Three-dimensional poro-elasto-plastic model for wave-induced seabed response around submarine pipeline

The evaluation of the wave-induced excess pore pressure around a buried pipeline is particularly important for pipeline engineers involved in the design of offshore pipelines. Existing models for the wave-induced seabed response around submarine pipeline have been limited to poro-elastic soil behavior and de-coupled oscillatory and residual mechanisms for the rise in excess pore water pressure. To overcome the shortcoming of the existing models, in this study a three-dimensional poro-elasto-plastic soil model with submarine pipeline is established, in which both oscillatory and residual mechanisms can be simulated simultaneously. With the proposed model, a parametric study is conducted to investigate the relative differences of the predictions of the wave-induced pore pressure with poro-elasto-plastic model. Based on numerical examples, it can be concluded that the poro-elasto-plastic behaviors of soil have more significant influence on wave-induced pore pressure of seabed around submarine pipeline. As the seabed depth increases, the normalized pore pressures decrease rapidly at the upper part of seabed, and then change slightly at the lower part of the seabed. Soil permeability and wave period have obvious influence on the wave-induced normalized pore pressure.     

Numerical analysis of liquefaction of porous seabed around pipeline fixed in space under seismic loading

Liquefaction of seabed under seismic loading is one of the main points that govern the overall stability of submarine pipeline. However, most previous investigations concerned only with free seabed and searched for seismic accumulative excess pore pressure by solving Terzaghi's consolidation equation containing pore pressure source term. It is not able to introduce two-dimensional structures such as submarine pipelines in one-dimensional problem, and it is also not able to obtain the distribution of seismic accumulative excess pore pressure in seabed around submarine pipelines by such a way. In this study, a FEM numerical analysis method for determining the liquefaction of sandy seabed around a buried pipeline under seismic loading is presented. The empirical mode of dynamic increase of pore pressure under undrained shearing induced by seismic loading is incorporated with two-dimensional dynamic consolidation equation and a numerical procedure based on FEM is developed to assess the accumulative excess pore pressure. By numerical computations, the accumulative process of pore pressure and liquefaction potential of seabed soil during seismic loading is evaluated. From a series of numerical computations based on the presented model with various parameters, the effects of soil characteristic parameters and pipeline geometry on seismic accumulative excess pore pressure around submarine pipeline and along the depth of seabed are explored in detail.     

Effects of cross-anisotropic soil behaviour on the wave-induced residual liquefaction in the vicinity of pipeline buried in elasto-plastic seabed foundations

In this paper, a two-dimensional integrated numerical model is developed to examine the influences of cross-anisotropic soil behaviour on the wave-induced residual liquefaction in the vicinity of a pipeline buried in a porous seabed. In the wave model, the RANS (Reynolds Averaged Navier–Stokes) equation is used to govern the wave motion. In the seabed model, the residual soil response in the vicinity of an embedded pipeline is considered with the 2-D elasto-plastic solution, where the phase-resolved shear stress is used as a source for the build-up of residual pore pressure. Classical Biot׳s consolidation equation is used for linking the solid-pore fluid interaction. The validation of the proposed integrated numerical model is conducted by the comparisons with the previous experimental data. Numerical examples show that the pore pressures can accumulate to a large value, thus resulting in a larger area of liquefaction potential in the given anisotropic soil compared to that with isotropic solution. The influences of anisotropic parameters on the wave-induced residual soil response in the vicinity of pipeline are significant. A high rate of pore pressure accumulation and dissipation is observed and the liquefaction potential develops faster as the anisotropic parameters increase. Finally, a simplified approximation based on a detailed parametric investigations is proposed for the evaluation of maximum liquefaction depth (z_L) in engineering application.     

Numerical modeling for wave–seabed–pipe interaction in a non-homogeneous porous seabed

An evaluation of the wave-induced pore pressures and effective stresses has been recognized by marine geotechnical engineers as an important factor in the design of marine pipelines. Most previous investigations for such a problem have considered the pipeline as a rigid material. Thus, the internal stresses within the pipeline have not been examined in the wave–seabed–pipe interaction problem. In this paper, we consider the pipeline itself to be an elastic material, and link the analysis of the pipeline with the wave–seabed interaction problem. Based on the numerical model presented, the effects of pipe geometry and variable soil characteristics on the wave-induced pore pressure and internal stresses will be discussed in detail. It is found that the internal normal stresses in the angular direction (σ_pθ) and shear stress (τ_p) within the pipe are much larger than the amplitude of wave pressure at the surface of the seabed.     

