary for safe connection of each element platform. 



Selection of the best float configuration from the view point of 

 motions may be rather difficult ; however, the best configuration 

 from the view point of wave -induced forces can be achieved by adjust- 

 ing the waterplane area of floats, since the wave-induced force on a 

 floating body can be reduced to almost zero at a certain frequency if 

 the underwater configuration is properly selected. This property 

 was discussed by Motora and Koyama at the 6th Symposium in 1966 

 and is also demonstrated in the authors 'study. 



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