An Electromagnetic Helmholtz-Coil Probe for Arbitrary Orientation Crack Detection on the Surface of Pipeline

Wei Li, Jiuhao Ge*, Yanyun Wu, Xiaokang Yin, Guoming Chen, Xinan Yuan, Jian Liu and Weichao Yang

Centre for Offshore Engineering and Safety Technology, China University of Petroleum, Qingdao, 266580, China

Subjected to hoop stress and land movement load, the orientation of cracks on the surface of pipeline may be axial direction, circumferen-tial direction or the others. In this paper, an electromagnetic Helmholtz-coil probe is presented to detect the oriented cracks on the surface of pipeline. A structure of equal-spaced TMR sensor array of the Helmholtz-coil probe is applied to scan the full circumference of the pipeline simultaneously in a single pass. The simulation and experiment results indicate that using the combination effect of the electric current pertur-bation and the magnetic �ux leakage, the oriented crack can be mapped clearly.　[doi:10.2320/matertrans.M2016419]

(Received November 28, 2016; Accepted December 27, 2016; Published March 25, 2017)

Keywords:　 helmholtz-coil, oriented crack, surface crack, pipeline

1.　 Introduction

Pipelines provide the safest and most economical form of transportation of crude oil, natural gas, and other petrochem-ical commodities compared to truck, rail cars, and tankers1). Suffering from the degeneration of materials, for example crack, corrosion, it is important to carry out nondestructive testing (NDT) to maintain high reliability of pipeline trans-portation.

Surface cracks, especially the stress corrosion crack (SCC), are one of the most harmful degradations considering their effect on structural integrity2). Subjected to hoop stress and land movement load, the orientation of surface cracks may be axial direction, circumferential direction or the others. Ultra-sonic testing is a widely used technique in pipeline detection. However, the need of couplant limits the detection in gas transportation pipeline3,4). As to magnetic �ux leakage, the leaking magnetic �ux is proportional to the opening of the crack, so it is not sensitive to axial tight SCC5–7). Magnetic particle inspection (MPI) is the most reliable NDT method in pipeline detection. While MPI is a very effective inspection technique, its �eld use can be costly when one considers the surface cleaning and operating time5). The eddy current (EC) could not make accurate size assessment of cracks8).

The aim of this paper is to propose an electromagnetic Helmholtz-coil probe for oriented cracks mapping and sizing on the surface of pipeline. This paper is organized as follows. In section 2, the model of the Helmholtz-coil probe is pre-sented and the perturbation of the magnetic �eld above the crack is analyzed through the �nite element software COMSOL. In section 3 the electromagnetic Helmholtz-coil probe is set up and the oriented crack in the range of 0° to 90° are mapped through experimental testing.

2.　 Simulation of the Electromagnetic Helmholtz-Coil Probe

An electromagnetic Helmholtz-coil probe consists of two critical components was presented in this paper: a Helm-holtz-coil excitation and a detecting sensor array. The Helm-

holtz-coil excited by alternating current (AC) was coaxial with the pipeline to induce a uniform current �eld along the circumference of the pipeline and a uniform magnetic �eld along the axis of the pipeline9). The tunnel magneto resistive (TMR) detecting sensor array was used to measure the mag-netic �eld signals.

2.1　 Finite element model of the electromagnetic helm-holtz-coil probe

The �nite element model of the electromagnetic Helm-holtz-coil probe was built through the COMSOL software, as shown in Fig. 1. The “magnetic �eld physics” was selected and the “impedance boundary condition” was applied on the surface of the pipeline. The dimensions of the model are shown in Table 1 and the characteristic parameters are shown in Table 2. The electric current and magnetic �ux density dis-tribution on the surface of pipeline without crack are shown in Fig. 2. As shown in Fig. 2, the uniform electric current �eld and magnetic �eld are induced on the surface of pipeline10,11).

