力-热载荷下双金属复合管的屈曲失效研究

采用有限元方法研究了力-热载荷下双金属复合管的屈曲失效行为,通过三维有限元建模考虑了双金属复合管的准静态复合成型制造过程中产生的残余应力,分析了温度及内压两个主要参数对双金属复合管屈曲失效的影响。结果表明,高温导致材料发生软化,抑制了双金属复合管的屈曲;弯矩、内压及热载荷联合作用下,复合管内介质温度降低,复合管弯矩达到最大值对应的曲率减小,而弯曲承载能力增大,外基管的椭圆率也增大;内压变化对复合管的弯曲承载能力和外基管的椭圆率影响较小。     

复杂载荷下双金属复合管的屈曲失效研究

本文采用有限元方法系统地研究了复杂载荷下双金属复合管的屈曲失效，三维有限元数值模型考虑了双金属复合管的准静态复合成型制造过程中产生的残余应力，分析了外基管直径、内衬管壁厚、内外管初始间隙、内衬管屈服强度、内压等因素对双金属复合管屈曲失效的影响。结果表明，加载路径、复合管的几何尺寸及内衬管的屈服强度对双金属复合管的屈曲性能均有较大影响，内充压力可以延迟内衬管的屈曲失效。     

Mechanical behavior and wrinkling of lined pipes

The paper focuses on wrinkling of lined pipes (sometimes referred to as clad pipes) under bending loading, where a corrosion-resistant thin-walled liner is fitted inside a carbon–steel outer pipe. The problem is solved numerically, using nonlinear finite elements to simulate liner pipe deformation and its interaction with the outer pipe. Stresses and strains are monitored throughout the deformation stage, detecting possible detachment of the liner from the outer pipe and the formation of wrinkles. The wrinkling behavior of elastic and elastic–plastic (steel) lined pipes under bending is examined. The results indicate that the lateral confinement of the liner pipe due to the deformable outer pipe and its interaction with the outer pipe has a decisive influence on the wrinkling behavior of the lined pipe. It is also shown that the behavior is characterized by a first bifurcation in a uniform wrinkling pattern, followed by a secondary bifurcation. The values of corresponding buckling curvature are determined and comparison with available experimental results is conducted in terms of wrinkle height development and the corresponding buckling wavelength. The results of the present research can be used for safer design of lined pipes in pipeline applications.     

弯曲及拉伸荷载作用下的钢管抗压溃性能研究

钢管在海洋油气资源开发中应用广泛,一般依据在位应用时的荷载进行设计,设计理论较为成熟。但在某些安装工况中的拉伸和弯曲荷载与在位荷载具有相同的量级,此时钢管的设计需要考虑安装荷载的影响。本文针对钢管在承受拉伸和弯曲荷载下的抗外压压溃性能开展研究。基于商业软件ABAQUS建立有限元模型,开展钢管压溃实验与带有弯曲荷载下的钢管压溃实验验证有限元模型的正确性,进而利用验证后的数值模型讨论拉伸、弯曲荷载和两者联合作用下对管道抗外压压溃性能的影响。结果表明,弯曲荷载是影响抗压溃性能的重要因素,弯曲荷载的增加都会使极限压溃值显著下降。管道所受拉伸荷载对抗压溃性能有一定的影响,但影响很小,在两者共同作用下极限压溃值随弯曲荷载的增加都出现先增大后减小的现象,而且极值的位置与拉伸荷载的大小和弯曲荷载有关。因此,在抗压溃设计中应重点考虑弯曲荷载对压溃的影响,可忽略拉伸荷载的影响。本文可为海洋管道的安装起到指导作用。     

Elastic-plastic secondary indeterminate problem for thin-walled pipe through the inner-wall loading by three-point bending

The inner-wall loading by three-point bending about thin-walled pipe is an elastic-plastic secondary indeterminate problem in the symmetrical three-roller setting round process. In this study, the shifting of the tangent point between the pipe and lower roller is ignored. The bilinear hardening material model is adopted, and the static equilibrium condition, physical relationship of elastic-plastic deformation, and deformation compatibility condition are taken into account. Based on the geometrical discrete idea, a semi-circular pipe is meshed equably into N micro-pipe-wall elements with same geometric parameters along the circumferential direction. Deformation characteristics of each element are calculated, and then the deformation history response of the whole pipe is resolved by the load increment method. The finite element model of static bending in three-roller setting round process is established by using the software package ABAQUS. The theoretical and simulated results show that the cross section of pipe has two positive bending regions and two reverse bending regions; the maximum bending curvature appears in the bottom section of pipe, the minimum bending curvature appears in the section corresponding to the tangent point of the pipe and lower roller. The quantitative relationships between the upper roller load, maximum(minimum) bending curvature and reduction are given. Finally, the reliability of theoretical calculation is proved by numerical simulation.     

