Reliability-based assessment of polyethylene pipe creep lifetime

Lifetime management of underground pipelines is mandatory for safe hydrocarbon transmission and distribution systems. The use of high-density polyethylene tubes subjected to internal pressure, external loading and environmental variations requires a reliability study in order to define the service limits and the optimal operating conditions. In service, the time-dependent phenomena, especially creep, take place during the pipe lifetime, leading to significant strength reduction. In this work, the reliability-based assessment of pipe lifetime models is carried out, in order to propose a probabilistic methodology for lifetime model selection and to determine the pipe safety levels as well as the most important parameters for pipeline reliability. This study is enhanced by parametric analysis on pipe configuration, gas pressure and operating temperature.     

Finite element modeling of hydrogen atom diffusion and distribution at corrosion defect on aged pipelines transporting hydrogen

Repurposing existing natural gas pipelines for hydrogen transport has attracted wide interests. However, the corrosion defect present on these aged pipelines can affect hydrogen (H) atom accumulation, potentially causing hydrogen embrittlement. In this work, a finite element-based model was developed by coupling solid mechanics and H atom diffusion to investigate the distribution of H atoms at a corrosion defect on a steel pipe segment under applied longitudinal tensile strains. The applied strain causes local stress (both Mises stress and hydrostatic stress) and strain concentrations at the corrosion defect, affecting the H atom diffusion and distribution. In the absence of the tensile strain, the H atoms, once entering the interior of pipe, diffuse uniformly into the pipe body along the radial direction driven by a concentration gradient. When a strain is applied on the pipe, the H atom diffusion is driven by hydrostatic stress. The maximum H atom concentration exceeds the initial concentration of H atoms entering the steel pipe, indicating the H atom accumulation at the corrosion defect. The applied tensile strain also affects the location where the H atoms accumulate. For both internal and external corrosion defects, more H atoms will be concentrated at the defect center when the defect length reduces and the depth increases.     

Measurement and simulation of residual stress in type 304 weld overlay stainless steel pipe

Repair by welding overlay is a commonly used method mainly employed to rebuild piping systems suffering from intergranular stress corrosion cracking (IGSCC). It is desirable that the overlay welding technique, by attaching an overlay weld to the pipe and sustaining a heat sink of flowing water inside the pipe, induces a compressive residual stress at the inner surface of the welded pipe for prevention of IGSCC. A better understanding of the effect of a welding overlay repair on the residual stresses at the inner and outer surfaces of weld overlay is thus required. To obtain this understanding, it is necessary to investigate the distribution of residual stresses on the welded pipe.

In this study, the hole-drilling strain-gauge method was adopted to determine the residual stresses at the inner and outer surfaces of the weld overlay pipe. The incremental drilling technique was used on pipes with outside diameters of 267 mm. In addition, the Weld 3 code was applied to simulate the residual stress distribution for comparison and verification with the measured results.

The results obtained from the experimental and from the computational methods are in good agreement. The residual stress at the inner surface of the pipe is compressive with a magnitude approaching the yield stress of the material; that at the outer surface is tensile, also with a magnitude close to yield stress but smaller than the compressive stress. The experimental residual stress magnitude is generally greater than that from computation. This observation can be attributed to several factors including applied mechanics, temperature distribution, original residual stress, strain gauge location, mechanical grinding and the oxidation layer.     

Prediction of the failure pressure for complex corrosion defects

A new method for predicting the failure pressure of corrosion defects in pipelines has been developed. The failure pressure of a plain pipe represents an upper limit for the failure pressure of a pipe with a corrosion defect. The failure pressure of a uniform depth, infinitely long groove, where the depth is equal to the maximum depth of the corrosion defect, represents a lower limit for the failure pressure of a pipe with a natural corrosion defect. The predicted failure pressure can be calculated from these limits using the weighted depth difference (WDD) method, which accounts for the defect geometry and any interaction with adjacent defects. The WDD method has been validated using the results of 40 burst tests of pipe sections containing real corrosion defects. The results indicate that this method provides more accurate burst pressure predictions than the currently accepted corrosion defect assessment procedures.     

