Mechanical Damage Detection With Magnetic Flux Leakage Tools: Modeling the Effect of Localized Residual Stresses

Battelle has developed dual field magnetic flux leakage (MFL) technology for the detection and characterization of mechanical damage to pipelines. The basic principle involves the use of a high magnetic field between 140 and 180 Oersted (11.1 to 14.3 kA/m) and the use of a low magnetic field between 50 and 70 Oersted (4 to 5.6 kA/m). At high magnetic field levels, the flux leakage signal is primarily influenced by changes in the geometry of a pipe wall. At low magnetic field levels, the MFL signal is due to residual stresses and metallurgical changes as well as geometry changes to the pipe caused by mechanical damage and wall thinning. A decoupling signal processing method developed by Battelle is used to isolate the portion of the mechanical damage signals due to metallurgical damage and residual stresses, which allows the characteristics of a dent-gouge feature to be more clearly differentiated. The decoupling method involves first scaling down the high field signal to the level of the low field signal, and then subtracting it from the low field signal. This produces a decoupled signal that is primarily influenced by the residual stresses and metallurgical changes caused by mechanical damage. Rosen has developed a tool to test the dual field technology and is evaluating tool performance by running the tool in a 30 inch diameter pipeline segment. The tool itself is composed of three separate modules coupled together: a high field unit downstream of a low field unit which is downstream of a caliper arm unit that is used to detect and characterize reductions in the internal diameter. The general and magnetic design of the tool, along with the scaling algorithm is discussed. Results from a pull test in a pipe section with dents whose geometry has been independently characterized are also discussed. This work is partially funded by the U.S. Department of Transportation, Pipeline and Hazardous Materials Safety administration (DOT PHMSA) and the Pipeline Research Council International, Inc. (PRCI).

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Mechanical damage detection using magnetic flux leakage tools: modeling the effect of dent geometry and stresses

NDT & E International ◽

10.1016/j.ndteint.2004.12.002 ◽

2005 ◽

Vol 38 (6) ◽

pp. 471-477 ◽

Cited By ~ 29

Author(s):

Vijay Babbar ◽

James Bryne ◽

Lynann Clapham

Keyword(s):

Magnetic Flux ◽

Damage Detection ◽

Mechanical Damage ◽

Magnetic Flux Leakage ◽

Flux Leakage

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Management of Pipeline Dents and Mechanical Damage in Gas Pipelines

Volume 2: Integrity Management; Poster Session; Student Paper Competition ◽

10.1115/ipc2006-10407 ◽

2006 ◽

Cited By ~ 2

Author(s):

David J. Warman ◽

Dennis Johnston ◽

John D. Mackenzie ◽

Steve Rapp ◽

Bob Travers

Keyword(s):

High Resolution ◽

Magnetic Flux ◽

Mechanical Damage ◽

Management Plan ◽

Magnetic Flux Leakage ◽

Pipeline System ◽

Channel Geometry ◽

Channel Deformation ◽

Integrity Management ◽

Flux Leakage

This paper describes an approach used by Duke Energy Gas Transmission (DEGT) to manage dents and mechanical damage as part of its overall Integrity Management Plan (IMP). The approach provides guidance in the process for evaluating deformation anomalies that are detected by high resolution magnetic flux leakage (HR-MFL) and multi-channel geometry in-line inspection tools, the process to determine which deformations will be selected for excavation, the process to conduct pipeline field excavations, assessments, and repairs for pipeline integrity purposes. This approach was developed, tested and fully implemented during pipeline integrity work over a two year program involving over 1,100 miles of HR-MFL and 900 miles of geometry in-line inspection. Integration of data from high resolution ILI tools (HR-MFL and multi-channel deformation tools) was used to identify and characterize dents and mechanical damage in the pipeline system. From subsequent field assessments and correlation with ILI results, the processes were refined and field procedures developed. The new guidance provided in the 2003 edition of ASME B31.8 was used as the governing assessment criteria.

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The Design of a Mechanical Damage Inspection Tool Using Dual Field Magnetic Flux Leakage Technology

Journal of Pressure Vessel Technology ◽

10.1115/1.1989349 ◽

2005 ◽

Vol 127 (3) ◽

pp. 274-283 ◽

Cited By ~ 2

Author(s):

J. Bruce Nestleroth ◽

Richard J. Davis

Keyword(s):

