Vortex-induced vibration of a near-bottom horizontal flexible pipe is experimentally investigated in a water flume. The non-intrusive measurement technique with high-speed cameras is employed to capture the oscillation displacements and solid-structure impacts simultaneously. It is found that a new equilibrium position is build up in the cross-flow direction due to the wall proximity. The pipe experiences a slow post-impact bouncing phenomenon. The mode transition with presence of multiple frequencies, becomes more complicated as the result of wall proximity and solid-structure impact.
Submarine pipeline spans are easily formed as results of local scour, pipeline crossings and uneven seabed (Fu et al., 2014; Munir et al., 2018). The length of free spans can reach 100 times of the pipeline diameter with a seabed clearance ranging from zero to two or threefold pipeline diameter (Yang et al., 2006; Gao et al., 2020). Such free spans exposed to currents possibly experience vortex-induced vibration (VIV) and a potential impact on the seabed. The flow around and VIV of a circular cylinder in unbounded condition have been studied extensively in the past decades due to the widespread presence in nature and engineering applications. The wake flow is dramatically altered when the circular cylinder is placed close to a wall, as the wake involves a complex interaction between the shear layers separated from the cylinder surface and the bottom wall. The understanding of the dynamic behavior of submarine flexible spans is still limited as the associated literature involving both the VIV of near-wall flexible pipe and solid-structure impact is scanty.
Tsahalis and Jones (1981) reported that the maximum vibration amplitude of a flexible pipe is limited due to the wall proximity. Asymmetry response envelopes of a flapping near-wall pipe were observed by Larsen (2002) using a nonlinear time-domain model. . Pontaza and Menon (2010) numerically illustrated the presence of seabed proximity strongly affects the VIV response and consequently the fatigue life of both crossing spans and pipelines laying on the seabed with two ends attached to terminations. Li et al. (2011) observed the mode transition from the fundamental dominant mode to the second one at three gap ratios (G/D, where G is the gap between the cylinder bottom and the plane boundary and D is the cylinder diameter) of 4.0, 6.0 and 8.0. Based on the three-dimensional (3D) direct numerical simulations, Ji et al. (2019) illustrated the response amplitudes are symmetric to the mid-span in both in-line and cross-flow directions for an inclined flexible cylinder placed with gap ratio of 0.8 at Re = 500. The oscillating cylinder may impact the wall boundary if G/D is small and the response amplitude is sufficient large. Such a solid-pipe impact possibly influences the overall subsequent response. Nevertheless, a very few relevant works have been reported. The bouncing back coefficient was reported by Zhao and Cheng (2011) as a determining factor in the oscillation of near-wall cylinder. Based on two-dimensional (2D) simulation results, three vortex shedding modes were identified: single-vortex mode, vortex-shedding-before-bounce-back mode and vortex-shedding-after-bounce-back mode.