Experimental study on local scour of free-spanning submarine pipelines considering the effects of vortex-induced vibration

Local scour can destabilize free-spanning submarine pipelines and promote vortex-induced vibration (VIV), while VIV can further enhance scour through changes in near-bed flow, forming a coupled feedback process. To investigate this interaction under laboratory constraints, an instrumented experimental platform was developed using a truncated flexible pipeline model tuned to a target natural frequency, so that regime-consistent reduced-velocity similarity could be achieved while retaining structural flexibility. The setup allows independent control of support stiffness, flow velocity, and gap-to-diameter ratio, enabling systematic quantification of coupled VIV–scour responses under different boundary conditions. Results show that an enhanced-response regime was observed around V r = 5–7, where vibration amplitudes are up to twice those outside this range. Support stiffness shifts the natural frequency and thus controls the occurrence and intensity of lock-in. Vibration also alters scour development, producing a deeper, more continuous and redistributed scour morphology under lock-in. A positive correlation between scour depth (Sd/D) and vibration amplitude (A/D) suggests that pipe vibration can enhance or modify local scour under the present boundary-controlled conditions. Boundary conditions therefore play a key role in VIV–scour coupling and may provide a basis for mitigating both vibration and seabed erosion.