Using ILI low field technology to reveal metallurgical anomalies

Transcription

1 Using ILI low field technology to reveal metallurgical anomalies by Todd Mendenhall and Tod Barker T. D. Williamson, Inc., Salt Lake City, UT, USA Pipeline Pigging and Integrity Management Conference Marriott Westchase Hotel, Houston, USA February, 2014 Organized by Clarion Technical Conferences and Tiratsoo Technical and supported by The Professional Institute of Pipeline Engineers

2 Proceedings of the 2014 Pipeline Pigging and Integrity Management conference. Copyright 2014 by Clarion Technical Conferences, Tiratsoo Technical (a division of Great Southern Press) and the author(s). All rights reserved. This document may not be reproduced in any form without permission from the copyright owners. 2

3 Using ILI low field technology to reveal metallurgical anomalies MAGNETIC FLUX LEAKAGE (MFL) technology has been used for a number of decades as a primary technology for inline inspection (ILI). Ongoing ILI developments have produced additional magnetic field technologies that have the potential to greatly augment the capabilities of ILI tools to identify and categorize threats to pipeline integrity. In particular, the residual field and low field technologies utilize second and third quadrant magnetic responses to identify metallurgical anomalies that can accompany mechanical damage or result from manufacturing imperfections. When low field technologies are combined with the traditional MFL and deformation (DEF) technologies in a concurrent data acquisition, a greater potential for identifying interactive threats and an improvement on the traditional ILI capabilities for identifying and characterizing pipeline features emerge. The combined data sets from these technologies highlight the unique contributions of low field data in identifying dent re-rounding, dents with metal loss, and differentiating cold working from corrosion metal loss. This paper presents an additional benefit of the low field data as part of a method for creating a pipe signature and magnetic fingerprint for individual pipe joints which can be used to facilitate a discrepancy analysis of pipeline component records. Low field technology offers what is perhaps the most effective dataset for identifying unique metallurgical anomalies that can indicate the history of a single segment of pipe. The demonstrated successes of low field technology confirm that the inclusion of additional magnetic field technologies within a single tool continues to provide significant advances in the quality and quantity of information that can be gathered with inline inspection tools. PIPELINES PROVIDE AN EFFICIENT and safe method for transporting materials, but they are not immune to threats or damage that may result in an incident that impedes or precludes product transmission. Part of a pipeline integrity program includes inline inspection (ILI) to identify emerging and new integrity threats. Traditional ILI tools have been equipped with magnetic flux leakage (MFL) and deformation (DEF) technologies to detect geometric metal loss and disturbances to the pipeline curvature, respectively. These technologies continue to be primary ILI technologies, but as pipeline integrity programs become more sophisticated and move beyond merely identifying anomalies to classification, improved characterization and prioritization, there is a need for additional ILI technologies. As multiple ILI technologies are applied to a pipeline, the identification and characterization of features will be accomplished with greater accuracy and the ability to discriminate between minor and significant threats is greatly enhanced. The greatest benefit in applying multiple ILI technologies to a single pipeline is achieved when the data from each technology can be accurately combined and correlated to all other datasets. The combination of multiple datasets is best achieved with a single tool run that can map and merge the data from each technology so that the unique perspectives of each dataset can be overlaid to provide a complete picture of the pipeline features. Background The standard MFL technology consists of a magnetic circuit that magnetizes the pipe wall axially and identifies metal loss by detecting magnetic flux leakage at the point of defect. In an effort to improve the metal loss detection and sizing capabilities of ILI tools, systems that utilize alternate magnetic field orientations have been developed. For example, circumferential field orientations are employed in Transverse Field Inspection (TFI) technology and oblique magnetic field orientations have been employed in the SpirALL MFL. Alternate field orientations offer additional views of metal loss features, but share a common approach in magnetically saturating the pipe wall in the region of inspection. Figure 1 shows an illustration of a magnetic hysteresis curve for permeable material, such as a steel pipe wall, and identifies the saturation region. The high levels of magnetization that are required to saturate a segment of pipe wall will produce a flux leakage signal at sites of volumetric metal loss, due to the localized reduction in flux carrying capacity. Consequently, these technologies produce what is referred to as the geometric signal since they identify geometric metal loss anomalies in the metal structure. 3

