On the performance of integral buckle arrestors for offshore pipelines

This paper presents the results of a study on the effectiveness of integral buckle arrestors for offshore pipelines. A series of full scale experiments were conducted where the pressure at which buckles propagating quasi-statically crossed arrestors of various lengths and thicknesses was established. The crossover pressures were used to establish the parametric dependence of the arresting efficiency (as defined in Kyriakides and Babcock, ASME Journal of Pressure Vessel Technology102 (1980) and Proceedings of Offshore Technology Conference (1979)) of such devices. Buckles penetrated the arrestor in two modes: the flattening mode, where the arrestor and downstream pipe collapse in the same manner as the incoming buckle and the flipped mode, where the sense of collapse of the downstream pipe is orthogonal to the incoming buckle. The mode switch occurred in the neighborhood of efficiency of 0.7.The process of quasi-static engagement of an arrestor by a propagating buckle, the temporary arrest of the buckle and the eventual crossing of the arrestor were simulated through a finite element model. The model is based on finite deformation kinematics, incorporates J₂-type plasticity with isotropic hardening, and allows for contact of the walls of the collapsed section of pipe upstream of the arrestor. The model was verified by simulating each of the 15 physical experiments conducted using the actual geometric and material characteristics of the test specimens. The crossover pressures of the simulations were within 5% from the measured values and the mode of crossover was predicted correctly as well. The model was subsequently used to extend the experimental parametric study of arrestor efficiency. Some limiting values of the parameters were established from the results and several design recommendations are made.     

On the arresting efficiency of slip-on buckle arrestors for offshore pipelines

Slip-on buckle arrestors are tight-fitting rings placed over the pipe at intervals of several hundred meters. They locally increase the pipe resistance to collapse by providing additional circumferential rigidity, and thus impede downstream propagation of collapse. This type of arrestor offers important advantages over other designs as it does not require welding. Alternatively, when split in two and by the addition of flanges, it can be clamped onto a continuous pipe. This is an essential characteristic for lines installed by the reeling method.It has long been known that such devices often cannot reach higher levels of arresting efficiency. The somewhat deficient performance is due to the fact that a propagating buckle can penetrate such devices via a folded-up U-mode at pressures that are lower than the collapse pressure of the pipe. Previous work on the subject from the 1970s dealt with relatively thin-walled pipes used in shallow waters. The subject has recently been revisited and bounds for this deficiency in performance have been established. The deficiency depends on the pipe D/t and on the steel grade. This paper presents the results of a more detailed study of such arrestors for pipes with lower D/t values (18–35) suitable for moderately deep and deep waters. The arresting efficiency has been studied parametrically through experiments and full scale numerical simulations. The results have also shown the previously developed efficiency bounds to be viable. A new empirical design formula has been developed. Used in conjunction with the efficiency bounds, it provides a reliable method for an expanded use of slip-on type buckle arrestors in many deepwater applications.     

Dynamic performance of integral buckle arrestors for offshore pipelines. Part II: Analysis

A design methodology for integral buckle arrestors for deepwater pipelines was presented in a previous study (Park TD, and Kyriakides S., International Journal of Mechanical Sciences 1997;39:643–69). It was based on experiments and analyses in which buckles engaged the arrestors quasi-statically. In this two-part paper series, the performance of the same arrestors is reevaluated under the more realistic dynamic buckle propagation conditions encountered in the sea. The experimental program described in Part I involves tubes with D/t=27.9 and arrestors with L_a /D=0.5. The quasi-static arresting efficiency of buckle arrestors is first established experimentally as a function of the arrestor thickness. The same arrestor designs are then tested again in constant pressure environments where buckles propagate at velocities of 400–1100 ft/s. Experiments are conducted using both water and air as pressurizing media. A typical test specimen involves a relatively long upstream section of tube welded to an arrestor and to a downstream tube. The buckle is initiated in the upstream tube, accelerates to steady-state propagation, engages the arrestor and is either arrested or crosses over. For each arrestor design several such tests are required in order to bracket the dynamic crossover pressure. For all cases considered, the dynamic crossover pressure was found to exceed the corresponding quasi-static value. The reasons for this enhancement in performance are discussed in Part II in the light of results from numerical simulations of this process.     

