Effect of traveling waves on Vortex-Induced Vibration of long flexible cylinders

Abstract

Offshore marine risers and pipelines, exposed to ocean currents, are susceptible to Vortex-Induced Vibration (VIV). Accurate prediction of VIV is necessary for estimating the fatigue life as well as for taking corrective measures to prevent the vibrations. State of the art response prediction methods work reasonably well for short flexible cylinders vibrating at frequencies corresponding to low mode numbers (below the tenth mode). However, for long structures, which respond above the tenth mode, lack of experimental data has until recently impeded progress. Results will be presented from recent field experiments conducted in the Gulf Stream and Lake Seneca, NY. These experiments have provided an opportunity for new insights about the VIV of long flexible cylinders, responding at high mode numbers. The experimental results also include insights on the use of VIV suppression devices such as helical strakes. The experiments reveal that the dominant response of long flexible cylinders is often in the form of traveling waves. High spatial density fiber optic strain gauge measurements are used to obtain estimates of the phase speed of the waves, the response amplitude and the added mass coefficient. The mean added mass coefficient for the bare cylinder is shown to be approximately one and the maximum response amplitude is found to be approximately one diameter. A Green's function, response prediction method, is introduced which is able to emulate both the standing and traveling wave properties observed in the experimental data.

(cont.) A novel approach to modeling the excitation force as a combination of standing and traveling wave components is shown to predict the measured response very well. The method is also able to account for high localized damping that result from the use of response suppression devices, such as helical strakes. Many marine risers are composed of nested concentric steel pipes. The relative motion of these concentric pipes in the presence of confined liquids introduces unusual dynamic properties, including the potential for beneficial effects as dynamic absorbers. Numerical and theoretical models are developed as a preliminary step in the design of dynamic absorbers for deep water risers.