Spatial Variation of Drag on Long Cylinders in Sheared Flow

A method is described for measuring the local drag coefficient on a long cylinder which exhibits vortex-induced vibration (VIV). Results are shown from a field experiment in which a long flexible pipe was instrumented with two-hundred and eighty fiber optic strain gauges. The measured local drag coefficients are compared to a commonly used drag coefficient prediction formula. The formula is shown to be useful as a tool for predicting the average drag coefficients for the whole cylinder but is not able to accurately capture local variation in D C . The local D C measurements also reveal the location of VIV source regions. INTRODUCTION Amplification of drag forces due to Vortex-Induced Vibration (VIV) has been studied for many years. Formulations for the amplifications were developed by Vandiver and others in the late 1970's and early 1980's. Recent laboratory experiments on a rigid cylinder undergoing VIV by Jauvtis & Williamson [2] indicate great variations in the drag amplification factor when reduced velocity is varied. Spatial variations in local reduced velocities are common in realistic ocean current environments where flexible cylinders are placed in sheared currents. However, the local drag amplification factors corresponding to the local reduced velocities have never been reported primarily due to the difficulty in measuring local drag forces in field experiments. In this paper, the authors, using strain data from field experiments performed off the coast of Miami in the Gulf Stream, present an approach to estimate the local mean drag coefficient Downloaded 15 Dec 2010 to 18.80.5.156. Redistribution subject to ASME Numerous researchers studying VIV have found that the drag forces are amplified when a flexible cylinder is placed in a current and it undergoes VIV. A large volume of research has been performed to develop methods for predicting drag coefficient amplification factor in the presence of VIV. This amplification factor is used to multiply the drag coefficient of a stationary rigid cylinder in fluid flow to yield a prediction of the drag coefficient for the vibrating cylinder. Several formulations have been derived for the amplification factor using both field experiments [4] and laboratory tests [1]. The most commonly used formulation in the oil and gas industry has been proposed by Vandiver [4] and is shown in Equation (1.1). An example of the amplification can be obtained by assuming a root mean square (RMS) amplitude to diameter ratio, ( ( ) / rms y z D ), value of 0.707, equivalent to a peak amplitude to diameter ratio of 1.0. Such amplitudes are commonly seen in lightly damped pipes undergoing VIV, as is common in the oil and gas industry. The amplification factor from Equation 1.1 is thus found to be 2.31, implying a greater than doubling of the drag forces due to VIV.