Distribution of drag coefficients along a flexible pipe with helical strakes in uniform flow

Abstract

The influences of helical strake geometry and vortex-induced vibration (VIV) on the drag coefficients of a long flexible pipe with helical strakes are investigated through uniform flow experiments with the Reynolds (Re) number ranging from 1.1 × 104 to 9.5 × 104. The strains caused by drag forces of a bare pipe and six different straked pipes are measured by fiber bragg grating (FBG) sensors. According to the inverse analysis method and Morison formula, the drag forces and coefficients at each section of the pipe models are identified. Then, the mean drag coefficients of the bare and straked pipes are compared. The results show that helical strakes can effectively stabilize the fluctuation of drag coefficients with Re numbers. An unexpected phenomenon is that helical strakes with pitches of 5 D can reduce the mean drag coefficients. Moreover, the mean drag coefficients of straked pipes are less sensitive to VIV response. Furthermore, an empirical mean drag coefficient prediction formula for straked pipes is proposed, which accounts for the VIV response and geometry of helical strakes. Its applicability is well verified via comparison between predicted and experimental results.