While the general fracture mechanics principles and methodologies for calculating fatigue lives are well documented and validated, their application in the prediction of pipeline system fatigue lives differed from field experience. The source and magnitude of the conservatism inherent in the calculated fatigue life estimates are an important element when establishing integrity management programs. Of particular interest are the fatigue life estimates used in integrity management programs for electric resistance welded (ERW) pipeline systems that may have pipe seam anomalies oriented along the pipe axis. BMT Canada Ltd (BMT) was contracted by Pipeline Research Council International (PRCI) to develop a pipeline material fatigue crack growth database and conduct full scale cyclic pressure fatigue tests to develop improved crack growth rate parameters. A pipeline material fatigue crack growth database was developed using 185 fatigue crack growth rate tests on 45 pipeline materials ranging in grade from X46 to X70 and in vintage from 1937 to 2014. The database included fatigue crack growth rate tests on 18 pipe body base materials (BM) and 27 ERW weld seam materials at two different, stress ratios (R), of R-ratio = 0.1 and R-ratio = 0.6. The sampled crack growth rates observed in the pipeline steels, tested in the project were 2 to 3 times lower than the crack growth rates recommended in BS 7910. This paper presents the proposed power (Paris) law fatigue crack growth equation parameters, C and m, developed in the study. Two full-scale cyclic pressure tests were carried out to validate the use of recommended crack growth rate parameters. Axial flaws were machined in the pipe body and weld center line (WCL). Fifty-one (51) flaws of different lengths and depths were machined. The crack growth rates were monitored during the cyclic pressure tests by recording crack mouth opening displacement (CMOD). The calibration curves for correlating CMODs with crack depths were developed and validated against finite element (FE) analysis. The fatigue crack growth rates observed in the full-scale tests were then compared with existing BS 7910 and API 579 formulations. The comparison confirmed that the BS 7910 approach results in very conservative estimates of fatigue crack growth rates for axial flaws. The BS 7910 stress intensity factor formulation overestimated the bulging correction for axially oriented flaws. The API 579 fracture mechanics-based fatigue crack growth formulation combined with crack growth rate parameters developed in this program provided improved estimates for fatigue life. The fatigue crack growth rates for line pipe and ERW weld seams developed in this project were shown to be less conservative and better predictors for fatigue crack growth and represent a valuable tool for pipeline integrity management. The use of this information will enable pipeline operators to focus remedial actions on features that have the lowest estimated fatigue lives.

Dents containing crack fields (colonies) were often observed in liquid pipelines. A recent PRCI research “ Study of the Mechanism for Cracking in Dents in a Crude Oil Pipeline ” showed evidence of corrosion fatigue cracking mechanism in dents and estimated the crack growth rate as a function of stress intensity factor using the measured spacings of fatigue striations from fracture surfaces based on the assumption that the formation of fatigue striations on a cycle-by-cycle basis. However, due to the lack of full-scale fatigue crack growth data, the success was limited in this study. This gap prompted PRCI to launch a full-scale experimental investigation of cracks-indents under cyclic pressure load in the simulated groundwater (NS4 solution) environment. The objective of the study is to determine the crack growth rate in dent as a function of stress intensity factor, the number of cycles to failure, and the failure modes of crack-in-dent. The investigation is aimed at establishing a framework for the remaining fatigue life prediction of cracks-in-dents in liquid pipelines. This framework would benefit liquid pipeline operators to manage the integrity of dents associated with corrosion fatigue cracking exposed to groundwater in a timely manner. A total of six pipe samples containing cracks in shallow dents excavated from a 24-inch diameter liquid transmission pipeline were selected for full-scale fatigue tests. The test system developed under the project consisted of (1) a computer-controlled hydraulic pressure cycling system, (2) an environment chamber containing NS4 solution mounted on the dent region to provide a simulated field environment condition, (3) real-time crack growth monitoring systems including direct current potential drop (DCPD) system, Clip gage, and Strain gage, and (4) a data acquisition system. The cyclic pressure range used in the fatigue test was between 78 psig (7.2%SMYS, minimum) and 780 psig (72%SMYS, maximum) with R = 0.1, which was based on historical operational pressure fluctuation data. A constant frequency of 0.0526 Hz was selected for the testing to ensure the frequency requirement for corrosion fatigue was met. In this paper, the objective, along with the background of this research, is discussed first. Then, the pipe sample preparation, experimental setup, and test results are presented. The fatigue crack growth rate as a function of the stress intensity factor is then discussed. Following this, the fatigue crack growth coefficients were estimated using the full-scale test data and FEA. Finally, the fatigue test results are summarized and presented the framework for the life prediction of corrosion fatigue cracks in shallow dents.