The effect of residual stress on the creep deformation of welded pipe

The finite element method has been used to study the creep deformation of pipe butt welds in the presence of an initial residual stress distribution. The study has used values for pipe dimensions, temperature (575°C) and internal pressure (70 bar) which correspond to the conditions being used in a pressure vessel testing programme which is investigating transverse weld metal cracking in CrMoV reheat pipe welds. Two sets of steady state creep data for the weld metal have been used, one to represent a weld weaker than the parent and the other one that was stronger. A residual stress distribution, consistent with experimental data, has been generated as àn initial condition for the analysis. The results are presented, discussing the effects of residual stress on both the total and creep strain accumulations for the two weld properties. The steady state creep laws used in the analysis do not allow for damage accumulation, but the possibility of such an effect is discussed using the Kachanov model. The results of the finite element analysis are compared with the experimental creep strain data from the pressure vessel testing programme.     

Residual stresses in girth butt welded pipes and treatments to modify these

A number of Type 304 stainless steel pipes are used in the primary cooling systems of nuclear plants. Intergranular stress corrosion cracks (IGSCC) were found at some welded joints in these piping systems due to very high tensile residual stress, sensitization of the material due to welding, and corrosive environment, all occurring simultaneously. Investigations have shown that at least one of the above factors must be eliminated to prevent IGSCC.

This report describes experimental results on the temperature variations during pipe welding by conventional techniques and by the heat sink welding (HSW) technique. The mechanism of residual stress generation due to welding is also discussed. The pipe used in these experiments was 4B Sch80 Type 304 stainless steel. It was found that the temperature distribution through the thickness of the pipes was almost uniform for the conventional welding technique, but had a very sharp gradient for HSW. In the pipe axial direction, the temperatures varied sharply for both welding techniques. This implies that the sensitization of metal due to HSW is lighter than that of conventional welding and that the residual stress on the inside surface of the heat sink welded pipe is compressive.

The induction heating stress improvement (IHSI) method has been investigated analytically and experimentally. In the IHSI method, a pipe is heated with an induction coil while cold water is pumped through it. This causes a temperature gradient throughout the pipe wall which generates high thermal stresses. This, in turn, generates compressive stresses on the inner surface of the pipe. This method is designed to eliminate tensile residual stresses near the weld heat affected zone on the inner surface.

Temperature analysis and subsequent thermoelastic-plastic analysis show that tensile weld residual stresses at a joint were changed into compressive stresses on the inner surface of a pipe. It was confirmed experimentally that these stresses suppressed fatigue crack propagation in the heat affected zone (HAZ) of a welded pipe. Therefore, the IHSI method is effective not only in preventing crack initiation but also in suppressing crack propagation.

As for the relaxation of residual stresses, no significant relaxation was measured when external loads were applied at as much as 80% of the yield strength in the experiments.     

埋地钢质燃气管道完整性评价方法

管道完整性评价是在役管道完整性管理的重要组成部分．检测技术则是完整性评价的重要环节。本文提出了采用外腐蚀直接评价方法进行管道外覆盖层检测及完整性评价的方法和步骤，并对各种检测方法的特点及选用原则进行了探讨，上述方法可以用于埋地钢质燃气管道的综合检测及完整性评价。     

Failure analysis of corroded high-strength pipeline subject to hydrogen damage based on FEM and GA-BP neural network

