Failure analysis of a natural gas pipeline

In the present work, the failure investigation of a 30 in. diameter gas transmission pipeline (API 5L X-60 grade steel) has been described. The failure was due to a longitudinal crack developing in the centerline of longitudinal weld joint. Mechanisms and morphology of crack initiation and propagation were studied through different tests including: thickness measurement, chemical composition analysis, metallographic inspection, mechanical property testing, scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). The experiments resulted to the conclusion that some factors associated with Sulphide Stress Cracking (SSC) and metallurgical defects cause the failure of weld joint pipe. Detailed examination revealed that these factors are inappropriate welding parameters, pitting corrosion on longitudinal weld, and hydrogen permeation to the weld metal.     

Failure analysis of girth weld cracking of mechanically lined pipe used in gasfield gathering system

Girth weld cracking of mechanically lined pipe was occurred after 75 days operation in a gasfield gathering system in China. Failure causes were analyzed based on operation histories, field documents, and laboratory tests. Results showed that the girth weld failure was mainly due to two aspects, girth weld martensite microstructure and external stress. The crack was initiated from sealing pass zone and filling pass zone, which is a hard and brittle martensite structure with hardness of HV 350–450. The failed pipe area had undergone the heavy rain for 2 days, pulling stress, bending stress, and shear stress generated by soil movement resulted in high stress concentration at girth weld. The girth weld cracking failure was initiated from outer carbon steel, and propagated along the weld-fusion line in intergranular mode, which is a typical stress corrosion cracking failure.     

321不锈钢焊接波纹管失效分析

应用光学显微镜、扫描电镜和能谱仪等手段对321不锈钢焊接波纹管表面腐蚀物的化学成分、焊缝区域的显微组织、腐蚀裂纹的形态特征等方面进行检验和分析。结果表明，波纹管局部电镀端内壁腐蚀导致漏气失效，即加工应力、焊接残余应力与含Cl^-或含Cl^-和溶解氧的腐蚀性介质的联合作用使波纹管局部电镀端产生了点蚀和应力腐蚀开裂；退火工艺设计的不合理和钎焊工艺控制不恰当使焊缝热影响区敏化而出现了晶间腐蚀。据此提出了降低和避免321不锈钢焊接波纹管因腐蚀而漏气的措施。     

Stress corrosion cracking of stainless steel pipes for Methyl-Methacrylate process plants

In a Methyl Methacrylate (MMA) plant, tree-like transgranular cracks were found near the weld of a pipe that had been used for transferring MMA material at 110 °C and 0.77 kg/cm². The pipe was made of ASTM A312 TP304 stainless steel.In this study, it was shown that the failure was due to the stress corrosion cracking (SCC) caused by the chloride that remained in the pipe. Corrosion pitting occurred on the inside surface of the pipe. The stress corrosion cracking started from the pits and grew out through the thickness. Concentrated chloride was found in the deposit stuck to the pipe in addition to the pre-process MMA materials. Many work-hardened grains were observed in the area of SCC, providing the evidence of high residual stress due to welding, which could serve as the driving force for the SCC. Recommendations are made for preventing further failure due to SCC in such cases.     

Application of volumetric method to the assessment of damage induced by action of foreign object on gas pipes

Gas pipes are submitted to three major types of damage: damages induced by impact of foreign objects, pits and craters produced by corrosion, weld defects produced during manufacturing. Damages induced by impact of foreign objects are considered as the most important problem for the gas pipes reliability and lead to dents, gouges or notches at the pipe failure. Geometrical effect of gauges or notches on brittle or elastoplastic failure of pipes submitted to internal pressure can be treated by the volumetric method. The volumetric method has been applied to a gas pipe submitted to internal pressure and exhibiting a longitudinal external and surface notch. We introduced a special procedure SINTAP, which allows one to compute the safety factor value. __________ Translated from Problemy Prochnosti, No. 4, pp. 109–117, July–August, 2006.     

Failure investigation of 321 stainless steel pipe to flange weld joint

Failure investigation was conducted on a refinery pipe-to-flange weld joint that suffered cracking. Both the pipe and flange are made of AISI 321 stainless steel. The flange was circumferentially welded to the pipe which is seam welded. The investigation revealed that both the circumferential and seam welds were in sound conditions, namely no evidences of sensitization, lack of weld penetration, and voids or porosities. Thus, welding practices were not suspected to be the cause of failure. The failure of the weld joint was found to have started at the δ-ferrite phase in the flange material and propagated through the circumferential and seam welds. The failure mode was concluded to be chloride stress corrosion cracking synergized by the presence of H₂S. The presence of corrosive compounds in the refinery stream and the residual stresses at the weld joint triggered active anodic dissolution of the δ-ferrite precipitates, resulting in cracking of the material.     

