Current federal regulations in the U.S. require excavation of plain dents identified through in-line inspection surveys based primarily on depth. Industry experience, and previous research, has shown that the depth of the dent, alone, is not sufficient to assess dent severity and that releases could occur at dents below the excavation threshold (Dawson, 2006). Canada’s National Energy Board released a safety advisory on June 18, 2010, to all companies under their jurisdiction regarding two incidents involving shallow dents. The safety advisory stated that all integrity management programs should be reviewed and updated where appropriate to address the threat posed by shallow dents. Similar incidents have raised awareness in the United States and elsewhere around the world. This paper focuses on the fitness for service of dents identified by in-line inspection surveys. The fitness for service assessment provides an estimated remaining life of a dent based on the geometry of the dent and current pressure cycling of the pipeline. Dynamic pressure cycling at each dent location is estimated using the upstream and downstream pressure cycle data, elevation, and distance along the pipe. The dynamic pressure cycle data at each dent is then converted into equivalent stress cycles based on the results of rainflow cycle counting. Maximum strain levels of the dents are calculated based on the geometry of the dent as determined by radial sensor measurements from the in-line inspection survey. The combination of assessment methods provides estimates of remaining fatigue life and peak strain which can be used for prioritizing the investigation and remediation of plain dents in pipelines. Finite element analysis (FEA) is performed for one dent to calculate the maximum strain levels and identify stress concentration areas. These results are compared with the values applied during the fitness for service assessment to validate the accuracy and conservatism of the calculation methods used. An idealized dent will be analyzed to investigate the strain calculations in ASME B31.8 and localize maximum strain values.
Annually or as events occur, operators submit data to various regulatory agencies about the operation, maintenance and extent of their assets. Many of these figures are used by the public, non-profit organizations and private companies to independently conduct assessments about operators, ranging from safety to quality assurance to scope and nature of product deliveries. The Pipeline Hazardous Materials Safety Administration (PHMSA), the National Energy Board (NEB), and other industry organizations have recently put an emphasis on more meaningful metrics by releasing guidelines and leading discussions at industry conferences and workshops. In order to derive more strategic accuracy and pertinence, Explorer Pipeline Company (Explorer) and Det Norske Veritas (U.S.A.), Inc. (DNV GL) have developed a procedural effort to develop meaningful metrics. Several derivative benefits come from this effort such as support for calculating cost-benefit / ROI figures for maintenance projects, justification for compliance-plus activities and, most importantly, a more informed perspective of operational risk. A renewed approach to this effort is to organize the more meaningful factors into three categories: (1) Metrics of job roles and tasks within Explorer’s Asset Integrity staff, (2) Other existing influential metrics (3) Regulatory metrics. Using this approach, Explorer defined well-targeted, unitized metrics, each with a meaningful basis. Explorer anticipates the development of these more meaningful metrics to support the transparency sought by regulators and other stakeholders, benchmark and continually evaluate our Asset Integrity program and possibly support the development of practical metrics for the pipeline industry.
Pipeline operators are often faced with excavating deformations caused by bottom-side indenters (e.g., rock dents). These dents are typically constrained by the rock, but during excavation, after the rock is removed, the dent is no longer constrained. Many operators have felt that it is prudent to perform in-the-ditch (ITD) non-destructive examination (NDE) techniques, such as liquid penetrant testing (LPT) and magnetic particle inspection (MPI), to determine if external cracking is present so that an appropriate repair method can be selected. Unfortunately, these external surface NDE methods do not identify the presence of internal cracking. Recent research [1], along with metallurgical analyses of cracks at bottom-side dents, demonstrates that the fatigue behavior of constrained dents is different than that of unconstrained dents, and that identifying the correct crack mechanism can be difficult. The paper discusses cracking mechanisms (e.g., stress corrosion cracking, fatigue, etc.) at bottom-side dents, ITD crack identification methods, differences between constrained and non-constrained dents, repair methods for dents, and presents a case study that uses NDE (MPI, unconventional LPT, and laser scanning) and destructive techniques (metallography, fractography, and hardness testing) to determine the metallurgical cause of a failure. The case study involves a pre-formed composite sleeve system that was used to repair dents in which correct installation procedures were followed but ultimately resulted in a delayed in-service failure. In hindsight, if ITD NDE methods were chosen based on our current knowledge of recent research, the operator may have been aware of the presence of cracking and selected a different repair method, and therefore would have likely prevented an in-service failure. This paper provides a case study to help increase awareness regarding how to properly evaluate cracking in dents. Operators should ensure that their excavation and repair procedures are updated to reflect the most current industry knowledge to help prevent a similar failure.
Dents in buried pipelines either caused by third party mechanical damage or introduced during pipeline construction remain a leading contributor to reportable pipeline releases. API Recommended Practice 1183 (API RP 1183) provides a modern, shape-based fatigue life assessment of pipeline dents that can be incorporated into a pipeline operator’s integrity management program. Specifically, in API RP 1183, dent shape information is obtained by analyzing in-line inspection (ILI) caliper data and is expressed using characteristic lengths and areas. Once obtained, these characteristic lengths and areas define not only the dent restraint condition, but also various fatigue growth parameters. API RP 1183 provides multiple screening techniques that are intended to identify dents that are non-injurious and therefore do not require additional detailed assessments or response actions. These screening techniques both increase in complexity and decrease in conservatism. Three screening processes provided in API RP 1183 are: (1) a table with lower bound, conservative estimates of fatigue life, (2) a pipe geometry and spectrum severity indicator (SSI) approach, and (3) a pipe geometry and operational spectrum approach. When combined with the shape-based fatigue life assessment, multiple analysis approaches are described. However, as more dents are being analyzed with the methods from the first edition of API RP 1183, discrepancies between the screening methods and the shape-based approach are being observed. The aim of this paper is to discuss those cases where the conclusions from the screening processes and the shape-based assessment are inconsistent. In other words, there are cases where the screening process “passes” a dent indicating the dent is non-injurious and can be monitored while the shape-based assessment dictates the dent is injurious and a response should be taken. After discussing the inconsistencies in these cases, the authors make recommendations on how operators should use API RP 1183 in its current form.
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