2.2　 Result analysisIn the simulation, the oriented cracks in the range of 0° to

90°, as shown in Fig. 3, were detected. The position of the probe was moved along the axial direction (z-axis) to simu-late the probe scanning process along a pipeline during exper-imental studies. The response magnetic �eld of the probe was measured at a 1 mm lift-off. Two components of the magnetic �eld, axial direction (z-axis) Bz and radial direction (y-axis) By, were measured as the characteristic signals, as shown in Fig. 4.

As shown in Fig. 4(a) and 4(b), with the increasing of the

* Corresponding author, E-mail: gejiuhao@163.com Fig. 1　FEM model the electromagnetic Helmholtz-coil probe.

Materials Transactions, Vol. 58, No. 4 (2017) pp. 641 to 645 ©2017 The Japan Institute of Metals and Materials

angle, the Bz signal changes from dip to peak gradually and the By signal remained a peak and dip. However, the order of the peak and dip in By signals varies with the increasing of crack orientation.

The electric current density around the 0° orientation crack on the surface of pipeline and magnetic �ux density around the 90° orientation crack in the x-y plane of the model are shown in Fig. 5.

It can be seen from the Fig. 5(a) that the electric current concentrating in the tips of the crack, which is expressed as

arrow, when the crack of 0° orientation is detected is the elec-tric current perturbation effect caused by discontinuous con-ductivity. As shown in Fig. 5(b), the magnetic �ux leaks in the edge of the crack, which is expressed as line, when the crack of 90° orientation is detected is the magnetic �ux leak-age effect caused by discontinuous permeability. These phe-nomenon also explain the variations of Bz and By signal in Fig. 4 that the Bz and By signals of the crack of 0°, 15° orien-tation are caused by the electric current perturbation effect and the signals of the crack in the range of 30° to 90° orienta-tion are caused by the magnetic �ux leakage effect.

An implication of this is the possibility that the combina-tion effect of the electric current perturbation and magnetic �ux leakage around the crack may have the advantage of de-tecting the oriented crack on the surface of pipeline.

Fig. 2　Electric current and magnetic �ux density distribution on the surface of the pipeline.

Fig. 3　Oriented cracks ranging from 0° to 90°.

Fig. 4　Bz and By signals, (a) Bz, (b) By.

Table 1　the size of the FEM model.

Model Diameter/mm Length/mm Width/mm Depth/mm Interval/mm

Pipeline(D/d) 65/45 300

Helmholtz-coil 80 4 40

Crack 20 1 5

Table 2　the characteristic parameter of the FEM model.

Number of turns Pipeline material Conductivity/S/m Permeability Excitation current/A Frequency/Hz

500 Carbon steel 1.12e7 4000 1 1000

642 W. Li, et al.

3.　 Experiments

3.1　 System set upThe structure of the Helmholtz probe is shown in Fig. 6.

The excitation coils were wound on the polymer frame. De-tecting sensors were equally-spaced installed in the polymer frame. Supports were used to �x the probe and keep the con-stant lift-off to different pipeline. The extension-type tape was used to �x the sensor array. A sensor array containing Tunnel Magneto Resistive (TMR) was employed to scan the full circumference of the pipeline. Considering the space of the Helmholtz-coil electromagnetic probe and actual manu-facturing dif�culty, a 24 equal-spaced sensor array was se-lected. An electromagnetic Helmholtz-coil detection system was built, as shown in Fig. 7. The excitation source produced an alternating current signal with the frequency of 1 kHz and the magnitude of 10 V. The turns of the coil were 1000 in to-tal. The current was transferred to the excitation coil through the power ampli�ers. The detecting sensor array picked up the magnetic �eld and translated it into electric signal. The signals were ampli�ed and �ltered in signal processing mod-ule. And then, the signals were converted into digital signal by an A/D convertor and sent to PC for signal processing. A detection software was developed to achieve defects recogni-tion. The Bz and By signals were shown in the display screen of computer. The Helmholtz-coil probe was �xed in an axial scan table, as shown in Fig. 7.

3.2　 Result analysisThe pipeline made from carbon steel was detected to test

the detectability of Helmholtz-coil probe to crack. The axial cracks, which are 1 mm width and 30 mm length, with the

depth in the range of 2 to 10 mm were machined on the sur-face of pipeline by Electric Discharge Machining (EDM), as shown in Fig. 8.

