Numerical investigation of local scour around a vertically vibrating subsea pipeline under steady flow

Numerical simulations are conducted to investigate the local scour below a vertically vibrating subsea pipeline for gap ratios ranging from −0.3 to 0.1 and a wide range of reduced velocities ranging from 0.5 to 15. Positive and negative gap ratios represent the cases where the bottom of the pipeline is above and below the flat sand surface, respectively. The novelty of the work is the frequency analysis of the vibrating pipeline specially as a function of time as scour and erosion progression occurs. Both scour rate and the vibration amplitude are found to change suddenly at a critical time for reduced velocities inside the lock-in regime. The existence of the critical time is caused by the change of the response mode due to scour. Two types of critical time are found: Type 1 critical time where the vibration transitions from synchronisation to desynchronisation and Type 2 critical time where the flow transitions from steady flow to vortex shedding flow. The variations of the vibration amplitude with the reduced velocity for all the gap ratios follow the same trend and a distinct lock-in regime is found, but the width of the lock-in regime reduces with the reduction of the gap ratio. For all the studied gap ratios, the maximum scour depth consistently occurs at the reduced velocity of U_r = 5, which is the lower boundary of the lock-in regime.