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

Abstract

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 56mm and a length of 5.36m. 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 1.4m/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 touchdown 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 0.5m/s. In these conditions the dimensionless peak frequency (based on the uniform normal incident velocity) was about 0.13.

Mode switching and standing-travelling waves in slug flow-induced vibration of catenary riser

“…Unlike a single-frequency vibration with an RMS amplitude of about 70.7 %, the peak RMS amplitudes in the x and z directions are about 52.4 % and 57.6 % of the corresponding maximum amplitudes, respectively, suggesting the coexistence of multiple modes. This multi-mode characteristic is also confirmed by a variable intersection of vibration envelops and a non-zero node of RMS amplitude along the span (Chaplin et al, 2005;Chaplin and King, 2018;Zhu et al, 2019c). The variation of instantaneous response mode versus a normalized time is plotted in Fig.…”

Vortex induced vibrations of wind turbine blades: Influence of the tip geometry

“…The re-appearance of the vortex shedding was also found in the numerical study of De Vecchi et al (2009), when a concave cylinder was forced to oscillate. Chaplin and King (2018) presented a set of experiments aiming at studying the VIV of a catenary riser with high concave curvature and very low bending stiffness. The authors observed a more complex structural response with respect to springmounted cylinders, accounting for broad-banded displacement spectra.…”

An experimental investigation of vortex-induced vibration of a curved flexible cylinder

“…Contributions of the travelling wave increased as the flow velocity was increased. Some other water tunnel experiments on VIV of a curved flexible cylinder looked into the response of the curved cylinder when the bottom end of the cylinder was completely lying on the floor of the tunnel (Chaplin & King 2018). The frequency content of the spanwise response showed a broad-banded, multi-frequency excitation, where the frequencies of oscillations were found to be lower than those usually observed for a vertical cylinder in flow.…”