海床CPT和井下CPT在砂土中贯入机理差异研究

海洋石油建设中的一个关键环节是对有关海域的工程地质条件作出准确评价。海上原位静力触探(CPT)是主要的原位勘察手段,与陆地CPT不同,其贯入方式主要有Seabed和Downhole两种形式。这两种CPT由于贯入方式不同,所得到的锥端阻力存在一定的差异。采用有限元方法对这两种形式的CPT锥端阻力和贯入模式进行计算分析。研究显示,CPT入土过程中,周围土体存在两种不同位移状态,即滑动状态和排挤状态。在入土初期,土体以滑动状态为主,土体中竖向应力随贯入深度的增加而递增。在入土一定深度以后,土体的排挤状态占主导地位,竖向应力变化趋于平稳。这两种位移状态的相互转化解释了Seabed CPT和Downhole CPT实测数据之间的差异。在贯入深度较大时,Seabed CPT受排挤状态控制,而Downhole CPT仍然受滑动状态的影响。     

板翼动力锚沉贯深度模型试验研究

板翼动力锚是依靠自重完成安装并靠自重和海床土的抗力来锚固的新型动力锚。板翼动力锚高速(15~25 m/s)贯入地基过程中涉及到高应变率、流固耦合、土体软化和大变形等难题,模型试验可避免上述计算困难,能直接得出不同的贯入速度所对应的沉贯深度。本文首先推导了模型相似关系,然后在常规重力条件下,进行了两组26个工况的板翼动力锚在均质黏土中动力安装过程的模型试验,根据试验结果确定了率效应参数的取值范围,并研究了每一项受力对沉贯深度的影响。最后提出了在均质黏土中预测板翼动力锚沉贯深度的经验公式。     

海上平台桶形基础模型压力压贯与负压沉贯试验研究

介绍了海上平台桶形基础模型（模型桶）压力压贯和负压沉贯的试验概况,对比分析了两者贯入力的巨大差异和产生的原因,给出了模型桶压力压贯中贯入深度与压力的关系,负压没贯负压与贯入深度,抽吸泵流量,基土渗流量,桶内土塞隆起之间的关系,试验结果表明,负压沉贯可以大大降低以砂质粉土为基土的土抗力,为在胜利油田类似基土海域海上平台应用桶形基础提供了试验依据,为海上现场导管架桶形基础平台的安装就位施工和控制提供了     

深水井口导管管土界面摩擦退化试验研究

深水井口导管在黏土中的贯入阻力与导管土体界面的黏性摩擦系数密切相关。在导管安装过程中,管土界面会产生大位移剪切,使得导管-黏土界面剪切力产生退化。本文针对传统剪切试验装置小位移剪切的限制,无法正确描述井口导管贯入时导管-黏土界面大位移剪切时的退化机理,采用GDS多功能界面剪切试验仪在恒应力条件下进行大位移单调剪切试验,研究法向应力对导管管土界面摩擦退化机理的影响规律。试验结果表明,大位移单调剪切时,界面剪应力随剪切位移先增大后减小,最终趋于稳定,界面表现出剪应力退化现象,随着法向应力的增大界面峰值应力与残余应力均增大;土样法向位移随着剪切位移、法向应力的增加而增大最终趋于稳定。最后在对试验数据分析的基础上,基于剪应力随无量纲位移的退化规律,提出了界面剪应力随累积位移退化的计算方法。并结合算例对比分析了井口导管在贯入80 m深度处,由于界面摩擦退化效应导致导管贯入阻力降低了28.48%。     

筒型基础贯入过程中的渗流分析

使用有限元计算软件ABAQUS对筒型基础负压贯入过程中的渗流场进行分析,讨论了筒型基础渗流场的特点,不同参数对筒型基础渗流的影响,结合实例分析多层土和单层土筒型基础周围渗流的差别。     

重力活塞取样器贯入深度研究

重力活塞取样器是一种重要的海底连续沉积物采集仪器,贯入深度和样品质量对海洋地质研究有着重要影响。通过对重力活塞取样器贯入过程建立能量守恒方程,得出贯入深度控制方程。已知取样器参数和土质类型条件,可计算取样管贯入深度,深度受取样器重量、体积、形状、取样管直径和沉积物类型等因素共同影响。已知取样器参数和贯入深度,可推测海底沉积物类型。     