4 Saturation Magnetizing Force (H) Figure 1. Hysteresis curve. The effectiveness of the high field level technologies is a result of the high flux density in the pipe wall that becomes very sensitive to volumetric metal loss and, consequently, very good at identifying such features. In a broad sense, the MFL technologies identify localized variations in flux carrying capacity or permeability. Permeability describes the relationship between the magnetic flux density and the magnetizing force and is typically represented as: B = μh (1) B: Flux Density μ: Permeability H: Magnetizing Force The permeability (μ) is expressed in (1) as a scalar and may seem to suggest that it is a constant within the material. This is true of some materials; however, the permeability in a carbon steel pipe wall can be slightly different at any point in the pipe wall, and may in fact be directionally dependent. This concept of variable permeability is captured in the more accurate representation of permeability as a tensor: B = μ H (2) The more subtle permeability variations within the material are not captured in traditional MFL technologies because these variations are not apparent at magnetic saturation. When the magnetic field levels are below the saturation level for the pipe material, the flux density is a function of both the H field and the permeability of the material. However, at the high field levels required for saturation the flux density is largely a function of the H field and any effects from subtle variations in the material are lost. One source of subtle permeability variation in pipeline steel is the residual stress level in the material. An illustration of the first quadrant of the hysteresis curve for stressed and unstressed carbon steel is shown in Figure 2. There is a distinct difference in the BH curves for relatively low magnetizing field levels; however, when the magnetizing field levels reach 120 Oe or higher (saturation) there is no discernible difference in the flux density in stressed and unstressed material. 4

5 Figure 2. First quadrant magnetization curves for stressed and unstressed carbon steel. (Simek 2006) It is apparent that a low field MFL technology would offer information about the pipeline material that neither DEF nor axial MFL would detect. For this reason, a low field technology is among the critical ILI datasets for improved feature discrimination and prioritization. Low field technology provides a nominal magnetizing field of 40 Oe compared to greater than 120 Oe for a traditional axial MFL. As illustrated in Figure 2, the flux density in the pipe wall at only 40 Oe is still considerable and flux leakage due to volumetric metal loss will still be present during an inspection. In order to distinguish the flux leakage due to permeability changes from the flux leakage due to volumetric metal loss, a technique referred to as decoupling is used. Decoupling has been discussed in detail by other authors and is a method for removing the geometric portion of the signal so that all that remains are the effects of permeability variation. The typical decoupling formula (Simek 2006) is expressed as: MFL MAG = MFL MIX SF MFL GEOM (3) MFL MAG : Portion of the signal resulting from permeability changes in the material (MAG) MFL MIX : Total low field technology signal including geometry signal and magnetic deformation (MIX) SF : Scaling factor, the high field and low field levels are normalized by scaling the high field data MFL GEOM : High field signal detects the volumetric metal loss or geometric deformation (GEOM) The subtle permeability changes within a material are referred to as magnetic deformation. These terms suggests that the permeability has been modified or altered from its original state in such a way that it can be distinguished from the material around it. The capability of low field technology to detect the magnetic deformation in a material makes it an attractive technology for ILI. One of the original purposes of low field technology for ILI was the detection of hard spots that are associated with residual stresses. The localized hardening of the pipeline material has been a concern for several years due to a number of pipeline incidents attributed to these types of anomalies. An ILI technology that can identify and characterize the magnetic deformation associated with hard spots may address potential regulatory concerns for pipelines that are susceptible to these types of anomalies. One of the challenges in using low field data to learn useful information about a pipeline is that residual stresses can result from any number of mechanisms. It is the context around the residual 5