火星熄灭器两相流数值模拟

针对某型号火星熄灭器,采用非定常雷诺时均（RANS）和离散型模型（DPM）对其内部流场和颗粒分离行为进行了数值模拟.计算了初始模型设计流速下的捕集效率和阻力损失;分析了内部速度场、压力场和颗粒运动轨迹特性.针对计算结果,对原火星熄灭器的结构进行了改进.采用相同的计算方法,对改进后的设计方案进行了仿真计算,结果表明提高了火星熄灭器的综合性能.该仿真计算方法能较好地预测火星熄灭器内部两相流的流动特性,对火星熄灭器性能预测和结构改进提供可靠依据.     

On the propagation pressure of long cylindrical shells under external pressure

Local imperfections induced in long tubes subjected to high external pressures can lead to local collapse, from which a propagating buckle can be initiated. This can result in catastrophic collapse of large sections of the structure. The propagation pressure is the lowest pressure at which such a buckle will propagate. For common structural metal tubes with diameter-to-thickness ratios of less than 100, the propagation pressure is typically half an order of magnitude lower than the collapse pressure of the intact tubes. As a result, the design of several tubular structures with external pressure loading requires that the collapse and propagation pressures be accurately known. This paper deals with the experimental and analytical challenges of establishing the propagation pressure. A special purpose three-dimensional analysis, in combination with experimental observations and results, is used to demonstrate a mechanism of initiation of propagating buckles in long tubes, to study the parametric dependence of the propagation pressure and to illustrate the effect of axial tension on the propagation pressure. Propagation pressures predicted with this analysis are used to evaluate the strengths and weaknesses of simpler models developed in the past.     

On the effect of axial tension on the propagation pressure of long cylindrical shells

Local imperfections induced to long tubes subjected to high external pressures can lead to local collapse from which a propagating buckle can be initiated. This can result in catastrophic collapse of large sections of the structure. The lowest pressure at which such a buckle will propagate, known as the propagation pressure of the tube, is typically half an order of magnitude lower than the collapse pressure of the intact tube. In a number of modern deep water applications, long tubular structures are subject to high axial tension in addition to external pressure. The paper describes the results of an experimental study in which the propagation pressure of long metal tubes was measured in the presence of a constant, axial tensile force. Tension was found to significantly reduce the propagation pressure. A parametric study of the problem, augmented by a simple model of the phenomenon, has yielded approximate expressions for the propagation pressure in the presence of axial tension.     

A comprehensive study on a propagating buckle in externally pressurized pipelines

Buckle propagation is a unique phenomenon occurring in deep-sea pipelines. In previous works, this phenomenon was investigated using a ring technique in which the pipeline was assumed to be in plane strain condition and the energies absorbed in membrane stretching and longitudinal bending were ignored. This paper presents a three-dimensional analysis of the buckle propagation phenomenon with an emphasis to address more complete factors that were not accounted for in the ring analysis. The analyses are based on the available solutions of the transition zone obtained in our previous works. A comprehensive mechanism for buckle propagation phenomenon is described from the point view of plastic stability theory for shells which enables the incorporation of the effects of transverse and longitudinal bending, membrane stretching and material strain hardening. The nondimensionalized buckle propagation pressure is represented in terms of yield coefficient, strain hardening coefficient and membrane stretching factor. It is found that a buckle once initiated in a pipeline may or may not propagate along the pipeline depending on its radius-to-thickness ratio. By comparing with various experimental results the theoretical predictions from this analysis are shown to provide very accurate estimations of the buckle propagation pressure for different materials with diverse geometric parameters and material properties. This paper points to the need for more complete information regarding the effects of transverse bending, membrane stretching and material strain-hardening on the buckle propagation pressure. Upon the requirement of application variations of the yield coefficient, strain hardening coefficient and membrane stretching factor with respect to the radius-to-thickness ratio are sketched out. This eliminates the need for recourse the curves and allows a fast and convenient resolution of buckle propagation pressure for certain pipeline. Most importantly, the present analysis offers the potential for future design of pipelines being at once more rationally and parametrically complete, and yet compact and simple to apply.     

