Structural integrity analysis of HDPE pipes for water supplying network

A problem of reliability of polymer pipes manufactured with the high‐density polyethylene (HDPE) for drinking water supply was considered. The safety of the pipelines network was analyzed with taking into account two main factors: arising of sudden overpressure (water hammer phenomenon) and presence of defects on pipe surface. Based on elastoplastic fracture mechanics and using of J‐integral approach, the numerical model of cracked pipe was developed and verified by direct experimental test. The formula for calculation of crack initiation pressure, which is the function of pipe size and size of external surface defect, was derived. The validity of this relation was proved for pipes of diameter 450 to 800 mm. It was also shown that the pressure peaks of the water hammer phenomenon are quickly damped because of the high deformability of the polyethylene pipes and the defect depth that is equivalent to 40% of pipe wall thickness can be considered as acceptable. The applicability of developed tools was demonstrated under analysis in the real state of South Tunisian water supply network.     

Accuracy and security analysis of transient flows in relatively long pipelines

Consideration is given, in this paper, to the accurate numerical solution of transient flows in relatively long pipelines caused by the water hammer phenomenon. The governing equations for such flows are a set of two coupled non-linear hyperbolic partial differential equations where the friction factor is a very important parameter. In these equations, the gravity force is introduced to take into account the pipeline inclination. The mathematical equations are solved in the time domain by the method of characteristics using linear integration. To reproduce correctly the transient flow, very small time increments were used in the constructed computer program. This program permits to get some systematic indications on the evolution and the damping of the pressure head waves due to a fast closing valve at the downstream end of a long pipeline. The study shows that results are different from those of short pipelines and small viscous fluids. To check the validity of the numerical model, computed results have been successfully compared with those found in the relevant literature. These results show that the gravity lift may have an important effect on the maximum pressures, which may become very important near the valve and cause failure of the pipeline especially in presence of defect. The safety factor, computed at equidistant sections of the line, determines the distance between the reservoir and the defect from which the failure may happen. For the considered application, results show that the pipeline is safe near the supply upstream tank.     

中空纤维超滤系统的水锤分析及防护

超滤系统的物理清洗是通过阀门频密开启和关闭、水泵的交替起动和停机来实现的,这使液体流速产生急剧变化;并引起很高的水锤压力,造成管道及膜元件损坏.分析了超滤系统出现关阀水锤及停泵水锤的成因,提出相应的防护措施.     

Revised burst model for pipeline integrity assessment

Failure of pipeline is often caused by corrosion. It affects the health, safety, environment and economy considerably. To prevent pipeline failure, it is necessary to focus on the corrosion and pressure of the structures. Burst pressure is a key factor to assess the integrity of a pipeline. There are three means to determine burst pressure. These are lab testing, evaluation criteria and the Finite Element Method (FEM) modeling. However, the results of the burst pressure assessment using evaluation criteria are too conservative compared to the results of an actual pipe burst experiment and FEM modeling.The objectives of this paper are to analyze the changing trend of burst pressure of pipelines with different defect dimensions; to compare the FEM results with those of the evaluated criteria (DNV model) to indicate the conservative property; to revise the DNV model and verify its validity with actual burst experiments. The revised DNV model predicts burst pressure more reliably that could result in better engineering design of pipelines.     

基于流固耦合的水力瞬变三维模拟及管壁动态应力分析

水力瞬变也称为水锤,是由于管道内压力瞬间升高,压力波在管道内以声速传递的现象。紧随着压力波,管壁上产生动态应力,进而导致管道失效。基于ALE流固耦合方法模拟由于阀门突然关闭导致的管道中流体冲击波,并与一维水锤冲击波理论值进行了对比。在此基础上分析了不同约束条件下,距离管道末端一定距离,由于管壁变形和振动而产生的三维动态应力。发现在水锤波传播过该位置一段时间之后,管壁应力才达到最大值,此外管壁周向应力在动态应力中影响是最主要的。     

Gouge assessment for pipes and associated transferability problem

The purpose of this work is to assess a gouge defect in a pipe submitted to internal pressure. To do that a method based on failure assessment diagram and more precisely on a Modified Notch Failure Assessment Diagram (NMFAD) which has been proposed as a mesofracture approach. The safety factor has been determine under conservative conditions i.e. for a X52 pipe steel having a relatively low fracture toughness and a severe gouge defect with high aspect ratio and high constraint. In addition a mesofracture approach of the fracture toughness transferability problem has been proposed. The crack (K–T) methodology has been modifying to create the (K_ρ–T_ef) two parameters fracture resistance criterion.     

