The comparison of dented pipeline displacement calculation methods

According to dent defects on pipeline, the depth criterion cannot meet demands of oil and gas industry, so dent evaluation system based on strain is proposed. In order to obtain strain value for evaluation, the profiles of dent defects must be drawn accurately to obtain accurate displacement. The linear superposition of general solution for beam, beam on Winkler foundation, bivariate polynomial function and modified cubic spline interpolation are used respectively to simulate and calculate actual displacement of dented pipeline, and compare with results of finite element. We can know from the analysis and comparison, the result of modified cubic spline interpolation is more approximate to actual displacement of dented pipeline, which is significant for further calculating strain value of pipeline and evaluating integrity of dented pipeline.     

Plastic damage analysis of oil and gas pipelines with unconstrained and constrained dents

Indentation resulting from mechanical damage is one of the main causes of pipeline rupture failure. The primary objective of this case study is to analyze the plastic damage of the pipelines with unconstrained and constrained dents using the existing plastic damage model, namely, the ductile failure damage indicator (DFDI) model. As part of this study, an attempt is made to clarify the influence of the constraint on the behavior of the pipeline. The investigation indicates that the plastic damage of the pipeline interior walls outweighs that of the pipeline exterior walls and cracks start frequently from the pipe interior walls. The position of the maximal circumferential plastic damage deviates from the deepest position of the dent when the critical displacement loading arriving due to the circumferential deformation of the pipeline. The total plastic damage of the unconstrained dented pipeline only depends on the initial displacement loading because the working pressure has very little influence on its behavior under the unconstrained condition. However, the total plastic damage of the constrained dented pipeline depends on the initial displacement loading and the working pressure because the working pressure has a significant effect on its behavior under the constrained condition. Under the same condition, the constrained dented pipeline is easier to crack compared to the unconstrained dented pipeline. In this paper, the model, results and the findings are summarized and discussed.     

Failure pressure of medium and high strength pipelines with scratched dent defects

It is very important in engineering design and integrity assessment of pipeline to accurately predict its failure pressure, especially for the pipeline with mechanical damages. This paper numerically investigates the failure pressure of medium and high strength pipelines with scratched dent which is on the outer surface of the pipe. Pipe materials of two different grades are chosen in the analysis which represent medium and high strength steel, respectively. Failure pressure of an intact pipeline with fixed ends is derived analytically. On the basis of the maximum plastic strain failure criterion put forward by previous scholars, failure pressure of finite element models containing dent and scratch defects is determined. Parametric studies are carried out to obtain the influencing rule of the dimensions of dent and scratch. The effects of scratch length and depth on failure pressure with various dent depths are obtained. Finally, a formula is fitted for predicting the failure pressure of pipelines with scratched dents on the basis of finite element results. Compared to burst test data from literature, the proposed formula is proved to be reasonable.     

海床土体减缓坠物对海底管道撞击作用的研究

海底管道受坠物撞击的损伤分析中,海床土体是不宜忽略的因素。基于耦合欧拉-拉格朗日算法（CEL法）,该文建立了模拟坠物撞击海底管道过程中土体变形的有限元模型,并进行了物理模型试验,二者结果吻合较好。针对粘土海床,分析了海床柔性、海床土质、管道埋深、摩擦及坠物形状对海底管道损伤的影响。研究表明:对于裸置管道,海床柔性使一部分撞击能量转化为管道的整体变形,减轻管道局部损伤;对于埋置管道,基于软件的二次开发,考虑了正常固结粘土及均质粘土两种情况,二者的安全埋深相差较大。综合考虑上述土质的影响,2 m的埋深可提供有效的保护;埋深超过1 m时,坠物与土体间的摩擦系数对管道损伤的影响更加明显;形状尖锐的坠物受到土体的阻力较小,对管道造成的损伤程度较大。不同形状的坠物撞击管道时,管道的变形特征存在差异。研究结果对管道的风险评估及安全埋深设计具有指导意义。     

Failure analysis of directional crossing pipeline and design of a protective device