The pipe string was moved along the axial direction at a speed of 10 mm/s and the Bz and By signals were measured and shown in Fig. 9. It can be seen from the �gures that all of the cracks on the surface of pipeline could be detected obvi-ously. Furthermore, the length of cracks can be obtained ac-curately. The amplitudes of the signal are proportional to the depth of the cracks.

Oriented cracks with the orientation in the range of 0° to 90° were machined on the surface of pipeline by EDM, as

Fig. 5　The electric current distribution and magnetic �ux density around the crack, (a) electric current density around the 0° orientation crack on the surface of pipeline, (b) magnetic �ux density around the 90° orienta-tion crack in the x-y plane of the model.

Fig. 6　Structure of Helmholtz-coil probe.

Fig. 7　Experimental setup.

Fig. 8　(a) Schematic diagram of pipeline and (b) Actual cracks in pipe strings.

643An Electromagnetic Helmholtz-Coil Probe for Arbitrary Orientation Crack Detection on the Surface of Pipeline

shown in Fig. 10, and the parameters of the cracks were 45 mm ×  1 mm ×  5 mm (length ×  width ×  depth). The pipe-line was scanned through the system in Fig. 7 at a speed of 10 mm/s. The data of the sensor array were obtained and drawn in the MATLAB. The C scan results are shown in Fig. 11. It can be seen from the results that the orientation of

Fig. 10　Pipeline with the cracks in the range of 0° to 90° orientation.

Fig. 11　C scan results of the oriented crack ranging from 0° to 90° with 15° interval, (a), (c), (e), (g), (i), (k), (m) are the Bz signals, (b), (d), (f), (h), (j), (l), (n) are the By signals.

Fig. 9　Bz and By signals.

644 W. Li, et al.

the cracks can be recognized clearly. Moreover, with the in-creasing of the crack orientation, the Bz signal varies from dip to peak gradually. The Bz and By signals of the crack of 0°, 15° and 45° orientation are caused by the electric current pertur-bation effect and the signals of the crack of 60°, 75° and 90° orientation are caused by the magnetic �ux leakage effect. Comparing with the results in simulation, we can see that the numerical and experimental results have the similar variation law and mechanism that using the combination effect of elec-tric current perturbation and magnetic �ux leakage, the arbi-trary orientation crack can be detected.

4.　 Conclusions

In this paper, a Helmholtz-coil probe has been proposed to detect oriented cracks on the surface of the pipeline. A FEM model of this probe is put forward through the COMSOL. Based on the model, the relationship between the crack orien-tation and Bz and By signal is analysis. Finally, the structure of the Helmholtz-coil probe with a sensor array is designed and the experimental studies are carried out to test the detectabil-ity of oriented crack. The results of the simulations and ex-periments show that: using the combination effect of the cur-rent perturbation and magnetic �ux leakage, the oriented crack on the surface of pipeline can be recognized clearly. The length of the cracks can be measured through the By sig-nals.

Although, the detection and length measurement of the ori-ented cracks through the probe in this paper have been proved, the depth of the oriented crack can’t be sized directly from the

Bz signals. In the future work, we will focus on the depth siz-ing algorithm of oriented cracks through the Helmholtz-coil probe. Moreover, in the experiment, the arti�cial cracks were detected. In the next stage, some actual cracks or closed SCC will be tested.

Acknowledgment

Wei Li and Jiuhao Ge contributed equally to this work. This work was funded by the National Natural Science Foun-dation of China (No. 51574276 and No. 51675536), the Shandong Provincial Natural Science Foundation (No.ZR2015EM009), Special national key research and develop-ment plan (No.2016YFC0802300 and No.2016YFC0303800), and the Fundamental Research Funds for the Central Univer-sities (No.16CX06017A, No. 15CX05024A and No. 14CX02198A).

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645An Electromagnetic Helmholtz-Coil Probe for Arbitrary Orientation Crack Detection on the Surface of Pipeline