The pipeline industry is currently taking several approaches to evaluate the integrity of dents, ovalities, or other geometric anomalies identified from in-line inspection (ILI). A primary threat associated with these features that operators should be concerned with is failure due to fatigue. In order to carry out a more accurate dent fatigue analysis, it is important to be able to quantify the amount of damage accumulated during the initial dent formation process and subsequent shakedown of the dent. Dents result from permanent deformation of the pipeline which leads to accumulation of plastic strain. Whether this permanent deformation was caused during initial construction (a backhoe striking the pipeline) or in service (changing underground soil conditions), the plastic strains that are observed will result in a decrease in the pipeline’s fatigue life. Pressure cycling has the potential to accumulate additional plastic stain, thus accumulating more fatigue damage. Eventually as the pipeline continues to be cycled, no additional deformation or accumulation of plastic strain will occur; this behavior is referred to as “shakedown.” Finite element analysis (FEA) can be utilized to quantify how much fatigue damage has been accumulated during the initial dent formation process and subsequent shakedown of the dent. When analyzing pipeline dents using FEA, importance should be placed on accurately simulating the dent forming process so that realistic plasticity effects can be captured. The process of calculating plastic stresses and strains during the dent forming process can be computationally expensive and result in numerical instabilities within the analysis. As a result, methods for simulating the formation and shakedown of a pipeline dent are continuously being refined. However, since it is difficult to determine exactly how these geometric pipeline anomalies were formed, the applicability and accuracy of such methods contains a great amount of uncertainty and is thus expensive (both from a cost and time standpoint) for an operator to validate. This paper will identify a new and innovative approach for using FEA to determine the amount of damage accumulated during the initial dent formation process and subsequent shakedown of the dent. This approach uses state-of-the-art FEA modeling techniques coupled with industry knowledge and experience to develop an accurate and efficient method for quantifying this damage. The knowledge gained during this analysis can be used in conjunction with a traditional rapid dent assessment methodology. A case study will be presented which demonstrates the impact that a direct calculation of this initial damage has on representative pipeline dent assessment analysis. By undertaking this additional analysis, operators will have the potential to eliminate unnecessary digs. Additionally, operators can be more confident that their resources are being applied to the highest priority features.

The occurrence of Hydrogen Induced Stress Cracking (HISC) is well documented in offshore pipelines constructed from duplex steel. The conditions required for HISC to occur are exposure to water, presence of a negative electrical potential from cathodic protection, and application of a high tensile stress. There is evidence that through wall HISC crack growth can occur in forgings when these conditions are present for just a few hours. Onshore pipelines may also be constructed from duplex steel to provide corrosion resistance, particularly flowlines carrying well fluids. Where the fluid has a high pressure or low temperature, high tensile stresses may occur. These stresses are further increased at localised stress raisers such as girth welds and attachment fillet welds. The presence of water in the soil and the use of cathodic protection is likely for most buried onshore pipelines. Therefore all the conditions necessary for HISC to occur may be present. Guidance for assessing the susceptibility of offshore duplex steels to HISC is available in DNV-RP-F112, based on laboratory testing and industry experience following failures caused by HISC. However, historically it has not been commonplace to use duplex material for onshore high pressure pipelines, so no standard guidance is available. A detailed engineering assessment is therefore required. However, many onshore design engineers may not be familiar with the HISC failure mechanism, and may not consider it at all when designing a pipeline using duplex material. This paper discusses recent experience in applying the design rules in DNV-RP-F112 in an onshore pipeline design project. Firstly the paper covers the conditions necessary for hydrogen generation to occur. It discusses the differences between onshore and offshore cathodic protection systems, with reference to the electrical potential required for hydrogen generation. It concludes that in some situations, control of the cathodic protection potential will be the sole barrier to hydrogen generation, and thus its reliability can be of critical importance. Secondly the paper describes the detailed analysis required to determine peak stresses and strains at localised stress raisers, for assessment against the limits provided by DNV-RP-F112. The definition of ‘peak’ stress and strain is discussed, since this is different from the definition more commonly used (for example in fatigue analysis). The HISC phenomenon is driven by dislocation movement due to applied stress, but DNV-RP-F112 provides limits in terms of both stress and strain. A method is presented to determine strains caused exclusively by stress and not by thermal expansion or the Poisson effect. It is concluded that some interpretation of the DNV-RP-F112 guidance is needed for application in an onshore environment. Further work, including testing, is required to demonstrate whether the DNV-RP-F112 guidance can be applied reliably onshore.