探头尺寸对FFP确定海底沉积物性质影响的原位试验研究

自落式贯入仪(Free-Falling Penetrometer, FFP)是一种新型海底沉积物原位测量设备,用于海底沉积物工程性质原位调查。本文基于自主研发的FFP设备在黄海海域开展现场贯入试验,分析探头尺寸对判断沉积物性质的影响。研究表明,在不同沉积物中探头尺寸对贯入深度、贯入阻力和沉积物类型判断有不同影响。在细粒沉积物中探头尺寸对贯入深度、贯入阻力影响较小;在根据贯入阻力变化划定分层时,小直径探头能反应贯入阻力更为细小的变化。在粗粒沉积物中探头尺寸影响更明显,大尺寸探头使贯入阻力变大,贯入深度减小。根据标准化深度和硬度因子可区分粗粒沉积物和细粒沉积物,在粗粒沉积物中探头尺寸增大,硬度因子减小,有利于细化沉积物类型。     

粉土中吸力桶沉贯阻力与土塞失稳机理研究

针对粉土中吸力桶在吸力沉贯中的两个关键性问题——沉贯阻力和内部土体稳定性,在自行研制的试验平台上进行了一系列吸力桶沉贯模型试验。试验结果表明,粉土中吸力沉贯时,Andersen所提供的基于CPT锥尖强度的阻力计算公式能较好预测阻力发展趋势,但计算结果偏小;而修正的承载力公式预测结果在沉贯前期与实测值较吻合,沉贯后期预测偏大。吸力沉贯前期,贯入阻力随深度稳步增长,直到贯入深度达到某个临界值后,阻力增长缓慢甚至保持不变。桶内粉土在吸力作用下的失稳机理与黏土或砂土不同,在渗透力作用下表现为从土层表面开始的自上而下管涌或渗流侵蚀。沉贯后期,端部土体在较高水头差下发生失稳并向桶内流动,造成了桶内土体密度降低,端阻力与内壁摩阻力降低,总贯入阻力主要由外壁摩阻力提供。并结合有限元模拟对上述实验现象的内在机理进行了解释。     

微型贯入试验与无侧限抗压强度试验结果的对比

本文探讨了探头锥尖角度和探头直径对微型贯入试验结果的影响及微型贯入试验结果与无侧限抗压强度试验结果的相关性。结果表明,探头锥尖角度和直径对贯入曲线性状没有影响,即贯入阻力随贯入深度线性增加,达到某一贯入深度时贯入曲线出现拐点,之后贯入阻力趋于稳定,定义此时的比贯入阻力为贯入强度。探头锥尖角度越大,达到贯入曲线拐点前贯入阻力随贯入深度增长越快;贯入强度随探头锥尖角度的增加先减小后增大,当锥尖角度为60°时贯入强度最小,锥尖角度为30°、90°和180°时的贯入强度与锥尖角度为60°时贯入强度的关系呈线性关系;探头直径对比贯入曲线的影响可忽略不计,但随着探头直径的增大,贯入强度呈线性减小趋势,探头直径3.4 mm的贯入强度比7.0 mm的贯入强度增大2.5%,比10.0 mm的贯入强度增大5.0%;可用贯入曲线拐点前直线段斜率换算无侧限抗压强度,也可利用贯入强度换算无侧限抗压强度;水泥土强度的形成条件对贯入曲线拐点前直线段斜率及贯入强度与无侧限抗压强度之间的关系没有明显影响。     

桩靴贯入黏土层时邻近群桩相互作用分析

为研究钻井船插桩对邻近平台群桩相互作用的影响,采用耦合欧拉拉格朗日(CEL)方法对桩靴贯入黏土层时邻近群桩中各桩荷载分担比、桩头附加位移及两桩相互作用系数进行了分析。首先通过对缩尺模型试验的数值分析,验证了CEL方法的可行性;然后进一步分析了桩靴贯入黏土层时对邻近群桩相互作用的影响;最后探讨了净间距、桩间距对群桩相互作用的影响。结果表明,在桩靴贯入中,前桩的荷载分担比大于后桩,且桩靴贯入至一定深度后,当净间距越小或桩间距越大时,前桩的荷载分担比越大、后桩的荷载分担比越小,但各桩的荷载分担比随桩靴贯入深度增加时的变化规律不变;净间距越大,桩头附加位移及相互作用系数越小;在桩靴贯入时,由于受群桩遮拦效应的影响,桩头附加位移及相互作用系数随桩间距的变化规律同插桩前有所不同,当桩间距大于3倍桩径时,随桩间距的增加而减小,当桩间距小于3倍桩径时,随桩间距的增加而增大。     

