Pulsed Eddy Current System Research Articles

Sparsity-Based Nondestructive Evaluations of Downhole Casings Technique Using the Uniform Linear Array

Borehole pulsed eddy-current (PEC) systems based on uniform linear multicoil arrays (ULMAs) perform efficient nondestructive evaluations (NDEs) of metal casings. However, the limited physical space of the borehole restricts the degrees of freedom (DoFs) of ULMAs to be less than the number of constraints, which leads to the difficulty of compensating for the differences in signals acquired by different receivers with different transmitting-to-receiving distances (TRDs), and thus limits the effectiveness of the ULMA system. To solve this problem, this paper proposes sparse linear constraint minimum variance (S-LCMV) for NDEs of downhole casings employing ULMAs. By transforming and characterizing the original PEC signal, it was observed that the signal power dramatically decreased with increasing Legendre polynomial stage, confirming that the signal was sparsely distributed over the Gauss–Legendre stages. Using this property, the S-LCMV cost function with reduced constraints was constructed to provide enough DoFs to accurately calculate the weight coefficients, thus improving the detection performance. The effectiveness of the proposed method was verified through field experiments on an 8-element ULMA installed in a borehole PEC system for NDEs of oil-well casings. The results demonstrate that the proposed method could improve the weighting effect by reducing the number of constraints by 70% while ensuring the approximation accuracy, which effectively improved the signal-to-noise ratio of the measured signals and reduced the computational cost by about 87.9%.

Circular-arc array for the pulsed eddy current inspection of thermally insulated pipelines

The inspection of corrosion under insulation (CUI) has been identified as a significant challenge in the petroleum and chemical process industries. As some of the most effective strategies, pulsed eddy current (PEC) techniques have proved effective for the measurement of the CUI of pipelines. In this paper, we propose a circular-arc array (CAA) to improve the measurement efficiency for the PEC inspection of thermally insulated pipelines. Based on the PEC system model for inspecting the CUI of pipelines, the magnetic field distribution of the CAA with multiple excitors was investigated. It is shown that the coverage of induced magnetic field gets much larger than that of the single excitor to realize high-efficiency measurements. Moreover, a sparsely distributed receiver array is designed to further improve the signal-to-noise ratio by eliminating the waviness effect due to multiple excitors. Finally, experiments were conducted, and the results demonstrated the effectiveness of the proposed method for the inspection of thermally insulated pipelines.

A Magnetic Shielding-Type PEC Sensor With a Canister Structure and Magnetic Core

The pulsed eddy current (PEC) technique exhibits considerable benefits over conventional eddy current techniques. The structure and dimensional parameters of PEC sensors considerably influence the PEC systems. Conventional PEC sensors are susceptible to background noise of a magnetic excitation field. To strengthen the magnetic gathering ability and improve detection sensitivity, a magnetic shielding shell structure was introduced to PEC sensors. In this study, a novel canister structure and magnetic core-based PEC sensor with magnetic shielding was designed. Finite-element simulation and experimental study were performed subsequently to optimize the design. A 3-D finite-element simulation model was established according to the eddy current detection mechanism and electromagnetic shielding principle. The influence of the PEC sensor structure and size on its detection performance was observed. Next, a 2-D axis-symmetric model was established, and the size of this PEC sensor was optimized by using the orthogonal approach. The results revealed that the detection performance of this PEC sensor could be improved by incorporating an appropriate magnetic shielding mechanism and through the optimization of the sensor size. The research results show that compared with the PEC sensor without magnetic shield, the PEC sensor with magnetic shield developed in this article can obtain higher signal amplitude and can be better applied to surface and subsurface defect detection.

Synthesized Magnetic Field Focusing for the Non-Destructive Testing of Oil and Gas Well Casing Pipes Using Pulsed Eddy-Current Array

In this article, we address the problem of synthesized magnetic field focusing (SMF) of the pulsed eddy-current (PEC) technology for the non-destructive testing (NDT) of oil and gas well casing pipes. The electromagnetic response excited by multiple transmitters was derived using the signal model of downhole PEC system. The magnetic field was shown to be adjustable by modifying the currents of the multiple transmitting array. Based on this property, magnetic fields were synthesized with the mean squared error (MMSE) criterion to be focused on the center of the transmitting array, and a weighted MMSE (WMMSE)-based SMF was also proposed to achieve an optimal synthetic magnetic field. Furthermore, the primal-dual interior-point method was used to solve the WMMSE optimization problem with the total and single transmit power constraints for the practical application. Simulations and experimental results showed that the SMF with the PEC array effectively focuses the eddy-current field on the center of the transmitting array.