The pipeline is a major approach to achieving large-scale hydrogen transportation. Hydrogen damage can deteriorate the material performance of the pipe steel, like ductility and plasticity reduction. Corrosion is dominating damage that impairs a pipeline's bearing capacity and structural reliability. However, previous research barely investigated the effect of hydrogen damage on failure behaviors, residual strength and interacting effect between adjacent corrosions of corroded high-strength pipelines transporting hydrogen. Besides, hardly any burst pressure model considers hydrogen damage. In this paper, several approaches, including the finite element method (FEM), regression analysis, the orthogonal test method, and the artificial neural network method, are applied to fill the gap. First, a series of finite element models with different geometric features and hydrogen damage is established to investigate the effects of hydrogen damage and corrosion on failure behaviors and residual strength. The results show that hydrogen damage can change the corroded pipeline's failure behaviors and reduce the residual strength. Second, based on the simulation results and regression analysis, a new burst model is developed to consider the hydrogen damage and improve the estimation accuracy. Third, based on the genetic algorithm (GA), a GA-BP neural network is established and trained for accurate and efficient residual strength estimation considering hydrogen damage. Furthermore, an orthogonal test is designed and performed to investigate the effects of critical parameters on the burst pressure of the corroded pipeline after hydrogen damage. The results indicate that hydrogen damage and corrosion length have similar contributions to the residual strength. Finally, the simulation results of pipelines with multiple corrosions show that hydrogen damage has a significant impact on the interacting effect between adjacent corrosions. The results obtained are valuable for further integrity management of steel pipelines carrying hydrogen.     

支吊架附重对管系应力的影响分析

管道支吊架在火力发电厂四大汽水管道中主要承担承受管道荷载、限制管道位移和控制管道振动等作用。针对某300MW机组的再热管道系统,分析支吊架附属重量对管道一次应力和二次应力的影响,对考虑支吊架附重和不考虑支吊架附重两种情况对管道一次应力及二次应力影响进行对比,得出大型支吊架的附属重量在管道的应力分析中不容忽视,对管道的应力分布有较大影响。     

Measuring and modelling residual stresses in butt welded P91 steel pipe including effects of phase transformations

Abstract

Residual stresses in a circumferentially butt welded steel pipe have been measured and numerically predicted. The pipe, containing the circumferential weld, has an outer diameter of 290 mm and a wall thickness of 55 mm, typical of components in power generation plants. An axisymmetric thermomechanical finite element (FE) simulation has been performed to obtain the residual stress field induced by the fusion welding of the pipe, taking solid state phase transformation effects into account and using temperature dependent material property data. Residual stresses have been measured using the X-ray diffraction and deep hole drilling techniques. Good correlation has been demonstrated with the predictions of the FE model. The paper demonstrates that a mixed experimental and numerical approach is useful for determining the residual stress distribution in welded joints.     

深圳市某河道截污管道近距离上穿既有大直径地下供水管道有限元分析

城市建设过程中各类地下管线交叉不可避免,由于地下管线材质、管径、使用功能、荷载不同,常规计算方法繁琐且不够直观。为此,通过有限元方法分析了深圳市某河道截污管道近距离上穿既有大直径地下供水管道问题,为既有管道安全和正常使用提供了直观的计算结果,亦为工程设计和施工方案提供了技术支撑。     

地下供水管井过滤管腐蚀结垢试验

我国每年水井工程建设量巨大,目前常用成井管材为金属井管,PVC-U塑料井管正处于推广应用阶段。为客观评价国内常用的几种供水井过滤管的腐蚀结垢和析出现象做了室内试验和实际井下检测。结果表明,金属井管腐蚀产生结垢物又加速腐蚀,而PVC-U过滤管不腐蚀不结垢,代替了传统金属管材具有成本低、能耗小、使用寿命长、维修次数少等优点。     

改善零件疲劳寿命的激光冲击强化技术

喷丸强化在工业界已得到广泛的应用,而激光冲击强化处理是最近几年来出现的表面改性新工艺。它利用激光冲击波技术使零件表层材料的亚结构得到改善和产生有益残余压应力,因此降低了生成疲劳裂纹源的比例和增加抗应力腐蚀的能力,从而延长零件的疲劳使用寿命。同传统的机械喷丸强化相比,激光冲击强化具有更大的优势。     

Experimental determination of stress corrosion crack rates and service lives in a buried ERW pipeline

