Experimental Study of Local Scour around Pipeline in Oblique Flow

The critical gap ratio of local scour around pipeline and variations of bed form in the action of oblique flow was studied by flume experiments. A tube was chosen as a simplification submarine pipeline in the experiment. A high accuracy instrument ADV was used to measure the parameters of the flow field and a topographic meter was used to measure the terrain. The incident directions of the flow were 30 degrees, 45 degrees, 60 degrees and 90 degrees, with 0 degrees representing flow parallel to the axis of the pipeline and 90 degrees representing flow perpendicular to the axis of the pipeline. The results showed that the form of sand ripples behind pipeline can be divided into transverse and longitudinal forms which were under the action of oblique flow. With the angle a between flow and pipeline was the smaller, longitudinal sand ripples become more obvious. There was only formed horizontal sand ripple while the angle alpha = 90 degrees and when a = 30 degrees, it had hardly any horizontal sand ripple. The critical gap ratio of the local scour G = e/D increased as the angle alpha (0 degrees-90 degrees) increasing. At the same velocity and gap ratio, with the angle was the greater, the local scour of the pipeline become more obvious.