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.     

Assessment of the flexural capacity of corroded steel pipes

Flexural capacity of corroded pipes can be determined analytically assuming a full plastic failure mode for the pipe. A set of generalized solutions for flexural capacity of the pipe can be developed if the shape of the corrosion is known a priori. The generalized solutions derived in this paper are able to account for the simultaneous action of internal pressure and axial force. For practical purposes, the generalized solutions thus derived are simplified into approximate closed-form equations using three idealized corrosion shapes, namely, constant-depth, elliptical, and parabolic corrosions. Numerical examples indicate that the closed-form approximate solutions provide good comparison with the generalized solutions. The closed-form approximate solutions are subsequently compared to experimental results from full-size tests of pipes with different corrosion depth and width. Parameter study conducted as part of this paper indicates that the shape of a corrosion defect has significant influence on the flexural capacity of the corroded pipes.     

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.     

Creep damage prediction of the steam pipelines with high temperature and high pressure

Creep is the significant factor that caused failure of steam pipelines with high temperature and high pressure in the period of long-term service. In this paper, the creep tests were performed at serviced temperature of 520 °C for 1.25Cr–0.5Mo pipe material, and the creep and fracture constants were obtained by fitting the creep test data. Based on the modified Karchanov–Rabotnov constitutive equation, compiled the user subroutine computing the damage of the pipe element or 3D solid element, the creep damage prediction was carried out by finite element methods using ABAQUS codes for the steam pipelines with high temperature and high pressure, which serviced in a petro-chemical plant, the damage distribution and maximum damage location of the pipelines were obtained, which is testified by metallographic examination result. Furthermore, the local creep damage analysis of a tapered pipe serviced for 100,000 h was also carried out because tapered pipes used in the main steam pipeline is one of weakness in the piping system. Damage distribution and evolution in the analyzed tapered pipe were obtained. The location with the maximum damage value was determined, which is coincident with cracking position of the actual tapered pipe.     

Sensitivity of pipelines with steel API X52 to hydrogen embrittlement

The following cases of hydrogen influence on pipeline metal were considered: gaseous hydrogen under internal pressure in notched pipes and electrochemically generated hydrogen on external pipe surface from soil aqueous environment. The burst tests of externally notched pipes under pressure of hydrogen and natural gas (methane) were carried out after the pipe has been exposed to a constant “holding” pressure. It has been shown that even for relatively “soft” test conditions (holding pressure p = 20 bar and ambient temperature) the gaseous hydrogen is able to penetrate into near surface layers of metal and to change the mechanism of local fracture at notch. The sensitivity to hydrogenating of given steel in deoxygenated, near-neutral pH NS4 solution under soft cathodic polarisation was studied and the assessment local strength at notches in pipeline has been made for this conditions. Here, the relationship between hydrogen concentration and failure loading has been found. The existence of some critical hydrogen concentration, which causes the significant loss of local fracture resistance of material, was also shown.     

The synergistic effects of hydrogen embrittlement and transient gas flow conditions on integrity assessment of a precracked steel pipeline

We are reporting in this study the hydrogen permeation in the lattice structure of a steel pipeline designed for natural gas transportation by investigating the influence of blending gaseous hydrogen into natural gas flow and resulted internal pressure values on the structural integrity of cracked pipes. The presence of cracks may provoke pipeline failure and hydrogen leakage. The auto-ignition of hydrogen leaks, although been small, leads to a flame difficult to be seen. The latter makes such a phenomenon extremely dangerous as explosions became very likely to happen. In this paper, a reliable method is presented that can be used to predict the acceptable defect in order to reduce risks caused by pipe failure due to hydrogen embrittlement. The presented model takes into account the synergistic effects of transient gas flow conditions in pipelines and hydrogen embrittlement of steel material due to pressurized hydrogen gas permeation. It is found that blending hydrogen gas into natural gas pipelines increases the internal load on the pipeline walls due to overpressure values that may be reached in a transient gas flow regime. Also, the interaction between transient hydrogen gas flow and embrittlement of API 5L X52 steel pipeline was investigated using Failure Assessment Diagram (FAD) and the results have shown that transient flow enhances pipeline failure due to hydrogen permeation. It was shown that hydrogen embrittlement of steel pipelines in contact with the hydrogen environment, together with the transient gas flow and significantly increased transient pressure values, also increases the probability of failure of a cracked pipeline. Such a situation threatens the integrity of high stress pipelines, especially under the real working conditions of hydrogen gas transportation.     