Fracture mechanics methodologies for calculating fatigue lives have been successfully applied by pipeline operators to estimate integrity reassessment intervals. Their application in the definition of pipeline system fatigue lives has been overly conservative in actual practice. The source and magnitude of the conservatism inherent in the calculated fatigue life estimates needs to be identified so operators have a better indicator of when reassessments should take place. The pipe life estimation is especially critical for Electric Resistance Weld (ERW) and Electric Flash Weld (EFW) pipeline systems with longitudinally oriented defects. Prior work on improving fatigue life was initiated through studies completed by Pipeline Research Council International, Inc. (PRCI) to evaluate the sources of differences between fatigue life estimates produced by industry fatigue analysis software and different metallurgists. Two significant sources of conservatism in the fatigue life estimation process were identified: the fatigue crack growth rate (da/dN) and the bulging correction factor applied to axial surface flaws. The experimental and numerical simulation techniques considering the impact of these factors on rate of fatigue crack growth of pipeline axially oriented defects are described in this paper. Finite element modeling was used to simulate pipe bulging in the presence of axial flaws. The effect of the pipe thickness, diameter and flaw geometry was compared with treatments included in existing defect assessment standards. The results illustrate that for longer and deeper flaws existing treatments over represent the local bending due to pipe wall bulging. This results in unnecessarily conservative (shorter) fatigue life estimates. The crack growth rate (da/dN) was measured in a compact tension specimen material fatigue testing program. The test results included a range of ERW and EFW pipe materials with varying vintages and grades. The measured fatigue crack growth rate for the materials tested was found to be lower than that recommended by existing industry standards. This adds to the over conservatism of current approaches. The numerical simulation and materials testing results and related recommendations presented in this paper are compared to existing codified treatments to quantify the level of conservatism inherent in the current state of practice. Recommendations are provided to enhance the precision and better manage conservatism in fatigue crack growth rate calculations. Increased accuracy serves to improve integrity management and would be of interest to pipeline operators, consultants and regulators.

In the field of defects assessment, mechanical damage severity analysis is an issue of interest for many pipelines operators [1]. Indeed, common literature fatigue criteria [2] [3] [4] [5] often give underestimated dent lifetimes compared with experimental results. This causes repairs which could be avoided, therefore more predictive criteria would generate a significant maintenance costs saving. This paper presents an improved method developed at CRIGEN (GDF SUEZ research center dedicated to gas and new energies) to assess dent defects subject to fatigue loadings. A realistic computational model based on the finite elements method has thus been developed. An analytic formulation of the stress concentration factor (SCF) giving more realistic results has then been set, allowing a classical fatigue analysis with S-N curves to be performed. A specific software plug-in has been developed to automate the creation of the finite elements models. In these models dents are created at a realistic pressure with regard to the pipe characteristics. Then damaged pipe is exposed to a pressure variation to determine the maximal SCF. After validation based on tests results, a simulated experimental design which covers diameters from 8 inches to 36 inches and steels from A37 to X70 has been set. Different indentation tools and different strokes have also been defined, resulting in 1080 simulations. As a conclusion, this paper shows the method leading to the analytic formulation of the SCF allowing the fatigue assessment.

Recent failures in seam weld pipe have raised concerns within the pipeline industry over the integrity of such welded pipe. Low-Frequency (LF) Electric Resistance Welded (ERW) pipe manufactured prior to 1970, in particular, can be susceptible to failures caused by hook cracks, lack of fusion and other planar defects should the weld area exhibit low toughness. Integrity management regulations and Pipeline operators are evaluating potential methodologies to address and mitigate the LF-ERW seam weld threat. A program has been initiated at Williams Northwest Pipeline GP (NWPGP) to address the integrity management of its pre-70s ERW pipelines. In this case study, as part of an overall integrity management program, a hydrostatic test and fatigue analysis based methodology for addressing the LF-ERW seam weld threat is presented. The methodology was applied to 15 pre-1970’s natural gas pipelines. The results and findings are summarized in terms of the integrity threat mitigation and maintenance strategies.

Pipeline operators often deal with the large numbers of dents that require further consideration, and are required to prioritize these dents for further investigation and repair. Code guidance is clear on the relative severity of dents based on a depth or associated with welds, corrosion, gouging or cracking. It allows the pipeline operators to prioritize their dig lists and limit the number of “investigative digs” by omitting plain and shallow dents. However, the dent depth criterion has limitations, experiences learned in the past have shown that a dent prioritization based on depth alone, may still leave a significant number of dents in the pipeline which may pose a threat, particularly from local static strain and fatigue. In recent years, strain and fatigue analyses have been included in the assessment of severity of dents in order to better prioritize and effectively repair dents which represent a threat to the structural integrity of pipelines. However, strain analyses require tedious calculation of dent curvature at each data point from ILI reported dent profile. When a large numbers of dents require to be prioritized by strain severity, using this detail calculation is impractical. Therefore, a screening methodology is required to reliably identify candidate dents that require detail strain assessment. In this paper, an aspect-ratio based screening methodology is developed and applied to over 7,000 dents reported by an In-Line Inspection (ILI) caliper tool for a 265 miles long pipeline section. A total of 263 shallow dents which could have injurious strain level were identified and ranked for detail strain and fatigue calculations. In-ditch investigation of 20 dents with LaserScan profiling showed that the developed screening methodology provides an effective tool to capture all the dents with strain equal and 6.5% at 95% confidence level.

