<p>The top-end dynamic boundary of the riser is generated by the motion of floating body in complex marine environment, inducing the oscillation of equivalent flow and structural tension, further changing the vortex-induced vibration (VIV) characteristics of the riser. In order to explore the specific effects of the top-end dynamic boundary on the VIV of the riser, the decoupled single-degree-of-freedom harmonic dynamic boundary, simulated by the motor module arranged on the top of the designed vertical VIV test device, is taken as the research point in this paper. By conducting the VIV tests of a riser model, the unsteady VIV behaviors induced by the combinations of in-line and cross-flow top-end dynamic boundaries with uniform flow, defined as superimposed coupling and orthogonal coupling respectively, are investigated. After the comparison and discussion of the key VIV characteristics, it is found that some unsteady phenomena involving time-varying frequency, mode transition, amplitude modulation as well as the appearance of high-order and low-order harmonics in vibration frequency are induced by the dynamic boundary. The superimposed coupling of dynamic boundary and uniform flow is mainly responsible for the periodically oscillatory features of cross-flow VIV, while the orthogonal coupling dominates the similar behaviors of in-line VIV. Meanwhile, the superimposed coupling presents a disturbing effect on in-line VIV to a certain extent. Both in-line and cross-flow top-end dynamic boundaries can significantly enhance the amplitude and frequency responses of the in-line VIV at medium and low reduced velocities, which is further strengthened by increasing parameters of boundary amplitude and frequency. It is suggested to take the effects of dynamic boundary related to the motion of floating body into account while evaluating the in-line VIV of flexible structures like risers in ocean engineering.</p>

An Experimental Investigation on Unsteady Viv Characteristics of a Riser Model Under Superimposed and Orthogonal Couplings of Top-End Dynamic Boundary and Uniform Flow

Paper Proceedings: https://asmedigitalcollection.asme.org/OMAE/OMAE2021/volume/85116

OMAE 2021

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