Reliability of pipelines with corrosion defects

This paper aims at assessing the reliability of pipelines with corrosion defects subjected to internal pressure using the first-order reliability method (FORM). The limit-state function is defined based on the results of a series of small-scale experiments and three-dimensional non-linear finite element analysis of the burst pressure of intact and corroded pipelines. A sensitivity analysis is performed for different levels of corrosion damage to identify the influence of the various parameters in the probability of burst collapse of corroded and intact pipes. The Monte Carlo simulation method is used to assess the uncertainty of the estimates of the burst pressure of corroded pipelines. The results of the reliability, sensitivity and uncertainty analysis are compared with results obtained from codes currently used in practice.     

Prediction of remaining strength of corroded pressurised pipelines

In this paper a deterministic model is developed which can be used to evaluate the remaining strength of corroded steel pipeline over time. This model also can be used to evaluate the maximum allowable failure pressure of corroded pipelines. The motivation for the development of the model is that during in-service conditions the strength of pressurised pipelines may be impaired, for example, from the failure of the protective coatings, paint surfaces or cathodic protection or as a result of externally applied mechanical damage or as a result of ageing (e.g. fatigue). Any of these may lead to the initiation of corrosion at the damaged area if it is not repaired. Once initiated, corrosion increases gradually both in extent and depth with increased exposure period. This results in the reduction of the remaining strength and hence a reduction in the carrying capacity of a pipeline. It also creates uncertainty about the future capacity. The proposed model is related to accepted methods for estimating the remaining strength of pipelines, but uses a simple corrosion model to estimate future remaining life. A sensitivity analysis can be used to investigate the effect of corrosion parameters on the pipeline carrying capacity. A simple example is given to illustrate the approach.     

Theoretical and experimental evaluation of the limit state of transit gas pipelines having corrosion defects

The limit carrying capacity of gas pipelines having corrosion defects were evaluated by means of an improved FEM analysis, together with the ANSI/ASME defect judgement and an experimental verification. When compared with our past presentation at the proceedings of the CAPE'93 Colloquium (dealing with reproduced defects), this paper evaluates the remaining lifetime of a pipeline having natural faults. The new improved analysis includes geometrical nonlinearities leading to a modification of limit state parameters and thus matching better with actuality. This presented work was initiated by the firm of Trans-Gas in Prague after having had a pipeline inspected proving some parts of it as being heavily corroded. The task in hand was to determine the remaining pipeline lifetime with the view to at least a two-year period.     

System reliability of corroding pipelines

A methodology is presented in this paper to evaluate the time-dependent system reliability of a pipeline segment that contains multiple active corrosion defects and is subjected to stochastic internal pressure loading. The pipeline segment is modeled as a series system with three distinctive failure modes due to corrosion, namely small leak, large leak and rupture. The internal pressure is characterized as a simple discrete stochastic process that consists of a sequence of independent and identically distributed random variables each acting over a period of one year. The magnitude of a given sequence follows the annual maximum pressure distribution. The methodology is illustrated through a hypothetical example. Furthermore, the impact of the spatial variability of the pressure loading and pipe resistances associated with different defects on the system reliability is investigated. The analysis results suggest that the spatial variability of pipe properties has a negligible impact on the system reliability. On the other hand, the spatial variability of the internal pressure, initial defect sizes and defect growth rates can have a significant impact on the system reliability.     

承压类压力管道钢管检验技术分析

随着社会经济的发展,中国的管道运输业得到了蓬勃发展,但是由于管线的增多、管龄的增长、腐蚀、磨损等自然或人为损坏等原因,承压管道泄漏事故及爆炸事故频频发生。论述了石油化工压力管道无缝钢管在使过程中质量检验的重要性,叙述了常见的缺陷形式、检验项目和检验方法。     

Evaluation of local thinned pressurized elbows

Erosion, corrosion or mechanical damage may cause local thinned areas (LTA) in piping and degrade its integrity. Acceptability criteria for LTA are available for straight pipes. However, for elbows, there are few studies. In this paper, the finite element method (FEM) is used to calculate the collapse load of a local thinned pressurized elbow and to analyse the factors influencing the collapse load. Based on the solution of the collapse load of LTA straight pipe, the stress level in an elbow without defects, and finite element analysis results, an analytical solution of the collapse load of LTA pressurized elbow is presented. A method to assess the acceptability of LTA in an elbow is given and compared with FEM results.     

Influence of old rectangular repair patches on the burst pressure of a gas pipeline

Seven full scale hydrostatic burst tests were carried out on pipes extracted from an API 5LX52 gas pipeline that contained rectangular and elliptical fillet welded patches and other repairs of different geometries. All breaks took place after widespread yielding. This analysis shows that the patches that generate greater risks are those that: (1) were attached to the pipeline at very low pressure, (2) were placed to repair large defects, (3) are rectangular, long in the direction of the pipe, and narrow, (4) the quality of the weld is doubtful. Based on data reported by In Line Inspection (ILI), of the four conditions mentioned above, only the third can be assessed in order to quantify risks and to schedule replacements.     

