弹性海底管道涡激振动与沙质海床三维局部冲刷耦合机理研究

The dynamic processes of the vortex-induced vibration (VIV) of free-spanning pipelines and the local scour of seabed are highly coupled, mutually promoted and jointly developed. However, due to the high complexity of the problem, researchers tend to ignore the VIV of pipelines and just investigate the local scour of seabed surrounding stationary pipelines, or overlook the local scour of seabed and merely study the VIV of pipelines over a fixed seabed. These will inevitably under-estimate the depth and scope of the scour holes and the VIV amplitude of the pipelines, which eventually lead to a great increase in the risk of fatigue failure of pipelines. Although, there are few studies which investigated the interaction between the two processes, they are all focused on the 2-dimensional scouring and VIV which is obviously different form the reality. In the proposed project, the 3-dimensional VIV of a flexible submarine pipeline over an erodible sandy seabed will be systematically investigated by applying CFD numerical simulations together with laboratory model tests. The effects of flow directions on the multi-mode responses of VIV of pipelines, together with the effects of the 3-dimensional spiral vortices on the local scour near the span shoulders, will be looked into. The coupling mechanisms of the VIV of flexible pipelines and the 3-dimensional local scour of sandy seabed will be investigated. Based on these, an improved computing model of VIV of submarine pipeline and local scour of sandy seabed will be presented by taking into account of the flow-pipeline-soil coupling mechanisms. The proposed project will provide a theoretical basis for the optimal structure designing and fatigue analysis of submarine pipelines.

悬跨管道的涡激振动和海床的局部冲刷高度耦合，两者相互促进，同步发展。然而由于问题的复杂性，已有研究或是忽略管道振动，仅研究固定管道周围的海床冲刷，或是忽略海床冲刷，仅研究定床条件下海底管道涡激振动，过低地估计了冲刷的深度、范围和振动的振幅，导致管道疲劳破坏风险大大增加。尽管有少量研究关注了两者之间的耦合，但却仅研究了二维的海床冲刷与管道振动，与实际情况不符。本项目针对可冲蚀沙质海床上的弹性海底管道三维涡激振动问题，采用CFD数值模拟为主、物理模型实验为辅的研究方法，分析来流方向对弹性海底管道涡激振动多模态响应的影响，揭示三维螺旋来流对跨肩处床沙起动的作用机制，探究弹性海底管道涡激振动与沙质海床三维局部冲刷的耦合机理，提出考虑流-管-土耦合作用的、改进的弹性海底管道涡激振动和沙质海床三维冲刷计算模型，为海底管道的优化设计和疲劳分析提供理论依据。

悬跨管道的涡激振动和海床的局部冲刷高度耦合，两者相互促进，同步发展。由于问题的复杂性，已有研究或是忽略管道振动，仅研究固定管道周围的海床冲刷，或是忽略海床冲刷，仅研究定床条件下海底管道涡激振动，过低地估计了冲刷的深度、范围和振动的振幅，导致管道疲劳破坏风险大大增加。本项目针对可冲蚀海床上的弹性海底管道三维涡激振动问题，建立了弹性海底管道涡激振动和海床三维局部冲刷的流-管-土完全耦合数值模型，开发了并行计算程序。开展了斜向来流作用下弹性海底管道涡激振动研究，证实了独立性原理对横流向振动和升力系数的预测较为准确，但对顺流向振幅和尾涡的预测有较大的误差。分析了海底管道的振动响应和尾涡模式随来流方向、初始间隙比和折合流速的变化，发现在近壁面条件下，管道振动轨迹呈现水滴形或椭圆形，流速较小时，管道顺时针振动，流速较大时，管道逆时针振动。考察了雷诺数对管道涡激振动响应和尾涡模式的影响，发现在TrSL3（完全湍流剪切层）雷诺数范围内，孤立管道涡激振动振幅高达2.3倍的直径，远超前人研究结论的1倍直径。开展了固定刚性管道周围沙质海床三维局部冲刷研究，发现管道后方尾涡模式依次出现三个阶段：二维脱涡、平行脱涡和倾斜脱涡阶段，升力相关性和脱涡模式有密切联系。分析了三维螺旋来流对跨肩处床沙起动的作用机制，发现正交来流时，跨肩处的螺旋来流是悬跨发展的主要原因。随着来流倾斜程度增加，来流的展向速度分量逐渐取代螺旋来流成为悬跨发展的主因。研究了初始间隙比、来流方向和谢尔兹数对沙质海床三维冲刷发展的影响，发现正交来流时，悬跨发展分为三个阶段：快速冲刷、缓慢冲刷和冲刷稳定阶段。而倾斜来流时，随着来流角度的增加，悬跨发展阶段趋于不明显。开展了弹性海底管道涡激振动和沙质海床三维冲刷研究，分析了弹性海底管道涡激振动与海床三维局部冲刷耦合机理，给出了近壁管道发生涡激振动的临界间隙比和考虑管道涡激振动时冲刷坑深度增幅的计算公式。