A full encirclement thin layer steel laminated sleeve system has been designed, developed, and optimized for pipeline integrity management applications. Development goals included the elimination of thixotropic concerns as well as the exclusion of the degradation of material properties of composite repairs. Elimination of cyclical fatigue of welded repairs and safety concerns associated with hot work were also considerations. The use of thin layer steel with a modulus matched to base pipe and steel’s homogenous isotropic properties enable axial calculations and evaluation of strain-based concerns. The thin layer steel laminated design results in extremely high fracture toughness and promotes intrinsic mitigation of potential future third party damage. The resulting system has demonstrated the reliable engineering data and analysis required for pipeline repairs and demonstrates applicability for the augmentation of existing pipes without defects. An Engineering Critical Assessment (ECA) has been completed. This ECA follows the industry’s precedents of ASME B31G and ASME PCC-2 Article 4 type assessments and provides operators with greater functionality. This ECA has been named the Leewis Augmentation Analysis (LAA) and is presented, reviewed, and discussed. Third party full scale ASME PCC-2 style burst testing has been completed. The results are presented. Highly instrumented tests were also conducted to determine an effective modulus of elasticity of the installed system as well as a determination of any delay in system acceptance of load. As installed, an effective modulus of 14 million psig (96526.60 MP)a with loading in layer 3 of the laminate at only 50 micro strain is reviewed. Long term creep and cyclical fatigue testing of the steel/adhesive laminate is presented and reviewed. 10 million cycles at 50% of ultimate lap shear has been achieved, which exceeds current industry practice by several orders of magnitude. The classic metal loss defect mitigation principle is reviewed and updated in light of these available technical advances. Finally, the implications for mitigation of both stress and strain dependent integrity concerns is discussed.

The safety of gas transportation pipelines under fatigue loading remains an important issue. The purpose of the present study is to better evaluate the fatigue crack growth (FCG) behavior by carrying out analysis/predictions and experiments in full-size pipeline steels. A full characterization was made using several samples of an X42 grade pipeline steel, to characterize the monotonic and the fatigue behavior. Fatigue tests on full-scale pipeline steels under pressure loading were performed. The potential drop (PD) method applied to pressurized pipes makes it possible to monitor and quantify both crack initiation and crack propagation. These tests served as a basis for numerical comparison. Crack propagation of the full-size pipeline steel was simulated by finit element analysis (FEA) using an adaptive re-meshing approach implemented as part of the Z-set/Zebulon software. Simulation allows predicting fatigue crack growth life on pipes using results of tests on specimens as an input.

Mechanical damage is one of the major threats to oil and gas transmission pipeline integrity, which has been the case now for decades. Although much work has been done in that context, due to the complexity of its effects mechanical damage severity remains difficult to quantify. Thus, work continues to better understand the failure mechanism and develop the means to screen damage severity. The present paper adopts a validated elastic-plastic finite element analysis (FEA) model to simulate mechanical dents in pipelines and to quantify the effects of damage through a broad parametric study. This considers the need for three-dimensional FEA models and the effects of FEA element type, soil constraint condition, indenter type, pipeline grade and initial pipe pressure on dent response. The FEA model is also used to assess the minimum wall thickness for which a dent has the minimal effect on pipeline integrity. Finally, application of the proposed FEA model is illustrated by successfully predicting the failure behavior of a dent in a full-scale fatigue test involving a modern pipeline steel.

