Experimental study on local scour around a forced vibrating pipeline in unidirectional flows

The interaction between a forced vibrating pipeline and an erodible seabed in unidirectional flows is experimentally investigated. The experiments were conducted under clear-water scour conditions with a water depth of 0.3m and averaged approach velocity of 0.261 m/s. The pipeline model with a diameter (D) of 3.5 cm, was subjected to a vertical sinusoidal motion of varying amplitudes (A(0) = 2 similar to 6 cm) and frequencies (f(0) = 0.1-0.6Hz). The initial gap (G(0)) between the lower pipe surface and the undisturbed flatbed level was fixed at 1D. The results show that the maximum scour depth increases significantly with both vibration amplitude and frequency, with the former having a more dominant influence. However, frequency tends to exacerbate the scour hole development more in the early scour stage. Distinct scour mechanisms are observed based on the different combinations of vibration amplitude and frequency. In the high A(0) conditions (A(0) > G(0)), the combined effect of pounding (when the pipe hits the seabed) and piston actions (rising and falling of the pipe) during the pipe descending period dominates the earlier stages of scouring, and the pick-up of sediment particles during its rising period controls the subsequent scour process. In the low A(0) conditions (A(0) < G(0)) when pounding does not occur, vortex shedding during the pipe-falling stage controls the development of scour hole when the frequency is low. When the frequency is high, however, the flow field associated with the pipe-rising period dominates the scour development. Different empirical formulas for the prediction of the maximum scour depth and width are proposed and compared in the present study.