【作者】 张小玲；

【导师】 栾茂田； 郭莹；

【作者基本信息】 大连理工大学 ， 岩土工程， 2009， 博士

【摘要】 近年来,随着海洋石油和天然气的开发,海底管线在世界范围内得到了广泛的应用,成为海上油气田开发工程中的一个重要环节。海底管线铺设在恶劣的海洋环境中,将承受各类环境荷载,其中地震荷载是主要的荷载之一。我国处于世界两大地震带(环太平洋带和中亚—地中海带)之间,是一个多地震国家,尤其是有着丰富的石油资源的渤海湾地区属于地震高发区,所以铺设海底管线必须考虑地震的影响。为了合理地评价海底管线在地震荷载作用下的稳定性,必须对地震荷载作用下管线周围海床中的动力响应和管线自身内部动应力进行计算和分析,这也是管线工程设计人员的一个重要课题。本文对于多孔介质海床和海底管线,分别采用Biot动力固结理论与弹性动力学理论,同时采用摩擦接触理论考虑两者之间的相互作用效应,采用有限元方法建立了海床—管线动力相互作用的计算模型,通过有限元数值计算确定了地震作用下海床中超孔隙水压力和管线内应力等动力响应。在数值计算过程中引入粘弹性人工边界来模拟地震波的传播,真实地反映了在地震波作用下海底管线的动力响应问题。通过计算发现,本文所使用的粘弹性人工边界在求解瞬态动力问题上比固定边界具有很大的优越性,更接近于精确解。土的变形模量、渗透系数和管线半径、壁厚对管线外表面由地震所引起的孔隙水压力、径向正应力和剪应力及内表面的环向正应力均有一定的影响。均质海床与成层海床对水平地震荷载引起的管线外表面的超孔隙水压力响应差别较大,而对于管线的内应力响应的结果差别不大。管线周围土体的孔隙水压力随着矩形管沟覆盖层宽度的增加而减小,随着矩形管沟覆盖层厚度的增加而增大。但在不同长度和不同厚度下孔隙水压力的变化趋势是相同的。由于仪器设备的限制,目前还没有一个适用于地震循环荷载的较为准确的孔隙水压力增长表达式。本文利用先进的“土工静力—动力液压三轴—扭剪多功能剪切仪”,针对相对密实度Dr=60%的福建标准砂,在平均有效固结应力取p′m0=100,200,300kPa三种情况,对3种初始固结比Kc=1.0、1.5、2.0的情况分别进行了的9组循环扭剪试验。根据对试验结果的分析,建立了针对饱和砂土海床在地震荷载作用下的孔隙水压力应力模式,分析了初始固结比和初始平均有效固结应力对孔隙水压力发展特性的影响,较好的模拟了地震荷载作用下砂质海床中孔隙水压力的产生、发展和消散过程。针对海底管线周围海床液化边值问题,本文推导了考虑海床土微元控制体内有引起超孔隙水压力累积的源分布q的多孔介质渗流连续性方程,并联立孔隙流体的平衡方程和不排水条件下的超孔隙水压力增长模式建立了推广的带有累积超孔隙水压力源项f的二维固结有限元方程；进一步运用加权余量法对其进行了数值求解,计算预测了海床中累积超孔隙水压力的空间分布与时程变化规律,通过具体计算探讨了土性参数和管线几何特性对地震所引起的管线周围海床中累积超孔隙水压力分布的影响。通过计算分析可以发现：随着土的变形模量和泊松比的减小,由地震所引起的海床中的累积超孔隙水压力均增大。土的渗透系数对由地震所引起的管线周围海床中的累积超孔隙水压力具有显著的影响,渗透系数的微小减小就会导致海床中累积超孔隙水压力的显著增大。管线半径和管线埋深对管线附近海床中的累积超孔隙水压力及累积超孔隙水压力比有一定影响,并且影响规律较为复杂。管线的存在会对管线附近海床中的累积超孔隙水压力产生一定影响,在远离管线处的海床区域,管线的存在对累积超孔隙水压力的影响可以忽略。为了更具体地研究海底管线及其周围海床的动力响应,针对于三维饱和砂性海床和浅埋于其中的海底管线建立了海床-管线动力相互作用的计算模型,采用大型有限元计算程序DIANA-SWANDYNEⅢ,通过数值方法确定了三维条件下饱和海床中孔隙水压力和管线内应力等地震动力响应,讨论了地震荷载方向、海床土的渗透系数、海床厚度和表面水深以及管线半径和埋深对沿管线外表面由地震所引起的超孔隙水压力、管线外表面处径向正应力和剪应力及管线内表面处环向正应力的影响。通过对比分析表明,在单向地震波和双向地震波的情况下,由地震作用所引起管线外表面处的径向正应力和管线内表面处环向正应力结果有较大差别；管线周围的超孔隙水压力随着海床土渗透系数的减小、管线埋深的增加而明显增大；管线外表面处径向正应力和剪应力随着海床土渗透系数的减小而有所减小,随着管线半径和埋深的增大在不同的部位有不同程度的增加。更多还原