An experimental investigation of T-beam stability under bending

This paper presents an experimental setup used to study the response and stability of T-beams subjected to pure bending. The setup consists of a four-point bending device instrumented to measure the moment and curvature in the beam. Special grips are used to attach the specimens to the bending device and enforce a no-twist condition at the ends of the beams. A special instrument is used to measure deformations induced by buckling along the axis of the beam. Results are presented for the cases of monotonic and cyclic bending of short beams that buckle in a local mode. The results indicate that the lateral deflection of the web of the beams develops a ripple relatively early in the bending process. Local buckles arise from localization of the amplitude of the ripple at a given point along the specimen.     

Fluid flow in a helical pipe

Without simplifying the N-S equations of Germano's^[5], we study the flow in a helical circular pipe employing perturbation method. A third perturbation solution is fully presented. The first- second- and third-order effects of curvature κ and torsion τ on the secondary flow and axial velocity are discussed in detail. The first-order effect of curvature is to form two counter-rotating cells of the secondary flow and to push the maximum axial velocity to the outer bend. The two cells are pushed to the outer bend by the pure second-order effect of curvature. The combined higher-order (second-, third-) effects of curvature and torsion, are found to be an enlargement of the lower vortex of the secondary flow at expense of the upper one and a clockwise shift of the centers of the secondary vortices and the location of maximum axial velocity. When the axial pressure gradient is small enough or the torsion is sufficiently larger than the curvature, the location of the maximal axial velocity is near the inner bend. The equation of the volume flux is obtained from integrating the perturbation solutions of axial velocity. From the equation the validity range of the perturbation solutions in this paper can be obtained and the conclusion that the three terms of torsion have no effect on the volume flux can easily be drawn. When the axial pressure gradient is less than 22.67, the volume flux in a helical pipe is larger than that in a straight pipe.     

5-1/2 FH钻杆接头极限承载能力研究

随着超深井、定向井、水平井、大位移井及大斜度井在石油钻井工程中的广泛应用,由井下复杂工况引起的钻杆接头过早失效问题日益突出,导致钻井周期增长,成本增加,成为制约钻井工程效益的主要因素之一.近年来,不少学者对钻杆接头进行了大量研究,多数采用二维轴对称模型,少数采用三维力学模型,但并未同时考虑螺纹升角和井眼弯曲作用等因素的影响,而钻杆接头的极限承载能力研究更是鲜见报道.针对上述问题,基于虚功原理、von Mises屈服原则及接触非线性理论,同时考虑螺纹升角和井眼弯曲作用,建立了钻杆接头的三维数值仿真模型与井眼曲率到加载弯矩的转换模型,研究了钻杆接头的上扣特性、井眼曲率对连接强度和密封性能的影响,考虑预紧力、弯曲载荷及动载安全系数,计算了钻杆接头的极限工作拉力和极限工作扭矩.研究结果表明:上扣扭矩使钻杆接头产生一定的初始接触压力,保证钻杆接头井下作业过程中的连接强度与密封性能;井眼曲率对钻杆接头井下作业过程中的连接强度与密封性能影响极大,常见的某些工况会导致钻杆接头的连接强度和密封性能丧失,考虑服役时的随机振动与冲击,常规的超深井、水平井、定向井、大位移井及大斜度井弯曲段钻杆接头的设计和选型应着重考虑井眼曲率的影响;针对设计的每种钻杆接头,都应考虑常见的井眼曲率和轴向拉伸载荷进行极限工作拉力和极限工作扭矩的精细化数值计算,以确保其安全工作.     