地震作用下可液化场地管道上浮动力响应及影响因素分析

地震作用下土体发生液化之后,由于超静孔隙水压力的产生和土体抗剪强度的降低,管道易发生上浮破坏。为研究管道上浮动力反应的影响因素,基于OpenSees有限元软件,通过目标反应谱和谱匹配等方法选取地震波,考虑不同管土特性和地震动特性,对地震作用下管道上浮动力反应进行了二维数值模拟。结果表明:土体相对密度、管径和管道埋深对管道上浮反应的影响较大,分别给出了土体相对密度、管径、管道埋深对管道上浮位移的影响规律及对应拟合公式;长持时地震动作用下,超静孔隙水压力消散较慢,管道上浮位移可达短持时地震动作用下管道上浮位移的2倍左右;近断层脉冲地震动作用下,管道上浮破坏和横向破坏两种破坏模式同时存在,且由于速度脉冲效应,管道横向破坏风险大于上浮破坏风险。     

Uplift response of buried pipelines in saturated sand deposit under earthquake loading

Pipelines buried in saturated sand deposits, during earthquake loading could damage from resulting uplift due to excess pore water pressure generation. Several studies have been made to better understand the uplift mechanism and evaluate the effectiveness of mitigating techniques through experiment, but little numerical works have been done to assess the influence of soil properties and field conditions in pipeline floatation. Especially for previously buried pipelines, in order to set the priority for seismic retrofit, evaluating the risk of floatation in each region could be a concern. In this paper, effects of several parameters including dilatancy angle and density ratio of natural soil, diameter and burial depth of pipe, underground water table and thickness of the saturated soil layer on uplift of pipe have been investigated. Results show the prominent role of burial depth in pipe response and that there exits an optimum level for drop of water table to reduce floatation.     

跨越断层埋地管线地震反应数值分析

跨越断层埋地管线在地震中的破坏是非常严重的,地震本身和管土相互作用体系中都存在很多不确定性因素,所以管线在断层运动过程中反应比较复杂。本文利用有限元理论和数值模拟手段,建立了管土作用模型,采用非线性接触问题研究方法详细地分析了管线由断层运动而产生的反应,对影响管线的各种因素进行了分析,包括位错量、跨越角度、断层运动形式、埋设深度、初始轴向力、断层裂缝宽度、填覆土质和管径。通过研究,得到一些初步结论。     

沉陷区域埋地管线数值模拟分析

场地的不均匀沉陷是导致埋地管线破坏的重要原因之一。本文考虑了材料非线性、几何非线性以及管土接触非线性,将管线计算分析模型模拟为四节点薄壳单元结构,周围填覆土体采用八节点六面体单元划分。管土相互作用模拟为三维刚性与柔性的面面接触单元结构,并采用线性位移加载来模拟土体的沉陷作用,对三维薄壳有限元模型进行数值计算分析。通过比较不同参数,如沉陷长度、沉陷深度、埋深、管径、径厚比、土特性等对管线的反应影响,得出管线在沉陷情况下的应力和应变的关系,通过算例分析,说明了该方法能更好地模拟管线的破坏过程,该方法将为沉陷区域埋地管线数值模拟提供理论分析依据。     

Mechanical behavior of buried steel pipes crossing active strike-slip faults

The present paper addresses the mechanical behavior of buried steel pipes crossing active strike-slip tectonic faults. The pipeline is assumed to cross the vertical fault plane at angles ranging between zero and 45 degrees. The fault moves in the horizontal direction, causing significant plastic deformation in the pipeline. The investigation is based on numerical simulation of the nonlinear response of the soil–pipeline system through finite elements, accounting for large strains and displacements, inelastic material behavior of the pipeline and the surrounding soil, as well as contact and friction on the soil–pipe interface. Steel pipes with D/t ratio and material grade typical for oil and gas pipelines are considered. The analysis is conducted through an incremental application of fault displacement. Appropriate performance criteria of the steel pipeline are defined and monitored throughout the analysis. The effects of various soil and line pipe parameters on the mechanical response of the pipeline are examined. The numerical results determine the fault displacement at which the specified performance criteria are reached, and are presented in diagram form, with respect to the crossing angle. The effects of internal pressure on pipeline performance are also investigated. In an attempt to explain the structural behavior of the pipeline with respect to local buckling, a simplified analytical model is also developed that illustrates the counteracting effects of pipeline bending and axial stretching for different crossing angles. The results from the present study can be used for the development of performance-based design methodologies for buried steel pipelines.     

