Enhanced Multi-Layer Fatigue-Analysis Approach for Unbonded Flexible Risers

This paper proposes an enhanced approach for evaluating the fatigue life of each metallic layer of unbonded flexible risers. Owing to the complex structure of unbonded flexible risers and the nonlinearity of the system, particularly in the critical touchdown zone, the traditional method is insufficient for accurately evaluating the fatigue life of these risers. The main challenge lies in the transposition from global to local analyses, which is a key stage for the fatigue analysis of flexible pipes owing to their complex structure. The new enhanced approach derives a multi-layer stress-decomposition method to meet this challenge. In this study, a numerical model validated experimentally is used to demonstrate the accuracy of the stress-decomposition method. And a numerical case is studied to validate the proposed approach. The results demonstrate that the multi-layer stress-decomposition method is accurate, and the fatigue lives of the metallic layers predicted by the enhanced multi-layer analysis approach are rational. The proposed fatigue-analysis approach provides a practical and reasonable method for predicting fatigue life in the design of unbonded flexible risers.

Analytical and Numerical Models to Predict the Behavior of Unbonded Flexible Risers Under Torsion

This paper presents analytical and numerical models to predict the behavior of unbonded flexible risers under torsion.The analytical model takes local bending and torsion of tensile armor wires into consideration,and equilibrium equations of forces and displacements of layers are deduced.The numerical model includes lay angle,cross-sectional profiles of carcass,pressure armor layer and contact between layers.Abaqus/Explicit quasi-static simulation and mass scaling are adopted to avoid convergence problem and excessive computation time caused by geometric and contact nonlinearities.Results show that local bending and torsion of helical strips may have great influence on torsional stiffness,but stress related to bending and torsion is negligible;the presentation of anti-friction tapes may have great influence both on torsional stiffness and stress;hysteresis of torsion-twist relationship under cyclic loading is obtained by numerical model,which cannot be predicted by analytical model because of the ignorance of friction between layers.

Helical Wire Stress Analysis of Unbonded Flexible Riser Under Irregular Response

A helical wire is a critical component of an unbounded flexible riser prone to fatigue failure. The helical wire has been the focus of much research work in recent years because of the complex multilayer construction of the flexible riser. The present study establishes an analytical model for the axisymmetric and bending analyses of an unbonded flexible riser. The interlayer contact under axisymmetric loads in this model is modeled by setting radial dummy springs between adjacent layers. The contact pressure is constant during the bending response and applied to determine the slipping friction force per unit helical wire. The model tracks the axial stress around the angular position at each time step to calculate the axial force gradient, then compares the axial force gradient with the slipping friction force to judge the helical wire slipping region, which would be applied to determine the bending stiffness for the next time step. The proposed model is verified against the experimental data in the literature. The bending moment-curvature relationship under irregular response is also qualitatively discussed. The stress at the critical point of the helical wire is investigated based on the model by considering the local flexure. The results indicate that the present model can well simulate the bending stiffness variation during irregular response, which has significant effect on the stress of helical wire.

Behavior of Unbonded Flexible Risers Subject to Axial Tension

Owing to nonlinear contact problems with slip and friction, a lot of limiting assumptions are made when developing analytical models to simulate the behavior of an unbonded flexible riser. Meanwhile, in order to avoid convergence problems and excessive calculating time associated with running the detailed finite element （FE） model of an unbonded flexible riser, interlocked carcass and zeta layers with complicated cross section shapes are replaced by simple geometrical shapes （e.g. hollow cylindrical shell） with equivalent orthotropic materials. But the simplified model does not imply the stresses equivalence of these two layers. To solve these problems, based on ABAQUS/Explicit, a numerical method that is suitable for the detailed FE model is proposed. In consideration of interaction among all component layers, the axial stiffness of an eight-layer unbonded flexible riser subjected to axial tension is predicted. Compared with analytical and experimental results, it is shown that the proposed numerical method not only has high accuracy but also can substantially reduce the calculating time. In addition, the impact of the lay angle of helical tendons on axial stiffness is discussed.

An Analytical Approach for Assessing the Collapse Strength of an Unbonded Flexible Pipe

This paper presents an analytical scheme for predicting the collapse strength of a flexible pipe, which considers the structural interaction between relevant layers. The analytical results were compared with a FEA model and a number of test data, and showed reasonably good agreement. The theoretical analysis showed that the pressure armor layer enhanced the strength of the carcass against buckling, though the barrier weakened this effect. The collapse strength of pipe was influenced by many factors such as the inner radius of the pipe, the thickness of the layers and the mechanical properties of the materials. For example, an increase in the thickness of the barrier will increase contact pressure and in turn reduce the critical pressure.