Buckle propagation in pipe-in-pipe systems.: Part I. Experiments

Pipe-in-pipe systems are extensively used in offshore pipeline applications in which thermal insulation of the pipeline is necessary. Typically, the space between the two pipes is either empty or contains insulation material which provide minimal mechanical support to the system. In deepwater applications, the carrier pipe must be designed to resist collapse due to the ambient external pressure while the design of the inner pipe is usually governed by the pressure of the hydrocarbons it carries. The integrity of the two-pipe system in the event of accidental collapse of the carrier pipe is an issue of concern.In Part I of this two-part report, the results of an extensive experimental study of the problem are presented. The experiments were carried out on two-inch diameter carrier tubes with D/t values of 24.1, 21.1 and 16.7 and inner tubes of several diameters and wall thicknesses. In most cases local collapse of the outer tube led to simultaneous collapse of the inner one. Subsequently, the collapse propagated simultaneously collapsing both pipes. The propagation pressure of the two-pipe system (P_P2) has been quantified parametrically. An interesting second mode of collapse propagation was also discovered in which the carrier pipe collapses leaving the inner one intact. Propagation of collapse affecting both pipes is still possible but usually at a higher pressure level. The pressure at which this switch takes place has been found to closely correspond to the propagation pressure of a carrier pipe with a solid rod insert (P_PS) of the same diameter as the inner pipe. The parametric dependence of P_PS has also been established experimentally. Part I finishes with a discussion of how these new critical pressures of pipe-in-pipe systems should influence the design of such pipelines.     

Axisymmetric wrinkling of snug-fit lined pipe

An analytical bifurcation solution is presented for axisymmetric wrinkling on a lined pipe under axial compression without internal pressure. The internal liner consists of corrosion-resistant alloy (CRA), it is not metallurgically bonded to the carbon steel backing pipe, and it is assumed to be in a snug fit condition: i.e. there is no gap between the liner and the backing pipe, but also no prestress that would lead to a positive contact or gripping pressure between the liner and the backing. The backing is assumed to be much thicker than the liner, so that wrinkling-related deformations of the backing pipe can be neglected.The solution indicates that the incipient wrinkling strain for the snug-fit pipe without any imperfections is the same as the incipient wrinkling strain for a single pipe with (5/3) times the wall thickness of the liner, and the same midsurface diameter, as determined by the solution of Batterman (1965) for the case of small strains, or Peek (2000a) for the case of finite strains. For the case when the liner-pipe friction is included the factor (5/3) increases slightly.A positive contact pressure due to prestress or internal pressure raises the wrinkling strain, whereas imperfections (e.g. at seam or girth welds) reduces it. The snug-fit solution accounts for neither, but nevertheless provides a useful reference wrinkling strain, and can be used to validate numerical solutions, and it gives a bifurcation modeshape and wrinkle length that can be used in numerical models to investigate post-bifurcation behaviour.     

海底管道的弹塑性稳定性和屈曲传播

本文根据我国南海海底输油气铺管的要求系统分析了海底管道在弯曲与外水压力共同作用下的弹塑性稳定性和屈曲传播.研究了管道的极值型屈曲和分枝型屈曲.在考虑管道的初始非圆度和材料的物理非线性的情况下提出了临界屈曲载荷的计算方法.综述和评论了屈曲传播现象的本质和各种计算方法.介绍了我们所进行的全尺寸管道实验.在分析理论结果时与现行的有关设计规程进行了比较和评论.     

DIVERGENCE INSTABILITY OF VARIABLE-ARC-LENGTH ELASTICA PIPES TRANSPORTING FLUID

A flexible elastic pipe transporting fluid is held by an elastic rotational spring at one end, while at the other end, a portion of the pipe may slide on a frictional support. Regardless of the gravity loads, when the internal flow velocity is higher than the critical velocity, large displacements of static equilibrium and divergence instability can be induced. This problem is highly nonlinear. Based on the inextensible elastica theory, it is solved herein via the use of elliptic integrals and the shooting method. Unlike buckling with stable branching of a simply supported elastica pipe with constant length, the variable arc-length elastica pipe buckles with unstable branching. The friction at the support has an influence in shifting the critical locus over the branching point. Alteration of the flow history causes jumping between equilibrium paths due to abrupt changes of direction of the support friction. The elastic rotational restraint brings about unsymmetrical bending configurations; consequently, snap-throughs and snap-backs can occur on odd and even buckling modes, respectively. From the theoretical point of view, the equilibrium configurations could be formed like soliton loops due to snapping instability.     

