Study on Transition of Stress Corrosion Cracking to Sharp Edge Corrosion for a Liquids Pipeline

Although stress corrosion cracking (SCC) growth is attributed to the synergistic effects of stress and corrosion, these two factors can just as easily become competing mechanisms, with stress cycles driving growth (hydrogen, the by-product of corrosion, may facilitate the growth), and corrosion working to blunt the crack tip and arrest growth. It follows that reducing the maximum pressure and cycling severity can slow down the crack growth or even stop it, and aggressive corrosion can further blunt the sharp crack tip. The Authors have observed, on a particular Polyethylene (PE) tape coated pipeline, instances where SCC has exhibited a propensity to corrode and convert into sharp edge corrosion. This is attributed to the combined effects of limited corrosion protection and low stresses. The focus of the paper is to assist operators in recognizing this phenomenon and integrate lessons learned into pipeline integrity management strategies.

Using Finite Element Analysis to Prioritize ILI Calls for Combined Features: Dents in Bends

Pipeline in-line-inspections (ILI) are used to assess and track the integrity of pipelines, aiding in identifying a variety of features such as: metal loss, dents, out-of-roundness, cracks, etc. The presence of these features can negatively affect the operation, integrity, and remaining life of a pipeline. Proper interpretation of the impacts these features may have on a pipeline are crucial to maintaining the integrity of a pipeline. Several codes and publications exist to assess the severity of these features under known operating conditions, either through empirical formulations or more detailed analysis, in order to aid the operator in determining a corrective action plan. These empirical formulations are generally applicable to assess a singular defect but require a more detailed assessment to evaluate combined defects (i.e. dent in a bend). These detailed assessments typically require a higher level numerical simulation, such as Finite Element Analysis (FEA). This detailed FEA can be quite costly and time consuming to evaluate each set of combined features in a given ILI run. Thus, engineering judgement is critical in determining a worst-case scenario of a given feature set in order to prioritize assessment and corrective action. This study aims to assess dent features (many associated with metal loss) occurring in a pipe bend to determine a worst-case scenario for prioritization of a given feature listing. FEA was used to simulate a field bend of a given radius and angle in order to account for residual stresses in the pipe bend. A rigid indenter was used to form a dent of the approximate length, width, and depth from the ILI data. Separate models were evaluated considering the dent occurring in the intrados, extrados, and neutral axis of the pipe bend to evaluate the worst-case scenario for further assessment. The resulting stresses in the pipe bend-dent geometry, under proper loading were compared to the same dent scenario in a straight pipe segment to develop a stress concentration factor (SCF). This SCF was used in the API 579-1/ASME FFS-1 Fitness for Service (API 579) [1] methodology to determine the impact on the remaining life of the combined features.

Determining Pipeline Depth of Cover at River Crossings by Data Integration

Having sufficient depth of cover ensures pipeline protection and is a regulatory requirement. Confirming the pipeline depth of cover on dry land is generally easy and produces accurate results. However, determining the pipeline depth of cover at a river crossing can be problematic because of accessibility difficulties and the increased measurement errors from aboveground surveys. The difficulty of determining the pipeline depth of cover at river crossings can be resolved by integrating both the aboveground survey data and the inline inspection data. By comparing both sets of data, errors from both above survey data and inline inspection data can be detected. This paper describes watercourse management, aboveground DOC surveys, and a spreadsheet based tool developed for both the quick verification of aboveground survey results, and the calculation of the true DOC at water crossings without needing to set new GPS tie-points on both banks of the crossing and running a new ILI.

Achieving North American Record for Longest Intelligent Inspection of a Natural Gas Pipeline

TransCanada was faced with a significant challenge to inspect a 941 km NPS 48 pipeline. The options for the inline inspection (ILI) were multiple segments which would cause an increased cost with new pigging facilities required and a delay to the ILI schedule or attempt to pig the longest natural gas pipeline section in North America. The extraordinary proposal would require a massive 48″ combination Magnetic Flux Leakage (MFL) tool to traverse a high-speed gas pipeline 941km from Burstall, Saskatchewan to Ile des Chenes, Manitoba, Canada. Given the alternative of the installation of 3 additional launcher and receiver stations and the risk to overall project schedule from extended inspection operations, TransCanada took the bold decision to perform an MFL inspection in a single pass. However, as expected, this option created a new set of challenges to guarantee first run success in one of the harshest environments for an ILI tool and in a line where the cleanliness condition was unknown. This last factor, was a critical concern as the volumes of debris that could be collected with the highly aggressive MFL tool brushes, could easily and very quickly have led to very significant debris build up during inspection that at best would likely cause degraded data leading to an unwanted re-run and at worst the possibility of a stuck pig and subsequent retrieval program. From a project perspective either occurance was considered to be mission critical — if either occurred there was no easy solution to collecting the much needed condition data of the pipeline. In July 2017, a successful VECTRA HD GEMINI inspection was completed. This paper discusses the main program risks, mitigation steps taken over and above a standard ILI run. Key considerations and actions taken relating to additional engineering and tool modifications to various components of the inspection vehicle itself will be discussed. Lastly, insight will be given into an extensive smart cleaning program developed with the ILI vendor, using a combination of mechanical cleaning associated and debris level assessment, specifically designed and tailored for the project to ensure that the pipeline was both ready for ILI and that cleaning had reached optimum for ILI so that full, high quality MFL data would be collected the first time.