Failure analysis of crossing pipelines in the operation process after the completion of construction was investigated. Dent and collapse are the common failure modes of crossing pipelines in bad geological strata. The deformation processes of dent behaviour and collapse behaviour of crossing pipelines were simulated. The results show that high stress distribution extends along the axial and circumferential directions under the boulder load, and the maximum equivalent plastic strain appears at the centre of the dent. The collapse process of the crossing pipeline can be divided into six phases. The cross-section changes from an oval shape to a “gourd” shape and then to an “8” shape as the external load increases. Before buckling appears, the decrease in the cross-section area of the crossing pipeline is small. However, the cross-section area increases as the surrounding soil pressure increases after buckling appears. When the cross-section shape reaches the “8”, the decrease in the cross-section area reaches a maximum value of 88.1%. To reduce the failure probability and improve the service life of the crossing pipeline, a protective device was designed for preventing damage. For the protective device, the dent rate of the protective pipeline decreases as the annulus pressure increases under the action of boulders. However, the dent rate of the crossing pipeline increases as the annulus pressure increases. Under the same stratum movement, the deformation of the crossing pipeline with the protective device is smaller than without the protective device. Therefore, the protective device can effectively protect the crossing pipeline and prevent a collapse accident. The protective device can be effectively used on any parts of the crossing pipeline in different locations due to its simple structure and convenient installation in bad geological strata.     

Assessing mechanical damage in offshore pipelines – Two case studies

Mechanical damage in the form of dents and gouges is recognised as a severe and common form of mechanical damage to pipelines. In general terms, dents and gouges reduce both the static and cyclic strength of a pipeline. The severity of a dent depends on a number of factors, including: the size and shape of the dent; whether it affects the curvature of a girth or seam weld; and whether it contains other defects, such as a gouge or a crack. An understanding of the issues that influence the severity of mechanical damage is required to ensure that the appropriate action is taken on finding such damage, and that all of the necessary information is gathered for conducting an assessment. For example, what inspection method(s) are appropriate, and when is it better to repair such damage, rather than assess it? This paper discusses some of the issues that may arise during the assessment of mechanical damage, such as: when is a feature really a dent; when does a superficial scrape become classified as a gouge; and the limitations of existing assessment methods. These issues are illustrated with two recent case studies of damage to offshore gas transmissions pipelines.     

准静态压痕力作用下复合材料层合板的凹坑深度预测方法

建立了一种在准静态压痕力作用下复合材料层合板凹坑深度的预测方法。首先采用基于应变描述的Hashin 和 Yeh 失效准则并结合有限元法, 对复合材料层合板在准静态压痕力作用下的失效过程进行渐进损伤分析, 获取一系列的材料性能退化信息。其次采用Sun的方法对局部损伤区材料的弹性参数进行等效处理。最后结合Turner的接触理论预测了凹坑深度-接触力曲线。计算结果表明, 基体开裂与分层是导致层合板开始产生凹坑的主要原因, 纤维断裂是导致凹坑深度急剧增加的主要原因。分层起始载荷、 最大接触力及各自相应的凹坑深度的预测结果与试验值具有较好的一致性。      

Stress-life fatigue assessment of pipelines with plain dents

This paper presents a new algorithm for assessing the fatigue life of dented pipelines. The proposed methodology was conceived according to the current stress-life fatigue theory and design practice: it employs S–N curves inferred from tensile test material properties and uses well established methodologies to deal with the stress concentration, the mean stress and the multi-axial stress state that characterizes a dented pipe. Finite element analyses are carried out to model the denting process and to determine the stress concentration factors of several pipe-dent geometries. Using dimensional analysis over the numerical results, a non-dimensional number to characterize the pipe-dent geometry is determined and linear interpolation expressions for the stress concentration factors of dented pipelines are developed. Fatigue tests are conducted with the application of cyclic internal pressure on small-scale dented steel pipe models. In view of the fatigue test results, the more appropriate S–N curve and mean stress criteria are selected.     

单层和双层足尺度海底管道抗冲击性能分析

海底管道在服役过程中除了受到常规荷载作用外,还会受到各种意外的冲击载荷作用而失效.为了研究承受横向冲击载荷作用下海底管道的动态特性,对三个单层和一个双层的足尺度管道进行了落锤冲击试验,获得了横向冲击作用下管道的破坏形态、冲击力时程曲线、位移时程曲线及应变时程曲线.建立了分析冲击荷载作用下海底管道失效过程的有限元模型,并...     

