A pipeline operator set out to assess the risk of circumferential stress corrosion cracking and to develop a proactive management program, which included an in-line inspection and repair program. The first step was to screen the total pipeline inventory based on pipe properties and environmental factors to develop a susceptibility assessment. When a pipeline was found to be susceptible, an inspection plan was developed which often included ultrasonic circumferential crack detection in-line inspection and geotechnical analysis of slopes. Next, a methodology was developed to prioritize the anomalies for investigation based on the likelihood of failure using the provided in-line inspection sizing data, crack severity analysis, and correlation to potential causes of axial or bending stress, combined with a consequence assessment. Excavation programs were then developed to target the anomalies that posed the greatest threat to the pipeline system or environment. This paper summarizes the experiences to date from the operator’s circumferential stress corrosion cracking program and describes how the pipeline properties, geotechnical program, and/or in-line inspection programs were combined to determine the susceptibility of each pipeline and develop excavation programs. In-line inspection reported crack types and sizes compared to field inspection data will be summarized, as well as how the population and severity of circumferential stress corrosion cracking found compares to the susceptible slopes found in the geotechnical program completed. Finally, how the circumferential SCC time-average growth rate distributions were calculated and were used to set future geohazard inspections, in-line inspections, or repair dates will be discussed.

The failure of a corroded pipe is generally controlled by the depth and the longitudinal extent of the metal loss area subjected to hoop stress. However, the failure of metal loss due to its circumferential extent under longitudinal stress is possible if significant longitudinal stress exists in the pipe or the metal loss has considerable circumferential extent and depth. If such circumstances exist, it is prudent to conduct a complementary analysis of pipe integrity to assess the potential for circumferential as well longitudinal failure. Most existing approaches for assessing circumferential metal loss, such as Miller’s equations, were derived by assuming the metal loss to be centered at the extreme stress position around the pipe circumference, i.e., the center of the metal loss is centered at the location of the maximum bending stress in the pipe. The assessment may be over-conservative if the metal loss area deviates from the extreme position related to the bending plane. Described in this paper is a new approach to assess the potential for circumferential failure of metal loss centered at an arbitrary angle from the location of maximum bending stress. The approach results in the same failure stress as existing models when the metal loss is centered at the location of maximum bending stress. The failure stress increases when the metal loss deviates from the location of maximum bending stress and reaches the maximum value when the metal loss is centered at the neutral axis. The equations of the model developed in this paper can be easily implemented into a spreadsheet tool for routine integrity assessment. Other considerations related to the assessment of circumferential metal loss are also discussed, including non-uniform corrosion, negligible corrosion, and the interaction of multiple corrosion areas in the same pipe cross section. The model developed in this paper can also be used to determine the cutoff line for plastic collapse in a failure assessment diagram (FAD) based approach for assessing circumferential cracks, such as API 1104 Appendix A and API 579.

This paper presents a tool for surface loading stress analysis that was developed in-house by TransCanada (TCPL). This tool utilizes fundamentals of the surface loading assessment method developed by Kiefner & Associates Inc. (KAI) for Canadian Energy Pipeline Association (CEPA), but incorporated many advanced functionalities to improve the accuracy, efficiency and transparency of the analysis. The new functions of the tool include the batch analysis, multiple angle analysis, generic/site-specific loading analysis, graphical display of stress distributions for refined assessment, user-defined impact factor and automated reporting for documentation of surface loading calculations. This tool also incorporated the improved numerical algorithm for longitudinal global bending stress considering the actual live load pressure distribution over a certain length of pipeline. The accuracy of the developed tool was validated by comparing it to the KAI tool. The improved algorithm for longitudinal global bending stress calculation reduces the conservatism of the longitudinal global bending stress compared to the original simplified method but does not sacrifice safety, which has been demonstrated by comparison with the experimental results. The new functionalities improved the business efficiency and maintains safety and regulatory compliance.