Stress corrosion cracking of pyrotherm reformer tube for steam-reforming hydrogen production

A section of Pyrotherm G 25/35 Nb reformer tube was rupture-failed in a steam-reforming hydrogen plant and analysed to identify the causes of failure. Examination of the internal surface of the pipe indicated signs of heterogeneous corrosion attack in localized areas near to the primary crack site. Some of these areas were associated with fissures, although they did not penetrate through the pipe wall. Measurement of the pipe wall thickness revealed that fair amounts of the material had been consumed by corrosion. Cross-sectional examination of the dissected pipe in areas showing signs of corrosion attack and fissures revealed the presence of radial macrocracks, originating from the internal surface, and numerous microcracks in the pipe interior. Most microcracks were formed along the grain boundaries of the spin-cast microstructure. Further examination of the macrocracked surfaces revealed the presence of a granular microstructure, indicative of a brittle failure mode. Based on the characteristics exhibited by the macrocracking, the rupture failure of the reformer tube is attributed to stress corrosion cracking (SCC). The SCC is believed to be produced through synergistic reactions amongst sulfur-containing derivatives in the natural gas (feedstock), hydrogen and superheated steam in the processed gases under a mechanically-stressed environment. The presence of the mechanical stress is attributed to the bending of the pipe caused by improper suspension design.     

Analysis of the integrity of welded pipes of gas mains by the FITNET procedures

We have evaluated the integrity of a rectilinear element of a welded pipe of a gas main after its operation for 40 years with the use of the FITNET procedures according to the Failure-Assessment-Diagram approach. We have determined the critical sizes of defects in the base (17G1S steel) and weld metal of a longitudinal welded joint of a reserve pipe. Two levels of the gas pressure in the pipeline (4.5 and 7.0 MPa) are taken as critical values. It has been shown that, in the absence of corrosion and hydrogenating influence of the transported medium, the base metal of pipes both in as-received state and after long-term operation is more susceptible to fracture due to reaching the critical stressed state than the weld metal.     

Creep rupture behaviour of circumferentially welded mod. 9Cr–1Mo steel pipe subject to internal pressure and axial load

The multi-axial creep strength of circumferential welds in power piping, including failure mode and failure life against a wide range of stress ratios and stress levels, was newly examined in this paper. The creep rupture behaviour of modified 9Cr–1Mo steel (9Cr–1Mo–VNb steel; ASME P91) pipe with a circumferential weld subject to combined internal pressure and axial load was experimentally investigated at 650 °C. The test results, with several kinds of stress ratios of macroscopic axial and hoop stress can be summarised as follows. Along with the increased stress ratio (axial/hoop), both failure location and failure mode changed from a base metal failure caused by hoop stress to an FGHAZ (fine-grained heat affected zone) failure due to axial stress. The stress ratio where the failure mode changed was ‘0.8’. Strength reduction in FGHAZ failure by axial stress should be considered in the structural design of circumferential welds. A series of FEM creep analysis was carried out to discuss the relation of the failure mode to both the local stress distribution and the damage. The failure mode variation along with the increase in the stress ratio is also discussed based on simplified ductile creep failure analysis.     

A study on axial cracking failure of drill pipe body

Frequently happening drill pipe failure accidents in oil and gas wells not only affect drilling speed, but cause enormous economic losses and many safety issues. Most of these accidents are transverse cracking of drill pipe body and pin thread or axial cracking of box thread. Based on the axial cracking failures of drill pipe body in an ultra-deep well in China, this paper give a systematic analysis of axial cracking failure in consideration of service condition, material quality and stress corrosion mechanism. Measurement and inspection are performed on macroscopic and microscopic morphology of crack surface, corrosion products and circumferential residual stress. Then stress corrosion cracking experiments against hydrogen sulfide is conducted. Finally, the critical stress value for sulfide stress corrosion cracking of the drill pipe material is obtained, and the mechanisms of axial cracking failure and corresponding preventive measures are proposed.     