Effects of the UOE/UOC pipe manufacturing processes on pipe collapse pressure

Large-diameter pipes used in offshore applications are commonly manufactured by cold-forming plates through the UOE process. The plate is crimped along its edges, formed into a U-shape and then pressed into an O-shape between two semicircular dies. The pipe is welded closed and then circumferentially expanded to obtain a highly circular shape. Collapse experiments have demonstrated that these steps, especially the final expansion, degrade the mechanical properties of the pipe and result in a reduction in its collapse pressure upwards of 30%. In this study the UOE forming process has been modeled numerically using a 2-D finite element model. The model can assess the effects of press parameters of each forming step on the final geometry and mechanical properties of the pipe. The final step involves simulation of pipe collapse under external pressure in order to quantify the effect of the forming variables on its performance. Examples of these variables are the radii of the forming dies, the chosen displacements of the dies, the compression strain in the O-step, the expansion strain, etc. An extensive parametric study of the problem has been conducted, through which ways of optimizing the process for improved collapse performance have been established. For example, it was found that optimum collapse pressure requires a tradeoff between pipe shape (ovality) and material degradation. Generally, increase in the O-strain and decrease in the expansion strain improve the collapse pressure. Substituting the expansion with compression can not only alleviate the UOE collapse pressure degradation but can result in significant increases in collapse performance.     

Finite element analysis of buckle propagation in pipelines under tension

Using a novel finite element technique, results are obtained regarding the effects of tension on buckle propagation in pipelines. The unique feature of the technique, which is otherwise built within the framework of large-deformation, elastoplastic finite element analysis, is that the steady-state nature of buckle propagation is exploited in the formulation. Parametric studies can be conducted easily, since the computational requirements of the procedure are low compared with those of conventional finite element analysis. In this paper, the technique and its implementation are reviewed briefly. In order to demonstrate the predictive capability of the technique, results for aluminum Al-6061-T6 are compared with experimental data for a wide range of values of the diameter-to-thickness ratio. Calculations are then carried out in order to examine the effects of tension on buckle propagation.     

高钢级管线管压缩变形能力研究

工作在地震、泥石流等地质灾害多发区的管线,必须承受较大的位移,其变形能力是非常重要的.针对两种钢级、两个系列(X70-1, X70-2, X80-1, X80-2)的四种管线管,采用有限元法、四结点壳单元对其压缩变形能力进行研究.结果表明压缩载荷作用下管体发生轴对称模式的屈曲,屈曲部位起皱,并有应力、应变集中.其中2系列管的屈曲应变及极限应变高于1系列管,二者在力学性能方面的主要区别在于其屈强比、形变强化指数、均匀伸长率不同,研究结果显示,降低管材屈强比、提高应变强化指数及均匀伸长率,可有效提高管线管的抗压缩变形能力.     

16MnR钢在不同应力比下的疲劳裂纹扩展的试验研究及模拟

采用3.8 mm厚带有圆形缺口的CT试样,研究了16MnR钢在不同应力比的恒幅循环载荷作用下的疲劳裂纹扩展。开发了一种基于疲劳损伤的方法来模拟疲劳裂纹扩展速率。将16MnR钢的循环塑性本构模型通过用户材料子程序UMAT嵌入到ABAQUS中。把有限元计算得到的疲劳裂纹尖端附近区域的弹塑性循环应力应变结果,代入到疲劳损伤模型中,得到每个加载循环在裂尖各点产生的疲劳损伤值。通过疲劳损伤准则,导出疲劳裂纹稳定扩展速率的计算公式。疲劳裂纹扩展试验验证了模拟结果。实验结果和模拟结果都表明,该试样厚度下,应力比对裂纹扩展速率几乎没有影响。     