Large deformation finite element analysis of the installation of suction caisson in clay

The suction caisson (or called suction anchor) which is considered as a relatively new type of foundation of offshore structures, has been extensively studied and applied for offshore wind turbines and oil platforms. The installation of the suction caisson is of great importance in the design and construction because it can bring about several issues and further influence the performance of holding capacity in safety service. In this paper, large deformation finite element (FE) analyses are performed to model the installation of suction caisson (SC) by suction and jacking in normally consolidated clay. The penetration of the suction caisson is modeled using an axisymmetric FE approach with the help of the Arbitrary Lagrangian–Eulerian (ALE) formulation which can satisfactorily solve the large deformation problem. The undrained shear strength of the clay and elastic modulus are varied with depth of soil through the subroutine VUFIELD. The numerical results allow quantification of the penetration resistance and its dependence on the installation method. The centrifuge test and theoretical solution are used for the FE model validation. After the validation, the penetration resistance, the soil plug heave, and the caisson wall friction have been examined through the FE model. Based on the numerical results, it is shown that the ALE technique can simulate the entire suction caisson penetration without mesh distortion problem. The installation method can play an important role on the penetration resistance, namely, the suction installation reduces the penetration resistance significantly compared to the purely jacked installation. With a further study on the suction case, it is found that as the final applied suction pressure increases, the soil plug heave increases, while the penetration resistance reduces with increase of the final suction pressure. The effect of the friction of internal caisson walls has been also investigated and a conclusion is drawn that internal wall friction has a significant contribution to the penetration resistance and it can be implicitly represented by varying coefficient of internal wall friction. As for the penetration resistance, both jacked and suction installation have great dependency on the internal wall friction.     

Bearing capacity and critical punch-through depth of spudcan on sand overlying clay

Spudcan may experience punch-through failure on strong over weak layered soils, such as sand overlying clay. A large deformation finite element method （LDFE） is used to simulate the penetration process of spudcan into sand overlying clay. The sand is simulated by smoothed hyperbolic Mohr-Coulomb model, and the clay is simulated by a simple elasto-plastic model which obeys Tresca yield criterion. According to the LDFE results of a large amount of cases, the effects of the strength, unit weight and thickness of the top sand layer, as well as the effect of the strength of the underlying clay on the spudcan punch-through behavior, are investigated. The critical depth occurring punch-through and the critical bearing capacity are presented in charts. Fitting equations to calculate the critical punch-through depth and the critical bearing capacity are proposed for the convenience of engineering practice.     

Physical and finite element modeling of lateral stability of offshore skirted gravity structures subjected to iceberg impact load

Static stability mainly against sliding of a typical, relatively large skirted gravity structure was investigated using three-dimensional finite element modeling. The numerical model was validated against centrifuge test results. A specific set of dimensions was chosen to model a typical skirted gravity structure in a centrifuge with two types of foundation soils: uniform saturated sand and a clay zone sandwiched between two sand layers. Soil shear strength parameters used in the finite element models were estimated from in-flight cone penetration resistance measurements obtained in the centrifuge. Numerical parametric studies were conducted using the validated finite element model. The parameters included were the depth and strength of the clay zone and the inclination of external load. It is shown that a relatively simple three-dimensional finite element model was effective in providing information that would be needed to design such a critical and expensive offshore structure. Basic Mohr-Coulomb strength parameters and moduli based on cone penetration resistance measurements and published empirical correlations were appropriate in modeling the soils in the finite element simulations.     

