海底管线肩部三维冲刷的研究

Submarine pipelines laid on the seabed disturb local flow fields, so sediment is often eroded away from underneath the pipelines, causing the pipelines to be suspended over a distance, which poses a great risk to the safe operation of the pipelines. Most of the research so far is limited to the two-dimensional scour around pipelines, which examines the flow field, sediment transport and bed deformation at the central part of the scour pit and in the vertical plane perpendicular to the pipeline. Moreover, the onset of scour and the development of the scour have only been studied separately. This project aims to investigate the three-dimensional scour phenomena through theoretical analyses, laboratory experiments and numerical simulations. In particular, it will gain insight into the mechanism of the scour development along the pipeline and establish a mathematical model capable of simulating both the onset and evolution of the scour by considering both bed shear stress and seepage-induced pressure gradient inside the bed. Finally, this project will reveal the general principles of the three-dimensional scour around offshore pipelines through parametric studies using the established mathematical model, including the situations when the pipeline vibrates. This research will enrich the research on the flow around a circular cylinder near a plane boundary, improve the mathematical description of the local sediment transport, and reveal the three-dimensional scouring mechanism. Moreover, The outcome will not only bridge the gap in the current understanding of the three-dimensional scour beneath pipelines, but also provide technical support for marine resource exploitation.

铺设在海底的管线干扰了局部的水流运动，经常导致管线底部的泥沙被冲走、使得管线出现局部悬跨而对管线的安全运行造成严重隐患。现有的工作大都局限于绕海底管线的二维冲刷现象，研究冲坑中部垂直于管线的截面内的流场、泥沙运动和床面变化，而且都只是孤立地研究冲刷起动的条件或者冲刷发展的过程。本项目将通过理论分析、模型实验和数值模拟，研究三维的管线冲刷问题，特别要研究冲坑在端部沿管线方向的扩展机理，并将同时考虑作用在泥沙颗粒上的床面剪应力和沙体中水力梯度所诱发的渗透力、建立模拟冲刷发生和发展全过程的数学模型。最后，通过大量的数值实验，总结、归纳三维冲刷在不同流动和管线参数组合下的一般规律，包括管线在振动情况下的三维冲刷。本研究将丰富近壁圆柱绕流的研究内容，完善描述局部泥沙运动的数学模型，揭示管线三维冲刷的机理。这将填补目前关于管线三维冲刷研究的空白，为海洋能源的开发提供技术支持。

研究了铺设在海床上的管道在水流作用下的三维冲刷问题。现有的工作大都局限于绕海底管道周围的二维冲刷现象，研究冲坑中部垂直于管线的截面内的流场、泥沙运动和床面变化，而且都只是孤立地研究冲刷起动的条件或者冲刷发展的过程。本项目通过理论分析、模型实验和数值模拟，研究了多种三维的管线冲刷问题，特别研究了冲坑沿管线方向的扩展、有限长度管线的冲刷和振动管线附近的冲刷，并同时考虑了作用在泥沙颗粒上的床面剪应力和沙体中水力梯度所诱发的渗透力、建立了模拟冲刷发生和发展全过程的数学模型。在计算中既采用了传统的基于网格的数值计算方法，又采用了现代的无网格计算技术。通过大量的数值实验，总结、归纳了冲刷在不同流动、床沙和管线参数组合下的发展变化规律，包括管线在振动情况下的三维冲刷。本研究丰富了近壁圆柱绕流的研究内容，完善了描述局部泥沙运动的数学模型，揭示了管线三维冲刷的机理。特别是在管道冲刷模拟中，开发了可以同时使用两套粒子的物质点算法，分别模拟水体和土体的运动。水体的运动包括床面上的自由水的流动和土体中的渗流流动，而土体包括沙床和做推移质和悬移质运动的泥沙。通过分析床面上土颗粒在水流剪应力和土体中渗流力联合作用下的运动，初步建立了水流-结构物-床面强耦合系统的分析方法，促进了工程水力学和工程泥沙学科的发展。