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深水海底管道典型失稳模式竞争机制及失稳预测
Alternative TitleMode-Competition and Predictions of the Instability of Deepwater Submarine Pipelines
师玉敏
Thesis Advisor高福平研究员
2019-11-28
Degree Grantor中国科学院大学
Place of Conferral北京
Subtype博士
Degree Discipline工程力学
Keyword海底管道,管土相互作用,竞争机制,随机场,侧向屈曲,可靠度分析
Abstract

深海长距离输运管道通常穿越不同海域,遭遇多种环境载荷和海床土体类型,因此合理评估海底管道结构稳定性和潜在风险对于管道铺设和安全运行保障具有重要意义。本文围绕管道侧向在位失稳、冲刷悬空、侧向整体屈曲问题开展研究,旨在获得海底管道失稳的耦合力学作用机理,揭示侧向失稳与悬空触发模式之间的竞争机制,建立考虑海床土性参数空间随机分布特性的长输管道细长结构的侧向整体屈曲失稳预测方法。

通过对我国南海某深水区表层沉积物柱状样的微观结构和物理力学性质的探究发现,深海沉积物的微观结构为絮凝或叠片状,呈现出高微生物含量、高孔隙比、低强度等典型特点。针对深海黏性土海床条件,建立了基于被动土压力理论的海底管道侧向失稳预测模型,得到了极限侧向土阻力及其被动土压力分量和滑动土阻力分量的显式表达式。开展流固土多场耦合分析,揭示了海流作用下管道侧向失稳与土体内部侵蚀诱导管道悬空两种典型失稳模式的竞争机制,构建了以无量纲管道嵌入深度、无量纲水下重量和临界流速三参量描述的管土相互作用系统失稳包络面。参量分析表明,当管道嵌入深度小于临界值时,管道下方土体发生侵蚀渗透破坏而导致管道发生悬空;而当嵌入深度超过该临界值时,管道下方土体侵蚀破坏将受到抑制从而使得管道侧向失稳更易被触发。

鉴于深水海底管道稳定性通常受到大陆坡斜坡海床的影响,开展了斜坡砂质海床上轴向管土相互作用研究。基于已有机械加载试验结果,分析获得了管道极限轴向抗滑力及相应位移的特征及规律。管土之间的圆弧接触面使得管土界面的径向压力积分高于管道水下重量而引起楔形效应,为此推导得到了针对砂土海床条件的楔形效应系数。进而构建了斜坡海床上管道极限轴向抗滑力的预测模型。

结合南海某深水区表层沉积物的勘探数据,对海床土性参数的空间随机分布特征开展了统计分析。基于随机场理论,采用局部平均细分算法实现了土性参数空间随机场的构建,提出了长输管道细长结构侧向整体屈曲失稳预测的半解析分析方法。开展了非均质海床上管道侧向整体稳定性的可靠度分析,蒙特卡洛模拟结果发现,管道侧向屈曲临界温度的均值略小于均质海床上的结果;且临界温度与不排水抗剪强度的变异系数之间近似满足线性关系。可靠度分析表明,采用确定性分析方法设计的管道将面临高概率整体屈曲的风险。

Other Abstract

A long-distance deepsea pipeline usually crosses different water areas, and encounters various environmental loads and seabed geological features. As sucha reasonable assessment of the pipeline stability and potential risks is of great significance for the laying and safe operation guarantee. This dissertation mainly focuses on the lateral-instability of pipe, the tunnel-erosion of seabed soil, and the lateral global buckling of submarine pipelines. Its aim is to obtain fluid (thermal)-structrure-soil coupling mechanisms of pipeline instability, to reveal the mode-competition mechanism between the lateral-instability of pipe and the tunnel-erosion of soil, and eventually establish a semi-analytical analysis method for predicting the lateral global buckling of long-distance pipelines, which can reflect random characteristics of the spatial distribution of seabed properties.

By analyzing the microstructure, the physical and mechanical properties of columnar surface sediments at certain deepsea locations of the South China Sea, we found that the microstructure of such deepsea sediments is flocculated or laminated, and that characteristic features of the sediments are high microbial content, high void ratio and low strength, etc. Based on the passive soil pressure theory, an analytical model is then developed for predicting the pipeline lateral instability on the clayey seabed. In the proposed model, the ultimate lateral soil resistance can be decomposed into a passive-pressure component and a sliding-resistance component. Through the flow-pipe-soil sequential coupling analysis, the competition mechanism between the lateral-instability of pipeline and the tunnel-erosion of the underlying soil is revealed. The instability envelope for the pipe-soil interaction system is further established, which can be described with three key parameters, i.e. the embedment-to-diameter ratio, the dimensionless submerged weight of pipe, and the corresponding critical flow velocity. Parametric study indicates that the tunnel-erosion is more prone to emerge than the lateral-instability for small values of embedment-to-diameter ratio. With increasing pipe embedment, the tunnel-erosion could be suppressed, and the lateral-instability thereby gets more preferential for occurrence.

Since the stability of deepsea pipelines is always affected by the sloping seabed at the continental slopes, the axial pipe-soil interactions along a sloping sandy seabed are investigated. By analyzing the results of the existing mechanical-actuator experiments, the basic characteristics and trends for the relationships between the axial anti-sliding capacity and its corresponding axial pipe displacement are obtained. Considering the fact that the integrated normal pipe-soil contact force would exceed the submerged weight of the pipe due to the effect of pipe curvature, a wedging factor for the sandy seabed is derived. An expression for the ultimate axial anti-sliding capacity is finally deduced.

According to the exploratory and experimental data of deep-sea surface sediments from the South China Sea, we carried out the statistical analysis of the spatial random distribution characteristics of sediments properties. Based on the random field theory, the construction of spatial random field model describing the soil properties is realized by employing the Local Average Subdivision algorithm. Then, a semi-analytical stochastic finite element model for predicting the lateral buckling of submarine pipelines is proposed, which incorporates the spatial random distribution characteristics of soil properties. With the proposed model, simulations and reliability analysis are performed for the lateral global stability of a submarine pipeline on heterogeneous seabed. The Monte Carlo simulation results demonstrate that, the mean value of critical temperature for a heterogeneous seabed is slightly smaller than that for a homogeneous seabed. Furtheremore, the critical temperature and the variation coefficient of undrained strength approximately satisfy a linear relationship. Compared with the present reliability analysis, the traditional deterministic analysis could predict the lateral buckling of the pipeline with a relatively high probability.

Language中文
Document Type学位论文
Identifierhttp://dspace.imech.ac.cn/handle/311007/80724
Collection流固耦合系统力学重点实验室
Recommended Citation
GB/T 7714
师玉敏. 深水海底管道典型失稳模式竞争机制及失稳预测[D]. 北京. 中国科学院大学,2019.
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