High pressure buried gas pipeline leakage temperature drop testing and theory analysis

Abstract

Great damages to environment will be caused once high pressure buried pipeline leakage leaks occurs. Distributed temperature sensing (DTS) is a good monitoring method of temperature change. Gas from buried pipe leaks is monitored by changes caused by the Joule-Thomson (J-T) effect. In this work, the temperature variation of buried pipeline leakage is analyzed by finite element method (FEM). The temperature drop of nitrogen at the leak hole is calculated by using the J-T empirical formula. In addition, the site test of buried high pressure nitrogen pipeline leakage is carried out. The results show that the temperature drop the leakage hole is obvious, and the temperature directly above the leakage hole decreases with the increase of the distance from the leakage hole. The larger leakage pressure, the greater the temperature drop. Therefore, the fiber needs to be placed closer to the pipeline to monitor the soil temperature change.     

Effects of the geometry of downstream pipes with different angles on the shock ignition of high-pressure hydrogen during its sudden expansion

To investigate the effects of the geometry of downstream pipes on the shock ignition and the formation of the shock waves during high-pressure hydrogen sudden expansion, a series of bench-mark experiments were designed and high-pressure hydrogen were released into five types of pipes with different angles (60, 90, 120, 150 and 180°). It was found that the geometry of downstream pipes had a significant influence on the shock ignition of hydrogen. The incident shock wave would be reflected at the corner of the pipes with angles of 60, 90, 120 and 150°. The intensity of the reflected shock wave is higher if the angle is smaller. In addition, the average velocity of the leading incident shock wave would decrease when it passed the corner of the pipe. Using a pipe with smaller angle significantly increases the likelihood of shock ignition and lowers the minimal required burst pressure for shock ignition. The overpressure of the incident shock waves inside the exhaust chamber (for the cases with the angles of 60, 90, 120 and 150°) decreases sharply. There are three flame propagation behaviors inside the exhaust chamber: flame quenching, flame separation and no flame separation. The results of this study have implications concerning designs for storage safety of hydrogen energy and may help get better understanding of shock ignition mechanism of high pressure hydrogen and effect of pipeline geometry on ignition.     

Enhancement of hydrogen charging in metal hydride-based storage systems using heat pipe

Heat transfer in metal hydride bed significantly affects the performance of metal hydride reactors (MHRs). Enhancing heat transfer within the reaction bed improves the hydriding rate. This study presents performance analysis in terms of storage capacity and time of three different cylindrical MHR configurations using storage media LaNi₅: a) reactor cooled with natural convection, b) reactor with a heat pipe on the central axis, c) reactor with finned heat pipe. This study shows the impact of using heat pipes and fins for enhancing heat transfer in MHRs at varying hydrogen supply pressures (2–15 bar). At any absorption temperature, hydrogen absorption rate and hydrogen storage capacity increase with the supply pressure. Results show that using a heat pipe improves hydrogen absorption rate. It was found that finned heat pipe has a significant effect on the hydrogen charge time, which reduced by approximately 75% at 10 bar hydrogen supply pressure.     

CFD investigation of pressure drop reduction in hydrotransport of multisized zinc tailings slurry through horizontal pipes

Zinc is an important metal useful in hydrogen production. Mining of zinc also involves zinc tailings disposal using pipeline transportation which is an energy- and water-intensive process. In this study, the multisized slurry of zinc tailings is modeled using CFD to investigate the flow field characteristics in a straight horizontal pipe fitted with swirl-inducing sections. The primary objective was to study the pressure drop by adding swirl-inducing pipe of different numbers of lobes to the conventional pipe. Nine configurations of multi-lobed swirl-inducing pipes were studied for transport characteristics of solids at three flow rates and three overall solids concentrations. Eulerian-Eulerian modeling with the RNG k-ε turbulence model is employed for simulations. The numerical model is validated for pressure drop against available experimental results, and excellent agreement is achieved. The study shows that 10-lobed swirl-inducing section fitted to the conventional pipe reduces the pressure drop by nearly 35%, offering the potential for further exploration.     