Failure analysis of thermowell weldment cracking

This study presents the root cause analysis of a weldment failure in a thermowell assembly operating in a natural gas processing plant. Laboratory investigations indicated inferior quality for the fillet weld joining the thermowell flange to the pipe supporting the thermowell to the main pipe. The fillet weld exhibited excessive concavity, lack of penetration and lack of fusion. This led to minute weldment cracking, which was exacerbated by flow-induced vibrations exceeding safe operational limits. This promoted small magnitude, high frequency stress cycling coupled with high mean stress at the poorly welded joint, which led to accumulation of high cycle fatigue damage and final fracture causing gas leakage. Mitigation plans included use of a shorter support pipe and revised safe operational envelope of the pipeline under consideration.     

The main mechanisms of stress corrosion cracking in natural gas trunk lines

Stress corrosion cracking in a pipe material presents a most critical hazard to natural gas trunk lines. The paper addresses the causes (mechanism) of crack formation and growth. Understanding of these issues will enable a proper implementation of methods for early identification, diagnostic, and prediction of this type of failure. A most comprehensive explanation of a joint influence of internal and external factors on the corrosion cracking can be provided by means of the probabilistic local electrochemical corrosion theory. It is based on the probabilistic nature of initiation of a local corrosion damage. A probabilistic approach is used to develop a concept of stress corrosion cracking. The factors responsible for the formation of local damage nuclei have been determined. The pipe manufacturing operations are shown to have a certain effect on the pipeline susceptibility to stress corrosion cracking. Modeling has been performed for the corrosion processes that may occur in a gas trunk line section subjected to a stress corrosion cracking hazard.     

A PROCEDURE FOR ANALYSIS OF LEAK BEFORE BREAK IN PIPES SUBJECTED TO FATIGUE OR IGSCC ACCOUNTING FOR COMPLEX CRACK SHAPES

Abstract A procedure is described which predicts the growth of an initial circumferential surface crack through a pipe wall and further on to final failure of the pipe. The crack growth mechanism can either be fatigue or stress corrosion. Consideration is taken to complex crack shapes, since especially the last growth mechanism often results in a substantially longer crack length on the inside of the pipe than on the outside for the initial leaking crack due to the distribution of weld residual stresses. The procedure has been implemented in a computer program that in an integrated fashion calculates crack sizes and mass leak rates as a function of time and also predicts when leakage and final failure occur. The information obtained makes it possible to judge if the concept of leak before break is fulfilled.     

Metallurgical investigations and corrosion behavior of failed weld joint in AISI 1518 low carbon steel pipeline

In this paper, failure analysis was carried out based on the available documents, metallographic studies and corrosion behavior of the welded joint pipe sample made of AISI 1518 low carbon steel. Nondestructive evaluations including penetration test (PT) and radiographic test (RT) were performed on the as-received pipeline and results indicated the presence of micro- and macro-cracks. Optical microscopic images and scanning electron microscopy (SEM) micrographs revealed various microstructures in the base metal (BM), heat affected zone (HAZ) and weld metal (WM). The microstructural variations may result in galvanic feature and lead to failure and rupture of the weld joint during the service. Microhardness measurements showed that hardness value was about 260 HV in the WM, while it declined in the HAZ and BM. Qualitative chemical analyses such as X-ray diffraction pattern (XRD) and SEM equipped with energy dispersive spectroscopy (EDS) confirmed the presence of corrosive media during weld joint rupture. Additionally, SEM and optical investigations indicated that micro-cracks were formed in HAZ due to residual stress as a consequence of improper welding condition. Surface fracture studies showed that the crack initiation, crack growth and finally crack propagation took place in the WM/HAZ interface. Electrochemical studies were conducted on the BM, HAZ and WM to investigate corrosion behavior of the failed joint sample. Finally, a proper corrosion mechanism is proposed based on the failure analyses and electrochemical studies.     

Failure analysis of a leak-off oil pipe from injection valves of an off-shore operating diesel engine

The failure analysis of a four-stroke 3000 kW off-shore operating diesel engine is presented. The failure occurred during engine normal operation period in the leak-off oil pipe from the injection valves, which experienced a fracture through the pipe wall and a diesel engine fire as a result. A detailed analysis of all elements which had an influence on the failure initiation was carried out, namely leak-off oil pipe vibration level, pipe stress level, presence of corrosion pits on the pipe external surface under the zinc coating and engine components temperature distribution in the failure zone. It was found that the crack initiation and propagation of the leak-off oil pipe from injection valves was driven by a fatigue mechanism, which was facilitated by loose pipe supports (excessive pipe vibration) and corrosion pits on the pipe surface that acted as stress concentrators. The contact of leak-off atomized oil due to the pipe through wall fracture with the hot engine exhaust muff (390 °C approximately) caused local fire of the diesel engine.     

