Experimental Investigation on Scour under a Vibrating Catenary Riser

Abstract

This paper investigates the impact of a vibrating steel catenary riser on scour in a steady current, motivated by the fact that pipeline scour studies are traditionally based on a horizontal stationary pipe. Laboratory experiments were conducted using a truncated riser subjected to a vertical sinusoidal motion of varying amplitude and frequency. By including a stationary test case for comparison, the results show that the scouring profiles for a stationary and a vibrating riser are fundamentally different. Using an acoustic Doppler velocimeter profiler to measure the flow velocities, the surrounding flow field was shown to be markedly modified by the riser vibration. The velocities fluctuated considerably at the vibration frequency, thereby promoting scour. The maximum scour depth increases significantly with both vibration amplitude and frequency, but frequency has a more profound influence. An explanation for this phenomenon is proposed based on various observed and measured results.