Laboratory measurements of vortex-induced vibrations of a vertical tension riser in a stepped current

This paper describes measurements of the vortex-induced vibrations of a model vertical tension riser in a stepped current. The riser, 28 mm in diameter, 13.12 m long and with a mass ratio (mass/displaced mass) of 3.0, was tested in conditions in which the lower 45% was exposed to a uniform current at speeds up to 1 m/s, while the upper part was in still water. Its response in the in-line and cross-flow directions was inferred from measurements of bending strains at 32 equally spaced points along its length. Cross-flow vibrations were observed at modes up to the 8th, with standard deviations of individual modal weights greater than 50% of the riser's diameter. Except at the lowest reduced velocities, the response included significant contributions from several modes, all at a frequency controlled by lock-in of the dominant mode. In the presence of multi-mode responses, drag coefficients were up to 120% greater than those for a stationary cylinder at the same Reynolds numbers, and like overall measures of in-line and cross-flow displacements, revealed a strong dependence on the modal composition of the motion.