Analyses of Multiple-Float-Supported Platforms in Waves 



Discussion of Interaction Test Results 

 Comparison of Articulated Model Tests 



The results, examples of which are given in Figure 47, de- 

 monstrate that tests with this model are repeatable. Although the 

 "scatter" of the data points even within a given test program (Tank 3 

 or Tank 2) is rather large, there is sufficient consistency between the 

 tests to say with confidence that the measured motions - in particular 

 the unexpected "tail wagging" - are characteristic of this articulated 

 model (and its associated apparatus, viz., pitch-restraining heave 

 most at Row 18 and low-tension spring lines at bow and stern). The 

 motions recorded in the tests in Tank 3 were not importantly influenc- 

 ed by tank side wall effects. 



The carpet plot of Figure 48 exhibits the dependency of the deck 

 motions on position along the length of the model and frequency. The 

 tail-wagging phenomena are shown clearly for all gigher frequencies. 

 The frequency range covered here corresponds to full-scale fre- 

 quencies for which significant wave energy exists for sea states with 

 significant wave height Hw-^g <15 feet. 



Correlation Between Heave Force Measurements and Motions 



The heave force results obtained in the large model (an ex- 

 ample of which is shown in Figure 49 and a carpet plot in Figure 51) 

 show important interaction effects on the vertical wave-induced force. 

 For instance, for f = 1.2 Hz, the force in the middle of the model is 

 42% greater than that at the bow, while the force at the stern is 36% 

 higher. It may also be shown that, for this frequency, the force at the 

 bow is 20% higher than those reported for the tests with the smaller 

 (five rows of five) array except that the outside rows (front and back) 

 are different from each other and different from isolated float results. 



The fore-and-aft asymmetry of the wave-induced heave force 

 suggests that the interaction may be influenced by either free- surface - 

 type frequency dependent effects or, perhaps, some viscous wake ef- 

 fects. If the interaction were purely potential in character and unaffect- 

 ed by wave diffraction effects (as is expected for slender bodies in re- 

 latively long waves), a linearized representation of the interaction ef- 

 fect on the vertical force due to waves on the j float due to the pre- 

 sence of the other floats might be expressed, formalistically at least, 

 as 



3jK j '° k/j 



843 