Transition welds joining pipes of unequal wall thickness are often used in gas and oil pipelines at road crossings, bends, fittings, and locations of class change. Pipeline operators have successively utilized a counterbore-tapered design for transition of unequal wall thickness. In the counterbore-tapered design, the wall thickness transition is moved away from a girth weld region, resulting in reduction of stress concentration near the girth weld, subsequently reducing the risk of hydrogen assisted cracking (HAC) and the driving force for fatigue crack growth. The counterbore length ( L ) is an important design parameter in the design of the counterbore-tapered joint. The aim of specifying the minimum counterbore length is to ensure that any local moments caused by the interaction of longitudinal stress and the dissimilar wall thickness are satisfactorily attenuated when they reach the weld area. While determining the minimum counterbore length, the present counterbore-tapered design criteria considers the attenuation of the bending stress induced by the local moments but does not consider the bending stress magnitude. In certain cases, although the bending stress attenuation length is long, the magnitude of the bending stress is very small. The long counterbore length creates practical challenges for boring the pipe due to the limitation of boring tools. Since the magnitude of the bending stress is very small, it is not necessary to fully attenuate the bending stress. In this work, the minimum required counterbore length ( L 0 ) criteria was revised. The revised counterbore length criteria considered the magnitude of the bending stress and the stress attenuation, hence avoided the unnecessary long counterbore length. The revised criteria was based on an analytical solution and supported by detailed finite element analyses. The analyses showed that the minimum counterbore length can be greatly reduced for those cases required unnecessarily long counterbore length with negligible or small increase in the stress concentration in the weld area. The present design method of counterbore-tapered joints allows only one value of taper angles at thickness transition, i.e., 14°. However, for some situations, creating the smoothness required with the 14° taper angle is difficult and an increase in the taper angle is preferred. In this work, it is shown that the stress concentration near the wall thickness transition and weld area for counterbore-tapered joints changes marginally by increasing taper angle from 14° to 30°.

Fibreglass reinforced plastic pipe (FRP) is the second most common type of pipe in the Canadian oil and gas industry, based on installed length. Industry methods to define risks and prevent failures are difficult because industry is still learning how these types of materials fail. Current industry failure records indicate that the failure rates for some of these materials are higher than steel failure rates. Unique details related to a specific FRP failure will be discussed in this paper. This failure occurred on an 8-inch OD FRP pipeline at the bottom of a riser. The failure resulted in a spill and a fire. The reasons for failure and fire initiation were analysed separately. The failure was a result of a combination of several types of stresses and material degradation. Both static and dynamic stresses contributed to the failure. • Ground settling resulted in high static bending stress of the last section of the pipeline connected to the riser elbow supported by the anchor. • The failure was in the last connection of the pipeline. Static tie-in stresses could have contributed to the failure. • Static stresses were evaluated using Finite Element Analysis (FEA) approach and found to be insufficient for the failure. • Dynamic stresses contributed to the failure. The failure happened soon after a power outage, when numerous wells were restarted, and several fluid surges may have occurred. • Material degradation associated with a specific orientation of glass fibres at the connection pup contributed to the failure. The failure sequence was established and different modes of fire initiation were analysed.

Magnetic Flux Leakage (MFL) inspection techniques for in-line corrosion monitoring of pipelines continue to evolve rapidly. Current R&D is aimed at improving the accuracy and reliability of defect sizing. Major issues are the variability and consequent need to characterize the magnetic properties of the pipes and the effects of line pressure, residual and bending stresses on MFL signals. Magnetic Barkhausen Noise (MBN) measurements are being used to study the stress-induced changes in magnetic anisotropy. Remote Field Eddy Current (RFEC) techniques are being investigated for detection and measurement of stress corrosion cracking in gas pipelines. Anomalous defect source models have improved the detailed explanation of crack defect interactions greatly.

For typical cross-country pipelines not expected to experience ground movement hazards, the longitudinal stresses experienced during lifting and lowering-in are typically the highest that they experience in their entire service life. Vertical bending stresses are produced by the curvature created from the upward lifting forces of the sidebooms and the downward force of the pipe weight. Horizontal bending stresses are produced due to the lateral movement of the pipe when the pipe string is moved from its support position on the side of trench to the center of the trench. It is critical to limit the stresses during lifting and lowering-in so that potential damage to the pipeline is avoided. For pipelines constructed using an engineering critical assessment (ECA) based flaw acceptance criteria, stresses must be controlled below the limit established during the development of the flaw acceptance criteria. However, there is little in the way of formal guidance in current codes and standards for controlling stresses during the lifting and lowering-in process. This paper is part of a long-term effort being sponsored primarily by Pipeline Research Council International (PRCI) to develop general construction guidelines that can be used to manage lifting and lowering-in stresses during pipeline construction. In this paper, the stresses during lifting and lowering-in on normally flat terrain were studied. The component of stress due to bending in the horizontal plane was determined through an analytical method. The component of stress due to the bending in the vertical plane was studied by finite element analysis (FEA). The FEA determined the stresses under various profiles. Recommended lifting profiles in the format of lifting height ranges were developed. The combined stress was then determined from the two components. In addition, FEA was used to simulate the lifting and lowering-in process of a pipe string including a field side bend. The results show that the side bend produces a very slight increase in the stress level. More work is being performed to investigate various other scenarios of field and pipe string conditions.

