Development and application of trenchless magnetic detector for oil and gas pipeline defect

Abstract

Trenchless detection technology for oil and gas pipeline defect is of great significance to remedy the shortcomings of in-pipeline detection and pipeline direct assessment technology. Pipeline defect detection technology can be expanded and the long-term service of the pipeline can be achieved due to the trenchless detection technology. According to the fluctuation range of ground magnetic field caused by typical pipeline defect, fluxgate sensor was selected as the sensitive element to detect the magnetic fluctuation of the pipeline in this paper. Excitation circuit, detection circuit and data processing circuit of fluxgate sensor were designed. Long-distance pipeline defect detection experiments under indoor and field conditions were carried out by using a self-made fluxgate detector. The indoor experimental results showed that the sensitivity of the magnetic detector is higher than that of the international advanced magnetic detector, and it can detect artificial pipe defects with a depth greater than 20% of the wall thickness at a distance of 1.6 $\sim$ 2.2 m (7 $\sim$ 10 times the pipe diameter). The field experimental results showed that the magnetic detector could detect corrosion thinning of pipeline with buried depth of 2.5 m (3.5 times diameter) effectively.

Keywords

Oil and gas pipeline trenchless inspection magneto-mechanical effect fluxgate

1. Index

In view of the important strategic significance of oil and gas pipelines and the seriousness of pipeline accident hazards, how to ensure the safe operation of oil and gas pipelines has always been the focus of global attention [1, 2, 3, 4]. In-pipe inspection and direct pipeline assessment are the most effective methods for maintaining pipeline safety [5, 6, 7, 8]. However, both have disadvantages. In-pipe inspection is subjected to stuck accidents due to the complicated geometrical conditions. The direct pipeline assessment indirectly evaluates pipeline corrosion by detecting anti-corrosion layer and cathodic protection system, which is difficult for excavation verification. Therefore, to ensure the long-term service of oil and gas pipelines, it is of great significance to investigate the trenchless detection technology and develop new methods of pipeline damage detection and evaluation.

2. Principle of pipeline trenchless magnetic detection

According to the force magnetic effect mechanism [9, 10], the ferromagnetic pipeline in the geomagnetic field is subjected to loads such as internal pressure and formation pressure, and its magnetization and magnetic permeability can be changed, which can affect the spatial distribution of the scattering magnetic field of the pipeline. Stress concentration caused by pipe-wall defects distort pipeline magnetic field on the ground, See Fig. 1 in [11]. The magnetic sensor is used to detect the scattering magnetic field of the pipeline, and the results of signal processing and feature analysis can be used to determine whether the pipeline has stress concentration and macro defects, and then evaluate pipeline damage.

Figure 1.

Non-excavation magnetic inspection principle of buried oil and gas pipelines.

The fluctuation of the magnetic field caused by pipeline damage is related to the material, pressure and depth of the pipeline. Since the magnetic field strength of the pipeline is attenuated by the cubic law of the distance, the scattering magnetic field of the buried pipeline distributed on the ground is very weak. Hence the development of high-sensitivity magnetic sensors to accurately detect the ground magnetic field distortion caused by pipeline damage is a prerequisite and can guarantee the successful implementation of non-excavation magnetic detection of pipelines.

3. Development of fluxgate detector

3.1 Micromagnetic sensor selection

Russia Power Diagnostics has an advanced pipeline trenchless detection technology [12]. Its latest ground magnetic detector TSC-5M-32 has a sensitivity of 0.1 A/m and can theoretically detect a magnetic field change of 125 nT. However, it is found that some of the ground magnetic field fluctuation caused by pipeline damage (such as corrosion thinning) was only tens of nanometers, which exceeded the detection range of existing instruments.

Micromagnetic sensors mainly include the types of SQUID, Hall, fluxgate, magnetoresistance and giant magnetoresistance [13, 14, 15]. Their characteristics are summarized as follows: SQUID type has the highest sensitivity, but it is expensive and needs to be used at low temperature; the fluxgate type has high sensitivity, small Volume, however slow response speed; Hall, magnetoresistance and giant magneto resistance are mature, miniaturized and easy to implement, but with large sensitivity errors. Micromagnetic sensors are the optimal choice for the pipeline trenchless detection, as no high sampling frequency is needed for the detection, and the ground detection method does not require miniaturization of the instrument. So a sensitivity of fluxgate sensor with 1 nT can completely meet the requirement [16, 17].