Assessment of Pipeline Spiral Weld Cracks Subjected to Internal Pressure

Spirally welded pipelines can make up significant portions of operator transmission systems, and may contain manufacturing anomalies that are susceptible to fatigue growth. Modifications to inputs of crack assessment models, such as CorLAS®, are required to account for the angle these cracks make with respect to the longitudinal pipe axis, given that these crack assessment models were developed for longitudinally orientated cracks. Two such modifications were investigated and are discussed in this paper. One approach considered the normal stress component perpendicular to the angled crack, for which a stress transformation calculator was developed. Another approach, adapted from API 579 and BS7910 standards, used an effective crack length calculated as the longitudinal projection of the full length of an angled crack. Failure pressures calculated using these approaches were compared to validated finite element (FE) results. For both modifications, the pressure capacity increased for angled cracks versus longitudinal cracks. The transformed normal stress approach resulted in non-conservative failure pressure predictions with respect to the FE models, whereas the modified crack length approach was conservative. Additionally, the extended finite element method (XFEM) was used to investigate the propagation behavior of angled cracks. It was found that the general tendency was for propagation parallel to the longitudinal pipe axis; however, when considering weld residual stresses, the crack propagation would be directed toward the direction of the spiral seam.

Development of Pinhole Corrosion Management Using MFL

While In-line Inspection Magnetic Flux Leakage (MFL) tools have been used for many years to successfully manage corrosion related threats, small pinhole-sized metal-loss anomalies remain a significant concern to pipeline operators. These anomalies can grow undetected to develop leaks and cause significant consequences. The physical dimensions of these anomalies, their proximity to and/or interaction with other nearby anomalies can challenge MFL’s detection and sizing capabilities. Other factors such as tool speed, cleanliness of the line and incorrect assumptions have an impact as well. For pipeline operators to develop effective and efficient mitigation programs and to estimate risks to an asset, the underlying uncertainties in detection and sizing of pinholes need to be well understood. By using magnetic modeling software, the MFL response of metal-loss anomalies can be determined, and the effect of a number of factors such as radial position, wall thickness, depth profile, pipe cleanliness and tool speed on MFL response and reporting accuracy can be determined. This paper investigates these factors to determine the leading causes of uncertainties involved in the detection and sizing of pinhole corrosion. The understanding of these uncertainties should lead to improvements in integrity management of pinhole for pipeline operators. This paper first investigates the physical measurement methodology of MFL tools to understand the limitations of MFL technology. Then, comparisons of actual MFL data with field excavation results were studied, to understand the limitations of specific MFL technologies. Finally, recommendations are made on how to better use and assess MFL results.

Application of Wide-Band Ultrasound for the Detection of Angled Crack Features in Oil and Gas Pipelines

This paper describes a novel technique for the detection of cracks in pipelines. The proposed in-line inspection technique has the ability to detect crack features at random angles in the pipeline, such as axial, circumferential, and any angle in between. This ability is novel to the current ILI technology offering and will also add value by detecting cracks in deformed pipes (i.e. in dents), and cracks associated with the girth weld (mid weld cracks, rapid cooling cracks and cracks parallel to the weld). Furthermore, the technology is suitable for detection of cracks in spiral welded pipes, both parallel to the spiral weld as well as perpendicular to the weld. Integrity issues around most features described above are not addressed with ILI tools, often forcing operators to perform hydrostatic tests to ensure pipeline safety. The technology described here is based on the use of wideband ultrasound inline inspection tools that are already in operation. They are designed for the inspection of structures operating in challenging environments such as offshore pipelines. Adjustments to the front-end analog system and data collection from a grid of transducers allow the tools to detect cracks in any orientation in the line. Description of changes to the test set-up are presented as well as the theoretical background behind crack detection. Historical development of the technology will be presented, such as early laboratory testing and proof of concept. The proof of concept data will be compared to the theoretical predictions. A detailed set of results are presented. These are from tests that were performed on samples sourced from North America and Europe which contain SCC features. Results from ongoing testing will be presented, which involved large-scale testing on SCC features in gas-filled pipe spools.

