I. Nondestructive Testing of Piping Components

1. Inspection of Steel Pipes for Piping

Pipe materials for underground pipelines include seamless steel pipes and welded steel pipes. For seamless steel pipes, liquid-immersion or contact ultrasonic testing is primarily used to detect longitudinal defects. Liquid-immersion testing employs line-focused or spot-focused transducers, while contact testing uses angled transducers or focused angled transducers that maintain close coupling with the pipe surface. Eddy-current testing can be applied to all types of metallic pipes to detect surface and near-surface defects. For welded steel pipes, welds are subject to radiographic sampling inspection or 100% inspection; in the case of 100% inspection, real-time X-ray imaging is typically employed.
                2. Bolts for Piping

For steel bolted components with a diameter greater than 50 mm, ultrasonic testing shall be used to detect metallurgical defects within the bolt shank. Ultrasonic testing shall be performed using either a single-element straight-beam probe or a dual-element straight-beam probe in longitudinal-wave mode.

II. Nondestructive Testing During Pipeline Construction

In welded pipe production, the conventional primary nondestructive testing configurations used internationally are those shown in Process Steps A, B, C, E, F, G, and H in Figure 1; whereas the main inspection stations currently employed in China’s production are those shown in Process Steps A, C, D, E, F, G, and H in Figure 1.

1. Ultrasonic Testing

Fully automated ultrasonic testing technology is currently widely used abroad for the inspection of girth welds in long-distance pipelines. Compared with conventional manual ultrasonic testing and radiographic testing, it offers distinct advantages in terms of inspection speed, accuracy of defect quantification, reduction of environmental pollution, and alleviation of operational workload.

The fully automated phased-array ultrasonic testing system employs a zonal partitioning approach, dividing the weld into multiple zones along the vertical direction. An electronic control system then directs the phased-array probe to perform zone-by-zone scanning. The inspection results are displayed in the form of dual-gate strip charts, complemented by TOFD (time-of-flight diffraction) and B-scan capabilities, enabling comprehensive analysis and evaluation of internal defects in the weld.

During fully automated ultrasonic on-site inspection, the situation is often complex. In particular, factors such as the precision of rail positioning, the calibration accuracy of test blocks, and the ambient scanning temperature can significantly influence the inspection results. Therefore, evaluating the inspection outcomes requires a comprehensive consideration of multiple factors and the systematic collection of diverse information to minimize errors.

2. Radiographic Testing

Radiographic testing typically employs X-ray circumferential exposure units or gamma-ray sources, with the radiation source conveyed into the pipe interior via an in-pipe crawler to position it over the girth weld. A single exposure is then made from the outside using radiographic film; however, film processing and evaluation require considerable time, often delaying pipeline construction schedules. Consequently, in recent years both domestically and internationally, specialized real-time X-ray imaging systems for girth-weld inspection have been developed.

3. Magnetic Particle Testing

Magnetic particle inspection of weld groove surfaces in welded pipes is rarely performed domestically. Abroad, automated inspection systems predominantly employ the wet method using fluorescent magnetic suspensions. These automated magnetic particle inspection systems utilize magnetizing coils to locally magnetize the groove surface in the wall-thickness direction of the pipe, while simultaneously spraying a fluorescent magnetic suspension onto the groove surface. A high-resolution imaging system installed at the inspection station captures images of the magnetized and suspended-magnetic-fluid-sprayed area and transmits them to a nearby monitor. By observing the monitor, operators can promptly detect magnetic particle indications and identify defects.

Magnetic particle inspection is suitable for detecting surface and near-surface defects in ferromagnetic materials; therefore, it cannot be used for flaw detection in non-ferromagnetic materials such as austenitic stainless steels and nonferrous metals. However, martensitic and precipitation-hardening stainless steels are magnetic and can be inspected by magnetic particle testing. This method can detect surface and near-surface defects such as cracks, inclusions, porosity, lack of fusion, and incomplete penetration; yet it is less effective at identifying shallow, wide surface pits, deeply buried defects, and laminations that occur at very small angles to the workpiece surface.

III. Nondestructive Testing Techniques for Steel Pipeline Bodies

Nondestructive testing of steel pipeline bodies primarily focuses on assessing the integrity of the pipe wall, including remaining wall thickness, pipeline defects, surface corrosion morphology, types of corrosion products, and corrosion depth. The following table lists currently commonly used pipeline inspection techniques and their respective inspection parameters.

1. Elastic Wave Testing

Elastic wave detection diagnoses and locates pipeline leaks by analyzing changes in pressure waves within the pipeline caused by the leak; it is generally categorized into acoustic waves, negative-pressure waves, and pressure waves. The primary operating principle involves using strategically placed sensors to detect the elastic waves generated by pipeline leaks and thereby perform leak detection and localization. The key to this technology lies in distinguishing between the elastic waves produced during normal operation and those generated during a leak. Currently, two approaches are employed: one uses hardware circuit delay for signal filtering, while the other combines structural pattern recognition with neural networks to differentiate the distinct waveforms associated with normal operation from those arising during an incident, thus enabling more effective monitoring of pipeline operation.

