Effect of Top Tension on Vortex-Induced Vibration of Deep-Sea Risers

Jie Zhang,Yougang Tang,He Guo,Yulong Li

doi:10.3390/jmse8020121

Abstract

With the increase of water depth, the design and use of the top-tensioned risers (TTR) are facing more and more challenges. This research presents the effect of top tension on dynamic behavior of deep-sea risers by means of numerical simulations and experiments. First, the governing equation of vortex-induced vibration (VIV) of TTR based on Euler-Bernoulli theory and Van der Pol wake-oscillator model was established, and the effect of top tension on natural vibration of TTR was discussed. Then, the dynamic response of TTR in shear current was calculated numerically by finite difference method. The displacement, bending stress and vibration frequency of TTR with the variation of top tension were investigated. Finally, a VIV experiment of a 5 m long flexible top-tensioned model was carried out at the towing tank of Tianjin University. The results show that the vibration displacement of TTR increases and the bending stress decreases as the top tension increases. The dominant frequency of VIV of TTR is controlled by the current velocity and is barely influenced by the top tension. With the increase of top tension, the natural frequency of TTR increases, the lower order modes are excited in the same current.

Highlights

In recent years, owing to the large demand of crude oil, offshore oil and gas explorations have been shifted to deeper water regions
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In order to study the effect of top tension on VIV of top-tensioned risers (TTR), the governing equation of VIV of TTR is established, and the dynamic response of TTR is calculated numerically by finite difference method (FDM) based on Van der Pol wake-oscillator model

Summary

Introduction

In recent years, owing to the large demand of crude oil, offshore oil and gas explorations have been shifted to deeper water regions. When the riser moves under the action of sea current, vortices are shed along its surface, resulting in the formation of an unstable wake region behind it. Vortex shedding normally takes place with different frequencies and amplitudes, induces a periodically varying transverse force on the riser (i.e., perpendicular to the direction of the current), resulting in a periodically transverse vibration known as the VIV [1,2,3,4]. The reference lift coefficient CL0 being usually taken as CL0 = 0.3 in the large range side forcing term F models the effect of the cylinder motion on the near wake, F of =.

Natural Frequencies and Mode Shapes

System Parameters and Basic Calculations

Test Introduction

Conclusions