海管浮重对砂床局部冲刷扩展的影响研究

This proposal is focusing on the influncing law of pipeline submerged-weight in pipeline-sandy bed local scour procedure. The distribution history and statistical characteristics of pore water pressure in sandy bed is studied by experiment, and the influnce of pipeline submerged-weight on sand critical break and propagation configuration. To be specific, we are going to carry out a series of experiments, in which the pore water pressure history and the flow field near equilibrium scour bed are measured using dynamic distributed pore water pressure gage and high speed PIV respectively. Based on the data measured and combined with the stress distribution in sandy bed from soil mechanics, the interaction pattern within sandy bed is conducted. Then the condition of sand critical break is drawn out. In this way, combined with the measured scour hole configuration and flow shear stress, the modified formula of sand critical break under the load of pipeline submerged-weight could be given out. Ultimately, a prediction model for sandy bed local scour propagation under pipeline submerged-weight could be provided for practical usages. When the mechanism of the pipeline influncing the scour propagation has been further understood, the discussion on maximum length or slope of scour hole could runs further. In summary, this proposal could provide theoritical support and basic validation data for this kind of research.

本项目拟针对海管-砂床局部冲刷过程中海管浮重载荷的影响规律这一关键问题，以冲刷实验中砂床的孔压分布演化及统计特征为切入点，探讨海管浮重对砂粒临界起动及冲刷扩展形态的影响。具体而言，本项目拟通过分布式孔压动态测量系统与高速PIV系统，对海管浮重作用下的砂床孔隙水压分布在冲刷过程中的演化趋势以及冲刷平衡时床面附近的流场进行精细测量；并以测量数据为基础，结合土力学理论方法得到的砂床内应力分布状况，讨论砂粒间的相互作用力方式，并讨论冲刷坑床面上砂粒临界起动的条件变化，结合实测的床面形态与水流切应力，给出海管浮重作用下的砂粒临界起动判据修正形式，最终提供预测海管浮重作用下的砂床局部冲刷扩展预测模型。为此类研究的发展和应用提供基础数据与有效思路。

海底管道（下文简称海管）附近砂床的局部冲刷是一种常见而又复杂的环境现象，其导致的海管悬跨可能会引起海管断裂，酿成巨大的经济损失与环境灾难。.1.应用土力学和泥沙运动理论，修正了外载荷作用下的泥沙临界起动判据。与传统的泥沙起动不同，载荷作用下的泥沙起动除需要克服自重外，还需要克服临近沙粒附加应力作用导致的摩擦力。根据土体内附加应力的空间分布特点，提出了简化海管跨肩载荷的方式。.2.建立了海管下方底床的三维冲刷扩展模型。模型能够预测海管浮重作用下底床的冲刷扩展平衡跨度和坡度。冲刷跨度随来流速度和海管长细比的增大而增大，随水深的增大而减小。冲刷坡度随来流速度的增大先减小，达到最大冲刷深度后逐渐增大。根据番禺气田的泥沙和水深实测参数，模型预测出与实测相吻合的冲刷坡度值。结合相关实验研究结论，实际的冲刷坡度应当远小于泥沙自然休止角。.本研究为工程上预测海管下方的最大冲刷深度和跨度这两个重要的环境条件参数提供了便捷、高效的预测方法，分析并澄清了冲刷扩展坡度大小的问题。