Vortex-induced vibrations of submerged pipelines under unsteady flow: an experimental investigation of in-line and cross-flow coupling

Underwater pipelines serve as critical infrastructure for oil and gas transportation across both offshore seabeds and river crossings. Their structural integrity is increasingly threatened by unsteady hydrodynamic loading during extreme events such as floods and storm surges, where pipeline vibration and seabed scour interact in a complex, coupled manner that can lead to catastrophic fatigue failure and marine pollution. While vortex-induced vibration (VIV) has been extensively studied under steady flow conditions, the coupled in-line (IL) and cross-flow (CF) VIV mechanisms of suspended pipelines under unsteady flow and concurrent scour remain poorly understood. This study presents an experimental investigation of the IL–CF coupling behavior of a suspended pipeline on an erodible bed subjected to unsteady flow, with flow rates ranging from 12.3 to 12.68 L/s and initial clearance ratios e0/D of 0.8, 1.0, and 1.2. A modal analysis method was employed to extract the displacement response from the measured strain data, followed by analysis of the vibration frequencies, amplitudes, motion trajectories, and frequency ratios. The results reveal that the scour depth is governed jointly by the inflow rate, pipe diameter, and clearance ratio, with the latter acting as the dominant control parameter through a transition from a self-limiting scour regime at small e0/D to a sustained scour regime at large e0/D. A selective amplification mechanism is identified: the CF amplitude increases by 76% as e0/D rises from 1.0 to 1.2, whereas the IL amplitude increases by only 8%, indicating partial decoupling of the IL and CF responses. The IL-to-CF frequency ratio locks onto approximately 2.0 for e0/D>1.0, characterizing a stable symmetric vortex shedding mode and a strong-coupling regime, while it scatters irregularly between 1.0 and 2.0 at e0/D = 0.8. The motion trajectories evolve correspondingly from nearly circular to CF-elongated ellipses. These findings suggest a critical range of e0/D≈0.8–1.0 separating a bed-constrained, weakly coupled regime from a free-shedding, strongly coupled regime, with e0/D≈0.8 marking the onset of regular vortex shedding and e0/D≈1.0 indicating the fully established strong-coupling state, although further refined tests are needed to pinpoint the exact boundaries of this transition zone.