Do We Need a Safe Excavation Pressure for Dented Pipelines: How Should it Be Defined?

Pipeline operators’ utmost priority is to achieve high safety measures during the lifecycle of pipelines including effective management of integrity threats during excavation and repair processes. A single incident pertaining to a mechanical damage in a gas pipeline has been reported previously which resulted in one fatality and one injury during investigation. Some operators have reported leaking cracks while investigating rock induced dents. Excavation under full operating pressure can lead to changes in boundary conditions and unexpected loads, resulting in failure, injuries, or fatalities. In the meantime, lowering operating pressure during excavation can have a significant impact on production and operational availability. The situation poses two conflicting objectives; namely, maximizing safety and maximizing operational availability. Current pipeline regulations require that operators have to ensure safe working conditions by depressurizing the line to a level that will not cause a failure during the repair process. However, there are no detailed guidelines on how an operator should determine a safe excavation pressure (SEP) level, which could lead to engineering judgment and subjectivity in determining such safety level. While the pipeline industry relies on well-defined fitness for purpose analyses for threats such as crack and corrosion, there is a gap in defining a fitness for purpose for dents and dents associated with stress riser features in order to set an SEP. Stress and strain based assessment of dents can be used in this matter; however, it requires advanced techniques to account for geometric and material nonlinearity. Additionally, loading and unloading scenarios during excavation (e.g. removal of indenter, overburden pressure, etc.) drive a change in the boundary conditions of the pipe that could lead to leakage. Nevertheless, crack initiation or presence within a dent should be considered, which requires the incorporation of crack geometry and application of fracture mechanics in assessing a safe excavation pressure. Recently, there have been advancements in stress and strain based finite element analysis (FEA) of dents coupled with structural reliability analysis that can be utilized to assess SEP. This paper presents a reliability-based approach to determine a safe excavation pressure for dented liquid pipelines. The approach employs nonlinear FEA to model dents interacting with crack features coupled with uncertainties associated with pipe properties and in-line-inspection information. A fracture mechanics-based limit state is formulated to estimate the probability of failure of dents associated with cracks at different levels of operating pressure during excavation. The application of the developed approach is demonstrated through examples within limited scope. Recommended enhancements and future developments of the proposed approach are also discussed.

Structure Assessment Using Instrumented Indentation: Strength, Toughness and Residual Stress

The instrumented indentation technique (IIT) is a novel method for evaluating mechanical properties such as tensile properties, toughness and residual stress by analyzing the indentation load-depth curve measured during indentation. It can be applied directly on small-scale and localized sections in industrial structures and structural components since specimen preparation is very easy and the experimental procedure is nondestructive. We introduce the principles for measuring mechanical properties with IIT: tensile properties by using a representative stress and strain approach, residual stress by analyzing the stress-free and stressed-state indentation curves, and fracture toughness of metals based on a ductile or brittle model according to the fracture behavior of the material. The experimental results from IIT were verified by comparing results from conventional methods such as uniaxial tensile testing for tensile properties, mechanical saw-cutting and hole-drilling methods for residual stress, and CTOD test for fracture toughness.

Development of an Industry Test Facility and Qualification Process for ILI Technology Evaluation and Enhancements: Performance Evaluation Phase

The project “Development of an Industry Test Facility and Qualification Processes for in-line inspection (ILI) technology Evaluation and Enhancements” aims to expand knowledge of ILI technology performance and identify gaps where new technology is needed. Additionally, this project also aims to provide ILI technology developers, researchers and pipeline operators a continuing resource for accessing test samples with a range of pipeline integrity threats and vintages; and inline technology test facilities at the Technology Development Center (TDC) of Pipeline Research Council International, Inc. (PRCI), a PRCI managed facility available for future industry and PHMSA research projects. An ILI pull test facility was designed and constructed as part of this project based on industry state-of-the-art and opportunities for capability improvement. The major ILI technology providers, together with pipeline operator team members, reviewed the TDC sample inventory and developed a series of ILI performance tests illustrating one of multiple possible research objectives, culminating in 16-inch and 24-inch nominal diameter test strings. The ILI technology providers proposed appropriate inspection tools based on the types of the integrity threats in the test strings, a series of pull tests of the provided ILI tools were performed, and the technology providers delivered reports of integrity anomaly location and dimensions for performance evaluation. Quantitative measures of detection and sizing performance were confidentially disclosed to the individual ILI technology providers. For instances where ILI predictions were outside of claimed performance, the vendors were given a limited sample of actual defect data to enable re-analysis, thus demonstrating the potential for improved integrity assessment with validation measurements. In this paper, an evaluation of the ILI data obtained from repeated pull-through testing on the 16 and 24-inch pipeline strings at the TDC is performed. The resulting data was aligned, analyzed, and compared to truth data and the findings of the evaluation are presented.