2. Magnetic Flux Leakage Testing

The operating principle of magnetic flux leakage (MFL) pipeline corrosion inspection equipment is to use the onboard magnets to generate a longitudinal magnetic circuit field around the pipe wall. When the pipe wall is free of defects, the magnetic flux lines remain confined within the wall and are distributed uniformly. However, if there are defects on the inner or outer wall, the magnetic flux path becomes constricted, causing distortion of the magnetic flux lines; some of these lines will then penetrate the pipe wall, resulting in magnetic flux leakage. The principle of MFL detection is illustrated in Figure 3.

The leakage magnetic field is detected by a probe tightly fitted to the pipe wall between the two magnetic poles, generating corresponding induced signals. These signals are then processed through filtering, amplification, analog-to-digital conversion, and other steps before being stored in the detector’s memory. Upon completion of the inspection, dedicated software is used to replay, process, and analyze the data for evaluation and identification.

Overall, the magnetic flux leakage inspection process can be divided into the four sections shown in Figure 4:

Advantages of magnetic flux leakage testing:
1. Easy to automate and offers high detection reliability;
2. Preliminary quantification of defects can be achieved;
3. During pipeline inspection, within the wall thickness range of 30 mm, both internal and external wall defects can be detected simultaneously.
4. Efficient, non-polluting, and automated inspection can achieve very high inspection efficiency. 5. Magnetic flux leakage testing is not suitable for detecting very narrow cracks, especially closed-type cracks.
6. It is not permissible to conduct direct quantitative analysis of the type or severity of defects.

               3. Ultrasonic Testing

Ultrasonic pipe inspection utilizes existing ultrasonic transducers to measure the time difference between the arrival of the ultrasonic wave at a defect and its return, thereby determining the distance between the defect and the pipe wall; the size and orientation of the defect are then determined by measuring the amplitude of the reflected echo signal and the transmission position of the ultrasonic probe.

Figure 5 is a schematic diagram of the ultrasonic testing principle, where Wt represents the normal wall thickness of the pipe and SO represents the standard separation between the ultrasonic probe and the inner surface of the pipe.

Advantages of ultrasonic testing technology:
1. Fast detection speed and low detection cost;
2. Large detection thickness and high sensitivity;
3. Relatively accurate defect localization;
4. High sensitivity to subtle closed-crack defects.

Disadvantages of ultrasonic testing:

1. Due to the limitation imposed by the ultrasonic wavelength, this inspection method offers relatively low detection accuracy for thin-walled pipes and is therefore suitable only for thick-walled pipes; moreover, it places stringent requirements on the medium inside the pipe.
2. When the defect is irregular, multiple reflected echoes will occur, placing higher demands on signal identification and defect localization.
3. Since ultrasonic wave propagation relies on a liquid medium and is readily absorbed by wax, ultrasonic detectors are not suitable for inspection of gas pipelines or oil pipelines with high wax content, thereby exhibiting certain limitations.

4. Electromagnetic Ultrasonic Testing

Electromagnetic acoustic transducer (EMAT) technology is a new nondestructive testing technique that emerged in the 1970s. This technology is based on Lorentz force, magnetostriction, and electromagnetic forces, and utilizes the principle of electromagnetic induction to generate ultrasonic waves.

The generation and reception of electromagnetic ultrasonics are based on the mutual conversion between electromagnetic fields and mechanical wave vibration fields, with the two physical fields interconnected via a force field.

According to physics, an alternating magnetic field induces eddy currents within a metallic conductor, and these currents in turn experience the Lorentz force in the magnetic field. Moreover, under cyclic stress, the metallic medium generates stress waves; when the frequency of these stress waves falls within the ultrasonic range, they are referred to as ultrasonic waves. Conversely, this effect is reversible: the returning acoustic pressure causes particle vibrations, which, under the influence of the magnetic field, in turn modulate the voltage across the eddy-current coil. Consequently, the signal can be detected by a receiving device, amplified, and displayed. Ultrasonic waves generated and received in this manner are known as electromagnetic ultrasonics.

               Compared with conventional piezoelectric ultrasonic transducers, the main advantages of EMAT are:
1. Non-contact detection, no coupling agent required;
2. It can generate waves in multiple modes, making it suitable for surface defect detection;
3. Suitable for high-temperature detection;
4. Low surface quality requirements for the workpiece being inspected;
5. Under the same functional requirements, EMAT flaw detection equipment uses fewer channels and probes than piezoelectric ultrasonic systems.
6. Exhibits strong sensitivity to natural defects, with distinct corner reflections at different angles of incidence, resulting in high detection sensitivity for surface cracks.
               Disadvantages of EMAT:
1. The transduction efficiency of EMATs is 20–40 dB lower than that of conventional piezoelectric transducers;
2. The distance between the probe and the test specimen should be as small as possible;
3. EMAT can only be applied to materials with good electrical conductivity.

