Experimental study of scour around a forced vibrating pipeline in quiescent water

Submarine pipelines are widely used for the conveyance of hydrocarbons, oil or other fluids in the offshore industry. Although much progress has been made on pipeline-seabed interactions in the past decades, an experimental study solely devoted to investigating interactions between the flow fields, scour and forced vibrating pipelines in a quiescent water environment still has not been conducted. In practice, this condition, to a certain extent, resembles a Steel Catenary Riser (SCR) in deep offshore environments in the absence of currents and waves. The present study aims to obtain an improved understanding of the mechanics of scour around a forced vibrating pipeline in quiescent water. Tests with the same initial gap ratio G0/D = 1 and two different induced frequencies were conducted. A circular cylinder with a diameter of 3.5 cm was used as the pipeline model. With the help of a high-speed camera, the details of flows, pipeline motion and scour process are vividly presented in this paper. The magnitudes of flow velocity, vorticities, and turbulence fluctuations are found to be strongly dependent on the frequency of the vibrating pipeline. The scour mechanism of the two tests is different because of the complex interactions between the flow fields and pipeline motion. For the low-frequency case, the flow field associated with the falling stage of the vibrating pipeline controls the formation of the scour trench. On the other hand, for the high-frequency test, the scour process is controlled by the flow field during the rising stage of the vibrating pipeline.