Uniform Circular-Array-Based Borehole Pulsed Eddy-Current System for Asymmetry Defect Inspection in Downhole Casings

The inspection of wellbore casings has been extensively investigated owing to the increasing concern for safety in oil and gas production. However, efficient techniques for inspecting asymmetry defects have not been achieved. In this study, we developed a uniform circular array (UCA) to address the problem of borehole pulsed eddy current (PEC) techniques for asymmetry defect inspection in downhole casings. Based on the borehole PEC system model, the UCA developed with multiple independent probes was designed to achieve asymmetry defect inspection, and the three-dimensional magnetic field data of borehole depths, circumferential azimuths, and sampling times could be obtained. Furthermore, a multichannel data acquisition circuit, which guarantees downhole operation at 150 °C, was developed for the synthesized UCA. Using azimuth dimension information from the synthesized UCA at a certain borehole depth, we obtained an inspection approach for the width and penetration depth of asymmetry defects in the circumferential and radial directions, respectively. Simulations and field experiments were conducted, and the results demonstrate the effectiveness of the proposed method in inspecting asymmetry defects.

Tunnel-magnetoresistive-based Pulsed Eddy Current Probe for Inspection of Corrosion under Insulation

The problem of corrosion in the industrial oil and gas pipes has been one of the major contributors in catastrophic structural failures. Among the various types of corrosion, corrosion under insulation (CUI) has been known to cause serious problems. Pulsed eddy current (PEC) non-destructive testing has shown its effectiveness in detecting hidden CUI. Most PEC systems have been developed by using an inductive coil as their sensing device, while some use a magnetic sensor which potentially offers better resolution. A new probe design based on a solid-state Tunnel magnetoresistance (TMR) is presented. TMR sensors offer higher sensitivities compared to other commercially available sensors. The performance of the probe is evaluated by using ferromagnetic mild steel plates that have thicknesses in the range of 8mm to 12mm at different stand-offs with a thin aluminium sheet under the probe. The different thicknesses represent different corrosion levels, while the stand-offs and thin aluminium sheet are to mimic the insulation of different thicknesses and the cladding in the real pipeline structure. The results show an overall mean absolute error (MAE) of 0.19 mm, which is better than our existing Hall-device-based PEC probe.

Development of a Cone-Shaped Pulsed Eddy Current Sensor

Compared with the conventional eddy current, pulsed eddy current (PEC) technology has the characteristic of abundant components in the frequency domain, which is a significant technological advancement. Improvements in the production and performance of PEC sensors are necessary for nondestructive testing. Sensor configuration parameters have significant influence on the development of PEC system. In this study, through finite element simulation and experimental investigation, different optimization methods for PEC sensors are compared and studied. According to the eddy current detection principle, a two-dimensional finite element simulation model was established to study the influence of the shape and size parameters of cone-shaped PEC sensors. Then a cone-shaped PEC sensor was optimized by the orthogonal methodology and the response surface methodology, respectively. To compare the effectiveness of the optimization methods, two optimized sensors were fabricated. It was demonstrated that the combination of finite element method and response surface methodology was superior for improving the detection performance of the PEC sensor compared with the orthogonal methodology. The result also indicated that the developed cone-shaped PEC sensor had a stronger defect detection capability than cylindrical-shaped PEC sensors. The proposed optimization design method provides a feasible reference method for the design of PEC sensors and corresponding parameters.

Parameter Analysis of Pulsed Eddy Current Sensor Using Principal Component Analysis

Pulsed eddy current (PEC) technique provides a means to inspect structures without surface contact. It is particularly useful when the structure's surface is rough or inaccessible, such as insulated pipes in pipeline. Probe parameters of a PEC system, especially the sensing and excitation coil diameters, can significantly affect the PEC system's performance. Thus, detailed analysis of these parameters is paramount in developing a PEC system. Currently, this is accomplished by establishing the trend of features with respect to the analyzed variables, e.g. sample thicknesses. However, prior to extracting these features, a number of configuration parameters have to be determined. For this reason, analyzing PEC performance over a range of coil diameter values is rather time-consuming as both the sensing and excitation coil diameters significantly affect the received signals. Principal component analysis (PCA) is proposed as an alternative to the feature extraction. The work here analyzes the trends contributed by the PCA scores for different values of sensing and excitation coil parameters. Results from both numerical simulations and experiments suggest that the sensitivity of the PEC probe is highly correlated with the excitation coil diameter, while the excitation-sensing coil distance is not significant in determining the sensitivity of the PEC probe. These findings are consistent with those reported in the literature, suggesting the potential of adopting PCA for an automated PEC performance analysis process.

