Measurement of Cable Support Point Motions due to Vortex-Induced Vibrations; Experiments with a Vertical Towed Cable.

To assess the threat of undersea cable failure due to abrasion caused by vortex-induced vibrations, a series of experiments were conducted in which a length of vertical cable with a clump weight at the lower end was suspended over various frictional supports and towed through quiescent water. The objective of the experiments was to measure magnitude of the cable motions at the supports during resonant vortex-induced vibrations of the cable span. Three types of frictional support were used; 1) a soil support, in which the cable was suspended through a layer of soil, 2) a rock support on which the cables were suspended over samples of vesicular basalt, 3) a twenty inch radius aluminum cylindrical support. The soil supports provided a support which restricted cable motion through soil pressure and intergranular friction. The rock support provided a single point of contact with the cable, allowing the support point to become a node, and the large radius support provided a support which constrained the cable past the point of first contact with the support. Two cables were used, one 1.25 inches in diameter which is a co-axial cable which has been in use for greater than twenty years, and the other 0.45 inches in diameter is a new fiber optic design proposed for future use. Results show that at 0.5 knots all support types limit the cable displacement amplitudes to levels near the resolution of the data acquisition system. The soil box demonstrates that the region in which abrasion may occur will be limited to a narrow zone near the soil cable interface where the soil is unconstrained. The experiments with the hard surface supports show that the dynamic displacements due to vortex-induced vibrations are not large enough to produce tension components tangential to the support which can overcome the static friction between the cable and the support. The motion which was observed at the hard surface supports was a complicated mixture of rolling and pivoting with the point of contact being a node in the transverse vibrations of the cable.