钙质砂颗粒破碎对管道与海床相互作用的影响机理研究

Offshore on-bottom pipelines are vulnerable to lateral buckling caused by thermal- and pressure-induced axial expansion, which may lead to severe economic lost and environmental hazard. A cost-effective solution is to allow for the buckling to occur in a controlled manner rather than to prevent it. This design approach requires an ever more accurate quantfication of the soil resistance experienced by the pipe during large lateral displacements so that a planned buckle can be initiated; and it is not practical to adopt a conservative assessment of soil response. Calcareous sands proliferate in the seabed of the South Sea and the East Sea, where construction of offshore pipelines is in high demand. These are highly compressible sands comprising the remains of shelly and skeletal marine organisms which tend to form a very loose fabric. The grains are always of high intra-granular voids and low hardness, thus are crushable under loading. The pipe-soil interaction in these sands can cause significant grain slippage and breakage, and consequently affects the bearing behavior of the pipe. .This project aims to investigate the effect of grain slippage and breakage on large deformation pipe-soil interaction on calcareous sands, and to propose assessment methods of the service performance for offshore on-bottom pipelines. This is to be achieved through studies described as follows. Firstly, in situ loading tests of calcareous sand samples are undertaken within an X-ray micro computed tomography (CT) scanner to obtain high resolution three-dimensional images. Then detailed morphological quantification of the sand is performed by analysing the sizes, shapes, contacts, etc. of the grains, based on an adaptive watershed segmentation (AWS) technique; and the grain motions and grain breakage under loading are assessed using a discrete-continuum imaging method developed based on AWS and the digital volume correlation (DVC) technique. The Mohr-Column constitutive model is then modified to account for the post-peak mechanical behavior of calcareous sand caused by grain breakage. Secondly, large-displacement full scale model tests are to be carried out to look into the vertical and lateral loading behavior of pipe segments with different diameters. Thirdly, the new constitutive model is to be incorporated into coupled Eulerian Lagrangian method in Abaqus, with the numerical model being validated against the model testing results and against each other. Finally, analytical equation models are to be developed to quantify the cyclic embedment of on-bottom pipelines during installation and the soil resistance experienced by the pipelines during large lateral displacements on calcareous sands.

海底管道在油气输送过程中受热膨胀易侧向屈曲, 可能造成管道破裂及严重的经济损失和环境污染。目前海底管道设计普遍采用可控屈曲准则，即允许管道按指定形式屈曲以释放热胀应力，这对大变形下管土相互作用的定量评估提出了更高要求。我国南海广泛分布有钙质砂，其颗粒形状极不规则、且内孔隙比高，在管土相互作用中发生颗粒破碎和构造重组，会严重影响管道的侧向大变形。.本项目的研究目标是揭示钙质砂颗粒破碎对管土相互作用的影响机制，并提出相应的评估方法：结合CT断层扫描技术开展钙质砂单元体压缩和剪切试验，基于图像分析获取颗粒形态表征模型及运动和破碎特性；揭示其与颗粒破碎相关的峰后力学行为, 修正现有超塑性本构模型；利用有限元大变形分析方法建立考虑颗粒破碎与构造重组的管土相互作用数值分析模型，开展足尺模型试验对其进行验证，提出管道安装深度、在位稳定性和水平大变形位移的分析方法。

本项目按照原计划进行，针对南海钙质砂微观构造表征和演变及其对管道与海床相互作用的影响机理问题，通过开展CT扫描下的钙质砂的原位侧限压缩试验、管道竖向受荷特性模型试验、管道-海床相互作用大变形数值模拟等手段开展研究，取得的主要进展如下：（1）实现了对南海钙质砂微观结构的定量表征，获取了颗粒典型形态参数的分布及土体构造重组与颗粒尺寸、形状、接触及应力水平的相关性，揭示了颗粒运动与破碎对钙质砂物理力学参数的影响机制；（2）通过钙质砂中管道竖向贯入模型试验，探索了颗粒破碎对竖向贯入阻力的影响规律；（3）建立了管道-海床相互作用大变形连续极限分析模型，揭示了管道水平循环振荡对海床的“犁”效应，提出了考虑施工扰动的海底管道自重安装深度分析方法和并建议了动力因子取值；（4）揭示了管-土作用最薄弱方向与海床面法向方向存在偏角的机理，发现了现有基于管土竖向抗力设计规范和分析理论在评估海底管道在位稳定性时偏不安全的缺陷，提出了表征这一效应的管土抗力修正模型；（5）提出了管道大变形水平屈曲过程中残余水平抗力的计算方法，建立了水平位移过程中管道-海床体系的V-H屈服包络面演变归一化模型。. 本研究课题共发表相关学术论文10篇（均标注该项目资助），包括《Géotechnique》和《J. Geoenviron. Geotech. Eng., ASCE》等本领域权威期刊论文8篇，EI论文2篇，公开国家发明专利1项，项目研究过程中共培养博士研究生3名，硕士研究生3名。依托该项目所建立的管道-海床大变形相互作用模型受到工业界重视，目前全球最大的海底管道设计与安装公司Technip FMC已委托项目负责人开展相关分析并于工程中应用。.