APPLICATION OF THE PROBABILISTIC FRACTURE MECHANICS METHOD OF PREDICTING THE FATIGUE LIFE OF TUBULAR JOINTS

ＡＰＰＬＩＣＡＴＩＯＮＯＦＴＨＥＰＲＯＢＡＢＩＬＩＳＴＩＣＦＲＡＣＴＵＲＥＭＥＣＨＡＮＩＣＳＭＥＴＨＯＤＯＦＰＲＥＤＩＣＴＩＮＧＴＨＥＦＡＴＩＧＵＥＬＩＦＥＯＦＴＵＢＵＬＡＲＪＯＩＮＴＳＮｉｅＧｕｏ－ｈｕａ（聂国华）ＷｅｎｇＺｈｉ－ｙｕａｎ（翁智远）...

The Stress Concentration of Multiplanar Tubular Joints of Offshore Structure

- The formulation for the stress concentration matrix of multiplanar tubular joint with braces subjected to combined load is established in this paper. The formulations for SCF values of multiplanar tubular joints are compared with those by Wordsworth and Efthymiou. The stiffening effect and load-interaction effect are discussed. A spatial offshore jacket with braces subjected to combined load is computed as a numerical example. The maximum stress of each multiplanar tubular joint and its location are found through SCF matrix. Finally, the difference between the maximum stresses of multiplanar tubular joints and their corresponding uniplanar tubular joints are discussed.

AN INVESTIGATION OF RANDOM FATIGUE STRENGTH OF K-TYPE TUBULAR JOINTS AND CRACK PROPAGATION BEHAVIORS

Random fatigue of welded K-type tubular joints subjected to axial or out-of-plane bending load is analyzed. By considering the sizes of initial surface cracks and material constants as random variables with some probabilistic distributions, incorporating the effect of the weld, five hundred random samples are generated. Statistical computational results of life of crack propagation and effect of change of crack shape are finally obtained and compared with experimental data available based on a regression analysis. Meanwhile, crack propagation behaviors are also investigated.

An Improved Formulation for the Ultimate Static Strength of Tubular Joints subjected to Combined Loads

"The effect of interaction of loads on the ultimate static strength of tubular joints of offshore fixed platforms, is a practical problem. But there is still absence of rigorous theory to explain available experimental data and empirical criteria for the static strength of tubular joints. The idea of yield at hot spot of tubular joints is introduced in this paper. The interaction equations of plastic capacity for the tubular joints under combined loads (two and three different kinds) are derived. Thereafter the Yura’s test data and empirical criteria of ultimate static strength for the tubular joints can be explained. The idea of classification of category of loads in accordance with experimental data and the present theory is suggested. Finally, the improved ultimate capacity equations for tubular joints are recommended. The physical significance of the coefficient of plastic reservation Qp is discussed.

Reinforcement Method of the Tubular Joints under Combined Loads Based on ANSYS Software

The jacket offshore platform structures working in the environment are subjected to various external conditions,such as wave loads,wind loads and corrosion of sea water.Therefore,the research on reinforcement of tubular joints has great practical value for the safety of offshore platforms.In this article,the finite element(FE)models of T-type tubular joint(T-joint)and K-type tubular joint(K-joint)are established by ANSYS software.Triangular rib reinforcement and collar plate reinforcement are used to reinforce the tubular joints.The reinforcement effects are assessed through the ultimate bearing capacity,and the influences of parameters of the rib and the collar plate on the ultimate capacity are analyzed.Besides,the effects of the two reinforcement methods are compared under the combined loads,and the results show that the reinforcement of the ribbed plate is more effective in resisting the deformation caused by bending moment,while the reinforcement of the collar plate is more effective to avoid the plastic damage caused by the axial pressure.