Analysis of collapse in flattening a micro-grooved heat pipe by lateral compression

The collapse of thin-walled micro-grooved heat pipes is a common phenomenon in the tube flattening process, which seriously influences the heat transfer performance and appearance of heat pipe. At present, there is no other better method to solve this problem. A new method by heating the heat pipe is proposed to eliminate the collapse during the flattening process. The effectiveness of the proposed method is investigated through a theoretical model, a finite element(FE) analysis, and experimental method. Firstly, A theoretical model based on a deformation model of six plastic hinges and the Antoine equation of the working fluid is established to analyze the collapse of thin walls at different temperatures. Then, the FE simulation and experiments of flattening process at different temperatures are carried out and compared with theoretical model. Finally, the FE model is followed to study the loads of the plates at different temperatures and heights of flattened heat pipes. The results of the theoretical model conform to those of the FE simulation and experiments in the flattened zone. The collapse occurs at room temperature. As the temperature increases, the collapse decreases and finally disappears at approximately 130 ℃ for various heights of flattened heat pipes. The loads of the moving plate increase as the temperature increases. Thus, the reasonable temperature for eliminating the collapse and reducing the load is approximately 130 ℃. The advantage of the proposed method is that the collapse is reduced or eliminated by means of the thermal deformation characteristic of heat pipe itself instead of by external support. As a result, the heat transfer efficiency of heat pipe is raised.     

On strength criteria of fillet welds

A comparison between two widely used strength criteria (ISO and Eurocode 3) for welding design is carried out in this paper. Limit analysis theorems are applied for analysis of collapse load of a fillet weld using von Mises yield criterion, generalizing a previous methodology in which Tresca criterion was used. Using von Mises criterion, lower bounds of collapse load can be found by solving a quartic equation, thus avoiding the numerical sweep needed with Tresca criterion. The results of limit analysis fit much better with those of Eurocode 3 criterion, and this criterion also reasonably fits experimental results for side-fillets, but experimental results on end-fillets agree with ISO criterion and not with Eurocode 3 criterion and limit analysis. A numerical finite element analysis shows that this apparent contradiction is originated by the influence of overlapping between the welded plates, which gives rises to the transmission of part of the internal moment through contact forces between plates and not through the throat. The results support the use of Eurocode 3 criterion instead of ISO, and this is confirmed by the analysis of a varied set of welded layouts using the former in a general numerical method developed previously, which gives results that show a very reasonable agreement with experimental results found in the bibliography.     

顶驱中心管螺纹力学行为模拟及优化研究

针对顶驱中心管螺纹粘扣、断裂等问题，基于弹塑性理论、von Mises屈服准则以及中心管材料本构模型试验研究结果,建立了扭矩作用下的二维、三维有限元模型，并通过实验结果与数值模拟结果验证有限元分析的可靠性，对比二维、三维的分析结果验证二维轴对称模型方法的可靠性，获得了中心管螺纹接头的应力分布规律和力学特性，指出紧扣扭矩对螺纹连接强度影响极大。在此基础上，结合现场失效的螺纹接头，对螺纹锥度、螺距以及有效螺纹牙长度3个参量进行敏感性分析，综合考虑连接强度以及生产成本推荐了参量最优值。     

Fracture criteria identification using an inverse technique method and blanking experiment

In order to optimize the blanking processes, it is important to identify the conditions within the deforming workpiece which may lead to fracture initiation and propagation. Within this framework, numerical simulations are widely used in industries to optimize sheet metal forming processes. However, in order to have a confidence in the results of such simulations, an accurate material model is required. The accuracy of a material model is affected by the constitutive equations and the values of the material parameters. In order to reduce the danger of fracture of metal parts during manufacturing processes, advanced optimal design requires knowledge of critical values of some fracture criteria of the material used. Experimental identification of fracture criteria are currently obtained by performing several complicated tests and long duration of experiments.This study presents a computation methodology allowing for the identification of critical values of fracture criteria in order to simulate crack initiation and propagation generated by shearing mechanisms, which are needed for metal blanking processes simulation. The approach is based on inverse technique using circular blanking experiments and finite element calibration model. The critical values of fracture criteria are obtained in such a way that the finite element force–penetration predicted curve fit the experimental plot deduced from blanking tests. The numerical results obtained by the simulation were compared with experimental ones to verify the validity of the proposed technique for fracture criteria identification.     