水域隧道地震响应分析

本文基于Biot动力固结理论和弹性动力学理论,考虑海床（土壤）的两相性、黏弹性人工边界及流（水）-固耦合作用,建立了隧道-土-流体相互作用的力学模型,讨论了P波作用下有无水的情况以及水深、水域隧道埋深、海床土性质和地震波入射角等因素对隧道及其周围海床应力的影响。结果表明:隧道周围海床土的孔隙水压力和隧道内应力随着水深的增加而增加;地震波特性和海床土特性对隧道的内应力和海床土的孔隙水压力均有较大的影响;海床土的渗透性和隧道埋深对隧道的内应力影响较小,而对隧道周围海床土的孔隙水压力影响较大;地震动的入射角对隧道的内应力和隧道附近土层的孔隙水压力均有较大影响。      

Finite element modelling for water waves-soil interaction

The soil permeability and shear modulus of many marine sediments vary with depth because of consolidation under overburden pressure. However, conventional theories for wave-induced soil response have assumed a homogeneous porous seabed, with constant soil permeability and shear modulus. This paper presents a finite element model for the wave-induced soil response in a porous seabed, with variable permeability and shear modulus as a function of burial depth. The soil matrix considered here is unsaturated and hydraulically anisotropic, and subjected to a three-dimensional short-crested wave system. The present finite element formulation is established by using a combination of semi-analytical techniques and the Galerkin method. The nodal effective stresses directly derived from the governing equations can be calculated accurately in the present model. Verification is available through the reduction to the simple case of homogeneous seabed. Three typical marine materials, course, fine sand and gravel, are considered in this study. The numerical results indicate that the soil permeability affects the wave-induced seabed response significantly especially for gravelled seabed, as does the soil shear modulus for sandy seabed.     

埋地管线-土横向动力相互作用等效弹簧系数解析解

基于一维柱面弹性波动理论,推导了埋地管线-土横向动力相互作用等效弹簧系数解析表达式,并对该表达式的影响因素,诸如频率(ω)、管线埋深(d)、管线半径(b)、场地波速(VP)、泊松比(μ)的影响程度进行了分析。分析结果表明,无量纲频率bω/VP取较小值时(<0.1),弹簧系数取值随着管线埋深与管线半径之比γ的增加而增大,但总是限制在2.0G～4.0G(G为土体剪切模量)之间,且泊松比对其影响很小,对于工程实际情况,弹簧系数可在该范围内取值。     

反应位移法在埋地腐蚀管线抗震分析中的应用

埋地管线作为油、气、水等的传输载体,是地下工程的重要设施之一。埋地管线特别是金属管线容易发生腐蚀现象,在地震灾害的影响下,含有腐蚀缺陷的管线容易发生泄漏和断裂事故,造成巨大的资源浪费和环境污染等损失。因此埋地腐蚀管线抗震性能研究的重要性也越发凸显。本文以埋地腐蚀管线为研究对象,探索管线腐蚀深度、宽度、腐蚀位置和腐蚀管线埋深等腐蚀参数对管线抗震性能的影响,旨在为管线的安全性和实用性提供理论参考,对管线是否继续使用、维护或更换和安全生产提供指导,以及为管线的抗震设计提供基础研究。本文运用反应位移法在有限元软件ANSYS上实现各参数下的埋地腐蚀管线的地震响应分析。     

砂土液化引起大位移对地下管道影响的非线性分析

地下管线是生命线工程的主要部分,已经成为现代工农业生产和城镇生活的大动脉。已有震害调查表明,饱和砂土液化引起的地基大变形(侧向变形和沉降)是导致强震区生命线工程震害的主要原因。采用三维非线性有限差分分析方法来研究砂土液化引起的大位移对地下管道的破坏特征,分析砂土液化的斜坡变形特征、孔隙水的演化过程。结果表明,砂土液化引起的大位移对地下管道有破坏作用,导致管道变形规律与其斜坡的位移规律相同,地下管线的变形随着振动频率和幅值的增加其非线性增大。     

波浪荷载作用下海底隧道-孔隙海床的动力分析

孔隙海床在波浪荷载作用下其有效应力减小、孔隙水压力增加,这将影响孔隙海床中隧道的稳定性,因此,研究在波浪荷载作用下孔隙海床与隧道的动力响应具有重要的工程意义。本文基于Biot的动力固结理论和弹性动力学理论建立波浪荷载作用下孔隙海床与海底隧道的动力分析模型,并同时考虑海底隧道与海床之间的接触效应、边界效应等对海底隧道内力的影响。最后通过变换海床的变形模量、渗透系数和海底隧道的半径、埋深等,观察其对海床土孔隙水压力和海底隧道内力的影响,为海底隧道的设计提供依据。     

