Metal Loss Defects Research Articles

PART 4: RUPTURE TESTS OF PIPELINE SEGMENTS CONTAINING LONG REAL CORROSION DEFECTS

shaped internal corrosion defects. Measurements were taken in order to determine the actual dimensions of each of the tubular specimens and their respective metal loss defect profiles. Tensile specimens were tested to determine the actual material properties. The failure pressures measured in the laboratory tests were compared with those predicted by four assessment methods: the ASME B31G method, 4 the DNV RPF101 method for single defects, 5 the RSTRENG effective area method, 7 and the DNV RP-F101 method for complex-shaped defects. 5

704 内圧荷重下における非き裂状欠陥を有する配管の破壊強度評価(OS13(1) エネルギー機器構造物の劣化現象の理解と健全性評価)

704 内圧荷重下における非き裂状欠陥を有する配管の破壊強度評価(OS13(1) エネルギー機器構造物の劣化現象の理解と健全性評価)

Use of the cylindrical instability stress for blunt metal loss defects in linepipe

Assessment of metal loss defects in gas pipelines can be analysed by a number of methods. In analyses with finite element methods a failure criterion is required. A material property is introduced, the cylindrical instability stress, which determines the plastic collapse of cylindrical pressure containing vessels. The use of this is extended to cover blunt metal loss defects. Some published finite element studies of defected vessels are re-analysed using this failure criterion. The cylindrical instability stress is a more accurate failure criterion for plastic collapse in pipelines and pressure vessels than commonly used measures such as flow stress, Specified Minimum Yield Stress plus 10 ksi or multiplied by 1.1. It can be used in determining burst pressure of defected and un-defected pressure vessels and piping.

Defect separation considerations in magnetic flux leakage inspection

The magnetic flux leakage (MFL) method is used for assessing the corrosion integrity of oil and gas pipelines. The interpretation of defect-induced magnetic signals has to take into account the inspection tool and pipeline steel characteristics. The MFL signals are very sensitive to the stress state and magnetisation level of the specimen. The presence of a material-loss defect influences both the stress and magnetisation distributions in the material. Moreover, clustering of the corrosion defects complicates this situation by superposition of the local stresses and by magnetic shielding in the region between defects. In this paper, the degree of interaction between two adjacent metal-loss defects is analysed as a function of the defects' centre-to-centre separation, residual and applied stresses, and magnetic flux density. The necessary centre-to-centre separation for two defects to be considered interacting is evaluated by extrapolation of the magnetic signal features of known defect geometries.

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

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.

Enhanced transient eddy current detection of deep corrosion

Recent developments in the use of transient eddy current methods have demonstrated the advanced ability to detect and characterise corrosion and cracks within complex structures, using a single acquisition scan. In addition, the use of Hall sensors to measure the magnetic field directly enables measurements at greater depths in metals than is achieved using a conventional coil sensor. However, the rapid increase in effective field size with depth in the structure means that there is always a trade-off between defect sensitivity and susceptibility to the presence of nearby edges, fasteners and other sub-structure. The ability to characterise the defect in terms of metal loss, size and depth can also suffer when defect sensitivity is increased. This paper reports an investigation to determine how the sensitivity could be increased for metal-loss defects deeper than 15 mm (0.625) in the case when the proximity of structural changes is not an issue. The essential requirement is to increase the contrast between good and defective structure by either reducing noise levels or increasing the strength of the field reflected from the defect. An increase in the reflected field can be achieved by either increasing the incident field strength, or changing its spatial characteristics. Results have shown that defect sensitivity can be increased using these methods to enable a considerable improvement in detectability of metal loss deep in thick aluminium structure.

Influence of Corrosion Products on Magnetic Flux Leakage Signals in Inspection of Far-side Metal-loss Defects in Oil Storage Tank Bottom Floors

Magnetic flux leakage testing (MFLT) is a reliable inspection method for far-side metal loss type flaws. Corrosion products of ferromagnetic substances are generated in wet environments, so may affect the application of MFLT to real tank bottom floors. This study investigated the influence of corrosion products on MFL signals. Rectangular grooves with various depths and widths were cut in test specimens and filled with various ratios of iron oxides as corrosion products. Magnetic flux leakage density distributions from the grooves were measured with a Hall probe. Analytical models for magnetic leakage field from flaws containing corrosion products were also considered using the three-dimensional finite element method. From the experimental and analytical results, the influence of corrosion products on MFL signals in inspections was discussed.

Magneto-optical visualization of metal-loss defects in a ferromagnetic plate: experimental verification of theoretical modeling.

Rare-earth iron garnet films with in-plane magnetic anisotropy grown on [111]-oriented substrates are promising for the visualization of magnetic leakage fields in nondestructive evaluation. Such magneto-optical films have to be specifically engineered, and we give an example of this technology. To assess the validity and accuracy of finite-element calculations of a magnetization assembly combined with the physical modeling of the image formation, comparisons between calculated and experimentally obtained magneto-optical images of metal-loss defects have been made. A convincing quantitative agreement is demonstrated. It is shown that both physical and computer modeling techniques allow for a predictive engineering design of the prospective applications and provide greater insight into the method.