Pipeline dents can be developed from the pipe resting on rock, a third party machinery strike, rock strikes during backfilling, amongst other causes. The long-term integrity of a dented pipeline segment depends upon parameters including pipe geometry, indenter shape, dent depth, indenter support, secondary features, and pipeline operating pressure history at and following indentation. US DoT and other standards include dent repair and remediation criteria broadly based upon dent depth, dent location (top or bottom side), pressure cycling (liquid or gas), and dent interaction with secondary features (weld, corrosion, cracks). These criteria are simple and easily applied, however, they may not direct maintenance appropriately and be overly conservative or, in some cases, unconservative. Previous IPC papers have discussed the full-scale dent fatigue testing and dent modelling efforts to support integrity management criteria development by collecting material and structural response during dent formation and pressure loading. The present paper will present the results of this extensive dent structural and fatigue life numerical simulation program using a validated finite element (FE) analysis process. The paper describes the numerical simulation technique, as well as, the development of the novel engineering tool for integrity management, eliminating the need for numerical simulation of individual dent features to assess the relative integrity threat they pose. The development of the engineering tool presented in this paper considers the dent formation, re-rounding and through life response to pressure fluctuations to evaluate the fatigue life of dent features. The results of these analyses are used to develop fatigue life trends based on dent shape, restraint condition and operating pressure. These trends were used to develop models to predict dent relative severity and life based upon ILI inspection dent shape data for single peak dents. Dent shape has also been used to determine the restraint condition of a dent and its influence on the dent feature fatigue life. The tools were developed to address many of the uncertainties inherent in existing regulatory repair and remediation criteria. Current and future applications of the integrity assessment model are described along with recommendations for further development and testing to support pipeline integrity management, industry guidelines and standards. The results of this research will be of use in improving integrity management decisions and support further development of industry guides and standards. As such the information presented in this paper will be of interest to pipeline operators, integrity management specialists, in-line inspection (ILI) organizations and regulators. The recommendations presented in this paper may be used to influence the direction of pipeline standards in their direction in the disposition of dent features.

Fatigue tests were carried out on ERW line pipes 200 mm and 300 mm in diameter with a severe gouge in dent type defect. Hoop stress fluctuation between 15% and 30% of the yield stress of the pipe materials was applied by cyclic internal pressure loading. By the parameter Q, a function of defect size and fracture toughness, fatigue behavior could have been well explained. Fatigue behavior differed above and below the threshold Q (Q th . When Q<Q th , crack propagation was initially associated with ductile crack growth, and therefore, fatigue life, N f , decreased to less than 1000 cycles. On the contrary, N f was larger than 1000 cycles when Q≧Q th . N f was predictable with a power law equation incorporating dent depth, gouge depth, and stress amplitude when Q≧Q th . A gouge in a dent with Q<Q th , requires immediate repair or replacement.

Mechanical Damage on pipelines is the most frequent in service failure cause, at least in Europe. The category of Mechanical Damage actually covers on one hand rock dents, on the other hand, combined “Dent and Gouge” defects resulting from an impact by a ground moving machinery. The latter are complex defects in terms of geometry and mechanical behavior. Improved understanding of toughness reduction in these defects and their vicinity, especially in the case of vintage pipelines, and its impact on burst pressure and remaining fatigue lifetime is crucial in order to ensure a high safety level at the best economic conditions for transmission pipelines. PRCI and DOT/PHMSA supported a large experimental program to investigate the burst and fatigue strength of such defects in modern pipes as well as in vintage pipes by producing very detailed reference experimental data in order to support improvement of current burst and fatigue strength models for these defects. This paper focuses on experimental results data for two vintage pipes removed from service, one 26″ in diameter, X52 grade (from US) with high carbon content and sulfur content involving very low toughness and another 24″ in diameter, X63 grade (from Europe) characterized by alternated pearlite and ferrite bands. In each pipe, two different types of combined “Dent and Gouge” defects were created, “shallow dent with shallow gouge” and “severe dent with moderate gouge”. Each defect was reproduced three times: • One for destructive metallurgical investigation; • A second one for highly instrumented burst test; • A third one for highly instrumented fatigue test. The defect profile was monitored during pressure increase until burst, and a camera recorded pictures at each fatigue cycle to detect crack initiation at the gouge bottom and its further propagation. The different features of the experimental work and results described in this paper are: • Material characterization; • Metallurgical and residual stress investigation on combined dent and gouge defects; • Burst tests conditions and results; • Fatigue tests conditions and results. • Comparison with results from modern pipes containing the same types of defects. All this experimental data is made available for an in-depth understanding of the “Dent and Gouge” type defects leading to improved models, as well as reference data for validating these improved models for burst and fatigue strength of these defects.