Threshold stresses and crack growth rates for in-service stress corrosion cracking (SCC) of two electrical resistance weld (ERW) seam welded pipes from two 45-year-old oil pipelines were experimentally assessed. Seventeen high-pH SCC tests were carried out, in both base and ERW weld metals, at two temperatures (73 and 45 °C). Tapered specimens were used for base metal, and constant section specimens were developed for ERW tests, in which original surface conditions were preserved. It was found that susceptibility of the ERW seam welds is much higher than for base materials, so that the welds define the length of the pipe that is susceptible to SCC. Threshold pressure estimates for SCC initiation were defined from tests at elevated temperature, service temperature, and literature correlations. Fabrication residual stresses were also measured and taken into consideration. SCC threshold pressures for these lines are controlled by the ERW welds; the pipe tracts that are considered to be susceptible to SCC are those that undergo a service pressure of at least 2.4 MPa. For the case under study, this represents about 70% of the length of the pipeline.     

A study on the mechanical stress relieving in a butt-welded-pipe

Heat transfer, thermal and residual stress distribution were numerically determined by using the finite element method for a butt-welded pipe. A mechanical stress relieving (MSR) treatment which was frequently used in the fabrication of pressure vessels instead of the post-weld heat treatment (PWHT) was also simulated to investigate its effect of reducing the residual stress in the welded zone by a mechanical loading. Stress redistribution in a butt-welded pipe during MSR loading was studied numerically in three different loading conditions. In this study, the inner loading case has the strongest effect on the residual stress reduction behavior among the three loading cases. The reduction quantity of the tensile residual stress near the weld centerline, when MSR treatment of the inner loading case is used after pipe welding, linearly increases when compared with the magnitude of the applied loads. When the MSR treatment is performed after pipe welding, the load case is a tensile stress if the reduction of the peak value of tensile stresses is strong. On applying a compressive MSR load to the weldment, the peak value of the compressive residual stresses are reduced quickly.     

Residual stresses in a CrMoV-2CrMo pipe weld: Part 1—The as-welded condition

There is a complete lack of reliable data concerning the through thickness distribution of residual stress in welds, both for the as-welded and the stress relieved conditions. As a result, the assessment of defects in welds and current stress relieving practice are largely based on surface measurements of residual stress.To improve this situation a technique has been established which permits the complete distribution of residual stress to be determined. This technique has been applied to a ferritic main steam pipe weld, in the as-welded condition. In establishing this technique, which uses experimental measurements combined with finite element analyses, three different methods of solution were investigated involving line loads, band loads and triangular loads. Only the last was found to give acceptable accuracy.The results of the analysis show that, in general, the residual hoop stresses over the outer two-thirds of the weld are tensile, being compressive over the inner third. Axial stresses are tensile over the outer third and compressive over the inner two-thirds.The approximations and assumptions used in the analysis are discussed, together with the implications that the results have on defect assessment and stress relieving.     

A generic approach for a linear elastic fracture mechanics analysis of components containing residual stress

A review of through thickness transverse residual stress distribution measurements in a number of components, manufactured from a range of steels, has been carried out. Residual stresses introduced by welding and mechanical deformation have been considered. The geometries consisted of welded T-plate joints, pipe butt joints, tube-on-plate joints, tubular Y-joints and tubular T-joints as well as cold bent tubes and repair welds. In addition, the collected data cover a range of engineering steels including ferritic, austenitic, C–Mn and Cr–Mo steels. The methods used to measure the residual stresses also varied. These included neutron diffraction, X-ray diffraction and deep hole drilling techniques. Measured residual stress data, normalised by their respective yield stress have shown an inverse linear correlation versus the normalised depth of the region containing the residual stress (up to 0.5 of the component thickness). A simplified generic residual stress profile based on a linear fit to the data is proposed for the case of a transverse residual tensile stress field. Whereas the profiles in assessment procedures are case specific the proposed linear profile can be varied to produce a combination of membrane and bending stress distributions to give lower or higher levels of conservatism on stress intensity factors, depending on the amount of case specific data available or the degree of safety required.     