【Abstract】 People intensify the exploitation of submarine petroleum and natural gas in recent years because of the needs of energy sources increasing, it makes submarine pipelines applied widely. But submarine pipelines buried in ocean environment usually carry all kinds of loadings, not only wave loading but also seismic loading. The seismic-induced pore water pressure of seabed and internal stress of pipeline are important for people to design submarine pipelines. China is located in the middle of two main earthquake zones of the world (the zone around Pacific Ocean and the zone of middle Asia-Mediterranean Sea) and earthquakes occur easily, particularly in the Bohai Sea which has rich petroleum resources, therefore we must consider the influence of earthquakes when we bury submarine pipelines. In order to evaluate the stability of submarine pipelines under seismic loading correctly, the computation of seismic-induced pore water pressure and effective stresses in saturated seabed is one of main projects for engineers to design submarine pipeline.In this paper, the governing equations of the seabed and pipeline are formulated based on the Biot’s consolidation theory and elastic dynamic theory respectively. The model of the seabed-pipeline interaction is established by using the friction contact theory and the FEM analysis method. The distribution of the seismic-induced excess pore water pressure along the pipeline outer surface and the dynamic response of submarine pipeline under seismic loading are studied by virtue of the FEM numerical computations. Through numerical analysis, viscous-elastic artificial boundary is taken into account to simulate the transmission of seismic wave from finite region to infinite region effectively. It actually shows the validity and accuracy of dynamic response of submarine pipeline under seismic loading. Based on the numerical computations, it is found that the viscous-elastic artificial boundary has more superiority than fixed boundary in transient dynamics and the result under viscous-elastic artificial boundary is closer to precise result. The soil characteristic parameters such as deformation modulus and permeability coefficients, the pipeline radius and wall thickness all have some influence on the seismic-induced pore water pressure and radial normal stress as well as shear stress along the pipeline circumferential outer surface and circumferential normal stress along the pipeline circumferential inner surface. The effects of homogeneous seabed and two-layered seabed on the seismic-induced excess pore water pressure along the pipeline circumferential outer surface are remarkable but the effects on internal stresses of pipeline are small. The seismic-induced excess pore water pressure along the pipeline surface decreases with the width of cover layer increasing and increases with the thickness of cover layer increasing, but the tendency of excess pore water pressure with different length and thickness is almost the same.Due to the limitation of equipments, there have no exact expression to describe the building-up pore water pressure which is applicable to seismic cyclic loading so far. In this paper, the advanced soil static and dynamic universal triaxial and torsional shear apparatus is employed to perform cyclic torsional shear tests subjected to cyclic loading. In order to obtain the building-up expression of pore water pressure of seabed under cyclic torsional loading, a series of cyclic torsional tests of different initial effective consolidation pressure (p’m0=100, 200,300kPa) and different initial consolidation ratio (Kc=1.0、1.5、2.0) are performed for saturated Fujian standard sand with relative density of Dr=60%. By the analysis of test results, the stress mode of pore water pressure in saturated sandy seabed under seismic loading is established, the effects of initial effective consolidation pressure and initial consolidation ratio on the pore water pressure are investigated, the generation, increasing processes of pore water pressure in sandy seabed under seismic loading is simulated preferably.According to the boundary value problem of liquefaction around a buried pipeline, the continuity equation for porous medium including accumulative excess pore water pressure source term distribution q in unit control volume of seabed is deduced, two-dimensional dynamic consolidation equation with accumulative excess pore water pressure source term f is established by incorporating with the mode of dynamic increase of excess pore water pressure under undrained conditions gained from tests to equilibrium equation of pore fluid; and it is solved by employing weighted residual method. By numerical computations, the effects of soil characteristic parameters and pipeline geometry on seismic-induced accumulative excess pore water pressure around the pipeline and along the depth of seabed are studied, the accumulation process of pore water pressure and liquefaction potential of seabed soil under seismic loading are evaluated in details.By computations and analysis it is found that the seismic-induced accumulative pore water pressures of seabed increase with the values of soil deformation modulus and Passion’s ratio decreasing. The effects of soil permeability coefficient on seismic-induced accumulative pore water pressure ratios of seabed around pipeline are remarkable. The seismic-induced accumulative pore water pressure ratios of seabed increase with the values of the permeability coefficient decreasing, the small reduction of permeability coefficient will lead to significant increment of accumulative pore water pressure ratios of seabed. Otherwise the radius and burial depth of pipeline have some influence on seismic-induced accumulative pore water pressure ratios of seabed around pipeline but they are more complicated. The existence of pipeline only influences the accumulative pore water pressure ratios of seabed nearby the pipeline. The effect of pipeline is very slight and can be neglected in the region of seabed far away from the pipeline.In order to further study dynamic response of submarine pipeline and the seabed around pipeline, a three-dimensional model of the seabed-pipeline interaction including buried pipeline is established based on the FEM analysis method. By using the extending DIANA-SWANDYNE III models, the effects of earthquake loading directions and soil characteristics, such as soil permeability, water depth, seabed depth and pipeline configuration, such as pipeline radius and pipeline buried depth on the seismic-induced excess pore water pressure along the pipeline outer surface and the dynamic response of the pipeline are examined. It can be concluded from numerical results:there is an obvious difference between the distribution of the seismic-induced excess pore water pressure and radial normal stress as well as shear stress along the pipeline circumferential outer surface and circumferential normal stress along the pipeline circumferential inner surface under different seismic loading, the seismic-induced excess pore water pressure around pipeline increases with soil permeability decreasing and pipeline buried depth increasing, radial normal stress as well as shear stress along the pipeline circumferential outer surface decrease with soil permeability decreasing, and increase in some extent at some points with pipeline radius and buried depth increasing.更多还原