Evaluation of stress concentration for planar tubular joints

In offshore tubular structures, a typical tubular joint may be subjected to three different types of basic loadings: axial, in-plane bending and out-of-plane bending, through its brace members. Each type will cause a different stress distribution at the joint intersection of structures. Moreover, the actual load condition of a tubular joint can be any combination of the above three basic load cases, for this reason, a combined loading was investigated in addition to these three basic loadings. This load is composed of an axial loading combined with a continuation of rotational bending loading obtained while rotate center of the brace around a circle. Different types of planar joints such as T, Y, X, K, DT, DY, DX, TY, TK, DTX, DTDY, and DTDK, with braces subjected to combined loading, were numerically analyzed to study the effect of those different cases of loading and different types of joints, on the stress concentration zone and values.

A Ring Model for Limit Analysis of Spatial XX Tubular Joints

-A ring model is developed to investigate the ultimate strength of tubular X and XX joints in the case that the brace is compressed. In the present analysis, the tubular joint is made of elasto-perfectly-plastic material, and the arch element of the chord section intersecting with the brace is assumed to be rigid. It is found that when 6 plastic hinges for X joint and 8 plastic hinges Tor XX joint appear in the ring, the limit state is reached, and by means of the equivalent ring width Be formula proposed in a previous paper by the authors, the ultimate strength of tubular X and XX joints subjected to compression can be obtained.

A review of stress concentration factors in tubular and non-tubular joints for design of offshore installations

Tubular structures are widely used in offshore installations,trusses,high rise buildings,towers for wind turbines,ski-lift installations,lightning,road pole signals etc.,owing to their excellent structural performance and attractive appearance.Stress concentration,especially in the welded joints of these structures,is an important design consideration particularly for fatigue design.In the context of tubular and non-tubular joints,this paper provides a review of the experimental and numerical studies that have been carried out so far to determine the stress concentration factor(SCF).Emphasis is also placed on the complexity of capturing different types of stresses in tubular/non-tubular joints for estimation of SCF.Present code provisions for evaluation of SCF are also discussed.Further,a few issues,which require significant research effort to advance our understanding and to improve the current design guidelines,have been identified.©2016 Shanghai Jiaotong University.Published by Elsevier B.V.

Prediction of Static Strength of Tubular T and X Joints Subjected to Axial Compression

-The formulation of ring analogy method for the prediction of static strength (ductile collapse) of tubular T, X joints under axial compression based on the limit analysis of the ring with some assumptions is presented in this papaer. The regression formula for the effective length of the chord based on test results is established by means of the least square method. The results computed by the present semi-analytic formula are compared with previous results and test data. They are quite close to each other. The accuracy of the present formula depends on the reasonable selection of the effective length of the chord, which requires numerous test data.

Fatigue crack in large-scale tubular joints for offshore structures

Tubular members have been used for constructing offshore structures. Large-scale fatigue tests of welded tubular K-joints under the balanced in-plane bending brace were carried out to investigate the fatigue behavior of API 2W Gr.50 steel produced by POSCO. The experimental results were verified by numerical approaches and compared with the IIW, DnV RP-C203 and API RP 2A-WSD design curves. The test results based on the hot spot stress were in agreement with the design curves. The SCF factor for tubular K-joint was also obtained. The numerical parametric study of the K-joint (using the finite element program) was described and its results were compared with experimental results. The stress effects of various parameters including α, β, γ, τ and θ on the stress in the K-joint were investigated. The stress distribution for each parameter was introduced. The study showed that the maximum stress of the joint varied according to the variation of joint parameters.

Location Results for Brace Spacing in Gap Tubular “K” Joints Using FEM

The 8-node iso-parametric thin shell element was employed in the study of stress concentrations in the welded tubular “K” joint. Element equilibrium equations were derived using isoparametric formulation based on thin shell theory. After assembly, the resulting system equations were solved using existing fortran programs. Numerical experiments were conducted to isolate and locate ideal gap (positions) for the two braces of the “K” joint. The nominal stresses were calculated from which stress concentration factors were obtained. The resulting stress concentration factors were presented both as tables and as figures. A good agreement between our solutions and those for model joints in the literature is good and acceptable. It was found that the wider apart the brace spacing is, the weaker the strength of the joint. It was also found that the best location for the braces occurs when the stress level changes sign either from positive to negative or vice versa at a critical sampling point.