PT Transportasi Gas Indonesia (TGI) own and operate a 536 km long natural gas pipeline in South Sumatra, Indonesia which transports natural gas from ConocoPhillips’s gas field in Grissik, South Sumatra to Chevron Pacific Indonesia’s station facilities in Duri. On 29th September 2010, an event occurred that resulted in a release of gas from the pipeline. In response to the emergency, the affected section of the pipeline was isolated by closing block valves upstream and downstream of the leak. The incident was brought under control by TGI on the same day and there were no reported injuries or fatalities. Failure was located in a girth weld on a 28 inch diameter pipe section, which had spread into the adjacent pipe material. Subsequently a failure investigation was requested by TGI. The investigation included a fracture examination and materials testing of the failed girth weld and parent pipe; a geotechnical investigation; and an engineering critical analysis (ECA) of the failure. This paper describes the multidisciplinary works undertaken to investigate the cause of the incident. The primary observation of this failure investigation is that no single factor contributed to the failure that occurred. The pipeline at the rupture location had been subjected to high bending stresses when the pipeline was laid and the stresses were exacerbated following consolidation and creep settlement of the underlying swamp material. The field joint coating had been compromised, leading to the formation of near neutral stress corrosion cracking (SCC). Initial cracking from the SCC had then extended to the point where the remaining ligament then failed by plastic collapse. It was judged that the settlement may also have been enhanced by a recent earthquake.

The problem of smoothing the spatial line based on position measurements of discrete points exists in cases where a) the positions of points are determined with some errors, b) the goal of smoothing is not a continuous position itself but the higher derivatives of it. It is a very common problem in many engineering applications. With respect to the pipeline industry this problem is very prominent at least in two cases but regretfully many researchers do not pay due attention to it at all. First , the Geopigs are widely used for the determination of spatial position of the pipe centerline points. This information inter alia may be (and in fact are widely) used for the calculation of the global centerline curvatures which are proportional to the global bending strains. Second , the maximum strain levels of the dents are calculated based on the local geometry of the dent as determined by radial sensor measurements from the in line inspection survey. Note, that in both cases mathematically the curvatures are the second derivatives of the function of global (pipeline) or local (dent) positions. The input information about the global X–Y–Z position of each consecutive point of axis line as well as the local radial position of the dent points are given with some error. This leads to a huge noise in predicted curvatures which can overrun the useful information. The amplitude of errors of calculation is inversely proportional to the squared distance between the points of measurement. The application of any smoothing procedure may lead to the loss of the useful information about real curvatures. Thus tradeoff between the smoothing of the noise and the loss of accuracy presents a big problem in the pipeline industry. Two quantitative parameters are introduced here to allow performing such a tradeoff. First parameter characterizes the standard deviation (also referred to as standard in the following) of the random value of the position measurement accuracy by the devices, ρ . Second parameter is the requested accuracy of the curvature determination and is defined in terms of the standard deviation of the bending stress, σ or strain, ε . The spatial beam on elastic foundation model is used to fit the measured point positions to the spatial curve. Its main characteristic is the specific compliance of the foundation α which is determined based on two above root-mean-square errors ρ and σ . The corresponding formulas and tables based on the solution for the elastic beam are obtained. The bigger the allowed error in bending stress σ the lesser is required compliance of the foundation, α . In turn this leads to the smaller value of characteristic wave length of solution and the possibility to retain more useful information about the actual short length stresses in the pipeline. Some practical examples of applications of the procedure are given.