Reliability analysis has become widely used as a method of accounting for uncertainty in the sizing of metal loss features in pipeline integrity management programs. As inline inspection (ILI) technology for crack detection becomes more widely available, the opportunity to use reliability methods in a manner similar to that already adopted for metal loss features presents itself. Nevertheless, the technical challenges to the application of reliability analysis of cracks are distinct from those that are relevant to the reliability analysis of metal loss features. Calculating the time-dependent threat of failure due to fatigue or corrosion fatigue must address different parameters than it would for metal loss features, and consequently this presents new challenges in developing statistical analysis tools. Such challenges include predicting operational pressure cycling, accounting for uncertainty in ILI crack sizing, and characterizing crack growth behaviour type. This paper provides an overview of some important parameters to be considered in reliability-based fatigue or corrosion fatigue analysis with some examples of how they have been addressed in work to date by Dynamic Risk Assessment Systems, Inc.

Liquids pipelines undergo pressure cycling as part of normal operations. The source of these fluctuations can be complex, but can include line start-stop during normal pipeline operations, batch pigs by-passing pump stations, product injection or delivery, and unexpected line shut-down events. One of the factors that govern potential growth of flaws by pressure cycle induced fatigue is operational pressure cycles. The severity of these pressure cycles can affect both the need and timing for an integrity assessment. A Pressure Cycling Monitoring (PCM) program was initiated at Enbridge Pipelines Inc. (Enbridge) to monitor the Pressure Cycling Severity (PCS) change with time during line operations. The PCM program has many purposes, but primary focus is to ensure the continued validity of the integrity assessment interval and for early identification of notable changes in operations resulting in fatigue damage. In conducting the PCM program, an estimated fatigue life based on one month or one quarter period of operations is plotted on the PCM graph. The estimated fatigue life is obtained by conducting fatigue analysis using Paris Law equation, a flaw with dimensions proportional to the pipe wall thickness and the outer diameter, and the operating pressure data queried from Enbridge SCADA system. This standardized estimated fatigue life calculation is a measure of the PCS. Trends in PCS overtime can potentially indicate the crack threat susceptibility the integrity assessment interval should be updated. Two examples observed on pipeline segments within Enbridge pipeline system are provided that show the PCS change over time. Conclusions are drawn for the PCM program thereafter.

The performance of older ERW pipelines has raised concerns regarding their ability to reliably transport product to market. Low toughness or “dirty” steels combined with time dependent threats such as surface breaking defects, selective corrosion and hook cracks are especially of concern in hazardous liquid pipelines that are inevitably subject to cyclic loading, increasing both the probability and rate of crack growth. The existing methods of evaluating seam weld flaws by hydrostatically testing the pipeline or In-Line Inspection (ILI) with an appropriate technology are well established. Hydrostatic testing, whilst providing a quantified level of safety is often impracticable due to associated costs, logistics and the possibility of multiple failures during the test. ILI technologies have become more sophisticated and as a result can accurately detect and size both critical and sub-critical flaws within the pipeline. However, the vast amounts of data generated can often be daunting for a pipeline operator especially when tool tolerances and future growth are required to be accounted for. For either method, extensive knowledge of the benefits and disadvantages are required to assess which is the more appropriate for a particular pipeline segment. This paper will describe advances in the interpretation of seam weld flaws detected by ILI and how they can be applied to an Integrity Management Plan. Signal processing improvements, validated by in-field verifications have enabled detailed profiles of surface breaking defects at seam welds for ERW pipelines to be determined. Using these profiles along with established fracture and fatigue analysis methods allows for reductions in the unnecessary conservatism previously associated with the assessment of seam weld flaws detected by ILI. Combining these results with other available data, e.g. dig verifications, previous hydrostatic testing records, enables more realistic and better-informed integrity and maintenance planning decisions to be made. A real case study conducted in association with a pipeline operator is detailed in the paper and quantifies the benefits that can be realised by using these advanced assessment techniques, to safely and economically manage their assets going forward.