Pulsed Eddy Current: Feature Extraction Enabling In-Situ Calibration and Improved Estimation for Ferromagnetic Application

Steel pipes in process plant applications are often covered with insulation or weather protection that make inspection difficult because the additional layers need to be penetrated to inspect the pipes’ structure. The pulsed eddy current (PEC) method was devised as a means of inspection through the surface layers. However, the performance of a PEC system is dependent on the electrical and magnetic properties of the pipe material, which are generally unknown. Therefore, the use of a calibration block from a different steel will give inaccurate results. The concept of calibrating using tau _0 values obtained during inspection has undoubtedly been discussed in the literature. However, no comprehensive work was dedicated to using |nabla |^{-1} to carry out calibration on inspected structure. The linear relationship of the |nabla |^{-1} feature with the thickness squared, d^2, is first established using analytical solutions, and the calibration is carried out using the feature values obtained in air and the reference signal. The performance of this technique is assessed and compared with the conventional tau _0 technique. Although both features exhibit similar immunity towards lift-off, tau _0 technique requires normalisation procedure, which contributes to determining more configuration parameters. Experimental results also suggest the relative advantage of using |nabla |^{-1} feature in both wall thickness estimation and influences of noises.

Effects of Coil Diameter in Thickness Measurement Using Pulsed Eddy Current Non-destructive Testing

Non-destructive testing (NDT) techniques are used in industry to evaluate the properties of a material, component or structure without causing any permanent damage. Among the techniques, pulsed eddy current (PEC) NDT is regarded as a new technique where a broadband pulse excitation is used, as opposed to single frequencies employed in conventional eddy current NDT. In this study, a 2D axisymmetric electromagnetic model of a PEC probe has been developed and it has been used to study the effects of the excitation coil diameter on the performance of PEC probes in sample thickness measurement. A PEC system has also been built to validate the model. Aluminium plates are used as the sample and they can be stacked up to replicate thickness from 1 mm to 10 mm. The results show that there is a very good correlation between the simulation and experimental results, with an average error of less than 10%. The results also suggest that the larger the diameter of the excitation coil, the deeper the penetration and therefore the larger the thickness measurement range. It has also been shown that although the larger diameters have deeper penetration, the smallest diameter has the highest sensitivity if normalization is not used. These conclusions indicate that coil diameter is an important parameter in a PEC probe design for thickness measurement applications.

Investigations on a pulsed eddy current system for flaw detection using an encircling coil on a steel pipe

Investigations on a pulsed eddy current system for flaw detection using an encircling coil on a steel pipe

Steel Corrosion Characterization Using Pulsed Eddy Current Systems

A pulsed eddy current (PEC) system has been used to characterize atmospheric corrosion on steel samples. International Paint has supplied coated and uncoated mild steel (S275) samples with marine atmospheric corrosion (exposure time between 1 and 10 months). The PEC response due to corrosion is a complex mix of many factors, including conductivity, permeability and material thickness variation, which are all taken into account through experimental studies and the extraction of signal features. Considering the conductivity and permeability variation in the corrosion layer or the actual rust region, two time-domain features, each representing the conductivity and permeability, are extracted and used to characterize corrosion. The relationship between PEC features and exposure time has been derived, which can be useful for corrosion rate measurement and early-stage corrosion evaluation and prediction. In conclusion, PEC testing has the potential for corrosion characterization and monitoring in areas such as the marine industry.

Pulsed eddy current differential probe to detect the defects in a stainless steel pipe

Pulsed eddy current (PEC) is an electromagnetic nondestructive technique widely used to detect and quantify the flaws in conducting materials. In the present study a differential Hall-sensor probe which is used in the PEC system has been fabricated for the detection of defects in stainless steel pipelines. The differential probe has an exciting coil with two Hall-sensors. A stainless steel test sample with electrical discharge machining (EDM) notches under different depths of 1–5 mm was made and the sample was laminated by plastic insulation having uniform thickness to simulate the pipelines in nuclear power plants (NPPs). The driving coil in the probe is excited by a rectangular current pulse and the resultant response, which is the difference of the two Hall–sensors, has been detected as the PEC probe signal. The discriminating time domain features of the detected pulse such as peak value and time to zero are used to interpret the experimental results with the defects in the test sample. A feature extraction technique such as spectral power density has been devised to infer the PEC response.

The Pulsed Eddy Current Differential Probe to Detect a Thickness Variation in an Insulated Stainless Steel

Non-destructive testing (NDT) plays an important role in the safety and integrity of the large industrial structures such as pipelines in nuclear power plants (NPPs). The pulsed eddy current (PEC) is an electromagnetic NDT approach which is principally developed for the detection of surface and sub surface flaws. In this study a differential probe for the PEC system has been fabricated to detect the wall thinning in insulated steel pipelines. The differential probe contains an excitation coil with two hall-sensors. A stainless steel test sample was prepared with a thickness that varied from 1 mm to 5 mm and was laminated by plastic insulation with uniform thickness to represent the insulated pipelines in the NPPs. Excitation coil in the probe is driven by a rectangular current pulse, the resultant PEC response which is the difference of the two hall sensors is detected. The discriminating features of the detected pulse, peak value and the time to zero were used to describe the wall thinning in the tested sample. A signal processing technique such as power spectrum density (PSD) is devised to infer the PEC response. The results shows that the differential PEC probe has the potential to detect the wall thinning in an insulated pipeline of the nuclear power plants (NPPs).