Characterisation of metal loss defects from magnetic flux leakage signals with discrete wavelet transform

Metal loss defects in a buried pipeline are detected by magnetic flux leakage technique. Characterisation of the defects and sentencing according to the severity is extremely important for organised maintenance of pipelines. In this paper we identify the parameters that characterise a defect and the features of magnetic flux leakage signal (MFL) that are affected by those parameters. We show that analysis of the MFL signal using wavelet transform scores over any other method of its kind and exposes the incompleteness of the other analysis techniques that have appeared in the literature, to date. A number of experiments were performed on a rotating drum test rig having defects of different shapes and sizes. The results from these experiments are presented and discussed in detail. Wavelet transform decomposition and reconstruction techniques were applied for denoising the raw data. We test the efficacy of discrete wavelet transform for denoising MFL signal and present a complete scheme of characterisation of defects from denoised MFL signal. We discuss the issue of defect classification and suggest that characterisation to specified accuracy, amounts to designing a classifier that assigns a defect into known classes whose shapes and sizes are defined a priory.

Remote-field eddy current signal analysis in small-bore ferromagnetic tubes : Mackintosh, D.D.; Atherton, D.L.; Sullivan, S.P. Materials Evaluation, Vol. 51, No. 4. pp. 492–495,500 (Apr. 1993)

A simple method for obtaining the depth of metal-loss defects in small bore ferromagnetic tubes by using the remote-field eddy current method is described. Research and analysis were carried out using a probe equipped with a solenoidal detector coil coaxial with the pipe, scanning uniform-depth metal-loss defects shorter than the overall probe length. On a voltage plane polar plot, which is similar to the voltage plane used in conventional eddy current analysis, the variations in signal amplitude and phase caused by metal loss trace out an elongated loop known as a defect-signal trace. The angle and length of the trace reveal the dimensions of the defect. The outside angle between the defect-signal trace and a line from the origin through the point representing no metal loss indicates the depth of metal loss. The length of the defect-signal trace indicates the cross-sectional area of metal loss. The experimental results are explained with reference to the skin-depth equation.

Through-transmission equations for remote-field eddy current inspection of small-bore ferromagnetic tubes : Mackintosh, D.D.; Atherton, D.L.; Puhach, P.A. Materials Evaluation, Vol. 51, No. 6, pp. 744–748 (Jun. 1993)

The remote-field eddy current (RFEC) method is widely used for nondestructive testing of ferromagnetic tubes such as those found in heat exchangers. An exciter coil generates an electromagnetic field that diffuses through the pipe wall, axially along the outside of the pipe, and back through the pipe wall to a detector coil. Phase and amplitude readings can be interpreted to characterize pipe defects. Skin depth theory is commonly used to calculate through-transmission for a cylindrical wave impinging on a conducting tube. The cylindrical through-transmission equations agree well with RFEC data. The application of the new equations to RFEC defect signal analysis is discussed. A case study of an RFEC scan of a metal loss defect is described. The through-transmission equations were found to hold at the defect. However, the RFEC scan data deviated slightly from the value predicted by the through-transmission equations. The deviation was attributed to a perturbation of the field on the outside of the pipe caused by the defect, an effect not considered in a through-transmission analysis.

Flux-leakage vehicles pass tests for pipeline inspection: Shannon, R.W.E.; Braithwaite, J.C.; Morgan, L.L. Oil and Gas Journal, Vol. 86, No. 32, pp. 47–59 (8 Aug. 1988)

Tests of pipeline-inspection equipment based on principles of magnetic flux leakage have shown it to be capable of routine maintenance and to provide technical and economic advantages over hydrostatic testing. The equipment accurately locates and describes metal-loss defects and enables a pipeline's condition to be determined before a feature becomes critical. Further, equipment capable of detecting and sizing a range of different stress-corrosion cracking (SCC) geometries and types has also been developed and prototypes successfully evaluated during live gas tests in operating pipelines. This discussion includes information on: preventive maintenance; new systems; inspecting heavier wall pipe; modeling dynamic behavior; stop-start behavior; analysis and reporting; crack detection; SCC incidence; and ultrasonic preformance demonstration using two test spools. Also discussed are applications for determining offshore pipeline stability and inspection economics.

Magnetic flux leakage signal analysis for metal-loss defect characterization: Nestleroth, J.B.; Lindsay, B.B.; Davis, R.J. International Petroleum Industry Inspection Technology II, Houston Texas (United States), 25–27 Jun. 1991. pp. 9–13. ASNT (1991)

Magnetic flux leakage signal analysis for metal-loss defect characterization: Nestleroth, J.B.; Lindsay, B.B.; Davis, R.J. International Petroleum Industry Inspection Technology II, Houston Texas (United States), 25–27 Jun. 1991. pp. 9–13. ASNT (1991)

Magnetic flux leakage signal analysis for metal-loss defect characterization

Magnetic flux leakage signal analysis for metal-loss defect characterization