6 stresses that allow for a reasonable classification of an identified pipeline feature. The common mechanisms for the creation of residual stresses in pipeline material include: Work Hardening The phenomenon of work hardening in steel has been thoroughly studied and the ability of this mechanism to produce residual stresses is well understood. As steel is deformed, a stress field is created in the material as the crystalline structure of the material is rearranged or distorted. For a limited range of deformation the material can return to its original state when the deforming force is removed. This limited range is referred to as the elastic range since the steel will spring back to its original state. As long as the steel is not permanently deformed the stress field has not exceeded the yield point or elastic limit and there is no perceivable history of the stress event in the material. If the deforming force is sufficient, the stress field in the material will exceed the elastic limit and there will be permanent alterations to the crystal structure and to the overall shape of the metal. When the deforming force is removed the steel attempts to spring back, but the alterations to the overall shape and the crystalline structure are too severe to reverse. Consequently, the spring-back produces tensile and compressive residual stresses in the severely deformed regions. In addition to the residual stresses, the steel hardens and is more resistant to subsequent deformation. The work done to deform the steel results in a hardening effect, hence the term work hardening. The presence of this hardening and the associated residual stresses preserve a history of the stress event. Quenching Steel mills use water sprays at various stages of plate rolling to descale, clean and cool the steel. The water spray in the cooling process is carefully controlled to achieve the desired temperature profiles, but imperfections in that process can result in localized rapid quenching which impacts the crystalline structure of the steel. Rapid quenching freezes the crystalline structure of the steel so that the diffusion of carbon atoms is inhibited and the crystalline structure becomes a supersaturated solution of carbon in iron. This excess carbon results in a highly strained Martensitic crystal lattice with residual stresses. The presence of the residual stresses provides a magnetic fingerprint in the form of regions of elevated coercivity. Welding Welding is involved in the creation of individual pipeline segments at the mill as well as the joining of individual segments in the production of a pipeline. The extreme heat input required for welding and the subsequent cooling create a heat affected zone (HAZ) around every weld. The HAZ may contain harder material with residual stresses depending on the preparation of the steel prior to welding, the composition of the parent material and the cooling rate after welding. Magnetic deformation detection One of the most useful low field capabilities has been the identification of the magnetic deformation caused by residual stresses that persist in regions of mechanical damage. The application of low field technology for this purpose has been discussed to some extent previously. A typical example of the contribution of low field data to classifying a feature is presented in Figure 3. These datasets come from a concurrent data acquisition with a single ILI tool, which allows for accurate data correlation. Previous ILI inspections reported this feature as a small plain dent, but the SpirALL MFL and low field provide additional information enabling characterization of the feature as a dent with metal loss with subsequent re-rounding. The low field data highlights broad areas of residual stress surrounding the actual metal loss and deformation anomalies. These broad impacts to the pipe produce distinctive residual stress patterns that are indicative of a dent with re-rounding while the SpirALL MFL identifies previously undetected metal loss associated with a gouge. The severity of the anomaly may be more accurately characterized by the multiple datasets. 6

7 Multiple ILI datasets of a single pipeline feature DEF MFL SpirALL MFL Low Field Figure 3. Multiple datasets from a single anomaly. In addition to the application of low field technology to mechanical damage, T.D. Williamson, Inc. has conducted internal testing of a low field MFL tool s ability to detect thermally induced hard spots. Some of the low field data from the in-house testing on heated and quenched samples is presented in Figure 4. The two hard spots in Figure 4 were created using different quenching methods. Both hard spots were heated to 1900 degrees F using an acetylene torch and one spot was allowed to air cool rapidly while the other spot was quenched with water. The air cooled anomaly may simulate a HAZ like those produced after welding and the rapidly quenched area may simulate a hard spot created at the mill during a cleaning or cooling process. The hard spots presented in Figure 4 are approximately the same size and are shown in both MFL and low field data. The differences in these hard spots are quite minor in the MFL data, but the low field presents a wave pattern in the rapidly quenched zone as opposed to the roughly circular feature in the air cooled zone. The depictions of the shape and intensities of hard spots associated with various quenching methods are much more distinct in the low field data. 7

