Particle image velocimetry investigation of the wake fields of a long flexible riser

Xu, Jie (2006) Particle image velocimetry investigation of the wake fields of a long flexible riser. Masters thesis, Memorial University of Newfoundland.

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Abstract

Vortex-Induced Vibration of marine risers has been receiving increasing interest from both industry and academics as offshore oil and gas exploration moves into deeper water regions. A regular vortex pattern is formed in the wake of cylindrical risers that interacts with the cylinder motion and is the main source that causes elastically mounted cylinders to vibrate. Wake field studies of Vortex-Induced Vibration (VIV) for a long flexible cylinder can help to better understand the physical mechanisms of the interaction between the body response and the wake vortex. The presented work consists of two major parts: some preliminary tests for the Digital Particle Image Velocimetry (DPIV) system of the Memorial University of Newfoundland (MUN) and the Vortex-Induced Vibration (VIV) wake field research using DPIV. These preliminary tests were conducted in small water tanks and the MUN towing tank. Some essential parameters and techniques using PIV were also investigated through these preliminary tests. The VIV wake field measurement experiments were performed in the ice tank, Institute for Ocean Technology (IOT). Vibration responses in both in-line and cross-flow directions of a long flexible cylinder, with a diameter of 47mm and length to diameter ratio of 181, in a free stream, were investigated at moderate Reynolds number in the range of 9400-47000. The DPIV system was employed to measure the wake velocity field and vorticity field behind the cylinder, simultaneously with acceleration measurements of the cylinder. The experimental results show VIV responses in both in-line and cross-flow directions at the span location z/L=0.43, and with a mix of vibration frequencies and amplitudes. With the increase of the flow speeds, vibration frequencies became higher in both in-line and cross-flow directions. The vibration frequencies of the cylinder were in a range of 0.64 Hz to 10.38 Hz over the range of Reynolds numbers. The amplitude did not obviously increase with an increase of Reynolds number. The amplitude to diameter ratio in the in-line and cross-flow directions covered a range of 0.10 - 0.41 and 0.24-0.95, respectively. The flow field measurement results reveal that for a certain value of the vibration frequency, f, the Reynolds number, Re, and the amplitude to diameter ratio, AID, three vortex modes '2S', '2P' and 'P+S' are observed in the near wake of the cylinder. At the lower Reynolds number, Re=9400, and with lower response frequencies from 0.61 Hz to 1.28 Hz, only '2S' vortex modes were observed in the experiments, and the '2S' vortex modes presented were stable. With an increase of the Reynolds number, at Re= 141 00 and with frequencies from 0.95 Hz to 2.74 Hz, two vortex modes, '2P' and 'P+S', were observed in the near wake at different times. The '2P' mode was dominant at this Reynolds number. Vortex modes '2S' and '2P' were observed at different times when the Reynolds number further increased, to Re=18800 and 23500 and with frequencies from 1.34 Hz to 4. 79 Hz. The vortex pattern '2S' played a main role in the wake at the two speeds. At higher Reynolds number, Re>23500, and with frequencies from 1.86 Hz to 10.38 Hz, '2S' and '2P' vortex modes were also observed at different times and the vortex patterns presented were unstable due to velocity fluctuation and quick diffusion of vortices in the wake. The percentage of the vortex pattern '2P' increases at higher speeds. The results also showed that the vortex modes '2S' and '2P' were repeatable with the same vibration responses of displacements and accelerations.

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