扁化效应对弯曲段套管抗挤强度的影响对比分析

针对弯曲段套管抗挤强度随井眼曲率变化规律问题，利用材料力学理论将弯曲段套管等价为均布载荷作用下的弯曲梁模型，根据拉梅厚壁筒理论和强度理论得到套管抗挤强度公式．借鉴扁化变形在圆管类结构中的研究思路，将其拓展到套管抗挤强度分析，对比分析是否考虑扁化效应下四种规格套管不同井眼曲率下的抗外挤强度．研究表明，套管轴向应力随井眼曲率基本呈线性变化；套管抗挤强度随井眼曲率的增加非线性下降．井眼曲率为2o/30 m时，是否考虑套管扁化变形得到的5 1/2"×10.54 mm P110套管抗挤强度误差为12.8 %；井眼曲率为12o/30 m时，误差46.9 %．井眼曲率为2o/30 m时，4 1/2"×9.65 mm TP140V套管抗挤强度误差11.9 %；井眼曲率为12o/30 m时，误差44.7 %．可见，考虑扁化变形后套管抗挤强度小于未考虑扁化变形情况；井眼曲率不大时，两种算法的差距较小，随着井眼曲率的增加，套管扁化变形对套管抗挤强度的影响不能忽略．     

含局部减薄弯头塑性极限载荷的数值分析

局部减薄是一种压力管道表面常见的体积型缺陷，它不仅会降低管道的承载能力，而且还可能引起管道破坏，导致严重的事故．采用数值分析方法，对内压和面内弯矩作用下含局部减薄弯头的极限载荷进行了研究，分析了载荷组合、缺陷形状、位置、尺寸对弯头极限承载能力的影响，讨论了导致弯头破坏的典型失效模式．提出了含局部减薄缺陷弯头的塑性极限载荷工程估算式．计算结果为含缺陷弯头的安全评定提供了理论依据．     

Turbulence-driven secondary flows in a curved pipe

If the torque exerted on a fluid element and the source of streamwise vorticity generation are analyzed, a turbulence-driven secondary flow is found to be possible in a curved pipe. Based on this analysis, it is found that the secondary flow is primarily induced by high anisotropy of the cross-stream turbulent normal stresses near the outer bend (furthest from the center of curvature of the bend). This secondary flow appears as a counterrotating vortex pair embedded in a Dean-type secondary motion. Recent hot-wire measurements provide some evidence for the existence of this vortex pair. To verify the formation and extent of this turbulence-driven vortex pair further, a near-wall Reynolds-stress model is used to carry out a detailed numerical investigation of a curved-pipe flow. The computation is performed specifically for a U-bend with a full developed turbulent flow at the bend entrance and a long straight pipe attached to the exit. Numerical results reveal that there are three vortex pairs in a curved pipe. The primary one is the Dean-type vortex pair. Another pair exists near the pipe core and is a consequence of local pressure imbalance. A third pair is found near the outer bend and is the turbulence-driven secondary flow. It starts to appear around 60° from the bend entrance, grows to a maximum strength at the bend exit, and disappears altogether at about seven pipe diameters downstream of the bend. On the other hand, calculations of developing laminar curved-pipe flows covering a range of pipe-to-bend curvature ratios, Reynolds number, and different inlet conditions fail to give rise to a third cell near the outer bend. Therefore, experimental and numerical evidence together lend support to the formation of a pair of turbulence-driven secondary cells in curved-pipe flows.Research supported by the Office of Naval Research under Grant No. N0014-81-K-0428 and by the David Taylor Research Center, Annapolis, Maryland, under Contract No. N00167-86-K0075.     