8 Air Cooled Rapid Quench Low Field MFL Figure 4. Low field data, TDW in-house testing. Low field technology has proven effective at detecting anomalies created by localized work hardening and localized quenching. Many of these successes have been detailed in previous papers and will continue to be highlighted in upcoming studies. However, the focus of this paper is on the ability of low field technology to reveal metallurgical anomalies that are characteristic of the pipe itself as opposed to particular types of mechanical damage. Magnetic fingerprints In addition to detecting localized anomalies, the low field magnetizer can also identify patterns of magnetic deformation in a section of pipe that are not associated with any specific defect. These patterns of magnetic deformation are often attributable to the manufacturing or milling processes of the pipe itself since several of the known mechanisms for imparting residual stresses to the pipeline material arise in those processes. The forming presses, shaping rollers, expansion fixtures, etc. at a specific pipe mill will cold work the pipe material in certain patterns. Additionally, the quenching processes or welding processes at a specific mill during a given era may impact the pipe material in a certain pattern. Figure 5. shows MFL data for portions of two segments of pipe on either side of a weld. The MFL view does not reveal any appreciable distinctions between these two segments. The low field data for this same section of the pipeline is presented in Figure 6. The pattern of permeability differences in the pipe segment on the left side of Figure 6 becomes a magnetic fingerprint for the pipe segment. The fact that the permeability pattern exists along the entire length of the pipe segment suggests that it is the result of the manufacturing process rather than a unique and localized phenomenon. The distinctive fingerprint may allow for a characterization of all pipe joints that share the 8

9 fingerprint. The characterization on its own does not predict grade, yield strength, or other physical properties of the pipe material; however, Non-Destructive Examination (NDE) techniques can establish or confirm these properties for a particular characterization. Figure 5. MFL data. Figure 6. Low field data. In order to be useful, low field technology must be able to provide magnetic fingerprints that are distinct from one pipe type to another. It is not enough to identify patterns of permeability variation, but those patterns must also be traceable to a known pipe identification. Low field data of adjacent pipe joints from an inspection of a 16 pipeline are presented in Figure 7. Subsequent evaluation identified the two pipe joints as a vintage 16 pipe and a modern or post pipe. Both pipe sections had the same wall thickness of approximately 0.250, exhibited the same high field MFL background gauss levels of approximately 250 gauss, and had no appreciable bore variations. The spiral striping of the modern pipe is not a result of spiral welded pipe, but was created from rollers during the manufacturing process. The low field distinctions between vintage and modern pipe are clear in Figure 7. As field verification information is correlated with the low field data, a database or library of low field signatures for various pipe features and pipe types can be developed which will maximize the value of low field data indications. The development of such a library of features and pipe signatures is currently underway at T.D. Williamson, Inc. 9

10 Vintage Pipe Modern Pipe Figure 7. Low field data. A more challenging test of low field technology on six samples of 8 pipe with known grade and producer was conducted to determine if the magnetic fingerprints in the various samples would be distinct enough to distinguish one pipe from another. Six pipe samples were tested: Joint 1: Manufacturer A Grade B Joint 2: Manufacturer B Grade B Joint 3: Manufacturer C Grade B, Type 1 Joint 4: Manufacturer D Grade B, Type 2 Joint 5: Manufacturer E Grade B Joint 6: Manufacturer F Grade A Low field data for joints 1 and 2 are presented in Figure 8. The permeability differences along the pipe manifest as a wave pattern in the low field data. The shape and frequency of the wave patterns are noticeably different in these two pipe joints, suggesting that the magnetic fingerprint could be used to distinguish between these types of pipe. 10

11 Pipeline pigging and integrity management conference, Houston, February 2014 Joint 1 Joint 2 Figure 8 - Joints 1 and 2. Low field data for joints 3 and 4 is presented in Figure 9. Again the differences in wave shape and frequency are evident and the possibility of using the distinctions for identifying the pipe type in subsequent inspections is promising. Joint 3 Joint 4 Figure 9 - Joints 3 and 4 Low field data for joints 5 and 6 is presented in Figure 10. The differences in wave shape and frequency are present, but an additional distinction in hue or darkness is quite evident. The overall darkness in the data (a product of the background flux density) is an indication of the general permeability of the pipe material. Previous studies have shown that the general permeability can be correlated with material properties including chemical composition, grain size, yield strength and 11