Assessment of failure pressure of a GFRP composite repair system for wall loss defect in metallic pipelines

The present paper is concerned with the failure pressure assessment of a newly developed GFRP matrix laminated composite repair of metallic pipelines with 80 % wall loss defect. The main motivation is to validate the experimental failure pressure with the theoretical one. Wall loss defects were manufactured into the tube specimen and the performance of the repaired pipe was assessed by hydrostatic tests as per ISO/TS 24817 standard. Results reveal that the theoretical failure pressure as per ISO/TS 24817 standard is found to be conservative. A wide range of failure pressure values was obtained using different criteria. Only RSTRENG 0.85* modified criterion provides a closer value towards the experimental one. However, the criterion selection for damage factor and flow stress condition is an open question as it gives dispersed values of failure pressure. The prediction of the failure pressure must be accurate without being overly conservative. Plastic deformation of the tube occurs towards the end of the tube, consequently, it is necessary to account for this behavior in failure pressure analysis for an accurate prediction. The application of loading‐unloading cycles before the failure burst test should be also accounted in order to predict the failure pressure.     

Root Cause Failure Analysis of a Domestic Pressure Cooker Through Metallurgical Characterization and Computational Simulation

Pressure cooker is a closed domestic pressure cooking vessel for use with external heat source and capable of maintaining nominal cooking pressure up to 1.0 kgf/cm² (100 kN/m² approximately) gauge nominal. In pressure cookers, despite the development of electronic controllers, the basic edition of such vessels are still equipped with fewer advanced safety functions due to economic constraints. Although the provision of pressure relief valves (PRV) is considered as one of the major protective features, however, pressure cooker failure accidents have been reported frequently. This paper describes the analysis of pressure cooker failure that failed prematurely after 1 year of service considering the design life of > 5 years. It was hypothesized that the root cause of pipe failure was either material degradation from exposure to an aggressive environment or an inherent defect in the pressure cooker. To test this hypothesis, a thorough visual examination of the exhumed failed section and the fracture surface was undertaken, followed by liquid penetrant testing, material identification, hardness testing, and metallographic analysis. Computational models of static and transient loading were also used to determine the stress distribution along the actual geometry of the failed cooker and to understand the main causes of recurrent failures. Visual and macroscale examination revealed significant body deformation at the lower dish-ended shell showing distorted locking grooves. It was also noticed that dirt and food particle, from the earlier cooking, were stuck in the pressure valves orifice. In addition, no evidence of metallurgical defect was observed. The inspection indicated that the cause of failure is primarily due to the choking of pressure relief value (PRV) and overpressure safety valves. Consequently, the pressure release occurred from the sealing side of the top lid, which resulted in its ejection.     

有压管道水击波动过程及优化控制的解析研究

为了实现阀调节对有压管道水击过程的精确控制,获得水击波动过程的解析解很有意义。该文在详细分析水击基本微分方程组及初边值条件的基础上,将波动方程在有限区间内的行波解应用于线性水击波动问题中。通过给定阀门处速度变化规律,引入曲线积分与路径无关条件,得到了阀门关闭过程中管道内无因次水击压强的精确解析解。应用Ritz法求解泛函极值,得到了使管道阀门处峰值压强为最小值时所对应的速度变化及相应的关阀规律。以此构建程序控制,就可最大限度地削减水击压强,以求通过阀门来实施对水击过程的主动控制。     

A physical probabilistic model to predict failure rates in buried PVC pipelines

For older water pipeline materials such as cast iron and asbestos cement, future pipe failure rates can be extrapolated from large volumes of existing historical failure data held by water utilities. However, for newer pipeline materials such as polyvinyl chloride (PVC), only limited failure data exists and confident forecasts of future pipe failures cannot be made from historical data alone. To solve this problem, this paper presents a physical probabilistic model, which has been developed to estimate failure rates in buried PVC pipelines as they age. The model assumes that under in-service operating conditions, crack initiation can occur from inherent defects located in the pipe wall. Linear elastic fracture mechanics theory is used to predict the time to brittle fracture for pipes with internal defects subjected to combined internal pressure and soil deflection loading together with through-wall residual stress. To include uncertainty in the failure process, inherent defect size is treated as a stochastic variable, and modelled with an appropriate probability distribution. Microscopic examination of fracture surfaces from field failures in Australian PVC pipes suggests that the 2-parameter Weibull distribution can be applied. Monte Carlo simulation is then used to estimate lifetime probability distributions for pipes with internal defects, subjected to typical operating conditions. As with inherent defect size, the 2-parameter Weibull distribution is shown to be appropriate to model uncertainty in predicted pipe lifetime. The Weibull hazard function for pipe lifetime is then used to estimate the expected failure rate (per pipe length/per year) as a function of pipe age. To validate the model, predicted failure rates are compared to aggregated failure data from 17 UK water utilities obtained from the United Kingdom Water Industry Research (UKWIR) National Mains Failure Database. In the absence of actual operating pressure data in the UKWIR database, typical values from Australian water utilities were assumed to apply. While the physical probabilistic failure model shows good agreement with data recorded by UK water utilities, actual operating pressures from the UK is required to complete the model validation.     

气液固三相管流耦合水击振动特性的参数影响分析

以气液固三相管流为研究对象,采用矢通量分裂法并结合Lax-Wendroff格式和迎风Warming-Beam格式,对浆液池-管道-阀门系统的耦合水击振动响应进行了数值计算,分析了含气率、固液密度比以及固液弹模比等参数对系统振动特性的影响。结果表明:增加含气率可有效地降低压力和应力波速,同时削弱流体压力波动和管道振动强度;当固液密度比增大时,管系振动强度随之增大,系统振动能量的增量主要集中在流体里,造成流体压能升高较快;随着固液弹模比的增加,管系压能和振动强度均增加,但增幅很小;当固液弹模比增加到某种程度后,其对系统压能和振动强度的影响可忽略。     

一种用于输油管道安全保护的高,低压泄压阀

为了防止长输原油管道因遇突然事故对机，泵，阀等设备及管道的损坏，以保障安全平稳地输油，在输油管道上装设安全保护装置是十分必要的。我们根据液压系统中的溢流阀原理，参照了输油管道上引进的高压阀，设计出一种新型的应用于长输原油管道上的安全保护装置－先导式电动高压阀及气－液阻尼式电动低压泄压阀。     

Analysis of shape and location effects of closely spaced metal loss defects in pressurised pipes

Metal loss due to corrosion is a serious threat to the integrity of pressurised oil and gas transmission pipes. Pipe metal loss defects are found in either single form or in groups (clusters). One of the critical situations arises when two or more defects are spaced close enough to act as a single lengthier defect with respect to the axial direction, causing pipe ruptures rather than leaks, and impacting on the pressure containing capacity of a pipe. There have been few studies conducted to determine the distance needed for defects to interact leading to a failure pressure lower than that when the defects are treated as single defects and not interacting. Despite such efforts, there is no universally agreed defect interaction rule and pipe operators around the world have various rules to pick and choose from. In this work, the effects of defect shape and location on closely spaced defects are analysed using finite element analysis. The numerical results showed that defect shapes and locations have a great influence on the peak stress and its location as well as the failure pressure of pipes containing interacting defects.     