Analysis of continuous buried pipelines for seismic wave effects

An analysis procedure for seismic wave propagation effects on straight continuous buried pipelines is proposed. It shown that ground strain due to surface waves can be substantially larger than that due to body waves. An elastic model a buried pipeline surrounded by equivalent soil springs indicates that frictional slip between the pipeline and the surrounding soil springs is likely for high ground strains. A method for estimating ground strain due to surface waves, based on data from the 1971 San Fernando earthquake, reviewed. An analysis procedure, which utilizes frictional forces near the soil-pipeline interface, is proposed for surfae wave effects on straight buried continuous pipelines. The proposed procedure is illustrated with an example.     

考虑土结相互作用的逆断层作用下埋地管道性能离心机试验

埋地管道地震作用下的破坏因素源于地震引起的永久地面变形(PGD),其中管道-土体间相互作用决定土体位移作用到管体的大小。利用离心机试验技术模拟埋地管道在逆断层大位移下的反应特性,重点讨论断层与管道的交角、断层位移大小、管土相互作用、管径和埋深五个参数对管道破坏的影响水平。实验结果表明:上述参数对管道断层作用的反应均有明显影响,其中断层的位移量、管土相互作用、埋深和管径的影响更为显著。本文的研究结果对于管道经过断层区的抗震设计有十分重要的意义。     

穿越逆冲断层的埋地管道非线性反应分析

穿越逆冲断层的埋地管道在地震作用下,容易发生局部屈曲或整体失稳等形式的破坏,研究逆冲断层作用下的埋地管道地震反应规律,对管道抗震设计及施工等具有重要的意义。本文将埋地管线及周围土体从半无限地球介质中取出,分别以空间薄壳单元和实体单元进行离散,采用非线性接触力学方法模拟管、土之间的滑移、分离及闭合现象;采用线性位移加载模拟断层的错动,考虑了系统初始应力状态的影响,对土体未开裂前的管土相互作用系统进行了拟静力数值分析;分析了位错量、土体刚度、埋设深度、径厚比及跨越角度对埋地管道反应的影响,得出了一些有益的结论。     

Pipe–soil interaction and pipeline performance under strike–slip fault movements

The performance of pipelines subjected to permanent strike–slip fault movement is investigated by combining detailed numerical simulations and closed-form solutions. First a closed-form solution for the force–displacement relationship of a buried pipeline subjected to tension is presented for pipelines of finite and infinite lengths. Subsequently the solution is used in the form of nonlinear springs at the two ends of the pipeline in a refined finite element model, allowing an efficient nonlinear analysis of the pipe–soil system at large strike–slip fault movements. The analysis accounts for large strains, inelastic material behavior of the pipeline and the surrounding soil, as well as contact and friction conditions on the soil–pipe interface. The numerical models consider infinite and finite length of the pipeline corresponding to various angles β between the pipeline axis and the normal to the fault plane. Using the proposed closed-form nonlinear force–displacement relationship for buried pipelines of finite and infinite length, axial strains are in excellent agreement with results obtained from detailed finite element models that employ beam elements and distributed springs along the pipeline length. Appropriate performance criteria of the steel pipeline are adopted and monitored throughout the analysis. It is shown that the end conditions of the pipeline have a significant influence on pipeline performance. For a strike–slip fault normal to the pipeline axis, local buckling occurs at relatively small fault displacements. As the angle between the fault normal and the pipeline axis increases, local buckling can be avoided due to longitudinal stretching, but the pipeline may fail due to excessive axial tensile strains or cross sectional flattening. Finally a simplified analytical model introduced elsewhere, is enhanced to account for end effects and illustrates the formation of local buckling for relative small values of crossing angle.     

地震断层作用下的埋地管道等效分析模型

地震作用下,活动断层附近的埋地管道易发生强度屈服、局部屈曲或整体失稳等形式的破坏,建立准确、高效的埋地管道在断层作用下的计算模型,对管道的抗震设计和震后安全状态评估具有重要的实用价值。本文采用非线性弹簧模拟远离断层处埋地管道的反应,基于管土之间小变形段管道处于强化阶段,提出一种改进的管土等效分析模型,进一步减小了管土之间大变形段的分析长度,从而提高了有限元分析效率。该模型采用ALA推荐的方法计算管土间的滑动摩擦力,可以考虑土体种类的影响;用Kennedy方法确定管道的计算长度。通过与精确模型比较,验证了管土等效模型的合理性和有效性。