12 toughness. So, while we may not be able to predict any one property based on the background flux density, the variations in that value can serve as part of the magnetic fingerprint. Joint 5 Joint 6 Figure 10 - Joints 5 and 6. A side-by-side comparison of all six test samples is presented in Figure 11. Wave shapes and frequencies are a result of the milling pattern which is presumed to be consistent for pipe of the same type from the same producer. Joints 1 and 5 have very similar wave patterns and were, in fact, determined to be the same pipe type (Manufacturer A Grade B). Joint 1 Joint 2 Joint 3 Joint 4 Joint 5 Joint 6 Figure 11 Low field data, six samples. The magnetic fingerprints from the pipe samples in this experiment were visually distinct and allowed for unique identification of each type of pipe. Of the available ILI data sets, the low field data set provides the most dramatic pipe signature. Nevertheless, these signatures are composed primarily of variations in wave shape, wave frequency, and to some extent background flux density. For the low field technology used in this particular experiment the variations are axial (with respect 12

13 to the pipe) variations. When low field technology is combined with additional ILI data sets the magnetic signature can become a more general pipe signature and can include circumferential permeability variations and even geometric characteristics. Material documentation process The generation of a pipe signature using low field MFL technology will be useful for material documentation processes that are part of some proposed regulatory initiatives. Material documentation processes will require pipeline operators to have and maintain certain material documentation records for each pipe joint in their pipeline. Proposed regulatory initiatives may require that operators implement a program to develop documentation for pipe joints that lack records to establish material properties. These programs would typically require cutouts and destructive tests for each of the undocumented pipe joints. Alternatives to physical testing programs that could provide positive material identification would offer significant advantages to the operator. Low field MFL technology provides an ability to group or classify pipe utilizing its magnetic fingerprint, and could play an important role in a material documentation process. An ILI program that includes low field technology may provide a method of assessing undocumented pipe joints by tying them to an established magnetic fingerprint and perhaps a more general pipe signature. The pipe signature may allow for the classification of pipe and an assignment to a group of pipe joints with known origin and properties. This analytical classification approach could reduce the number of in ditch verification digs that would otherwise be required for compliance with proposed regulatory initiatives. Summary and conclusion Traditional MFL technology is widely accepted as a primary technology for inline inspection, but its capability is limited primarily to the identification of volumetric metal loss in pipelines. Continuing development efforts have produced additional MFL technologies that are capable of identifying other types of anomalies which improve the assessment of threats to pipeline integrity. Among the most effective of the additional MFL technologies is the low field technology which can identify metallurgical anomalies associated with permeability changes in the material. Although the benefits of low field technology have previously been discussed primarily in the context of classifying localized phenomenon such as quenching or mechanical damage, the ability of the technology to identify more global magnetic patterns in a pipe also have clear benefits. The global magnetic patterns observed in the low field data constitute a magnetic fingerprint that is characteristic of the milling processes and production era for a specific pipe joint. The magnetic fingerprint may be combined with other ILI data sets including oblique field MFL and deformation data to produce a more descriptive pipe signature with additional facets that may be unique to specific types of pipe. Thus, the combination of multiple data sets that include low field technology provides a very effective tool not only for the classification and identification of localized threats but also for the general identification of the pipe itself. As pipeline operators strive to meet regulatory guidelines, the capabilities of the low field equipped multiple dataset tool will become increasingly valuable. Future material documentation needs will make the pipe identification capabilities of such a tool extremely useful. The demonstrated successes of low field technology confirm that the inclusion of additional magnetic field technologies within a single tool continues to provide significant advances in the quality and quantity of information that can be gathered with inline inspection tools. 13

14 Acknowledgments The authors would like to acknowledge the technical contributions made by TDW employees Adrian Belanger, Chris Goller, Jed Ludlow Chuck Harris, and James Simek in acquiring some of the data and concepts used within this paper. References 1. Goller, Chris, James Simek, and Jed Ludlow. "Multiple Data Set ILI For Mechanical Damage Assessment." IPC Alberta: ASME, Jiles, David. Introduction to Magnetism and Magnetic Materials. Boca Raton: Taylor & Francis Group, Nestleroth, J. B. Variation of Magnetic Properties in Pipeline Steels. Interim Report, Washington, DC: U.S. Department of Transportation, Pollard, Lee, Adrian Belanger, and Tim Clarke. "Managing HIC Affected Pipelines Utilizing MFL Hard Spot Technology." Corrosion Houston: NACE International, Simek, J. C. "Detecting Mechanical Damage." PipeLine and Gas Technology, June 2006: