Flow-Induced Vibration of an Underwater Lazy Wave Power Cable in Unidirectional Currents

Abstract

This paper describes measurements of the flow-induced vibration of an instrumented model cable in a lazy wave configuration immersed in unidirectional currents in the 2 m deep FloWave Facility at the University of Edinburgh.  The cable model, designed to represent a dynamic power cable used in offshore renewable energy structures for electricity transmission, has an external diameter (D) of 31 mm and a mass ratio of 1.09. The current speed was varied from 0.1 to 0.9 m/s and its direction was set at 0, 90, and 180 degrees relative to the initial longitudinal axis of the cable. An underwater Qualisys motion capture system measured the in-line (IL) and cross-flow (CF) displacement components at 36 locations along the length of the submerged cable. Local displacements, response frequencies, and travelling wave modes are determined for reduced velocity Ur ϵ (5.29, 47.69), and Reynolds number Re ϵ (103, 104). It is found that the root mean square (RMS) values of the displacement components exhibited an increasing trend with reduced velocity reaching 0.40D in the in-line direction and 0.45D in the cross-flow direction. For reduced velocity in the range from 5.29 to 10.58, the cable exhibited single frequency vibrations. For Ur > 10.58, the cable experienced broad-banded, multi-frequency responses. Along the cable, certain locations were found to execute distinct circular, elliptical, nearly linear, and figure-of-eight orbits at low Ur. A sudden phase shift was observed along the cable length, related to unsteady vortex-induced vibration (VIV), which effectively prevented lock-in occurring at high Ur..

Keywords: Dynamic power cable, Vortex-induced vibration, Lazy wave, Hydro-elastic cable, Unidirectional current, Reduced velocity