Structural reliability assessment of cracked pipes: The role of probability of detection data

The structural reliability of industrial pipes, including those in the nuclear, oil, and gas industries, has a significant impact on the safety of people and the environment. This work aims to develop a computational structural reliability model method in conjunction with the failure assessment diagram method and the user‐defined probability of detection curves of non‐destructive testing are used. The concept of “reliability factor of a repair” is proposed. Then, the effects of the pipe inspection considering different user‐defined probability of detection curves and different values of the reliability factor of a repair on probability of failure are discussed. The main results include the identification of cases where performing repairs do not guarantee an improved reliability, as well as the consequences of considering the repair as a “perfect process” which result in non‐conservative assessments.     

Mechanical Behavior and Evaluation of Dented Pipe Caused by Cylindrical Indenter

Journal of Failure Analysis and Prevention - The defect of pipe dent has a great threat to the safety of oil and gas pipe transportation. In this paper, the numerical calculation model of pipe...     

Mathematical simulation of the strength of a pipe with three-dimensional defect

We develop a mathematical model of plastic fracture of a thin-walled pipe with three-dimensional part-through “smooth” defect and compute the degree of weakening caused by this defect. The defect is simulated by an elliptic (in plan) slot appearing in the pipe. It is necessary to find the level of internal pressure under which the material in the zone of the defect passes into the limiting state. or sufficiently deep defects whose sizes are much smaller than the radius of the pipe, the problem of determination of the stress-strain state in the zone of the defect under given internal pressure is reduced to the solution of a nonlinear system of two one-dimensional integral equations. This system is solved by using an original algorithm and the limiting pressure is found within the framework of the proposed mathematical model. Under certain assumptions concerning the distribution of stresses in the zone of the defect, we deduce a formula which expresses the limiting pressure via the sizes of the defect and the strength characteristics of the material. The results of theoretical analysis are compared with numerical data obtained by the method of finite elements. PNVP “Integrator”. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 1, pp. 65–70, January–February, 1998.     

埋地PVC管纵向断裂失效预测概率模型

针对存在初始裂纹缺陷的埋地PVC管在荷载作用下的纵向断裂失效问题,基于管道环向受力特性和裂纹扩展经验公式建立失效预测模型,并在此基础上运用蒙特卡罗法(MC)对模型和参数的不确定性进行随机模拟,给出管道失效风险率随服役时间变化的规律。计算结果表明:MC数值模拟结果与实测值吻合良好,说明失效预测模型的合理性;裂纹增长是一个由慢到快的过程,初始裂纹越大,失效时间越短;失效风险率在开始的20年内达到最高,随后逐渐减小;所有参数都作为随机变量考虑时与只有初始裂纹为随机变量时相比,失效风险率提高近一倍;管道初始裂纹、内压、残余应力和壁厚是影响管道失效时间的四个重要因素,管道内压和残余应力增加都导致管道失效率变大,准确的给出管道初始裂纹的分布对PVC管纵向断裂失效预测尤为重要。能够减小PVC管初始裂纹缺陷的生产工艺和施工技术可以有效提高管道的使用寿命。     

Improved estimation of ultra-deep water pipe collapse pressures by using extreme value theory

We examine safety margins between measured pipe collapse pressures and maximum external hydrostatic pressure exerted on pipeline on the seabed. We analyze collapse pressures measured on specimen rings cut from pipes, which are valid proxies for testing full length pipes. Our novel application of statistical models motivated by Extreme Value Theory provides a firmer basis for extrapolating collapse pressures than current practice which assumes a Gaussian distribution, and which gives over-conservative safety margins compared with our method. Our more accurate assessment of collapse pressures could give large cost savings in the manufacture of thick-walled pipe for ultra-deep water pipelines.     

Prediction of residual pipeline resource taking into account the operation loading conditions

An effective energy approach to the evaluation of the residual service life of a pipe of oil pipeline containing a crack on its inner surface for the two-frequency loading mode of biaxial tension-compression has been proposed. The two-frequency variations of pressure in the pipe are caused by the turbulence of the oil flow (high frequency), opening and closing of the gate valves, and the shutdowns of the pumps (low frequency). __________ Translated from Problemy Prochnosti, No. 1, pp. 44–52, January–February, 2009.