3.2 Three-axis fluxgate sensor

The fluxgate sensor consists of a magnetic core, an excitation coil and an induction coil [18, 19, 20, 21, 22]. The magnetic core is a soft magnetic material with high magnetic permeability and low coercivity. Under the action of the high frequency magnetic field of the excitation coil, the induction coil generates the even-harmonic component which changes with the environmental magnetic field, and the even-harmonic component is measured by the high-performance conditioning circuit to achieve the detection of low frequency weak ambient magnetic fields.

The magnetic sensor can be placed on a non-magnetic horizontal table, which can rotate 360 degrees with the ground surface as reference, for measuring the vertical component of the geomagnetic field. $H$ 1, $H$ 2, $H$ 3, and $H$ 4, the vertical components of the geomagnetic field at angles of 0, 90, 180, and 270 degrees can be recorded and the slip angle of the sensor can be calculated according to Eq. (1):

$\displaystyle\alpha=\arcsin\left({\frac{H_{\max}-H_{\textit{avg}}}{H_{\textit{% avg}}}}\right).$ (1)

Here, $H_{\max}=\max\left\{{H_{1},H_{2},H_{3},H_{4}}\right\}$ , $H_{\textit{avg}}=\left({H_{1}+H_{2}+H_{3}+H_{4}}\right)/4$ .

The smaller the slip angle, the better the perpendicularity of the sensor magnetic axis to the bottom surface. A H62 copper shell with a flat bottom can be covered on the senor for protection. Adjusting the core direction of the sensor on the non-magnetic horizontal platform so that the magnetic axes of the magnetic sensors are perpendicular to the corresponding bottom surface. Packing three well-tuned sensors together and ensuring that they are perpendicular to each other, to achieve the measurement of three-dimensional magnetic field. The three-axis fluxgate sensor is shown in Fig. 2.

Figure 2.

Three-axis fluxgate sensor.

The technical parameters of the three-axis fluxgate sensor are shown in Table 1.

Table 1

Technical parameters of the three-axis fluxgate sensor

Size	Range	Conversion temperature coefficient	Accuracy
22 $\times$ 22 $\times$ 66 mm	$\pm$ 100 $\mu$ T	1 nT/ ${{}^{\circ}}$ C	1 nT
Calibration error	Response frequency	Stability	Output signal
$<$ $\pm$ 0.5%	3 kHz	2 nT/H	30 mV/ $\mu$ T
Preheating time	Assembly error	Operating temperature	Storage temperature
15 mins	$<$ 1 ${}^{\circ}$	$-$ 70 ${{}^{\circ}}$ C $\sim$ $+$ 85 ${{}^{\circ}}$ C	$-$ 40 ${{}^{\circ}}$ C $\sim$ $+$ 85 ${{}^{\circ}}$ C

3.3 Design of the control circuit

3.3.1 Overall design

The fluxgate sensor control circuit is made of an excitation circuit, a detection circuit, and a data processing circuit. The excitation circuit includes a signal generation circuit, a double frequency circuit and a power amplification circuit. The detection circuit includes a band pass filter circuit, a phase shift circuit and a phase sensitive detection circuit, is shown in Fig. 3.

The fluxgate output signal contains noise caused by either unbalanced turns of the fluxgate windings or non-parallel cores. To filter the noise, the fluxgate inverting amplifier is placed as a first-order band-pass frequency selective amplifier whose center frequency is double of the excitation frequency, so that the input second harmonic obtains the maximum gain and suppresses noise and non-second harmonic component. The second harmonic voltage is subjected to phase sensitive detection after frequency selective amplification. The phase sensitive detection output has a DC voltage proportional to the external magnetic field, and the voltage polarity is in the direction of the external magnetic field. When the output of the phase sensitive detection is a negative step signal, the integral filter outputs a skewed forward integrated voltage that is fed back to the sensor winding via the resistor and produces a compensation current. The direction of the compensation magnetic field is opposite to the external magnetic field, and the magnetic field of the sensor is close to zero due to mutual cancellation, so the second harmonic voltage and the phase sensitive detection output DC voltage are zero. If the external magnetic field changes, the balance is destroyed, and a second harmonic voltage and a phase sensitive detection output voltage are generated. The voltage can be used as to measure the external magnetic field after being integrated and filtered.

Figure 3.

Signal control flow of the fluxgate sensor.