Lateral load performance and seismic demand of unbonded scrap tire rubber pad base isolators

The scrap tire rubber pad(STRP)made by natural or synthetic rubber and high strength reinforcing cords exhibits substantial vertical stiffness and horizontal flexibility,and these properties can be regarded as suitable for seismic isolators for structures.The use of environmentally burdensome scrap tires as STRP isolators might be convenient as an efficient and low-cost solution for the implementation of aseismic design philosophy for low-to-medium rise buildings,especially in developing countries.Finite element analyses of unbonded square and strip-shaped STRP isolators subjected to a combination of axial and lateral loads are conducted to investigate its lateral deformation performance under seismic loading.The rubber of the isolator is modelled with Mooney-Rivlin hyperelastic and Prony viscoelastic materials,including the Mullins material damage effect.The influence of the length-to-width ratio and bearing height on the isolator performance is assessed in terms of the force-displacement relationship,horizontal stiffness,damping,and isolation periods.It is shown that the dependence of stiffness on the length-to-width ratio is significant in the longitudinal direction and minor in the transverse direction.The STRP isolators following the proposed design criteria are shown to satisfy the performance requirement at different levels of seismic demand specified by the ASCE/SEI 7-2010 seismic provisions.

Analysis and calculation of factors on curvature ductility of unbonded prestressed concrete beams

In consideration that behavior of curvature ductility of interior support directly influences the degree of moment modification of unbonded prestressed concrete (UPC) continuous structures, constitutive relationships of concrete, non-prestressed reinforcement and prestressed reinforcement used for nonlinear analysis are given. Through simulation analysis on simple beams subjected to single loading at the middle of the span, the law of factors influencing curvature ductility, such as global reinforcing index, prestressing degree, effective prestress, strength of concrete and grade of non-prestressed reinforcement are explored. Based on these researches, calculating formula of curvature ductility coefficient of UPC beams is established, which provides basic data for further research on plastic design of UPC indeterminate structures.

Fatigue Behavior of Unbonded Partially Prestressed Concrete Beams with Flexure

A study on fatigue behavior of unbonded partially prestressed concrete beams is presented. Model tests have been carried out in static loading and cyclic compressive loading on 15 beams with flexure. The ratios of the lower limit to the upper limit of fatigue load are 0.5 and 0.3 respectively, and the frequencies of cyclic loading are 8 Hz and 4.5 Hz respectively. The experimental results of the strains of the concrete and steel bars, the deflection of test beams, and the crack width of normal section are analyzed. According to statistics and analysis of test results, the corresponding calculation models are developed and presented.

预制拼装铁路重力式桥墩的抗震性能研究

为适应中国铁路建设向高烈度震区快速发展的需要,解决传统预制拼装桥墩墩柱及承台连接位置薄弱等问题,提出一种灌浆波纹管连接的模块化预制拼装桥墩体系,通过设置承台与墩身塑性区域共同浇筑及墩底局部无黏结段增强桥墩的抗震性能。制作1个局部无黏结整体现浇铁路重力式桥墩模型和1个局部无黏结预制拼装铁路重力式桥墩模型开展拟静力试验,并结合有限元分析,进行预制拼装铁路重力式桥墩抗震性能研究。结果表明:局部无黏结预制拼装桥墩整体连接性能稳定,可通过墩底塑性区域破坏与墩身及节段间的摇摆实现共同消能,其破坏模式表现为墩底塑性区域的弯曲破坏,未发生破坏位置转移现象;局部无黏结预制拼装桥墩等效塑性区域高度比整体现浇桥墩降低,抗侧向水平承载力与耗能能力提升显著,位移延性能力良好,可适应更大加载位移,最终累积耗能增长64.3%;结构接缝位置连接稳定可靠,同等加载位移下等效刚度基本一致,抗震性能得到明显提高;预制节段划分对预制拼装铁路重力式桥墩抗震性能的影响不大。

