Support requirements for unburied spans on existing pipelines can be difficult to assess. An approach for investigating the adequacy of support of unburied spans is presented. It begins with a screening based on theoretical longitudinal stress evaluation. An estimate of the allowable free span lengths between supports is calculated, considering the weight of the pipe and its contents, longitudinal stresses from internal pressure, thermal stresses, and steady wind loading. Existing span lengths are compared to the allowable length and any segments exceeding the limit are flagged for further study. Review by finite element analysis and comparison with survey data for the identified spans is then performed. Comparison of the FEA model deflections and survey data can help determine whether “pre-existing stresses” or plastic deformations are present, due, for example, to installation procedures or ground movement. The FEA results provide a factor of safety that can be used to help identify potential safety issues and prioritize mitigation efforts.

This paper presents findings from a study conducted as part of a joint industry effort involving engineers from Williams Midstream, Stress Engineering Services, Inc., GL Noble Denton, and Saipem America. The purpose of this study was to evaluate the severity of damage inflicted to Williams’ subsea 18-inch × 0.875-inch, Grade X60 Canyon Chief Gas Export Pipeline due to an anchor impact at a water depth of 2,300 feet. The phases of work included an initial assessment after the damage to the deepwater pipeline was detected, evaluating localized damage via finite element analysis based using in-line inspection data, full-scale destructive testing including burst tests, and final efforts included the design and evaluation of a subsea-deployed repair sleeve. The study included modeling Saipem’s repair sleeve design accompanied by full-scale destructive testing. Strain gages were used to measure strain in the reinforced dent beneath the sleeve, that were then compared to prior results for the unrepaired dent test results. The work associated with this study represents one of the more comprehensive efforts conducted to date in evaluating damage to a subsea pipeline. The results of the analysis and testing work provided Williams with a solid understanding on the behavior on the damage inflected to the pipeline and what level of performance can be expected from the repaired pipeline during future operation. After the engineering analysis and testing phases of this work were completed, the deepwater pipeline was repaired.

A significant amount of effort has been expended in the area of advancing pipeline dent remaining life assessment methods beginning in the late 1980s and extending to the current day. Initial research efforts were primarily empirical in nature while more recent research efforts have incorporated finite element modelling. Coupled with advancements in assessment techniques, the capabilities of advanced in-line inspection (ILI) tools have increased to a point where they can provide consistent, reliable information that is suitable for dent assessments. As a result of these advancements in assessment models and ILI tools, operators can now perform remaining life assessments using ILI data, and a multitude of remaining life assessment models are available, including solutions from the European Pipeline Research Group (EPRG), Pipeline Research Council International (PRCI), American Petroleum Institute (API), and finite-element based approaches. In addition to these remaining life assessments, many operators routinely perform strain-based assessments based on guidance from ASME B31.8. To date, there have been few studies comparing the various assessment methods on large numbers of dents, and as a result, significant questions persist as to the conservatism inherent in each method. In addition, the EPRG and PRCI methods are largely based on full-scale testing and finite-element models performed with idealized indenter shapes while actual pipeline dents typically exhibit complex shapes and interactions between multiple dents. Each model also has limitations and advantages that are discussed in this paper, such as ease of use and how pipeline geometry and weld association are considered. This paper provides a robust comparison of selected dent assessment methodologies on 220 actual dents from a 24-inch pipeline with depths ranging from 0.6–4.5% OD, and 32 dents from a 30-inch line with depths ranging from 1–2.5% OD. The assessment includes both top and bottom of line dents and investigates the influence of restraint on remaining life. The results presented in the paper are based on high-resolution ILI caliper data collected during two in-line inspections. Furthermore, the paper provides statistical comparisons between strain and remaining life methodologies and also between the various remaining life assessments. The paper also provides a comparison of the restraint parameter from the PRCI model with calculated stress concentration factors from finite-element models. The paper provides a first of its kind comparison of the various methods and discusses how the work may be extended to other pipe diameters and wall thicknesses.

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.

Manufacturing flaws are common on low frequency electric resistance welded pipe (LF-ERW), such as hook cracks and lack of fusion. One less commonly identified flaw is a lamination that terminates at the ERW weld. This interaction results in a unique flaw geometry, occurring mostly at the pipe’s mid-wall thickness but can vary in severity along the length of the affected pipe joint. This paper discusses the significance of this flaw geometry, the method of formation, and the impact of typical operations (fatigue cycling and hydrostatic testing) on the strains along the indication. First, a comprehensive set of metallurgical examinations were performed, including metallography along the flaw. Examination included light microscopy, scanning electron microscopy, electron dispersive x-ray spectroscopy, and sub-load hardness testing (HV0.5) along the flaw. This set of examinations helped identify the flaw and how it formed. Second, several tensile straps were prepared across the indication to better understand how the feature behaves during the operation of the pipeline. Each tensile specimen was monitored using a digital image correlation (DIC) system with a focus on the ERW seam weld. Tensile straps were prepared and tested to understand the influence of a hydrotest on normal operations by completing the following three phases of simulated operations: (1) normal operations with fatigue cycling at stress ranges equivalent to operating pressures between 6% SMYS and 56% SMYS (MOP), (2) a spike hydrostatic test at stresses equivalent to an operating pressure of 80% SMYS, and (3) normal operations with fatigue cycling at stress ranges equivalent to operating pressures between 6% SMYS and 56% SMYS (MOP). This sequence of testing permitted comparison of the strains at the flaw before and after the simulated hydrotest conditions. The comparison between the strains recorded via the DIC system showed an increase in strain of approximately 40% across the feature in the sample subjected to the spike test, which included an irrecoverable strain of approximately 0.06%. The evaluation process described in this paper provided a valuable understanding of the impacts of a hydrostatic test on manufacturing flaws. This type of evaluation can be used to better understand the effect of spike hydrostatic testing on other common planar flaws including ERW hook cracks, ERW lack of fusions, and stress corrosion cracking.