Pipeline dents fatigue life prediction is a subject of high interest for pipelines operating companies. Empreinte is an in-house developed pre and post processor to ABAQUS Finite Element Calculations dedicated to pipeline integrity assessment. Empreinte was first developed and experimentally validated for dents assessments under static loading conditions. As oil but also gas transmission pipelines are submitted to cyclic loading conditions (internal pressure variations, shutdowns, temperature variations …), it was decided to introduce a fatigue life criterion in Empreinte based on the Dang Van theory assuming that local mesoscopic stresses drive fatigue crack initiation. Full scale tests performed for PRCI projects PR-201-927, PR-201-9324 and MD-4-2 were used to validate the proposed fatigue assessment methodology: - the first full scale fatigue test was performed in 1994 on an X52 pipe. For this test, limited material and test data were available. - the second full scale fatigue test was performed in 2007 on an X52 pipe. For this test, material characterization (in particular tensile tests with full stress strain curves) and test data (strain gages measurements, indenter geometry …) were available. Fatigue life assessments were performed following three main steps: 1. using available data: non linear kinematic hardening constitutive laws were identified for the two pipes materials; 2. finite elements elastic-plastic modeling of the denting processes were carried out; 3. fatigue calculations were performed following a new approach using Dang Van criterion for which the parameters were determined from literature data. The elastic shakedown assumption allowed the determination of the local stress cycle from the macroscopic stress cycle. The fatigue criterion integrating the combined influences of shear and hydrostatic stresses was checked on all points of the pipe. Good agreement between experimental and calculated fatigue lives and fatigue crack initiation points was reached. This opens a promising way to assess pipeline defects fatigue life. Efforts are now focused on the standardization of a testing method to identify the Dang Van criterion of a pipeline material at least in air environment.

The target of inline crack inspection is normally related to the detection of axial cracks (weld cracks, SCC etc.) as axial cracks are usually expected under typical loading conditions in pressurized pipe. Ultrasonic crack inspection tools for this type of cracks have been available for over 20 years and have become a standard application in the ILI business. However, under certain conditions circumferential cracking may occur and the inspection technique needs to be modified accordingly. Especially under offshore conditions with limited pipeline accessibility not only the early detection of crack-like defects is required but also a precise depth sizing is important in order to minimize the risk of crack-related pipeline failure. In order to ensure a high probability of detection together with state-of-the-art depth sizing, a 10″ inline inspection tool was developed for the detection of circumferentially orientated weld cracks. The tool combines the advantages of the pulse-echo technique on the detection side with the excellent capabilities of the TOFD (time-of-flight-diffraction) technique for accurate sizing. Both techniques are implemented into a tethered tool where the pulse-echo unit serves for fast screening while the relatively slow TOFD-unit is used for sizing of any detected crack-like features. In order to qualify the new tool for a special offshore application (inspection of circumferential cracking at welded anode pads) extensive testing was performed using a 10″ test pipeline prepared by Statoil. The test line contained 64 artificial weld defects for reference purposes as well as an unknown number of fatigue cracks in the weld area which were generated by fatigue testing of the corresponding pipes. The tests were full blind tests with no advance knowledge on locations and sizes of the cracks. The sizes of the fatigue cracks (length, depth) were determined by destructive examinations carried out after inline testing. The subsequent comparison of the destructive results with the non-destructive results showed that the specification of the tool with regard to detection and sizing was fully met. In the paper, the inspection concept and the setup of the new tool are described, and the results of the qualification tests are presented.

A collection of results of fatigue tests undertaken on full scale pipeline specimens containing unconstrained plain dents is presented. A total of 61 test results are reported, comprising dents of different shapes and depths, introduced in pipe sections with nominal diameters of 12″ and 24″, D/t ratios varying from 18.6 to 77.2 and made of API 5L X42, X46, X52, X60, X70 and X80 steels. The fatigue test results are used to compare five plain dent fatigue assessment methodologies. The assessment methodologies most widely used in the industry, namely, API 1156 and EPRG, are evaluated; each one of these has two different versions, evaluated independently. A dent assessment methodology previously proposed by the authors is also is included in the study. The classic Palmgren-Miner’s cumulative fatigue damage rule is employed in order to correlate the results, since all tests were carried out with two different pressure cycles. Only two of the evaluated methodologies presented a correlation with the fatigue tests that justifies recommending their application in the industry.