层间摩擦系数对非黏结柔性管抗拉铠装层轴压屈曲失效模式的影响研究

在柔性管运输、铺设安装和服役过程中,由于其抗拉铠装层钢带为柔性细长螺旋结构,当外部载荷超过临界值时,抗拉铠装层钢带容易发生屈曲失效。考虑非线性材料、几何大变形、层间接触和摩擦等非线性效应的影响,运用ABAQUS有限元软件建立了63.5mm典型非黏结柔性管8层完整结构模型,研究了在抗拉铠装层不同层间摩擦系数的情况下,非黏结柔性管在轴向压缩载荷作用下的力学性能和屈曲失效模式。研究结果表明,层间摩擦系数对非黏结柔性管的抗压刚度和抗拉铠装层屈曲失效模式有较大影响,层间摩擦系数以0.1和0.2为分界点,抗压刚度和屈曲失效模式均呈现出“三段式”变化规律。研究成果可为海洋非黏结柔性管的结构设计和屈曲失效评估提供技术参考。

An evaluation of force-based design vs.direct displacement-based design of jointed precast post-tensioned wall systems

The unique features of jointed post-tensioned wall systems, which include minimum structural damage and re-centering capability when subjected to earthquake lateral loads, are the result of using unbonded post-tensioning to attach the walls to the foundation, along with employing energy dissipating shear connectors between the walls. Using acceptance criteria defined in terms of inter-story drift, residual drift, and floor acceleration, this study presents a multiplelevel performance-based seismic evaluation of two five-story unbonded post-tensioned jointed precast wall systems. The design and analysis of these two wall systems, established as the direct displacement-based and force-based solutions for a prototype building used in the PREcast Seismic Structural Systems （PRESSS） program, were performed at 60% scale so that the analysis model could be validated using the PRESSS test data. Both buildings satisfied the performance criteria at four levels of earthquake motions although the design base shear of the direct displacement-based jointed wall system was 50% of that demanded by the force-based design method. The study also investigated the feasibility of controlling the maximum transient inter-story drift in a jointed wall system by increasing the number of energy dissipating shear connectors between the walls but without significantly affecting its re-centering capability.

无粘结环锚预应力衬砌张拉工艺足尺模型试验研究

针对预应力衬砌张拉过程中易出现应力集中而导致张拉裂缝的现象,通过开展洞外地面足尺模型试验,提出了分级分步张拉顺序和质量控制指标。首先根据钢绞线摩阻测试计算出摩擦系数,随后应用上述张拉顺序和质量控制方法进行自动张拉,并采用磁通量传感器和混凝土应变计分别对钢绞线和内衬混凝土的应力变化开展实测。现场张拉试验结果表明:①钢绞线沿程预应力损失约16.3%,采用的双层钢绞线在实践中可保证较高的有效预应力;②对锚具槽进行分级分步张拉,可使内衬混凝土获得较均匀的封闭环向压应力环,平均压应力达-10.9 MPa;③提出的3项控制指标可进行量化管理,结合自动张拉技术可有效地保证张拉质量。

后插无粘结钢绞线U形管承压型抗拔桩工程应用

后插无粘结钢绞线U形管承压型抗拔桩技术是一种新型研发的抗浮技术,该技术具有桩身混凝土在使用周期内始终处于受压状态,充分发挥了混凝土的抗压性能,桩身不产生裂缝,无需进行裂缝宽度验算,耐久性好,使用范围广、工期快、造价低等优点。本文结合工程应用实例,详细介绍了后插无粘结钢绞线U形管承压型抗拔桩工程施工工艺,为后续类似工程的高质量施工提供参考。

无粘结预应力混凝土结构抗震性能研究分析

无粘结预应力混凝土(unbonded prestressed concrete,简称UPC)结构具有结构恢复性能好、摩擦系数小、施工方便等特点,故其被广泛应用在高层建筑、大跨度工程中。研究从UPC结构中柱节点的特性、节点研究的重要性、其国内外研究发展状况和目前抗震性能的分析方法等角度,对UPC结构中柱节点抗震性能的相关研究进行了分析和归纳。结论总结如下:目前UPC结构的研究主要是针对无框架边节点以及梁的试验研究,而对UPC结构中柱节点的试验研究甚少。

自复位混凝土剪力墙研究进展

自复位剪力墙在遭受地震时能够实现大位移而不会有明显的残余变形或大量损伤,适用于地震后的快速恢复结构。为研究自复位混凝土剪力墙的发展趋势,文中介绍了国内外自复位混凝土剪力墙结构的相关研究进展,对相关研究进行展望和总结,表明在自复位混凝土剪力墙中加入耗能部件或选择合适的配筋方式,可以改善墙体的力学性能并控制损伤。

