a plumb bob, clamped in position, and then calibrated as described 

 above. The experimental arrangement is shown in Figure 21. 



For each bottom clearance, six angles of orientation (0°, 15", 30°, 

 45°, 60°, and 75°) were tested. Fifteen runs with different wave con- 

 ditions were made for each bottom clearance and orientation angle. 

 These runs covered four wave periods ranging from 1.4 to 2.6 seconds, 

 with waves generated at four heights for each period, ranging to a maxi- 

 mum of about 0.7 foot (21.3 centimeters). Eight bottom clearances were 

 tested, ranging from 0.001 foot, 1/16 inch, 1/8 inch, 3/16 inch, 1/4 

 inch, 1/2 inch, 1 inch, and 2 inches. 



The above experiments were done using a 3-inch-diaraeter pipeline 

 model. The tests were then repeated using a 2- and 4-inch-diameter 

 pipeline. The 1- and 2-inch clearances were not tested because the lift 

 forces at these clearances proved insignificant in the previous tests. 

 Also, the tests at an orientation angle of 75° were eliminated, since 

 the previous experiments demonstrated that the vertical forces measured 

 at this angle were insignificant, and too small to be measured with any 

 accuracy. Aside from these changes, the 4-inch-diaraeter pipeline was 

 tested at the same bottom clearances, orientation angles, and wave con- 

 ditions as the 3-inch-diameter model. The 2-inch-diameter model was 

 tested at the same bottom clearances and wave conditions, but only three 

 of the five orientation angles (0°, 30°, and 60°) were tested. 



The Stillwater depth in the wave tank was held constant at 3 feet 

 throughout the three-dimensional experiments, but since the base of the 

 pipeline model was located 2-7/16 inches (6.19 centimeters) above the 

 tank bottom, the effective Stillwater depth over the pipeline base was 

 2.797 feet (85.25 centimeters). The definition sketch for the three- 

 dimensional experiments is shown in Figure 22. 



4 . Data Reduction . 



The wave force data were taken on a two-channel strip-chart recorder 

 with the paper advancing at a speed of 25 centimeters per second. One 

 channel recorded the forces while the other channel simultaneously re- 

 corded the wave surface profile directly over the center of the pipeline 

 test section, thus allowing direct correlation of the two records. 



The two-dimensional experimental data were digitized manually using 

 a Gerber digital data reduction system connected with a card punch to 

 automatically punch the digitized values on computer cards. Using a 

 variable linear scale, each force record was first divided into 20 

 equally spaced intervals per wave, each interval representing a time 

 interval of T/20, where T is the wave period. Each force record was 

 digitized at these points over an interval of two consecutive waves 

 (beginning at the wave crest) , thus giving 40 values for the analysis 

 and averaging the wave forces over two wave cycles. 



53 