The response of honeycomb sandwich panels under low-velocity impact loading

This paper describes the results of an experimental investigation and a numerical simulation on the impact damage on a range of sandwich panels. The test panels are representative of the composite sandwich structure of the engine nacelle Fan Cowl Doors of a large commercial aircraft. The low-velocity impact response of the composites sandwich panels is studied at five energy levels, ranging from 5 to 20 J, with the intention of investigating damage initiation, damage propagation, and failure mechanisms. These impact energy levels are typically causing barely visible impact damage (BVID) in the impacted composite facesheet.A numerical simulation was performed using LS-DYNA3D transient dynamic finite element analysis code for calculating contact forces during impact along with a failure analysis for predicting the threshold of impact damage and initiation of delaminations. Good agreement was obtained between numerical and experimental results. In particular, the numerical simulation was able to predict the extent of impact damage and impact energy absorbed by the structure. The results of this study is proving that a correct numerical model can yield significant information for the designer to understand the mechanism involved in the low-velocity impact event, prior to conducting tests, and therefore to design a more efficient impact-resistant aircraft structure.     

飞机整体翼梁结构裂纹扩展试验与分析

对飞机整体翼梁结构进行裂纹扩展试验,应用ANSYS有限元软件对试验件结构进行裂尖应力强度因子分析,利用Paris公式对其进行寿命估算,研究飞机整体翼梁结构裂纹扩展的基本特征和规律,并进行试验与计算分析的对比.试验及分析结果可为飞机整体翼梁结构损伤容限设计及评定提供依据.     

Collapse mode transitions of thin tubes with wall thickness, end condition and shape eccentricity

Transition of deformation mode shapes of round aluminum tubes from axisymmetric concertina to non-axisymmetric diamond mode have been studied with varying tube wall thickness, boundary conditions and tube shape eccentricities. Quasi-static axial compression experiments were carried out on as received aluminum tubes and tubes with wall thickness eccentricity, incorporated by off center machining. Tubes were of D/t=29 and L/D=1.4. The numerical simulation of the collapse phenomenon has been undertaken using a static non-linear finite element analysis in ANSYS with a fine mesh discretization of the tube domain and small incremental displacements as load steps. Convergence studies for the finite element model with respect to load step size and mesh density have also been established. The numerical results are found to compare well with the experimental load compression and energy absorption responses both for the axisymmetric concertina and non-axisymmetric diamond collapse modes. Having validated the numerical model with experiments, it has been used to undertake a systematic study of the load–deformation characteristics, energy absorption response and collapse mode transition of the tubes in varying configurations of wall thickness, shape and inplane boundary condition eccentricities. Dependence of tube collapse characteristics and collapse mode transitions on such eccentricities have been discussed.     

矩形蒸汽灭菌器强度数值模拟与试验研究

真空压力蒸汽灭菌器由于具有良好的灭菌性能,在医疗器械领域得到了日益广泛的应用。由于外加强带圆角矩形截面灭菌器结构特殊,GB 150—1998中规定的应力公式不能适用,需运用数值方法对其进行强度计算和校核。因此,在试验研究基础上,本文建立了一种具有较高精度的矩形灭菌器三维有限元模型,获得了灭菌器在设计压力下的应力分布结果。通过比较表明,有限元结果和试验结果吻合得较好,最大应力均发生在圆角附近区域。有限元结果为制定矩形截面灭菌器相关标准提供了一个参考依据。