Laboratory measurements of the vortex-induced vibrations of an untensioned catenary riser with high curvature

Measurements of the vortex-induced vibrations of an instrumented catenary model riser with high curvature and very low bending stiffness are presented and discussed. The riser had a diameter of 56 mm and a length of 5.36 m. It was tensioned only by its own weight and by drag and was tested in a recirculating water channel 2m deep at flow speeds up to 1.4 m/s in both directions, i.e. with the upstream anchorage on the floor of the tank and the downstream one at the surface (defined as positive current cases), and then vice versa (negative currents). In many cases much of the riser rested on the tank floor with the touch-down point determined by the flow-induced forces on it. Attention is focused mainly on the frequencies of cross-flow vibrations after careful analysis of the effects of gravitational contamination on acceleration measurements. Response spectra were found to have a higher bandwidth than those of tensioned risers perpendicular to the flow and often exhibited more than one distinct peak. Dimensionless peak frequencies (having been normalised with respect to the diameter and a characteristic velocity) were much lower than that associated with a Strouhal number in the region of 0.2. In positive currents (based on the normal incident velocity at the top of the riser, where it was near vertical) it was generally uniform over the length of the riser at about 0.11. In negative currents, away from the tank floor, the riser's profile was a straight line whose inclination was such that the normal incident velocity was independent of the current speed when this exceeded about 0.5 m/s. In these conditions the dimensionless peak frequency (based on the uniform normal incident velocity) was about 0.13.