3.3.2 Specific design

The excitation circuit provides both an excitation signal for the fluxgate sensor and a reference signal for the phase sensitive detection circuit. The ICL8038 chip is used to generate a sine wave and a digital square wave of 2 KHz frequency, wherein the sine wave is an excitation signal and the digital square wave is a reference signal of the detection circuit. The CD4064 type phase-locked loop and CD4017 type chip are used to design a double frequency circuit. The 2 KHz square wave signal passes through this double frequency circuit and its frequency becomes 4 KHz.

A phase sensitive detection circuit is designed by using CD4051 analog switch chip and integral circuit chip. The second polarity signal is phase-shifted after getting through the band-pass filter circuit, in this condition, a phase shift circuit is required, to achieve the same frequency and phase with the digital square wave reference signal. Under the control of digital square wave, CD4051 realizes half-wave rectification by periodic gating and cutting, and outputs a stable DC signal through the integration circuit to detect the direction and magnitude of the magnetic field.

The multi-channel network port type acquisition card is used to collect the voltage signal of the 18-way fluxgate sensor in real time, and is converted into a digital signal via ADC, then transmitted to the upper computer through the network cable. The A/D converter adopts 24-bit ADS1274, with 111dB signal-to-noise ratio in compressed mode. It adopts high precision working mode and uses frame synchronization protocol to realize data interaction with MCU chip. The ADC circuit design is shown in Fig. 4.

Figure 4.

ADC circuit design.

Figure 5.

The power supply circuit design.

MCU circuit is powered by LT176333 regulator chip and its power supply voltage is 3.3 V. The ADC circuit is powered by LT17635 regulator chip, and its power supply voltage is 5 V. The power supply circuit design is shown in Fig. 5.

The fluxgate sensor control circuit layout is shown in Fig. 6.

Figure 6.

Fluxgate sensor control circuit.

Figure 7.

Fluxgate detector.

3.4 Magnetic detector technical parameters

The self-made fluxgate detector, consisting of fluxgate sensors, a data acquisition board, a power supply and a magnetic analyzer analysis software, as shown in Figs 7 and 8. The main technical parameters are: excitation voltage $\pm$ 5 V, accuracy 1 nT, detection channel 18, range $\pm$ 1 Gauss, bandwidth 3 kHz at $-$ 3 dB, internal noise 100 PT (1 Hz), output signal 30 mV/ $\mu$ T, zero offset error $\pm$ 50 nT, conversion temperature coefficient $\pm$ 1 nT/ ${}^{\circ}$ C, scale temperature coefficient $\pm$ 100 ppm/ ${}^{\circ}$ C, calibration error $\pm$ 0.5%, assembly error $<$ 1 ${}^{\circ}$ , stability 2 nT/H. It adopts 220 VAC and $\pm$ 12 VDC and provides TTL voltage signal, 100 M Ethernet interface and standard RJ45 interface. The communication distance with the host computer is 100 meters.

Magnetic analyzer analysis software was designed as well for testing multi-dimensional magnetic field. This software has some functions including detector motion control, display of the detecting results, final test report generation, et al. For example, for the motion control part: the movement type, the step length, the step number and the stabilization time can be input. The step length can be given by distances/angles, or the input pulse, the step number is determined by the starting point, terminal point and the step length, and the moving velocity can be changed accordingly. Based on the recording date of each step, two-dimensional or three-dimensional graphics can be displayed. A test report based on the above test information can be generated as well.

4. Magnetic detector experiment

4.1 Magnetic detector indoor experiment

The indoor experiment was used to test the detection effect of the self-made magnetic detector on the pipeline defect, and the result was compared to the Russian power diagnostic company’s TSC-5M-32 measuring instrument. The test pipe is $\Phi$ 220 $\times$ 10 mm, and the material of is steel Q235 without demagnetization treatment, and the pipe pressure is 6 MPa. The chemical compositions and mechanical properties of Q235 steel are shown in Tables 2 and 3.

Table 2
Chemical composition (wt%) of tested pipe

Material	C	Si	Mn	$P$	S
Q235	0.14–0.22	$\leqslant$ 0.35	$\leqslant$ 1.4	$\leqslant$ 0.045	$\leqslant$ 0.05

Table 3

Mechanical properties of tested pipe

Material	Elastic modulus (GPa)	Yield strength (MPa)	Ultimate strength (MPa)
Q235	203	250	375–460

Figure 8.

Magnetic analyzer analysis software.

