The response curves of the circular, semi-circular, and crescent 

 shaped models are shown in Figures 6, 7, and 8, respectively. One would 

 expect a peak response when the wave frequency matched the platform's 

 natural pitch frequency. But the responses of all three platform models 

 peaked at wave frequencies higher than the natural frequencies. The 

 circular cylinder platform has a wide spread of data points (Figure 6) . 

 Unfortunately, the peak of the response curve cannot be defined because 

 of the lack of data points near f/f^ - !• However, the peak responses 

 of the non-symmetric legged platforms fall definitely beyond f/fn = 1- 

 This indicates that the model resonance frequencies are different from 

 the natural pitch frequencies of models measured in still water. 



All five response curves were replotted on Figure 9 for comparison. 

 In general, all tests resulted in "downstream" drifts. However, there is 

 a trend that indicates "upstream" platform motion can be achieved in low 

 frequency waves. The velocity of such drift, however, is not significant. 

 Upstream drift was observed in very long waves for several unrecorded 

 tests in which the drift velocities were too small to be recorded. 



The dimensionless drift velocity, Vj/V^^, measures the efficiency of 

 each test geometry in converting wave energy into propulsion force. The 

 ranking with respect to peak values of drift velocity are; test geometry 

 4 the highest, geometry 1 the second, and geometry 2, 3, and 5 the third, 

 fourth and fifth, respectively. Based on the hypothesis discussed in the 

 background section, the drift velocities for geometry 2 and 4 were expected 

 to be high and drifts against wave direction were predicted for geometries 

 3 and 5. The circular model was expected to be stationary. The high 

 velocity of geometry 4 and some upstream drift in geometry 5 have somewhat 

 confirmed the theory. The high velocity drift of the circular model and 

 the failure of upstream drift by geometry 3 indicate that some force other 

 than the hydrodynamic drag force possibly the inertia force predominated 

 the wave force . 



These results have indicated that non-symmetric platform legs may 

 provide definite advantages over the conventional circular legs. Further 

 investigation was therefore justified. The next step was to conduct a 

 more elaborate model test to measure the peak wave forces on larger models 

 for more effective evaluation of non-symmetric leg geometries, 



PRINCIPAL TESTS - WAVE FORCE MEASUREMENTS 



Three different models of a MOBS platform leg, 1 to 60 scale in length 

 were fabricated from 4-inch and 6-inch PVC pipes. The length of each model 

 was 8 feet long. Again, a circular, a semi-circular, and a crescent shaped 

 cross-section were selected for these models. The dimensions of model 

 sections are shown in Figure 10. A base plate and stiffening collar were 

 attached at the upper end of each model for easier and faster mounting and 

 removal from the support. 



The model tests were conducted in the Ocean Laboratory of the Offshore 

 Technology Corporation, Escondido, California. The test tank resembled a 

 deep swimming pool 50 feet wide, 150 feet long, and 15 feet deep (Figure 11) 



11 



