Sorting of sand grains around submarine pipelines under the oblique wave

Non-equilibrium sediments transport near the submarine pipeline results in local scour, thus endangering the operation of the pipeline. To investigate the sorting of sand grains in local scour hole under regular wave action, an experimental study is performed in the wave basin. The 1:15 slope with prototype sand, the median diameter of which is 0.219 mm, is tested against waves coming at 45 degree. The experimental results indicate that the evolution of the scour width, as well as the scour depth, can be divided into two stages: the primary stage and the secondary stage. The highest degree of bed coarsening is found to occur behind the pipeline rather than under the pipeline; bed coarsening grows with the increase of wave period, that is, the sand in the scour hole, compared with initial sand bed, will be refined under action of the short-period wave and coarsened by the long-period wave. However, the relationship between the wave height and the perturbation of bed sediment is not significant.