Magnetic Field ◽

Magnetic Flux ◽

High Magnetic Field ◽

Mechanical Damage ◽

Magnetic Flux Leakage ◽

Metal Loss ◽

Unexpected Result ◽

Lower Field ◽

Study Results ◽

Flux Leakage

This paper describes the design of a new magnetic flux leakage (MFL) inspection tool that performs an inline inspection to detect and characterize both metal loss and mechanical damage defects. An inspection tool that couples mechanical damage assessment as part of a routine corrosion inspection is expected to have considerably better prospects for application in the pipeline industry than a tool that complicates existing procedures. The design is based on study results that show it is feasible to detect and assess mechanical damage by applying a low magnetic field level in addition to the high magnetic field employed by most inspection tools. Nearly all commercially available MFL tools use high magnetic fields to detect and size metal loss such as corrosion. A lower field than is commonly applied for detecting metal loss is appropriate for detecting mechanical damage, such as the metallurgical changes caused by impacts from excavation equipment. The lower field is needed to counter the saturation effect of the high magnetic field, which masks and diminishes important components of the signal associated with mechanical damage. Finite element modeling was used in the design effort and the results have shown that a single magnetizer with three poles is the most effective design. Furthermore, it was found that for the three-pole system the high magnetization pole must be in the center, which was an unexpected result. The three-pole design has mechanical advantages, including a magnetic null in the backing bar, which enables installation of a pivot point for articulation of the tool through bends and restrictions. This design was prototyped and tested at Battelle’s Pipeline Simulation Facility (West Jefferson, OH). The signals were nearly identical to results acquired with a single magnetizer reconfigured between tests to attain the appropriate high and low field levels.

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Understanding Magnetic Flux Leakage Signals From Dents

Volume 2: Integrity Management; Poster Session; Student Paper Competition ◽

10.1115/ipc2006-10043 ◽

2006 ◽

Author(s):

Lynann Clapham ◽

Vijay Babbar ◽

Alex Rubinshteyn

Keyword(s):

Magnetic Flux ◽

Pipe Wall ◽

Experimental Studies ◽

Mechanical Damage ◽

Magnetic Behaviour ◽

Magnetic Flux Leakage ◽

Element Analysis ◽

Aspect Ratios ◽

Stress Relieving ◽

Flux Leakage

The Magnetic Flux Leakage (MFL) technique is sensitive both to pipe wall geometry and pipe wall stresses, therefore MFL inspection tools have the potential to locate and characterize mechanical damage in pipelines. However, the combined influence of stress and geometry make MFL signal interpretation difficult for a number of reasons: 1) the MFL signal from mechanical damage is a superposition of geometrical and stress effects, 2) the stress distribution around a mechanically damaged region is very complex, consisting of plastic deformation and residual (elastic) stresses, 3) the effect of stress on magnetic behaviour is not well understood. Accurate magnetic models that can incorporate both stress and geometry effects are essential in order to understand MFL signals from dents. This paper reports on work where FEA magnetic modeling is combined with experimental studies to better understand dents from MFL signals. In experimental studies, mechanical damage was simulated using a tool and die press to produce dents of varying aspect ratios (1:1, 2:1, 4:1), orientations (axial, circumferential) and depths (3–8 mm) in plate samples. MFL measurements were made before and after selective stress-relieving heat treatments. These annealing treatments enabled the stress and geometry components of the MFL signal to be separated. Geometry and stress ‘peaks’ tend in most cases to overlap — however stress features are most prominent in the dent rim region and geometry peaks over central region. In general the geometry signal scales directly with depth. The stress scales less significantly with depth. As a result deep dents will display a ‘geometry’ signature while in shallow dents the stress signature will dominate. In the finite element analysis work, stress was incorporated by modifying the magnetic permeability in the residual stress regions of the modeled dent. Both stress and geometry contributions to the MFL signal were examined separately. Despite using a number of simplifying assumptions, the modeled results matched the experimental results very closely, and were used to aid in interpretation of the MFL signals.

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Detection of Mechanical Damage Using the Magnetic Flux Leakage Technique

2004 International Pipeline Conference, Volumes 1, 2, and 3 ◽

10.1115/ipc2004-0006 ◽

2004 ◽

Cited By ~ 2

Author(s):

Lynann Clapham ◽

Vijay Babbar ◽

James Byrne

Keyword(s):