Buried medium and small diameter piping at road crossings and under access roads within plant or oil terminal boundaries that is not protected by any sort of casing presents inspection difficulties because access normally calls for a selective or total excavation of such piping. Often, only when an oil or gas leak is suspected to originate in the area of a road crossing, is such piping excavated and inspected. Sometimes such a finding will stimulate inspections at other selected road crossings as well as failure analysis of the pipe where the oil or gas leak was found. One of the root causes of piping failure at road crossings and under access roads may be the overburden (the soil cover above the buried pipe) and cyclic traffic load stresses that are increased in the carrier pipe because of local areas of reduced wall thickness, mechanical damage such as a dent or surface cracking. In-service corrosion or damage of the uncased pipe at road crossings can lead to integrity problems since the original design assumptions or safety margin for such piping may no longer hold. This paper reviews a typical approach to analyze buried pipe design at road crossings and adds selected API RP 579 procedures to illustrate the analysis of fatigue loadings. A brief note on common inspection practices and techniques for buried medium and small diameter piping at road crossings and within plant limits is also included.

This paper studies the fatigue problem of risers in marine platforms located in the Bay of Campeche, Mexico. Considering the future wave loads on these systems, the lifetime of these structures is obtained. Possibly, these systems have problems of fatigue in the zones where large stress concentrations appear. The risers fatigue analysis with a regular pipeline section is carried out taking into account operational and environmental load conditions. Due to operation, among others, the risers are subject to internal pressure, thermal expansion, and the type of fluid that transports. In order to take into account the environmental conditions one it is considered that the riser is subject to waves, current and ground movement, which were suitably modeled in the analysis. The supports on the platform, in the riser zone, were considered. They take into account the movements and their corresponding lateral stiffness. To evaluate the load cycles to which the risers are subject, for the study zone, the annual rates of occurrence and all the sea states have been considered. In addition, all the possible heights and directions of the wave that might be present in the Bay of Campeche have been considered the accumulated damage caused by the diverse cycles of stresses by means of the Palmgren Miner rule are assessed. This study includes the analysis of representative number of risers with different diameters and recent inspections. This allows us to know, in an approximated way, the present condition of the risers; it an give us suitable information for the calculation of the fatigue reliability and therefore define the present levels of structural safety of these facilities.

Extruded outlet headers are furnished as special fittings to the various fittings specifications. These specifications provide the controls for materials and manufacturing necessary to produce the fittings. Some of these specifications are complete while others refer to ASTM (or CSA) standards for part of the control. The piping codes provide the rules for design of the outlets. Most of the codes have a special section covering the reinforcing rules for extruded outlets. These rules require control of dimensions and radii, and require that the reinforcing be integral with the body of the fitting and cover the complete circumference. The outlet radii must be produced by forming, rather than by machining of grinding. For qualified fittings, the amount of reinforcing required can be reduced for outlet diameters smaller than the run. Some of the piping codes require integral reinforcing for the more critical applications. The piping codes also provide Stress Intensification Factors (SIF’s) to be used for piping fatigue analyses. Although the codes give SIF’s for many types of outlets, they do not provide guidance for properly reinforced outlets made per the rules for extruded outlets. Taylor Forge (TF) has run tests over the years demonstrating that properly reinforced extruded outlets, will not only pass burst tests as required by fittings specifications, they also have SIF’s approaching those listed for tees. (B31.3 recently increased the SIF’s for tees with smaller crotch radii, and TF tests indicate that properly reinforced extruded outlets are better than indicated by the new rules for tees.) Reasons to consider extruded outlet headers for pipeline service are: • Versatility of size, spacing, number of outlets, etc.; • Low stress concentrations, fatigue resistant construction; • Radiographable butt weld connections; • Design and analysis rules are code qualified; • Field work is minimized.

Pipeline dents are common occurrences that have a potential integrity threat to the system. Dents are typically found through in-line inspections, and historically, low-resolution in-line inspection geometry tools were used to find the locations of dents. These tools gave little information about shape, orientation or other dent features. Newer ‘high-resolution’ tools give a much clearer picture of the dent shape, location, orientation and location of welds. This information has been previously unavailable and has enabled dent integrity assessment with much greater accuracy and confidence. However this still leaves the question of how to best address the information from older, low-resolution inspection tools. In the past, CSA Z662 required that all dents with a deflection greater than 6% or that contained stress concentrators, including welds, had to be repaired. In the newly published 2003 edition of CSA Z662, dents can be assessed by an engineering assessment to determine their acceptability. Historical evidence has shown that dents less than 6% can also be subject to failure under certain conditions, and is indicated in the notes of CSA Z662-03 10.8.2.4.2. Dents that contain stress concentrators, including corrosion, welds and cracks must be given special consideration, however often little information is available for the dent from solely a geometry tool. TransCanada PipeLines Limited has been involved in the development of a dent assessment methodology for several years. Based on the 2003 revisions to CSA Z662, TransCanada has started to implement a dent integrity management program. This paper discusses the approach taken by TransCanada: to create a database of dent features, classification of dents, finite element analysis (FEA) to determine cyclic stress spectra, fatigue analysis, validation through dig programs, and the management of these features from a system integrity standpoint.