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IPSO-GEV优化的腐蚀油气管道剩余寿命预测

针对埋地管道腐蚀的随机性及极值类型选择不当而引起的拟合误差等问题,构建了基于IPSO(免疫粒子群优化算法)-GEV(广义极值分布)优化的油气管道最大腐蚀深度预测模型。首先,采用GEV分布拟合极值数据,利用1PSO优化GEV分布函数的参数并确定极值分布类型,以此确定整条管道的最大腐蚀深度;然后,建立基于可靠性理论的腐蚀裕量模型,来预测管道的剩余寿命;最后,以国内两条腐蚀管道为例验证模型的预测精度。实例结果表明:经IPSO-GEV优化的管道最大腐蚀深度预测模型不受限于数据的具体分布,且预测模型的预测精度较高,管道的剩余寿命预测合理。     

在役腐蚀管道动态非概率可靠性分析简

 腐蚀失效是压力管道失效的主要形式之一,研究腐蚀管道的可靠性具有重要理论意义和应用价值.在对腐蚀管道可靠性分析时,概率可靠性模型和模糊可靠性模型对于数据信息的要求较高.而在掌握不确定性信息很少情况下,为了充分利用管道的不确定性信息弥补原始数据的不足,可将腐蚀管道可靠性分析中的材料屈服强度、管道直径、缺陷深度和操作压力等不确定参数视为区间变量,基于区间模型建立一种在役腐蚀管道动态非概率可靠性模型,给出了腐蚀管道剩余寿命预测的简便方法.结合工程实例计算与分析,表明了文中所提出方法的可行性和合理性,并在此基础上,分析了管道的壁厚、缺陷深度、实际压力和腐蚀速率这些区间变量的不同变异系数对非概率可靠性指标的影响,分析结果表明非概率可靠性指标对管道壁厚的变异系数最为敏感.     

Influence of foreign object impact mode on fatigue performance of 2198‐T8 Al‐Li alloy thin sheets for fuselage

The 2198‐T8 is the advanced aircraft aluminium‐lithium alloy used for airframe construction. Based on the digital image correlation technique, the postimpact fatigue performance of 2198‐T8 aluminium‐lithium alloy sheets was analysed. The foreign object impact mode includes a single impact, continuous twice‐impacts at the same position and impacts at two parallel positions. For each mode, the collapse behaviours with different impact energies and insert dimensions were investigated. The results could show that residual fatigue life is sensitive to the impact mode, and through digital image correlation (DIC) nephogram analysis, on the surface of dented specimens, there exists the redistribution of strain field. For three types of impact modes, the concept of average damage volume is introduced to fit the effect of dent depth and width on residual fatigue life; the goodness of fit for the whole average damage volume is 89.5%. The crack initiation and propagation of the postimpact fatigue specimen will be affected by the stress concentration and microstructure, and the local multiaxial strain status around the dent will accelerate the crack propagation.     

In-service repair of main pipelines by welding

A new approach to the repair of main pipelines by welding without removing them from service is proposed. The approach is based on four points: ranking of defects by the degree of failure risk; safety of welding works on pipeline under pressure; use of different variants of repair by welding; serviceability evaluation of welded repair constructions. The use of this approach makes it possible to minimize pipeline failure risk and to predict pipeline serviceability after repair by welding.     

Assessment of requirements on safety management systems in EU regulations for the control of major hazard pipelines

The European Commission has carried out a thorough review and assessment on whether pipelines conveying dangerous substances need a level of control similar to chemical installations, based on consideration of the hazards and risks and on accidents world-wide. The study examines whether sufficient controls already exist within the European Union (EU) Member States, considering the control philosophy of the 'Seveso II' Directive and the need to support development of pipeline networks across the EU taking into account the principles of sustainability. Community legislation would be supported by a set of criteria for the classification of Major-Accident Hazard Pipelines. Such criteria are examined for the onshore transmission pipelines in the EU. The Commission study has shown that there is a large variation in the degree to which Member States have comprehensive 'major-accident hazard' legislation on pipelines, and that existing legislation rarely addresses elements of the Safety Management System. The assessment, based on a proposed Pipeline Safety Instrument, indicates that there will be added value in a European Instrument drawing on the principles of Seveso II Directive concerning Safety Management System requirements.     