There have been several recent weld failures either during the initial post construction hydrostatic tests, or immediately following construction. Girth welds typically do not fail as a result of internal hoop loads without the contribution of loads due to out side forces. External loading should be considered during design, welding procedure development, construction, and pipeline operations. This paper presents one example where a girth weld failed as a result of preexisting 1940’s weld imperfections and recent, 1980’s, external loading. This analysis of the girth weld failure in the 30-inch pipeline included an initial failure analysis, a fracture mechanics analysis, and a finite element analysis that integrated the pipe-soil interaction, as well as localized stresses associated with weld imperfections. A critical part of this study was to evaluate how changes in soil conditions associated with a drought followed by soil saturation associated with rainfall, contributed to lack of local support and increased overburden loads associated with the saturated soil. The failure analysis of the ruptured girth weld and surrounding pipe concluded that the failure of the girth weld was caused by increased bending loads imposed on the pipeline after recent construction activities, and that the fracture initiated at a lack-of-penetration/fusion imperfection that was 20 1/4-inches long and 0.110 inches deep. A coupled investigation using finite element and fracture mechanics analyses verified numerically that with reduced-strength soil, stresses were generated in the girth weld of sufficient magnitude to cause a fracture. Temperature, terrain, and fatigue were considered, but were not deemed to significant enough to affect the stresses or other conditions that resulted in the failure. The overriding observation of this study is that no single factor contributed to the failure that occurred. Rather, the girth weld failure was the result of weld imperfections that generated elevated stresses due to excessive loads imparted to the pipe due to settlement associated with non-compact backfill associated with excavation work. Had the pipe not displaced vertically due to localized soil conditions, it is unlikely that the pipeline would have failed. The recent excavation activities were adequate for normal soil conditions; however, dry soil at the time of construction resulted in lack of compaction and excessive moisture just prior to the failure that generated in differential settlement and heavy overburden, combined with lack of penetration imperfection in the girth weld in question, resulted in generating excessive bending stresses that contributed to the eventual failure of the pipeline.

Historic pipeline construction utilized miter joints to enable small directional changes in pipeline routing, and this legacy construction remains in today’s pipelines. Current codes and regulations impose a limit on the maximum miter angle to less than three degrees of the total pipeline direction change, for pipeline operating with pressure over 30-percent SMYS (Specified Minimum Yield Stress). In anticipation of an operational pressure increase, an experimental and simulation effort was undertaken recently to determine the stress amplification due to miters in 30-inch diameter, 0.5-inch thick gas transmission pipelines. Experiments were conducted on six miter joints ranging in miter angle from 0° to 8° degrees of total pipeline direction change. Three of the miter joints were removed from the field (1950’s original installation), while the remaining three were specifically fabricated for the testing. All the miters considered were X42 pipeline steel. The miter pipe joint specimens were tested with pure pressurization, pure bending, and combined pressure and bending using a custom designed loading apparatus. Hoop and axial strains were measured using internally and externally mounted strain gauges. Pressure, as well as four point bending loads and deflections were recorded. One 3.8° field miter specimen was tested to burst. Experimental data, analytical solutions, and finite element results are compared at the miter joint section for the three loading cases. The study is limited to pipe radius to thickness ratio values of 30, and hence the results presented in this study are useful near this value. Results showed that miter joints increase stresses in the vicinity of the miter joint for pressure and/or bending loads. The peak stresses are on the exterior at the intrados. The pressure induced peak stress values increase proportional to the miter angle, and bending further increases the miter stress magnitudes. The ovalization effects significantly compromise the use of linear superposition of pressure and bending stresses even though material behavior remains elastic. Findings from this study demonstrate that in-situ miters on the pipeline in question do not compromise the integrity of the line, and stress additions for small angles over three degrees are comparable to stress risers occurring from normal pipeline features. The results of this work are important for performing structural integrity assessments and for making informed regulatory decisions for mitered pipeline operation.

Pipe bend is very complicated element for the structural integrity assessment. Up to day there is no conventionally adopted technique for limit load calculation of pipe bend even without any defect. The problem is that at application of outer bending moment the pipe bend cross section ovalizes and the process of deformation can be described only with accounting for the geometrical nonlinearity. The paper deal with limit load calculation for pipe bend with axial part-through defect for particular case when circumferential stresses originated both from inner pressure and outer bending moment dominate over axial stresses from the moment and axial force. Two extreme cases are considered at start. First one is the action of the inner pressure only. The “Institute for Problems of Strength limit load model” (IPS model) can be applied here without any restrictions. The second case is consideration of circumferential bending stresses which have appeared due to ovalization from the outer bending moment. The model of the transmission of stresses from the defected region to the undamaged regions is suggested and the resulting formula for the stress concentration (or strength reduction) coefficient is obtained. At last the simultaneous action of both loadings is considered. As result the analytical formula for the reference stress calculation which is similar in appearance to that of API 579 for accounting for membrane stress as well as bending stress is suggested. The only difference is that strength reduction coefficients are considered for both the membrane stresses from inner pressure and bending stress from ovalization. This differs from API 579 approach where the influence of the defects length on the bending stresses is not taken into account.