A combined caliper and tri-axial MFL in-line inspection (ILI) reported a bottom side 2.7%OD dent associated with 76% metal loss. The reported dent depth is well below the 6% limit while strain analysis of this ILI dent profile showed a maximum equivalent strain of 17.4%, which exceeds the 6% strain limit for gas pipeline. Due to the high dent strain level, the raw signals of metal loss were revisited, which indicated this associated metal loss appears to be a crack rather than corrosion. In-field investigation revealed that this dent is indeed associated with branched cracks both at internal and external pipe surface but no leak was detected. The primary objective of this case study is to determine the cause for cracking in the dent. As part of this study, detailed investigation was performed including LaserScan based strain analysis, lab pressure-cycle testing and fracture surface examination. An attempt was made to quantify the plastic strain damage of this dent and its susceptibility to cracking using the existing plastic damage models, namely, ductile failure damage indicator (DFDI), strain limit damage (SLD) and minimum elongation limit criterion. The investigation showed that the internal cracks were formed at the time of indentation while the external cracks formed by spring-back (elastic rebounding) due to the removal of rock constraint. Full size fatigue testing of this cracked dent showed leak failure modes rather than rupture. In this paper, the approach, results and the findings are summarized and discussed.

External interference on gas and oil transmission pipelines is consistently reported as leading cause of leaks in Europe and USA as identified in the EGIG and PHMSA incident databases. External interference due to ground working machinery strikes, rock strikes during backfilling, etc. on buried pipelines result mainly in dent and gouge defects. The long-term integrity of a pipeline segment damaged by a dent and gouge defect is a complex function of a variety of parameters, including pipe material properties, pipe geometry, defect geometry linked to indenter shape, aggression conditions. The complexity and extreme variability of these dent and gouge defect shapes and pipe materials lead simple assessment models to scattered predictions, hinting towards an insufficient description of real structural and material behavior. To improve knowledge beyond the numerous studies led in the past, and to provide a sound foundation for developing and validating mechanistic models for predicting burst and fatigue strength of such defects, a large experimental program was funded by PRCI and US DoT and further coordinated with a complementary EPRG program. The experimental program part dealing with combined “ Dent and Gouge ” defects is covered for modern pipes (24″ OD, X52 and X70) by PRCI project MD-4-1: realistically created (with a Pipe Aggression Rig) defects submitted to full scale burst and fatigue tests, in addition to extensive characterization. This work interfaces with modeling to predict the immediate burst strength and fatigue resistance of such damage in two PRCI projects denoted MD-4-3 and MD-4-4 respectively. This paper gives an overview of some of these activities: PRCI project MD-4-1 results: material characterization, full scale burst and fatigue tests on Dents with Gouges, as well as detailed explanations concerning the initial approach to model burst and fatigue life of these defects, as developed byr PRCI project MD-4-4. The final outcome of the expected knowledge improvements about the mechanical strength of dent and gouge combinations will be applicable by pipeline operators, in order to enhance integrity management systems designed to manage the threat associated with mechanical damage.

Dents in buried pipelines can occur due to a number of potential causes; the pipe resting on rock, third party machinery strike, rock strikes during backfilling, amongst others. The long-term integrity of a dented pipeline segment is a complex function of a variety of parameters, including pipe geometry, indenter shape, dent depth, indenter support, pressure history at and following indentation. In order to estimate the safe remaining operational life of a dented pipeline, all of these factors must be accounted for in the analysis. The paper discusses the full-scale dent testing being completed to support the development of pipeline integrity management criteria and is a continuation of the work discussed in previous IPC papers [1,2]. The material and structural response of the pipe test segments during dent formation and pressure loading has been recorded to support numerical model development. The full scale experimental testing is being completed for pipe test specimens in the unrestrained and restrained condition using different indentation depths and indenter sizes. The dents are pressure cycled until fatigue failure in the dent. This paper presents typical data recorded during trial including indentation load/displacement curves, applied pressures, strain gauges along the axial and circumferential centerlines, as well as dent profiles. The use of the full-scale mechanical damage test data described in this paper in calibrating and validating a finite element model based integrity assessment model is outlined. The details of the integrity assessment model are described along with the level of agreement of the finite element model with the full scale trial results. Current and future applications of the integrity assessment model are described along with recommendations for further development and testing to support pipeline integrity management.

