ABSTRACT

Using an array of high speed cameras and underwater tracking method, we experimentally investigated the flexible cylinder with buoyancy modules undergoing VIV (module to bare cylinder diameter and length ratio of 3.2 and 1; module aspect ratio of 1; coverage ratio of 100% and 50%). It reveals interesting results that the riser will experience bifrequency and bi-modal vibration due to the excitation from both buoyancy modules and bare cylinder. Based on the coverage ratio and flow condition, when buoyancy modules induced motion dominates the response, the model experiences a small value of reduced frequency down to 0.085 and large vibration amplitudes up to 3.8 riser diameter. Besides, the generation of traveling wave patterns, combined crossflow and in-line trajectory and a comparison between flexible cylinder model and corresponding rigid model hydrodynamic database will be presented.

INTRODUCTION

Mitigating vortex induced vibration (VIV) is one of the key design considerations for the offshore riser and pipeline system. VIV may lead to large fatigue damage and hence system failures. Therefore a significant research effort has been devoted into the understanding of VIV mechanism, such as Xu (2013). One of the key factors that decide VIV response is the structure shape, and, as a matter of fact, sometimes even a small change of the shape will largely affect the vortex formation pattern and hence coupled structural response.

For risers used in the deep water (larger than 1000m), staggered buoyancy modules will be often installed to help providing additional lift force and therefore avoiding the excessive tension load in the riser, discussed by Li, et al. (2011). Meanwhile, buoyancy modules have also been widely used to maintain a lazy wave format for the riser system, shown by Jhingran, et al. (2012). However, because of its larger diameter (normally 2.5-5 times larger than riser diameter), the vortex formation may largely change behind the structure hence the different VIV response for the riser with buoyancy modules, compared to the bare risers. Questions therefore have been raised by Lie, et al. (1998), Vandiver, et al. (2003) and Rao, et al. (2013), such as whether the dynamic response will be dominated by the riser induced or buoyancy modules induced motion, or how the different diameter ratio, length ratio and coverage ratio between the buoyancy modules and riser will affect the dynamic response, etc.

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