Non-Contact Thickness Gauge for Conductive Materials Using HTS SQUID System

Eddy current testing using high- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TC</i> superconductive (HTS) superconducting quantum interference devices (SQUID), was developed to realize non-contact thickness gauge measurement of conductive materials, due to their high-sensitivity to magnetic fields generated by eddy currents. A HTS SQUID pulsed eddy current system with Fourier transform analysis is proposed and demonstrated. Aluminum plates (10 x 10 mm) with thicknesses in the range of 1 to 10 mm were used as test samples. The intrinsic responses <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</sub> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> ) of the samples were successfully extracted. The slopes of these responses were estimated by linear fitting, and plotted as a function of sample thickness. The <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</sub> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> ) slopes decrease with increasing thickness of the test samples. The analysed data suggests that this system can be used as a non-contact thickness gauge for conductive materials.

Study of magnetic sensors for pulsed eddy current techniques

In this study we compare the suitability of a Hall device and a GMR sensor as the sensing device in pulsed eddy current systems. The comparative study is carried out by using both experimental results and manufacturers' data. The factors analysed are sensitivity, noise level, and operational bandwidth. Conclusions are given following the analysis of the data.

A system identification approach to the modelling of pulsed eddy-current systems

The purpose of this article is to present a novel approach in the modelling of pulsed eddy-current systems. Frequency-domain finite element analysis is used to build up a z-domain, finite impulse response (FIR) model. The FIR model allows excitation to be varied without need for the field analysis to be performed again. Furthermore, the proposed approach to the modelling of the system function allows unequal spacing of the frequency samples. This enables greater accuracy of the modelling in the frequency range where the magnitude or phase response of the system varies greatly. The proposed approach was validated on a pulsed remote-field eddy-current system in a ferromagnetic tube. Excellent agreement between predicted and measured response was achieved.

Pulsed eddy current system for dynamic inspection of defects

Pulsed Eddy Current (PEC) techniques have been investigated and applied for field applications. On-line inspection is essential for many manufacturing processes and other NDE applications. This paper investigates the dynamic behaviour of pulsed eddy current techniques and the feasibility of their use in an on-line inspection system. After discussing the PEC principles and their signal signatures, an approach on improving the dynamic response of PEC is introduced and tested. The results have illustrated that the proposed PEC system and their feature extraction can be used for real-time inspection such as NDT for manufacturing processes

Pulsed Eddy-Current Measurements of Corrosion and Cracking in Aging Aircraft

ABSTRACTPulsed, or transient eddy-current methods are an effective tool for quantitative characterization of hidden corrosion and cracking in multi-layer aircraft structures. Eddy currents are the method of choice for this task, since they penetrate multiple layers of metal, whether or not the layers are mechanically bonded. The pulsed eddy-current technique is an important advance over conventional eddy-current methods because it rapidly acquires data over a wide range of frequencies, thereby providing more information than a conventional, single-frequency eddy-current instrument. We have combined a pulsed eddy-current instrument with a portable two-axis scanner to produce an instrument capable of rapidly scanning aircraft lap splices in situ, producing pseudo-color images that reveal hidden corrosion or cracking. A unique feature of time-domain eddy-current data is the ability to selectively filter clutter from the image by time-gating the pulsed signal. Time-gating permits the user to select the inspection depth, thereby eliminating interference from upper layers, air gaps, lift-off variation and fasteners. By using a theoretical model of the pulsed eddy-current system, it is possible to interpret the data quantitatively, yielding quantitative maps of corrosion damage. Some of the same advantages of the pulsed eddycurrent technique apply to the characterization of hidden fatigue cracks as well, although the tieory for crack signals is less advanced.

Pulsed eddy-current inspection of thin-walled stainless steel tubing : Oak Ridge national laboratory, Oak Ridge, Tennessee (United States), DE88-001492, 118 pp. (Sep. 1987)

A pulsed eddy-current system has been developed for production inspection of small, thin-wall, non-ferromagnetic tubing. To detect and accurately size both outer and bore-side flaws required an operating frequency higher than available from present commercial equipment. A pulsed eddy-current instrument was designed and constructed that used 3.2 MHz square waves, with a bandwidth of 20 MHz. The system is able to reliably detect flaws as small as 0.015 mm (0.6 mils) on either the inner or outer surface of the tube. A computer controls the scanning of the tube, as recording, analyzing, and plotting the data. The computer programs and instrument details are given in the report.