Pipeline is the common mode for transporting oil, gas, and various petroleum products. Aging and corrosive environment may lead to formation of various defects such as crack, dent, gouge, and corrosion. The performance evaluation of field pipelines with crack defect is important. Accurate assessment of crack depth and remaining fatigue life of pipelines with crack defect is vital for pipeline’s structural integrity, inspection interval, management, and maintenance. An experimental based research work was completed at the University of Windsor for developing a semi-empirical model for estimating the remaining fatigue life of oil and gas pipes when a longitudinal crack defect has formed. A statistical approach in conjunction with fracture mechanics was used to develop this model. Statistical analysis was undertaken on CT specimen data to develop this fatigue life assessment model. Finite element method was used for determining the stress intensity factor. The fatigue life assessment model was then validated using full-scale fatigue test data obtained from 762 mm (30 inch) diameter X65 pipe. This paper discusses the test specimens and test data obtained from this study. Development and validation of the fatigue life assessment model is also presented in this paper.

In August 2009, there was a 2.5 m long rupture in the longitudinal seam weld of a crude oil pipeline in France. The failure caused a spillage of approximately 2000 cubic metres in a protected area. This rupture caused the authorities to withdraw the permit to operate a 260 km long section of this pipeline. The pipeline had a similar failure in August 1980. The 1980 failure was attributed to a fatigue crack initiating at the inner side of the longitudinal weld. There was evidence of ‘roof topping’ along this weld. Penspen Ltd., UK were contracted by the pipeline operator Societe du Pipeline Sud Europeen (SPSE) to carry out an independent investigation into the cause of the 2009 failure, review and confirm the actions needed for safe short term operation to allow internal inspection, and determine a safe future life for the pipeline. The failed pipe specimen was not immediately available for inspection. An initial analysis indicated that purely an analytical evaluation would not provide conclusive results, due to variability in material properties, geometry and loading. It was decided that a better understanding of the behaviour of defects in the pipe, and the fatigue performance, was required. A detailed laboratory programme of burst and fatigue testing on a section of line-pipe was recommended. The tests were carried out on a combination of ‘defect-free’ ring specimens, and ring specimens with initial Electric Discharge Machined (EDM) slits to represent crack-like defects. These tests showed: 1. BURST TESTS: The failure pressures of the burst tests could be predicted using a recognised industry model. Roof topping, laminations and inclusions, and toughness variations were found to have no noticeable effect on the defect size at failure. 2. FATIGUE TESTS: The fatigue test results showed that defect-free rings had a fatigue life one order of magnitude longer than those containing an EDM slit, and that the fatigue life of a defect-free ring could be predicted using a standard S-N method. In addition, it was found that crack growth was conservatively predicted using standard fatigue fracture mechanics. The fatigue test results showed that the 1980 and the 2009 failures were caused by a combination of cyclic pressure loading, roof topping, and a pre-existing weld defect (probably present when the pipeline went into service in 1972). This paper provides an overview of the failure investigation, with a discussion of the ring testing, supporting tests, and fracture surface inspection.

In spite of current world economic climates, recognition that alternative energy sources to the traditional fossil fuels has to be explored and understood. One potential energy source being researched and developed is hydrogen gas. Currently the most economical method of transporting large quantities of hydrogen gas is through steel pipelines. It is well known that hydrogen embrittlement has the potential to degrade steel’s mechanical properties when hydrogen migrates into the steel matrix. Consequently, the current pipeline infrastructure used in hydrogen transport is typically operated in a conservative fashion. This operational practice is not conducive to economical movement of significant volumes of hydrogen gas as an alternative to fossil fuels. The degradation of the mechanical properties of steels in hydrogen service is known to depend on the microstructure of the steel. Understanding the levels of mechanical property degradation of a given microstructure when exposed to hydrogen gas under pressure can be used to evaluate the suitability of the existing pipeline infrastructure for hydrogen service and guide alloy and microstructure design for new hydrogen pipeline infrastructure. To this end, the microstructures of relevant steels and their mechanical properties in relevant gaseous hydrogen environments must be fully characterized to establish suitability for transporting hydrogen. Previously data from a US Department of Energy/private sector funded project to evaluate four commercially available pipeline steels alloy/microstructure performance in the presences of gaseous hydrogen was presented in 2010. Interest in this previous work from industry and the ASME B31.12 Hydrogen Piping and Pipeline Systems codes and standards committee resulted in additional funding for continued evaluation of additional pipeline steel alloys/microstructures in the presences of gaseous hydrogen. Samples from API grades X52 (1960’s and current vintage designs), X70 (1980’s and current vintage) and X80 along with various samples from an X52 induction bend pipe and one pressure vessel steel A516 Gr 70 are being evaluated. Microstructural characterization, fracture toughness and fatigue testing in the presence of gaseous hydrogen at 800 psig and 3,000 psig are being conducted. This paper will describe the fracture toughness results achieved to date on various commercially available pipeline steels used in the existing North American pipeline infrastructure in the presence of gaseous hydrogen at pressures relevant for transport in pipelines. Microstructures and fracture toughness performances will be compared between these in this study along with those published previously. In addition, recommendations for future work related to gaining a better understanding of steel pipeline performance in hydrogen service will be discussed.