Generation and Application of A Standardized Load-Time History to Tubular T-joints in Offshore Platforms

Marine structures are mostly made of metals and always experience complex random loading during their service periods. The fatigue crack growth behaviors of metal materials have been proved from laboratory tests to be sensitive to the loading sequence encountered. In order to take account of the loading sequence effect, fatigue life prediction should be based on fatigue crack propagation(FCP) theory rather than the currently used cumulative fatigue damage(CFD) theory. A unified fatigue life prediction(UFLP) method for marine structures has been proposed by the authors' group. In order to apply the UFLP method for newly designed structures, authorities such as the classification societies should provide a standardized load-time history(SLH) such as the TWIST and FALSTAFF sequences for transport and fighter aircraft. This paper mainly aims at proposing a procedure to generate the SLHs for marine structures based on a short-term loading sample and to provide an illustration on how to use the presented SLH to a typical tubular T-joint in an offshore platform based on the UFLP method.

异形单层网壳结构的整体稳定性研究及关键节点分析

单层网壳结构属于缺陷敏感结构,整体稳定问题是结构设计的关键.单层网壳结构可以采用空间相贯节点,有必要对节点进行三维实体有限元分析,在此基础上研究节点的力学性能.本文总结了单层网壳结构整体稳定分析的方法,归纳了空间相贯节点三维实体有限元分析方法;针对异形单层网壳结构,研究了结构的静力性能、结构网格与建筑形状对结构承载能力的影响,确定了结构的弹性稳定承载力和弹塑性稳定承载力;对空间相贯节点进行了有限元分析,得到节点处应力的分布情况,并研究了节点隐蔽部位有无焊缝对节点力学性能的影响.结果表明:异形单层网壳的曲率变化和结构网格对关键部位处杆件受力影响较大;稳定承载力对单层网壳结构起控制作用,对此异形单层网壳进行弹塑性稳定分析,结构安全系数达到24;隐蔽部位焊缝对节点承载力的影响较大,设置隐蔽部位焊缝后,节点处各杆件的最大应力可减小20%以上.

新型钢管混凝土梁柱节点力学性能试验研究

为研究内加强环式圆钢管混凝土柱与矩形钢管混凝土梁这种新型连接节点的力学性能,设计了缩尺钢管混凝土梁柱节点试件,开展了相同梁柱节点试件的静力加载试验和低周往复加载试验。通过研究该节点试件的破坏模式、荷载-位移曲线以及拟静力加载试验的骨架曲线、核心区剪切变形等,分析了节点试件的承载能力、延性和耗能能力,全面考察了同一梁柱节点在静力加载和低周往复加载两种工况下的受力性能和破坏模式。结果表明:钢管混凝土梁柱节点试件核心区强度较强,破坏模式主要为梁端破坏,低周往复加载试验时试件梁柱连接处附近的梁端钢板发生拉裂破坏和钢板鼓曲,静力加载试验时试件梁端焊缝发生拉裂破坏;试件延性较好,加载过程中经历了弹性、弹塑性和塑性发展阶段。最后提出了该类钢管混凝土梁柱节点核心区的抗剪强度计算公式。

海洋平台结构损伤涡流热成像检测激励线圈设计

海上平台作为海上石油开发的重要设施,处在海水浸泡、冲击等恶劣环境中,很容易出现损伤,影响其安全性。管节点作为高应力集中的部位是海上平台结构中更易产生损伤的地方,因此对管节点进行检测十分重要,为适应海上平台管节点这种特殊结构的损伤检测,利用涡流热成像技术,考虑管节点结构、在管节点表面感应出的涡流分布情况等因素,设计出了圆台形线圈、三角形线圈、扁平双线圈三种形式的线圈,并通过COMSOL、SolidWorks完成了相应的仿真模拟,为了符合实验室需求,又构建出了等比例待测工件及线圈的实际模型,通过实验模拟、MATLAB数据处理验证了仿真模拟的准确性及所设计线圈的有效性。结果表明,扁平双线圈对海上平台管节点的加热效果更佳,能够用于对其损伤缺陷的检测。