The hemispherical artificial defects of $\Phi$ 4, $\Phi$ 6 and $\Phi$ 8 are milled on the outer wall of the pipeline by ball cutter, which are used to simulate the corrosion pit of the pipeline. The positions of the corrosion pits are shown in Fig. 9. The photo of indoor experiment is shown in Fig. 10.

Figure 9.

Positions of the corrosion pits.

Figure 10.

The photo of indoor experiment.

The distance between the magnetic detector and the pipeline is 1.6 m. The results of the pipeline damage test are shown in Fig. 11. It shows that these three artificial defects are reliably detected.

Figure 11.

Self-made magnetic detector indoor test results (detection distance: 1.6 m).

As shown in Fig. 11, around 1.8 $\times$ 10 ${}^{4}$ nT variation of the magnetic field can be found when detecting corrosion pit1(in Fig. 9), and around 2 $\times$ 10 ${}^{4}$ nT variation can be detected at pit2, as well as 1.3 $\times$ 10 ${}^{4}$ nT variation in pit3. However, the variation can not obviously observed when using TSC-5M-32 detector from Russia Power Diagnostics due to the low resolution (as shown in Fig. 12). Comparison results show that our self-made sensor can have a better performance at the same condition

The distance between the self-made magnetic detector and the pipeline is increased to 2.2 m, and the pipeline defect detection result is shown in Fig. 13. The three artificial defects can still be reliably detected, and as the detection distance increases, the contrast of the detection signal decreases.

Figure 12.

TSC-5M-32 detector indoor test results (detection distance: 1.6 m).

Figure 13.

Self-made magnetic detector indoor test results (detection distance: 2.2 m).

4.2 Magnetic detector field experiment

In one oil depot of Sinopec, the in-service oil and gas pipeline was tested using the self-made magnetic detector. In-service pipeline parameters: dimension $\Phi$ 711 $\times$ 10 mm, design pressure 1.6 MPa, working pressure 1 MPa, carbon steel material, crude oil medium, polyurethane insulation shell, 8 mm thick insulation layer, buried depth 2.5 m.

Figure 14.

The photo of field experiment.

Figure 15.

Self-made magnetic detector field test results (detection distance: 3 m).

The distance between the magnetic detector and the buried pipeline is 3 m, the sampling distance is 200 mm/point, and the length of the detection pipe is about 20 m (Specifically, the pipes are composed of two sections, one is a new pipe segment with a wall thickness of 10 mm, no corrosion; the other is an old pipe segment with corrosion). Ultrasonic phased array inspection was used in advance, and it was found that pipe wall thickness became thinner in some parts because of uniform corrosion. 6.7 mm to 5.4 mm wall thickness reduced can be obtained from the ultrasonic phased array inspection. The photo of field experiment and the detected data of the pipeline magnetic field are shown in Figs 14 and 15 respectively. The effective detection data is from the sampling point 30 to 130; the pipe section of sampling point 30–60 corresponds to the new pipe segment; the pipe section of sampling point 61–120 corresponds to the old pipe segment. The thinning of the wall thickness caused by corrosion will enhance the leakage effect of the axial magnetic field of the pipeline and increase the ground magnetic field (from 20,000 nT to 50,000 nT). Therefore, the axial magnetic field of the pipeline can reflect the severity of corrosion reduction of the pipe wall. The radial magnetic field of the pipeline produces a sudden change where the wall thickness changes, but it does not reflect the details of the wall thickness variation; the circumferential magnetic field of the pipeline is not sensitive to the variation of the wall thickness of the pipeline.

5. Conclusion

Based on the specific application, fluxgate sensor is selected as the detecting component. A magnetic detector suitable for trenchless detection of oil and gas pipelines is developed, and pipeline damage trenchless tests were carried out. The experimental results show that the self-made fluxgate detector can detect the distribution of corrosion damage in buried pipelines, and the sensitivity of the self-made fluxgate detector is higher than that of the magnetic detectors mainly used in the world.

For ultrasonic testing, digging up a portion of the ground has been conducted to moving the sensors through the pipeline. Compared to ultrasonic testing, the magnetic detector designed in this paper can detect the corrosion of the pipeline without trenching. Time and money can be greatly saved with the usage of the magnetic detector.

Footnotes

Acknowledgments

The authors acknowledge the Science Foundation of Xinjiang Provincial Department of Education (Grant: XJEDU2017M047).

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