The present work aims to evaluate the microstructural mechanisms associated with the initiation of fatigue damage in steels used in the petroleum industry. Microdeformations and residual stresses (macrostresses) are evaluated by X-ray diffraction in real time during alternating bending fatigue tests performed on samples taken from an API 5L X60 grade steel pipe. Microdeformations are evaluated from measurements of the full width at half maximum (FWHM) of the diffraction peak and residual stresses are estimated from the peak displacement. The evolution of microdeformations shows three regular successive stages of changes. The amplitude of variation of each stage is intensified with increasing stress amplitude, while the duration is reduced. A similar evolution is found for residual stresses, whose stages of changes have nearly the same durations as those of microdeformations. Changes in density and distribution of dislocations are observed by transmission electron microscopy combined with the technique of focused ion beam. To understand the role of the initial structure, fatigue tests on annealed samples are performed under the same test conditions. Again, three stages of changes are observed, but with an increase in microdeformations instead of a decrease during the first stage due to the initial state of the dislocation network. The results are very encouraging for the consideration of microstructural changes measured by X-ray diffraction in the development of a future indicator of fatigue damage initiation in API 5L X60 grade steel pipes.

Pipes and tubular members are used in offshore applications as structural elements such as columns or in transport pipelines, risers etc. When subjected to dynamic loads, weld defects or geometrical stress raisers can initiate fatigue cracks causing the columns or pipelines to fail prematurely. In order to investigate the fatigue behaviour of pipe joints, a resonant bending fatigue setup was designed, suitable for testing pipes within a diameter range from 6 ″ to 20 ″ . In this setup the pipe, filled with water, is subjected to a dynamic excitation force with a frequency close to the natural frequency of the filled pipe. The force is applied using a unique drive unit with excentric masses. The pipe is supported in the nodes of its natural wave-form, so that no dynamic forces are transmitted to the setup. The deformation of the pipe is measured at discrete locations using an optical 3D dynamic measuring system. Through thickness fatigue cracks can be detected by pressurizing the water in the pipe and applying a pressure gauge. In this paper some unique aspects of the design of the resonant bending fatigue setup are discussed by presenting the results of a semi-analytical model used for calculating the deformation and bending stress in the excitated pipe and by comparing these results to the deformation measurements made by the dynamic measuring system. The working principles of the setup are illustrated by showing the preliminary test results for a 12 ″ diameter X65 steel pipe with a wall thickness of 12.7mm. It is demonstrated that the model predicts the behaviour of the pipe in the setup very accurately.

Dents in buried pipelines can occur due to a number of potential causes; the pipe resting on rock, third party machinery strike, rock strikes during backfilling, amongst others. The long-term integrity of a dented pipeline segment is a complex function of a variety of parameters, including pipe geometry, indenter shape, dent depth, indenter support, pressure history at and following indentation. In order to estimate the safe remaining operational life of a dented pipeline, all of these factors must be accounted for in the analysis. The goal of the full scale experimental program described in this paper is to compile a database of full scale dent test results that encompasses many of the dent types seen in the field, including plain dents, dents interacting with girth and long seam welds, and dents interacting with metal loss features, in both the unrestrained and restrained condition. The dents are pressure cycled until a fatigue failure occurs in the dent. Typical data recorded includes indentation load/displacement curves, applied pressures, pipe wall OD strains along the axial and circumferential centerlines, and axial and circumferential dent profiles. The full scale tests are being performed on behalf of PRCI and US DoT. This paper is intended to show the matrix of dents considered to date and present a representative summary of the data recorded. In addition to presenting the full scale test program and resulting data, this paper summarizes ongoing efforts to develop a validated pipeline dent integrity assessment model. The model under development makes use of the aforementioned full scale experimental data, to validate a finite element model of the denting and re-rounding process for a variety of dent scenarios (i.e. depths, restraints, indenter sizes). The paper discusses the efforts under way to develop and validate the finite element model with the goal being to estimate the fatigue life. The paper is an extension of work discussed in a previously presented IPC paper [1].