Visualization of the Onset of Scour under a Pipeline in Waves

Abstract

This paper visualizes the onset of scour under a pipeline in regular waves and studies the mechanism in this process. After the test started, the sand particles near the downstream mudline began to oscillate periodically, probably due to the distribution pattern of the seepage hydraulic gradient under the pipeline as well as asymmetric waves. As the wave height gradually increased, the sand under the pipeline began to oscillate at increasing amplitude and the coverage of oscillation extended upstream. Some sand rushed out from the bottom of the pipeline, and the sand oscillated almost symmetrically around the bottom line of the pipeline thereafter. The oscillation amplitude of the sand particles continued to rise, probably due to a decrease in the seepage path under the pipeline and loosened sediment in the oscillation. The scour onset occurred after more sand rushed out from under the pipeline. The visualization results reproduced the delay in the scour onset reported previously, and related the delay to the aforementioned increase in sediment oscillation amplitude. This visualization can improve the understanding of the scour onset mechanism, and can serve as a guide to further investigate the critical conditions of scour onset and scour control countermeasures for offshore structures.