Curvature effects on axially compressed buckling of a small-diameter double-walled carbon nanotube

The curvature effects of interlayer van der Waals (vdW) forces on axially compressed buckling of a double-walled carbon nanotube (DWNT) of diameter down to 0.7 nm are studied. Unlike most existing models which assume that the interlayer vdW pressure at a point between the inner and outer tubes depends merely on the change of the interlayer spacing at that point, the present model considers the dependence of the interlayer vdW pressure on the change of the curvatures of the inner and outer tubes at that point. A simple expression is derived for the curvature-dependence of the interlayer vdW pressure in which the curvature coefficient is determined. Based on this model, an explicit formula is obtained for the axial buckling strain. It is shown that neglecting the curvature effect alone leads to an under-estimate of the critical buckling strain with a relative error up to −7%, while taking the average radius of two tubes as the representative radius and the curvature effect leads to an over-estimate of the critical buckling strain with a relative error up to 20% when the inner radius downs to 0.35 nm. Therefore, the curvature effects play a significant role in axially compressed buckling problems only for DWNTs of very small radii. In addition, our results show that the effect of the vdW interaction pressure prior to buckling of DWNTs under pure axial stress is small enough and can be negligible whether the vdW interaction curvature effects are neglected or not.     

高铬铸铁钻井泵缸套之磨损机理与新型缸套的研究

本文对不同地区使用换效的１０只高铬铸铁钻井泵缸套严重磨损表面进行了金相分析、扫描电子显微镜形貌分析和磨屑的铁谱分析，进而对缸套的磨损失效过程和起主导作用的磨损机理作了探讨，提出了具有一定理论价值和实用意义的新观点。文章根据缸套材质的匹配试验结果指出，缸套的含Ｃｒ量以１０％（ｗｔ）左右为最佳，既能保证其具有良好的耐磨性而延长使用寿命，又能减少Ｃｒ的用量而使成本降低１７．８５％。按照本文提供的数据资料     

SIMPLE RESISTIVE MODELS FOR STEADY FLOW THROUGH UNIFORM AND TAPERED-STIFFNESS COLLAPSIBLE TUBES

An extremely basic model is postulated and examined numerically, to find out which aspects of observed steady-flow collapsible-tube behaviour are predicted and can be explained. The model in simplest form states that the tube has a fixed viscous resistance per unit length when not collapsed, and a higher one when collapsed. Collapse occurs where the falling internal pressure in the streamwise direction causes negative transmural pressure to pass a fixed threshold set by tube wall stiffness. This model suffices to explain (i) the sigmoidal dependence of pressure drop on flow rate when external pressure is fixed, (ii) the weak dependence of pressure-drop on flow rate when downstream transmural pressure is fixed and (iii) the weak dependence of flow rate on pressure drop when upstream transmural pressure is fixed. The effects of incorporating more realistic collapse behaviour (finite compliance once the tube buckles, varying compliance once opposite walls are in contact) on these dependencies are examined. The model is also used to explore the several qualitatively distinct configurations that may be taken up by a tube which varies in stiffness along its length.     

Development of a bluff body wake under the combined influence of curvature and pressure gradient

The present experimental investigation deals with the behaviour of a wake generated by a square cylinder developing in a curved diffuser, a curved duct, a straight duct and a straight diffuser having a same pressure gradient as in the curved diffuser. This enables a systematic study of the effects of curvature and pressure gradient on wake development. It is seen that the curvature makes the wake asymmetric; the wake half width increases on the inner side and decreases on the outer side; the inner side being the region between the centreline and the wall closer to the centre of curvature and the outer side being the region between the centreline and the other wall. It causes a higher entrainment in the inner side as compared to the outer side. An adverse pressure gradient, on the other hand, causes a higher wake growth and velocity defect but reduces the rate of decay of the velocity defect. These are not altered significantly when the curvature and pressure gradient effects are combined. The curvature enhances the Reynolds stresses and the kinetic energy on the inner side and suppresses them on the outer side which makes their profiles asymmetric. These profiles become more and more asymmetric with increase in the streamwise distance. When the effects of curvature and adverse pressure gradient are combined, the profiles become further asymmetric.Department of Aerospace Engineering     

Influence of concentrated lateral loads on the elastic stability of cylinders in bending

The character of the instability and the degradation of the moment-carrying capacity are found by Mylar model experiments for cylinders in bending when subjected to concentrated lateral loads. Lateral loads can seriously degrade the moment capability of cylinders. Critical combinations of moment and lateral load cause two distinct modes of failure—collapse and snapping. Collapse modes exhibit buckles which cover the compression half of the cylinder and are critical for large values of moment and small values of lateral load. Snapping modes of failure involve a single dimple and exist for smaller values of moment and larger values of lateral load.