Mean Drag Force of Flexible Riser Under Bidirectionally Sheared Flow

Abstract

There exists a special bidirectionally sheared flow field due to solitons in the South China Sea. An experimental study of the mean drag force of a flexible riser undergoing vortex-induced vibration under bidirectionally sheared flow is conducted. Modal analysis and the beam theory are applied to processing the strain signal measured in the experiment to obtain the mean drag force. The initial displacement, mean drag force, and mean drag coefficient are investigated, and the amplification of the mean drag coefficient is confirmed under bidirectionally sheared flow with comparable amplification to linear shear flow. Meanwhile, a unique phenomenon that the shear force reaches an extreme value in the center of the riser under bidirectionally sheared flow is found in the experiment. The shear force coefficient is proposed and fitted to obtain an empirical formula for the extreme value of the central shear force of the flexible riser under bidirectionally sheared flow, which will provide external load input for the design of the riser in the South China Sea.

Keywords

• vortex-induced vibration
• bidirectionally sheared flow
• mean drag
• shear force