Q690高强钢管焊接残余应力数值模拟

该文重点研究Q690焊接高强钢管截面纵向残余应力分布模式,利用商业有限元分析软件ANSYS对其进行数值模拟,运用其中的“生死单元”和热-结构耦合分析功能来模拟焊接过程,以管径与壁厚为变量,设计3组共12个有限元模型。通过对模型的数值分析,得到每个模型的截面纵向残余应力分布形式,经过对比分析得出其规律,并提出Q690焊接高强钢管截面纵向残余应力的简化分布模式。结果表明,其分布模式与Wagner提出的分布模式接近,但Q690高强钢管的纵向残余压应力峰值与其屈服强度的比值仅为0.2,要明显小于普通钢管纵向残余压应力峰值与其屈服强度的比值0.35。     

Failure analysis of left barrel of an aircraft engine

This investigation analyses the failure of left barrel used in engine of an aircraft. The barrel was subjected to detailed examination using standard non-destructive and destructive testing methods in order to identify the cause of failure of left barrel. The experimental results include fractography, chemical analysis, coating analysis and optical microstructure. The investigation concludes that failure is caused by the formation of corrosion pits which act as stress raiser and fatigue has assisted in propagation of the crack.     

Effect of core bar inserted into weld faying part to obtain an ideal pipe joint with non-generating inner flash via friction welding

In this study, the effect of core bar inserted into weld faying part to obtain an ideal pipe joint with nongenerating inner flash via friction welding is described. A steel pipe with inner and outer diameters corresponding to 8.0 mm and 13.5 mm was used, and the weld faying surface was machined to a groove shape of a flat (butt) type. The core bar of various materials was inserted in the weld faying part of the pipes, and those pipes were welded with a friction speed of 27.5 s^-1 and friction pressure of 30 MPa. The core bars did not decrease inner flash when joints were fabricated with a core bar of some metallic materials with melting points below that of steel; thus, they were melted during the welding process. The joint with an alumina core bar did not decrease inner flash and was crushed by generating an inner flash. However, a commercially pure tungsten (CP-W) core bar was successfully achieved for decreasing the inner flash. Additionally, all joints with a CP-W core bar did not exhibit the tensile strength of the base metal and a fracture in the base metal, when they were fabricated during the same time, the friction torque reached the initial peak. The joint exhibited a fracture in the base metal when it was fabricated with a CPW core bar and a taper groove shape that was proposed in the previous study. Furthermore, the core bars were easily removed from the joints; thus the joint with almost no inner flash was successfully obtained. To reduce the inner flash of pipe joints, they should be fabricated with a CP-W core bar inserted into the weld faying part with a taper groove shape.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-020-00319-w     

Failure analysis of Integrated Nozzle Actuating System (INAS) cooling hydraulic pipeline

During post test disassembly of a developmental aero engine, a hydraulic pipeline was found fractured from near weld joint. The pipeline was manufactured from a α–β Titanium alloy and was welded using Tungsten Inert Gas Welding (TIG) technique. The pipeline was in service for approximately 100 h. Fractographic features of the surface revealed transgranular cracking mode with presence of corrosive elements near the outer wall. Crack branching was also observed on the pipeline near to the fractured surface. Evidences of corrosion pits along with microcracks were also present near to the fractured surface. From the evidences, it was corroborated that the pipe failure was due to synergistic effect of corrosion fatigue and stress corrosion cracking.     

Failure of weld joints between carbon steel pipe and 304 stainless steel elbows

A number of weld joints between carbon steel (CS) pipe and type 304 stainless steel (SS) elbows constituting a gas piping system of a petrochemical unit developed cracks after a relatively short period of usage, resulting in leakage. The gas flowing through the pipe, was hydrogen rich at a temperature of 45 °C and a pressure of 16 kg/cm². Light optical metallography and scanning electron microscopy, combined with energy dispersive X-ray spectroscopy, inductively coupled plasma and microhardness testing were used to determine the most probable cause of failure. Analysis showed that the cracks originated at the interface between the CS pipe and the SS root weld. A narrow band between the CS pipe and SS weld exhibited a hardness of Rockwell C 60 suggesting the formation of martensite due to C segregation at welding temperature and subsequent quenching during cooling. The ferritic region of CS adjacent to the weld was decarburized and was devoid of pearlite; corroborating C diffusion. The weld region was diluted comprising mainly of Fe with small amounts of Cr and Ni. Cracking is thought to have initiated at the hardened region. However, the failure might have been aided by hydrogen rich medium and soft C-depleted ferrite region.