Magnetic Flux ◽

Experimental Studies ◽

Mechanical Damage ◽

Central Peak ◽

Magnetic Flux Leakage ◽

Element Analysis ◽

Stress Effects ◽

Stress Relieving ◽

Stress Dependent ◽

Flux Leakage

Since magnetism is strongly stress dependent, Magnetic Flux Leakage (MFL) inspection tools have the potential to locate and characterize mechanical damage in pipelines. However, MFL application to mechanical damage detection faces hurdles which make signal interpretation problematic: 1) the MFL signal is a superposition of geometrical and stress effects, 2) the stress distribution around a mechanically damaged region is very complex, consisting of plastic deformation and residual (elastic) stresses, 3) the effect of stress on magnetic behaviour is not well understood. This paper summarizes recent results of experimental and modeling studies of MFL signals resulting from mechanical damage. In experimental studies, mechanical damage was simulated using a tool and die press to produce dents of varying depths in plate samples. Radial component MFL measurements were made before and after selective stress-relieving heat treatments. These annealing treatments enabled the stress and geometry components of the MFL signal to be separated. Geometry and stress effects generate separate MFL peaks — the geometry effects lead to central peak regions while the stress effects produce ‘shoulder’ peaks. In general the geometry peaks tend to scale with depth, while the shoulder peaks remain approximately constant. This implies that deep dents will display a ‘geometry’ signature while shallow or rerounded dents will have a stress signature. Finally, the influence of other parameters such as flux density and topside/bottomside inspection was also quantified. In the finite element analysis work, stress was incorporated by modifying the magnetic permeability in the residual stress regions of the modeled dent. Both stress and geometry contributions to the MFL signal were examined separately. Despite using a number of simplifying assumptions, the modeled results matched the experimental results very closely, and were used to aid in interpretation of the MFL signals.

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Characterization of Mechanical Damage Through Use of the Tri-Axial Magnetic Flux Leakage Technology

Volume 2: Integrity Management; Poster Session; Student Paper Competition ◽

10.1115/ipc2006-10454 ◽

2006 ◽

Author(s):

Vanessa Co ◽

Scott Ironside ◽

Chuck Ellis ◽

Garrett Wilkie

Keyword(s):

Magnetic Flux ◽

Mechanical Damage ◽

Magnetic Flux Leakage ◽

Metal Loss ◽

Field Investigations ◽

Non Destructive ◽

Flux Leakage

Management of mechanical damage is an issue that many pipeline operators are facing. This paper presents a method to characterize dents based on the analysis of the BJ Vectra Magnetic Flux Leakage (MFL) tool signals. This is an approach that predicts the severity of mechanical damage by identifying the presence of some key elements such as gouging, cracking, and metal loss within dents as well as multiple dents and wrinkles. Enbridge Pipelines Inc. worked with BJ Services to enhance the knowledge that can be gained from MFL tool signals by defining tool signal subtleties in dents. This additional characterization provides information about the existence of gouging, metal loss, and cracking. This has been accomplished through detailed studies of the ILI data and follow-up field investigations, which validate the predictions. One of the key learnings has been that the radial and circumferential components of the MFL Vectra tool are highly important in the characterization and classification of mechanical damage. Non-destructive examination has verified that predictions in detecting the presence of gouging and cracking (and other defects within dents based on tool signals) have been accurate.

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API 579 Level 3 Assessment of Dents Using High-Resolution ILI Data

Volume 2: Structures, Safety and Reliability ◽

10.1115/omae2011-50349 ◽

2011 ◽

Cited By ~ 1

Author(s):

Chas Jandu ◽

Mike Taylor ◽

Suji Narikotte

Keyword(s):

High Resolution ◽

Magnetic Flux ◽

Single Channel ◽

Mechanical Damage ◽

Magnetic Flux Leakage ◽

Metal Loss ◽

Operating Pressure ◽

Data Set ◽

Level 3 ◽

Flux Leakage

In-line Inspection (ILI) surveys are periodically performed to determine the condition of the pipeline. Typical ILI surveys involve Magnetic Flux Leakage primarily to determine metal loss and simple single channel Calliper surveys to determine any signs of geometry imperfections. Additional surveys such as high-resolution multi-channel Calliper deformation tools are occasionally used to accurately record imperfections to enable a more accurate assessment of the integrity of the pipeline containing the imperfection. Such tools have had limited employment, and therefore little experience exists of using the data obtainable for the detailed assessment of defects. This paper presents a study of such a case. As part of an In-line Inspection (ILI) of an offshore pipeline, a high-resolution deformation survey recorded numerous dent anomalies which had potentially resulted from a single dragged anchor incident before the pipeline was trenched. This data set was correlated to Magnetic Flux Leakage inspection data to confirm external mechanical damage. Pipeline sections having anomalies that were either found close to girth welds, or had associated corrosion defects were automatically selected for repair. The remaining anomalies were assessed in order to determine their acceptability for the maximum allowable operating pressure using the approaches detailed in API-579. Due to the sharp nature of some of the dents, elastic-plastic finite element analyses (FEA) were performed using denting profiles generated from the calliper data of the ILI run. API-579 level 3 assessments were then carried out using the FEA results. This paper details the high-resolution deformation tool findings and the approach used in order to assess the fitness-for-purpose of the pipe with the recorded anomalies.

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