薄壁墙式墩结构装配式方案及其抗震性能分析

针对既有柱式墩预制拼装技术难以应用于截面横纵比大的薄壁墙式墩的问题,提出了一种节段水平并联的新型装配式桥墩结构方案,预制节段采用双向无黏结预应力筋、灌浆套筒及混凝土剪力榫混合连接.依托某国际机场飞机荷载宽幅桥梁工程实例,基于ABAQUS平台分别建立了新型装配式桥墩和现浇桥墩的精细化有限元模型,通过拟静力分析对比研究了2种桥墩抗震性能的差异,并讨论了预应力筋配筋率以及剪力榫埋深对新型装配式桥墩抗震性能的影响规律.结果表明:新型装配式桥墩与现浇桥墩在纵向水平荷载作用下呈现墩底塑性铰充分发展的弯曲破坏形态,而在横向水平荷载作用下则呈现弯剪破坏形态;相较于现浇桥墩,装配式桥墩纵桥向与横桥向水平承载力分别提高68.5%和43.0%,但残余位移分别降低49.2%和67.8%;增大竖向预应力筋配筋率可显著提升装配式桥墩的水平承载力,而增加水平向预应力筋配筋率和剪力榫埋深则对限制节段接缝处的相对变形作用明显.

无黏结预应力胶合木T梁受弯性能

胶合木梁刚度小、承载力低且易发生脆性受拉破坏,为了改善其短期受弯性能,提出了一种胶合木T梁和无黏结预应力相结合的新型构件。通过对4根落叶松胶合木T梁的四点弯曲试验,包括1组未增强试件和1组无黏结预应力筋增强试件,对比分析了两组试件的破坏形态和破坏机理,探讨了无黏结预应力对其受弯性能的影响。结果表明:施加0.25fptk(fptk为钢绞线抗拉强度标准值)预应力后,胶合木T梁未产生受拉破坏,而为腹板剪切破坏;相比未增强试件,无黏结预应力筋增强试件的受弯极限承载力提高了38.2%,抗弯刚度提高了22.5%,延性提高了45.8%;施加预应力还能有效减小木材缺陷对胶合木梁受弯性能的影响。

自复位预制节段桥墩抗震性能试验研究

为提高装配式预制桥墩的抗震性能以及解决震后可修复的问题,本文基于后张无黏结预应力筋的连接技术,提出了一种新型自复位预制节段桥墩(self-centering precast segmental bridge pier,SPSBP)。该新型自复位预制节段桥墩由预制构件(盖梁、节段、基础)、可更换耗能角钢和连接钢板组成。本文设计并完成了三组SPSBP试件的拟静力试验,研究初始预应力、耗能角钢厚度以及长度三个参数对SPSBP试件抗震性能的影响。试验结果表明:该新型结构形式具有较好的自复位以及耗能能力;当加载到最大位移比时,试件变形主要集中于耗能角钢,主体预制结构并未发生明显破坏,表明了结构的完整性;提高初始预应力能有效提高结构的承载力等抗震性能并减小残余变形;增加耗能角钢厚度,可以提高SPSBP耗能能力以及承载力;耗能角钢长度对SPSBP性能影响不明显。

无粘结预应力UHPC梁剪切性能试验研究

为研究无粘结预应力无腹筋UHPC梁的剪切力学行为,设计了1片矩形截面UHPC梁并进行了剪切试验。试验采用1000 t电液伺服长柱压力试验机对试验梁进行了单点加载,测试了UHPC梁的挠度、梁腹位置主应变、加载点截面正应变和钢绞线应力增量等数据,并将抗剪承载力试验结果与法国标准和瑞士标准UHPC梁抗剪承载力计算结果进行对比。结果表明:在剪跨比λ适中时(λ=1.86),无粘结预应力无腹筋UHPC梁发生剪压破坏;由于UHPC中钢纤维的桥接作用,试验梁在斜裂缝出现以后并未立即失去承载能力,仍然可以承受继续增大的荷载;试验梁在发生破坏时加载点下方截面底缘混凝土拉应变达到2700με,表明UHPC材料具有较好的抗变形能力;现有法国标准和瑞士标准对于UHPC梁抗剪承载力的计算方法均偏保守,未能考虑剪跨比和纵向钢筋消栓作用对抗剪承载力的影响。