Long flexible pipelines used in the offshore industry can be subject to vortex-induced vibration (VIV). Drilling and production risers, tension-leg platform tendons, pipeline spans as well as umbilicals are the most affected by VIV which leads to increases in their hydrodynamic loading and reduction in their service life due to fatigue. The most undesirable form of VIV for the offshore industry is termed lock-in. Lock-in is used to describe an elastic structure's ability to control the shedding process in a bandwidth around its resonant frequency and for flexible cylinders occurs as single mode, single frequency response. Associated with lock-in is a larger amplitude of excitation increasing unwanted effects. Lock-in always occurs for some velocities in uniform flow conditions; however, in spatially sheared flow conditions the occurrence of lock-in is more complicated. In the ocean, risers are generally exposed to a shear current profile over their depth and hence establishing the boundaries of lock-in and determining the necessary conditions to avoid it is a critical area of research. A long flexible cylinder exposed to ocean currents is known to undergo vortex-induced vibration. In a spatially sheared flow the response of a riser to VIV can vary from single mode lock-in to multimodal. A new experimental facility was designed and built to investigate the above-mentioned areas. The facility consisted of a long flexible cylinder in either a uniform or a simplified vertically sheared flow. The instrumentation consisted of direct local fluid force measurement at two locations on the cylinder as well as accelerometers spaced along the cylinder axis. The simplified shear flow was a 2-slab flow, with each slab having uniform velocity. Test conditions included forcing the cylinder simultaneously at resonance in both regions to investigate modal competition issues and multimodal response patterns. Resonant VIV excitation of two different modes simultaneously, was conducted which revealed single mode lock-in of the higher frequency through an unexpected mechanism. The higher frequency mode's damping region underwent inline excitation at four times the predicted shedding frequency that provided a power-in effect to support the dominant mode's cross-flow response.
On shear flow single mode lock-in with both cross-flow and in-line lock-in mechanisms
Journal of Fluids and Structures ; 22 , 2 ; 197-211
2006
15 Seiten, 10 Bilder, 1 Tabelle, 13 Quellen
Article (Journal)
English