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This paper explores the Reynolds number dependence of the Vortex-Induced Vibration (VIV) of flexible marine risers. Emphasis is placed on revealing the trends that exist between the Strouhal number and the Reynolds number and between the dimensionless amplitude (A/D) and Reynolds number. Data is drawn from recent towing tank experiments which used flexible cylinders of three different diameters. The 38m long pipes were exposed to uniform and sheared currents. The Reynolds number range extended from approximately 5,000 to 220,000 -well into the critical regime-with the larger diameter pipes responding in up to the 13 th mode and the smaller diameter pipe responding well above the 20 th mode. The results and trends from this set of experiments are compared to previous results from laboratory and field experiments. INTRODUCTIONThe Reynolds number, Re, is a very important dimensionless parameter in most fluid dynamics problems including VIV.

This paper presents results from two field experiments using long flexible cylinders, suspended vertically from surface vessels. The experiments were designed to investigate vortex-induced vibration (VIV) at higher than tenth mode in uniform and sheared flows. The results of both experiments revealed significant vibration energy at the expected Strouhal frequency (referred to in this paper as the fundamental frequency) and also at two and three times the Strouhal frequency. Although higher harmonics have been reported before, this was the first time that the contribution to fatigue damage, resulting from the third harmonic, could be estimated with some certainty. This was enabled by the direct measurement of closely spaced strain gauges in one of the experiments. In some circumstances the largest RMS stress and fatigue damage due to VIV are caused by these higher harmonics. The total fatigue damage rate including the third harmonic is shown to be up to forty times greater than the damage rate due to the vibration at the fundamental vortex-shedding frequency alone. This dramatic increase in damage rate due to the third harmonic appears to be associated with a narrow range of reduced velocities in regions of the pipe associated with significant flow-induced excitation.

Vortex-Induced Vibrations (VIV) are an important source of fatigue damage for risers in the Oil and Gas industry. Results from resent VIV experiments by Vandiver et al. [1] indicate significant dynamic strain energy at not only the Strouhal frequency, but also its harmonics. In certain regions of the pipe, these higher harmonics accounted for more that half of the measured RMS strain and increased fatigue damage by a factor exceeding twenty. However, the state-of-the-art in VIV prediction only accounts for the vibrations at the Strouhal frequency. Preliminary results from a second set of experiments, described in this paper, confirm the importance of the higher harmonics in fatigue life estimates of pipes. Further, the authors formulate an approach to incorporate the higher harmonics in VIV related fatigue design. Finally, the authors identify the estimation of the higher harmonics, in both location and magnitude, as an important area of ongoing research, the results of which will be required to implement this proposed method.

Despite of considerable research activity during the last decades considerable uncertainties still remain in prediction of Vortex Induced Vibrations (VIV) of risers. Model tests of risers subjected to current have been shown to be a useful method for investigation of the VIV behavior of risers with and without suppression devices.In order to get further insight on VIV of risers, an extensive hydrodynamic test program of riser models subjected to vortexinduced vibrations was undertaken during the winter 2010 by Shell Oil Company.The VIV-model test campaign was performed in the MARINTEK Offshore Basin Laboratory. A new test rig was constructed and showed to give good test conditions. Three different 38m long riser models were towed horizontally at different speeds, simulating uniform and linearly varying sheared current. Measurements were made In-Line (IL) and Cross-Flow (CF) of micro bending strains and accelerations along the risers. The test program compromised about 400 tests, which give a rich test material for further studies.In the present paper the test set-up and program are presented and selected results are reported. INTRODUCTIONThe key objectives of the riser VIV model test campaign were:

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper presents the initial results from VIV (Vortex-Induced Vibration) field testing of a long, flexible, model riser at high mode number. The experiments were designed to better understand the dynamic behavior of a long riser in uniform flow responding at mode numbers from 10 to 25 in cross-flow vibration. The 1.31 inch, (0.0333 m), diameter riser model was made from fiberglass line pipe, manufactured by Fiberspar Corp. Two model configurations with length of 201 feet (61.26m) and 401 feet (122.23m) were towed behind a vessel in a deep lake in upstate New York. Motion was recorded with twenty four evenly spaced internal tri-axial accelerometers. Observed reduced velocity and RMS displacement response levels are reported. Mean drag coefficients and hydrodynamic damping derived from measured data are compared to calculated values from formulas commonly used in engineering design of offshore systems.

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This paper addresses a practical problem: "What portion of fairing or strake coverage may be lost or damaged, before the operator must take corrective measures?" This paper explores the effect of lost fairings (the exposure length) on VortexInduced Vibration (VIV) of flexible cylinders. The source of data is a recent model test, conducted by SHELL Exploration and Production. A 38m long pipe model with varying amounts of fairings was tested. Response as a function of percent exposure length is reported. Unexpected results are also reported: (i) the flexible ribbon fairings used in the experiment did not suppress VIV at speeds above 1 m/s; (ii) Above 1 m/s, a competition was observed between VIV excited in the faired and bare regions of the cylinder, (iii) Unusual traveling wave behavior was documented-waves generated in the bare region periodically changed direction, and exhibited variation in VIV response frequency.The results of these tests showed that (1) the excitation on the bare and faired regions could be identified by frequency, because the faired region exhibited a much lower Strouhal number; (2) as expected, the response to VIV on the bare region increased with exposure length; (3) the response to VIV on the faired region decreased with exposure length.