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.

This paper describes recent experimental work on the measurement of absolute levels of biaxial stress in ferromagnetic pipeline materials. A prototype in-line inspection tool has been constructed and has subsequently undergone evaluation in a series of pigging trials to determine its accuracy in recording of stress values. Recently attention has been raised by a number of groups working on the development of techniques intended to utilize the dependence of the magnetic response of ferrous materials to applied stress in order to directly measure stress. Most of these techniques have been based on the Barkhausen effect. This paper discusses the use of an alternative technique based on other magnetic properties that have been shown to allow derivation of a quantifiable relation between the level of stress present in material and the magnetic response. This technique, named MAPS by its developers, MAPS Technology, has been employed with considerable success out-with the pipeline industry. An earlier paper described the technique used to make the measurements. The response of pipe-wall materials to an applied alternating magnetic field is measured, and knowledge of the variation of magnetic properties of the material used to determine stress values. In order to prove the suitability of this technique to pipeline applications, a prototype inspection pig has been designed and constructed, and a series of tests performed in a 24 ″ test facility, constructed specifically for the purpose. The test facility design allowed for two modes of operation. The first of these allowed pigging through the 50m of test pipe, with an arrangement that allowed bending stresses to be applied to the pipe. The second mode of operation enabled internal pressure to be applied as the tool was pulled through the test section. The pig based system successfully gathered stress measurements in both axial and hoop directions. The pig measurements were subsequently compared to theoretical values of stress and to measurements taken with strain gauges and other instrumentation fitted to the test facility.

One of the major technical challenges in constructing natural gas pipeline is how to cope with cold region pipeline engineering aspects caused by freezing and thawing of soil around the pipeline. A pipeline running through discontinuous permafrost is subject to the potential risk of an unacceptable deformation, which is caused by thaw settlement or frost heaving at the boundary of permafrost and non-permafrost. It is important for a design engineer to predict the behavior of soil-pipeline interaction and make an adequate assessment of safety of pipeline in such portion. Although extensive efforts have been made to document those aspects, relatively little research has been carried out to comprehensively study the behavior of pipeline in response to short- and long-term change of thermal and mechanical properties of permafrost. In order to understand the complex behavior of natural gas pipeline and surrounding soil in cold regions, a full-scale experimental gas pipeline was constructed near Fairbanks, Alaska and had been studied intensively. The research project was carried out from the year of 1999 to 2004 under the sponsorship of Japan Science and Technology. The changes of ground thermal regime, vertical movement of the pipeline and the induced bending stress in the pipes were studied. The research team including researchers from Japan and the U.S collected and analyzed the field measurements from the test site. In this paper, the major findings and lessons learned from the project will be presented together with the result of numerical simulations related to the experiment.

Williams Gas Pipeline requested that Stress Engineering Services provide engineering services to analytically assess issues that occurred in a 30-inch pipeline due to cracked girth welds. The focus of the analytical effort was to simulate the lifting during the pipeline construction process. Using finite element methods, nominal bending stresses were calculated considering a specific set of criteria including 80-foot pipe joints, skid locations, and a given root pass weld thickness. These bending stresses were used to calculate a total stress that included stress concentration factors (SCFs) due to hi-low weld mismatch and weld profile geometry. For conservatism, residual stresses were also included in computing the total stress. As an example, the results showed that if a residual stress of 50 percent yield and a total SCF of 4.5 are assumed, the lift height should be limited to approximately 9 inches for a root pass weld thickness of 0.0625 inches. In this calculation, a stress limit of two times yield was used. Once the final stress was computed considering the range of variables, a closed-form equation was developed that integrated the variables of interest for the 30-inch pipe. This equation was used to compute data points to guide Williams in establishing permissible lift heights for a range of SCFs for the welds. A stress limit of two times the yield strength was selected as the limiting value. The details of the work presented in this paper will assist pipeline operators in developing a protocol that balances the need for rapid construction and ensuring that proper girth welds are fabricated.