Effect of pipe-wall gap ratio on the flow-induced vibration of a submarine pipe span conveying flow

This paper reports the flow-induced vibration (FIV) of a near-wall flexible pipe conveying single-phase flow with gap ratio (G/D) ranging from 1 to 5. The flexible pipe possesses a length-to-diameter ratio of 75, and the experiments were conducted in the reduced velocity U_rrange of 1.6–17.56 in a water flume through the non-intrusive optical measurement. The experimental results indicate that the equilibrium positions of the pipe are altered in both in-line (IL) and cross-flow (CF) directions, depending on the effects of bottom wall and internal flow. With the reduction of G/D, the IL amplitude is generally amplified while the CF amplitude is reduced. The larger the offset of equilibrium position, the smaller the response amplitude. As G/D decreases, the U_rrange for mode transition is broadened, and the onset U_rof higher mode becomes larger. The effect of G/D is more prominent in mode transition cases, in which the spatial frequency competition is enhanced with the decrease of G/D. As G/D reduces, the strong coupling occurs more frequently with prolonged pipe length, especially for the large-amplitude segments. Although the weak coupling dominates the majority of pipe span, the coupling is enhanced with the decrease of G/D.