Experimental Research On 3D Scour Below Submarine Pipelines Under The Oblique Waves

The nearshore seabed is undulating and the waves are not orthogonal to the pipe axis frequently.The characteristic of three-dimensional scour below wave on pipeline was changed necessarily by slope and the difference of wave propagation direction.Therefore,further study of three-dimensional scour mechanism of submarine pipeline which under the action of oblique wave on the slope seabed of nearshore has the important scientific and practical significance.In this paper,the wave basin under the regular wave action with 45 degree incident angle,in which the median particle size of 0.219 mm on 1:15 slope is adopted.In order to analyzes the hydrodynamic characteristics around the pipeline,seaded morphology evolution,development process and shape features of three-dimensional scour,and sediment transport of scour pit mechanism,the change of three-dimensional scouring of submarine pipeline in different wave elements,angle and buried depth are considered.The results show that the hydrodynamic characteristic around pipeline has the obvious three-dimensional feature when the waves transmit to the shore obliquely,firstly.The wave height which on the left side of the wave propagetion direction is higher than whi ch on the right side and the existence of pipeline increases the height difference between left side and right side when the water depth along pipeline is the same,while the flow velocity is larger around pipeline in the pipeline where the water depth is small than where the water depth is big and the existence of pipeline increases the flow velocity behind pipeline greatly when the water depth along pipeline is different.Secondly,the evolution of seabed also becomes obvious different.Due to the reflection of pipeline to wave,the big sand ripple which is parallel to the pipeline and the small sand ripple which is orthogonal to wave propagation direction are appear at the same time when a connected scour pit do not form before and after the pipeline.Thirdly,the local scour of pipeline has three-dimensional scouring feature obviouly.When pipeline is orthogonal to wave propagation direction,the water depth along pipeline is different because of the complexity of the slope.The local scour of pipeline has a significant malconformation and the wave cycle has much effect onthe scour malconformation at this time.Changing the pipeline Angel to make the water depth along pipeline consistent,the local scour of pipeline has weak malconformation,but the degree of malconformation does not affected by the wave height and period.Through the analysis of the scour process,the fact that malconformation is form at the rapid development stage of scour is found.Finally,the sediment transport of scour pit is manifest as non-uniform sediment transport.The place which is the highest degree of sandbed coarsening around the pipeline is not just below pipeline,but lower back of pipeline and the degree of sandbed coarsening is increase with wave cycle.