Safe burst strength of a pipeline with dent–crack defect: Effect of crack depth and operating pressure

A defect that contains both dent and crack, often known as dent–crack defect, may lead to a failure due to rupture or leak in the pipe wall. Hence, the pipeline operator becomes concerned about the performance and safety of the pipeline when a dent–crack defect is diagnosed. A research was recently completed at the Centre for Engineering Research in Pipelines, University of Windsor to study the effect of the crack depth on the safe pressure capacity (burst strength) of 30-inch diameter and API 5L X70 grade linepipe. This study found that the dent–crack defect with crack depth of 70% of wall thickness can reduce the pressure capacity by 55%. This paper discusses the test specimens, test setup, test procedure, test results, and data obtained from finite element analyses.     

Gas transmission pipeline failure probability estimation and defect repairs activities based on in-line inspection data

Degradation of an underground pipeline during its service life leads to reduction of the pipe wall thickness. Periodic in-line inspections are performed by onshore pipelines operators to detect corrosion anomalies and size their depth and width. In the paper, a simple analytical method of burst pressure calculation for a straight pipeline repaired with a composite sleeve was investigated. Repair activities after an in-line inspection of a gas transmission pipeline were considered in this research to assess the pressure of a pipe with a flaw reinforced with a different number of layers of a fiber-based polymer sleeve. Det Norske Veritas criteria and formulation of a limit state function were applied to determine the burst pressure and corresponding failure probability of a pipeline with a large number of single and non-interacting part-wall defects. The Monte Carlo method was selected for estimation of pipeline failure probability and cumulative failure probability due to the external corrosion considering fluid pressure fluctuations in dynamic flow effects with respect to statistical distribution of input parameters were examined in this research. The proposed novel 3-step approach was validated on the real data collected from an onshore high pressure gas pipeline DN 700, MOP 5.5 MPa provided by two magnetic flux leakage inspections repeated at an interval of twelve years. A probabilistic methodology was applied to evaluate the part-wall external corrosion defects and their deterministic linear growth with time and repair activities on gas transmission pipelines were analyzed. The results of this study shall help maintenance engineers solve the problems of an optimal strategy in reliability-based high pressure gas pipelines management.     

Failure criterion of buried pipelines with dent and scratch defects

The excavation of buried pipelines shows that combined dent and scratch defects exist at the bottom of the pipe due to the extrusion and scraping action of rocks, which is likely to decrease its pressure bearing capacity. This paper experimentally investigates the tensile strength of CT80 steel plates with dent and scratch defects. Results show that the tensile strength is affected to varying degrees by dent depth, scratch depth and the indenter diameter. Finite element analysis (FEA) is then carried out to propose the failure criterion by comparing with experimental results. It is thought that failure happens when the equivalent stress of the intermediate node in the direction of the remaining thickness of the defect center of the model reaches the true tensile strength of the material. With this criterion, large numbers of finite element calculations can be developed to study the failure pressure of buried pipelines with combined dent and scratch defects.     

输油管道杂散电流检测评价与防护实践

杂散电流干扰对油气管道造成很大的危害和破坏,如何有效地检测评价并制定合适的防护对策是油气管道杂散电流防护的关键。基于杂散电流不同检测方法和排流应用实践,开展了输油管道杂散电流综合检测评价及防护研究。结果表明:采取全线检测与专项检测相结合方式,综合运用CIPS(密间隔管地电位测试)、SCM(杂散电流智能测试仪)、电位梯度、电位和电流监测等多种检测评价方法,结合现场开挖分析,可准确确定管道杂散电流干扰类型及对管道的影响;检测的输油管道受到较严重的直流杂散电流干扰及一定的交流杂散电流干扰而发生局部腐蚀;通过管地电位测量对杂散电流排流整治措施进行了效果分析与评定,建议了管道杂散电流干扰防护对策。提出应加强管道杂散电流源头控制,同时综合运用多种检测、监测技术,有效分析各种干扰因素和腐蚀状况,制定科学合理的治理方法和防护对策,为管道完整性管理提供依据。