船舶汽轮机蒸汽管路流动噪声源的声类比计算研究

针对某船舶动力系统蒸汽调节管路在不同阀门启闭状态下的流动噪声源空间分布,开展了详细的计算气动声学数值研究工作。采用剪切应力输运湍流模型(SST,shear stress transport)获取了管内复杂三维蒸汽流场分布;结合Curle声类比模型和Proudman声类比模型分别计算得到管路壁面偶极子噪声源和管内流体域四极子噪声源分布特性。研究结果表明：相比于阀门关闭状态下的一进一出“T型”和“Z型”管路,阀门开启后的两进一出“h型”管路内流动噪声源强度上升明显;偶极子噪声源主要分布在管路汇聚处与出口弯管区域壁面,与此区域流体剪切应力相关;由于出口弯管直径突然变小,且流体流向发生变化,导致此处管内流场湍流度较高,故而产生了较为强烈的四极子噪声源。     

Committee Report-recommended Practice on Specific Aspects of Cable Installation in Power-Generating Stations

In the late 1940's high voltage pipe-type feeders were first installed in the United States. Since that time, over 2360 circuit miles of underground high voltage pipe-type cables have been installed throughout the country. Con Edison has approximately 652 circuit miles of high pressure pipe-type cable on its system, operating at 69, 138 or 345 kV. The typical pipe-type cable system is comprised of a steel pipe, containing 3 cables, with splices located at intervals of approximately 2000 feet. The pipe is filled with dielectric fluid which is maintained at a nominal operating pressure of 200 psig. Pressurization on the feeder is maintained automatically by pumping plants. For the 345 kV system, these plants sometimes include cooling capability. As the pipe type cable system grows older, leaks of dielectric fluid develop. The major causes of leaks are corrosion, contractor damages, effects of stray currents and localized pipe wear due to vibration. Quick detection and location of dielectric fluid leaks, particularly without the need to deenergize the feeder, is of prime importance to the utility industry. Raychem Corporation has been involved with the development of sensor cables for the detection and location of fluid leaks such as water and gasoline. This technology has been enhanced to address the problem of pipe type cable dielectric fluid leaks and a new system has been developed. The new system uses a sensor cable which is buried in the trench with the pipe type cable.     

Mechanical methods of improving resistance to stress corrosion cracking in BWR piping systems

Pipelocks and the mechanical stress improvement process (MSIP) have been applied in BWR plants. Pipelocks restore the integrity of the weldments with identified cracks. MSIP removes residual tensile stresses from weldments, thus preventing initiation of cracks or retarding growth of pre-existing flaws in piping systems. The first 12-in pipelock installed at Commonwealth Edison's Quad Cities plant was inspected after operating for 18 months. Pipelocks for 10-in, 12-in and 28-in reactor safe-ends were fabricated for Carolina Power & Light's Brunswick plant. MSIP was applied at Commonwealth Edison's Dresden and LaSalle BWR plants. Extensive qualification has been completed for MSIP under US Nuclear Regulatory Commission and Electric Power Research Institute sponsorship. Sectioning of the pipe wall by Argonne National Laboratories provided stress distribution before and after MSIP for the 12-in and 28-in pipes. Measured ‘as welded’ tensile stresses before MSIP were within the range 30–50 000 psi. Compressive stresses after MSIP at the inside surface of the weldment reached more than 30 000 psi in both hoop and axial directions. The axial compressive stresses extended to the middle plane of the wall. Hoop stresses remained compressive through the wall. The stresses were uniformly distributed around the circumference of the pipe. J.A. Jones Applied Research Company completed an evaluation of MSIP applied to a precracked weldment between 28-in pipe and elbow. The pipe was squeezed to about 1·7% in the presence of cracks 25%, 50% and 90% through-wall. High compressive stresses were measured after MSIP. The cracks did not extend and could be identified after completion of the process by the usual UT technique. The use of mechanical methods becomes especially adequate for reactor safe-ends including bi- or tri-metallic joints. The use of overlay technique or induction heat stress improvement is more difficult due to high thermally induced strains at the strong discontinuity interface between materials of different thermal expansion. Basic concepts and practical application of mechanical methods to inhibit stress corrosion attack are described.