Some indication of the reproducability in the field tests is given by Fig. 3.16 which shows the 

 (scaled) frequencies measured at two different positions along the Kevlar cable during two different 

 runs. The agreement between the two test runs is excellent. The resonant strumming displacements 

 for two runs with the Kevlar cable in = 2) and one run with the Uniline cable (« = 5) are plotted as a 

 function of the reduced velocity V,. in Fig. 3.17. The measured displacements are shown on a relative 

 scale. The data for the individual runs are divided by the maximum measured displacement for that 

 run. The strumming results in Figs. 3.16 and 3.17 again clearly show the frequency and displacement 

 amplitude resonances that are characteristic of the vortex-excited oscillations. As in the laboratory 

 experiments discussed in Sections 2 and 3.1 and the towing tank results plotted in Fig. 3.10, the 

 resonant cross flow displacements for the field tests occur at reduced velocities in the range K, = 5 to 

 8, with maximum strumming displacements in the range V, = 6 to 7. 



3.4 Large Scale Field Experiments. FISHBITE is the name of a marine cable experiment conducted 

 by Softley, Dilley and Rogers in 1976 (56). A wire rope 12 mm (0.47 in.) in diameter and 500 m 

 (1640 ft) long was hung from a ship anchored in 1960 m (6430 ft) of water at the Tongue of the 

 Ocean, located at 77° 52' W and 25° 10' N. The tidal flow varied both temporally and spatially from 0.1 

 to 0.4 m/s (0.2 to 0.8 knots). A current meter was attached at the halfway point, but no other lumped 

 masses were attached to the cable. The cable response was measured at the top end only and the cable 

 parameters resulted in a modal spacing of 0.025 Hz. 



The response typically included more than one hundred modes between 8 and 12 Hz, with a 

 center frequency of 10 Hz. An rms acceleration spectrum from the report of Softley et al. is given in 

 Fig. 3.18. From a study of these data Kennedy and Vandiver (55) have noted that the rms response at 

 the measurement point on the upper end of the cable was limited to less than one cable diameter. 

 They attribute the bandwidth of the cable response to spatial and temporal variations of the current at 

 the test site. No lock-on was observed during any of the FISHBITE experiments. 



SEACON II was a major undersea construction experiment the chief goal of which was the meas- 

 urement of the steady-state response of a complex three-dimensional cable structure to ocean currents. 



62 