Failure assessments of corroded pipelines with axial defects using stress-based criteria: Numerical studies and verification analyses

Conventional procedures used to assess the integrity of corroded piping systems with axial defects generally employ simplified failure criteria based upon a plastic collapse failure mechanism incorporating the tensile properties of the pipe material. These methods establish acceptance criteria for defects based on limited experimental data for low strength structural steels which do not necessarily address specific requirements for the high grade steels currently used. For these cases, failure assessments may be overly conservative or provide significant scatter in their predictions, which lead to unnecessary repair or replacement of in-service pipelines. Motivated by these observations, this study examines the applicability of a stress-based criterion based upon plastic instability analysis to predict the failure pressure of corroded pipelines with axial defects. A central focus is to gain additional insight into effects of defect geometry and material properties on the attainment of a local limit load to support the development of stress-based burst strength criteria. The work provides an extensive body of results which lend further support to adopt failure criteria for corroded pipelines based upon ligament instability analyses. A verification study conducted on burst testing of large-diameter pipe specimens with different defect length shows the effectiveness of a stress-based criterion using local ligament instability in burst pressure predictions, even though the adopted burst criterion exhibits a potential dependence on defect geometry and possibly on material's strain hardening capacity. Overall, the results presented here suggests that use of stress-based criteria based upon plastic instability analysis of the defect ligament is a valid engineering tool for integrity assessments of pipelines with axial corroded defects.     

Hydrogen related degradation in pipeline steel: A review

To support our increasing energy demand, steel pipelines are deployed in transporting oil and natural gas resources for long distances. However, numerous steel structures experience catastrophic failures due to the evolution of hydrogen from their service environments initiated by corrosion reactions and/or cathodic protection. This process results in deleterious effect on the mechanical strength of these ferrous steel structures and their principal components. The major sources of hydrogen in offshore/subsea pipeline installations are moisture as well as molecular water reduction resulting from cathodic protection. Hydrogen induced cracking comes into effect as a synergy of hydrogen concentration and stress level on susceptible steel materials, leading to severe hydrogen embrittlement (HE) scenarios. This usually manifests in the form of induced-crack episodes, e.g., hydrogen induced cracking (HIC), stress-oriented hydrogen induced cracking (SOHIC) and sulfide stress corrosion cracking (SSCC). In this work, we have outlined sources of hydrogen attack as well as their induced failure mechanisms. Several past and recent studies supporting them have also been highlighted in line with understanding of the effect of hydrogen on pipeline steel failure. Different experimental techniques such as Devanathan–Stachurski method, thermal desorption spectrometry, hydrogen microprint technique, electrochemical impedance spectroscopy and electrochemical noise have proven to be useful in investigating hydrogen damage in pipeline steels. This has also necessitated our coverage of relatively comprehensive assessments of the effect of hydrogen on contemporary high-strength pipeline steel processed by thermomechanical controlled rolling. The effect of HE on cleavage planes and/or grain boundaries has prompted in depth crystallographic texture analysis within this work as a very important parameter influencing the corrosion behavior of pipeline steels. More information regarding microstructure and grain boundary interaction effects have been presented as well as the mechanisms of crack interaction with microstructure. Since hydrogen degradation is accompanied by other corrosion-related causes, this review also addresses key corrosion causes affecting offshore pipeline structures fabricated from steel. We have enlisted and extensively discussed several recent corrosion mitigation trials and performance tests in various media at different thermal and pressure conditions.     

Effect of spatial correlation on the failure probability of pipelines under corrosion

Corrosion in pipelines has been probabilistically modeled. However, the potential effect of spatial correlation of corrosion defects, in several segments of a pipeline, on its failure probability has not received much attention.In this paper, several degrees of spatial correlation are assumed for the corrosion in determined segments of a pipeline and their effects on the global reliability are examined.The pipeline is assumed to be a series system. The failure mode is considered to be controlled by the stresses due to internal pressure and the presence of corrosion. Component reliability is calculated by First Order Second Moment approximations. First order bounds are used to define the limits for the global failure probability by assuming first, either no correlation (independent pipeline segments) and, secondly, perfect correlation between segments.Then, second order bounds are estimated to improve the calculation of the failure probability by including the correlation coefficients mentioned above.The correlation degree between failure modes at two pipeline segments increases with the degree of correlation of the corrosion initial depths located at these segments. Also, for a correlation coefficient between corrosion depths larger than 0.6, its contribution to the correlation between failure modes becomes significant and, therefore, should be accounted for.When the specific correlation degree between corrosion defects at adjacent pipeline segments is considered in the calculation of an example pipeline failure probability, this probability is narrowly bounded between 0.58 and 0.59, as compared to the usual practice where this correlation is assumed to be either 0 or 1 for which the failure probability is bounded between 0.49 and 0.79.The formulation may be used to set optimal maintenance schedules for pipelines under corrosion.