Numerical study of effect of fluke inclination on behavior of drag anchor in clay with linearly increasing shear strength under unidirectional and combined loading

Drag anchor is a widely used anchor type in offshore engineering for the mooring system. The prediction of the anchor trajectory installation and the final position is important for anchor selection in design. The existing method using yield envelope method for trajectory prediction ignored the shallow anchor behavior but applied the deep yield envelope from a deeply embedded horizontal fluke in uniform clay for the whole drag-in installation process. However, the anchor fluke embedment depth and inclination angle change continually during installation in clay with linearly increasing shear strength soil profile in practice. Studies on the effect of fluke inclination angle on the anchor behavior in clay with such non-uniform soil profile under unidirectional and combined loading are important and necessary for the improvement of the yield envelope method to ensure a reasonable prediction. The current 2D finite element studies investigate the anchor behavior for inclined fluke in clay with linearly increasing shear strength under unidirectional vertical, horizontal and rotational loadings first. Then the effects of the fluke inclination angle, soil non-homogeneity and embedment depth ratio on the shallow yield envelopes are investigated. It is found that the effect of fluke inclination angle on the vertical capacity factors for anchor in clay with non-uniform and uniform soil profile is largely different. The resultant large impact on the yield envelopes shown here illustrates the importance of considering the fluke inclination angle and soil non-homogeneity in the prediction of anchor trajectory using yield envelope method.     

Application of T-bar in numerical simulations of a monopile subjected to lateral cyclic load

A series of centrifuge tests were performed to investigate the response of a free-head monopile due to cyclic lateral loading in normally consolidated clay. By linking the maximum reaction-force point of the final cycles in all tests with various amplitudes, a postcyclic reaction-force curve is obtained, which can be used to assess the postcyclic remolded lateral stiffness. To numerically analyze the tests, a strength degradation model of the clay is calibrated by the T-bar cyclic test. However, this model is T-bar-dependent, which is unable to capture the degrading behavior of the monopile stiffness. Thus, a modification approach is proposed based on the upper bound theory, and the modified model is further combined with finite element analysis to simulate the cyclic behavior of the model pile. The simulation results show similar degrading trend and consistent postcyclic remolded lateral stiffness with the model tests. This further demonstrates that the remolded lateral stiffness mainly depends on the soil remolded strength, which is one of the parameters calibrated by the T-bar tests. Based on this finding, a simplified numerical analysis is proposed, which can predict the postcyclic reaction-force curve by performing one monotonic loading instead of modeling the whole process of cyclic loading.     

桶形基础负压沉贯特性分析

介绍了桶形基础负压沉贯的室内试验,中间现场试验,应用有限元法对负压沉贯的渗流场分析,负压对桶形基础沉贯阻力的影响,对土壤特性的影响,负压大小对桶形基础沉深,沉速的影响,研究了桶形基础在海上的可行性。     

Effect of Rate-Dependent Soil Strength on Cylinders Penetrating Into Soft Clay

This paper presents a predictive model for undrained penetration of cylinders into soft seafloor soils. The penetration depth will depend on the velocity of the cylinder as it touches down at the seafloor, and the net deceleration of the cylinder as it is acted on by forces of self-weight, soil buoyancy, and soil-shearing resistance. The soil-shearing resistance force increases as a function of penetration depth and, due to the dependence of undrained shear strength on strain rate, penetration velocity. This paper presents finite element (FE) simulations that quantify both effects and form the basis of a simplified soil-resisting force model. Strain-rate effects are modeled within a framework of rate-dependent plasticity, with shearing resistance increasing semilogarithmically with increasing strain rate above a certain threshold strain rate. With all forces acting on the cylinder, estimated penetration depths are predicted from simple equations of motion for a single particle. Comparisons to laboratory tests involving penetration of cylinders into soft reconstituted marine clay show reasonable agreement between model predictions and measurements.     

剪土环对筒型基础负压沉贯下沉过程中渗流场的影响研究

筒型基础沉放施工中,随着入土深度的增加,筒体所受的侧摩阻力越大,当达到一定程度时,将导致基础无法继续下沉.为了避免这种情况的发生,在实际的施工中可以在锚体上设置剪土环以减小侧摩阻力的影响.论文针对海上吸力锚基础这一新型海洋平台基础形式研究中面临的吸力锚负压沉贯下沉中设置剪土环及剪土环对其渗流场的影响这一问题,通过对吸力锚渗流场的有限元分析,运用有限元软件ANSYS对锚体周围土体渗流场进行建模分析,利用有限元计算的结果来分析剪土环对渗流场的影响.