钢管混凝土扁柱-钢梁节点抗震性能有限元分析

为避免钢框架结构中梁柱节点凸出墙体,文章提出一种钢管混凝土扁柱与钢梁节点,采用ABAQUS有限元模拟软件建立4个不同连接位置的钢管混凝土扁柱-钢梁节点的有限元模型,分析节点在低周往复荷载下的破坏模式和抗震性能;结果表明,所提出的节点构造可以实现梁柱刚性连接,所有节点均发生梁铰破坏,抗震性能和耗能能力良好。进一步研究轴压比、竖向隔板厚度和连接构造对节点抗震性能的影响规律,结果表明:随着轴压比的提高,柱端峰值承载力随之下降;减小柱内竖向内隔板的厚度会导致柱端承载力的降低;将盖板连接改为竖向加劲肋连接后,能有效缓解节点处柱壁的应力集中现象。

螺栓球节点对钢管杆件屈曲特征值的影响

利用ANSYS软件建立螺栓球节点与钢管杆组合构件的有限元模型,采用接触模拟法计算螺栓球钢管构件的屈曲特征值,分析螺栓球节点作为杆件两端约束时对杆件临界荷载产生的影响;参数化研究螺栓半径、套筒壁厚、封板厚度、螺栓头半径和套筒长度等因素对不同长细比杆件屈曲特征值的影响;基于杆件屈曲的欧拉公式,通过回归分析获得考虑螺栓球节点参数与杆件长细比影响杆件的屈曲特征值计算公式。结果表明:随螺栓半径和套筒壁厚增大杆件屈曲特征值显著增大,螺栓头半径对其的影响可忽略不计,封板厚度的增加和套筒长度的减小也能使杆件屈曲特征值有所增加;随杆件长细比的增大,以上各因素对杆件屈曲特征值的影响均逐渐减小,同时杆件长细比越大螺栓球节点对杆件的约束越强;利用回归分析获得组合构件屈曲特征值公式的计算值与有限元结果接近,随节点参数与杆件长细比的变化趋势一致,该公式能够较精准地计算杆件屈曲特征值。

一起风电场固体绝缘管母故障的综合分析

介绍了一起风电场主变低压侧固体绝缘管母中间接头故障导致全场失电的案例。结合现场保护动作的情况及一次设备检查情况,通过对故障的管母进行解剖分析,查明了本次故障原因。针对本次故障暴露出的问题,提出了对应的防范措施,对于新能源电厂在固体绝缘管母的设计选型与日常运维具有一定的指导意义。

Parametric Equation of Stress Concentration Factor for Circular X-Joints Under Axial Loads

In engineering practice,tubular X-joints have been widely used in offshore structures.The fatigue failure of tubular X-joints in offshore engineering is mainly caused by axial tensile stress.In this study,the stress concentration factor distribution along the weld toe in the hot spot stress region for tubular X-joints subject to axial loads have been analyzed by use of finite element method.Through numerical analysis,it has been found that the peak stress concentration factor is located at the saddle position.Thereafter,80 models have been analyzed,and the effect of the geometric parameters of a tubular X-joint on the stress concentration factor has been investigated.Based on the experimental values of the numerical stress concentration factor,a parametric equation to calculate the stress concentration factor of tubular X-joints has been proposed.The accuracy of this equation has been verified against the requirement of the Fatigue Guidance Review Panel,and the proposed equation is found capable of producing reasonably accurate stress concentration factor values for tubular X-joints subject to axial loads.