5. Eddy Current Testing

Eddy current testing is currently one of the most widely used nondestructive testing techniques for pipelines, and it can be categorized into conventional eddy current testing, transmission eddy current testing, and far-field eddy current testing. Conventional eddy current testing is limited by the skin effect and is suitable only for detecting surface or near-surface defects; in contrast, transmission eddy current testing and far-field eddy current testing overcome this limitation, as their inspection signals exhibit equal sensitivity to both the inner and outer walls of the pipe. Among these methods, far-field eddy current testing stands out for its ease of integration with automated inspection systems (due to electrical signal output), high inspection speed, suitability for surface inspections, broad applicability, safety and convenience, and minimal consumable requirements. As a result, it has garnered significant attention in developed countries and is extensively employed for the inspection of in-service pipelines.

Advantages of eddy current testing:
1. High detection speed, low detection cost, and simple operation;
2. The probe and the test workpiece do not need to be in contact, and no coupling medium is required;
3. During testing, the results can be obtained either as direct electrical signal outputs or displayed on a screen.
4. Enables high-speed automated inspection and allows for permanent data recording.
Disadvantages of eddy current testing:
1. Applicable only to conductive materials; difficult to use for test specimens with complex shapes.
2. Can only detect surface and near-surface defects in materials or workpieces;
3. The inspection results are not intuitive and it is difficult to determine the type, nature, shape, and size of defects;
4. During inspection, numerous interfering factors can easily lead to spurious indications.

6. Laser Detection

The laser inspection system primarily comprises three key components: a laser scanning probe, a motion control and positioning system, and a data acquisition and analysis system, all of which leverage the fundamental principle of optical triangulation. Compared with conventional eddy-current and ultrasonic methods, laser inspection (or profilometry) offers several distinct advantages: high inspection efficiency, superior measurement accuracy, dense sampling, high spatial resolution, non-contact operation, the ability to deliver quantitative results, and the capability to generate cross-sectional views at any location along the inspected pipeline, as well as axial unfoldings and three-dimensional representations.

However, laser-based inspection methods can only examine the surface of an object; to obtain a comprehensive understanding of the test object, it is essential to integrate multiple nondestructive testing techniques and leverage their respective strengths while mitigating their weaknesses.

7. Pipeline Robot Inspection

A pipeline robot is a mobile device designed to traverse the interior of pipelines. It can be equipped with one or more sensors and, under remote control by an operator, perform a range of pipeline inspection and maintenance tasks, making it an ideal automated pipeline inspection system. A complete pipeline inspection robot typically comprises a locomotion platform, a vision system, a signal transmission system, a power system, and a control system.
The primary operating mode of pipeline robots is as follows: guided by vision and pose-sensing sensor systems, they identify the pipeline environment, approach the inspection target, and employ a variety of sensing technologies—such as ultrasonic sensors and magnetic flux leakage sensors—to acquire and interpret data, thereby automatically completing the inspection task. The core components of such systems are the pipeline environment recognition system (vision system) and the mobile platform. Currently, pipeline robot technology abroad has reached a relatively mature stage; these systems not only perform pipeline inspection but also offer functions for pipeline maintenance and repair, making them comprehensive pipeline inspection and maintenance solutions.
               IV. Pipeline External Coating Inspection Technology

1. PCM Detection Method

The core of the PCM (Multi-Frequency Pipe Current Detection) evaluation method is to control the ICI current injected onto the pipeline surface by monitoring the strength of the remote-controlled ICI current signal. By analyzing the patterns of current variation detected, the location and degree of aging of defects in the external anti-corrosion coating can be determined. The current applied to the pipeline generates a corresponding electromagnetic field, with the field strength being directly proportional to the magnitude of the applied current; moreover, as the transmission distance increases, the current signal gradually attenuates. When there is a defect in the external coating, the current flows through the defect point into the earth, causing a sudden increase in the rate of current attenuation at that location, thereby enabling the precise identification of the defect’s position.

However, the PCM method struggles to distinguish closely spaced pipelines, and significant measurement errors occur at pipeline crossings, bends, and in the presence of AC interference.

2. DCVG Detection Technology

The principle of DCVG (Direct Current Voltage Gradient) testing is that, when a DC signal is applied to a pipeline, a voltage gradient develops between the damaged, exposed points in the pipeline’s anti-corrosion coating and the surrounding soil. Near these damaged, exposed locations, the current density increases, and the voltage gradient correspondingly increases as well. In general, the exposed area is directly proportional to the voltage gradient. DCVG detection technology is based on this principle.

When performing DCVG measurements, a current interrupter should be installed on the cathodic protection output to facilitate signal observation and interpretation. During the measurement process, readings are taken along the pipeline at 2-meter intervals directly above the pipe.

The advantages of DCVG include its ability to accurately locate defects in the anti-corrosion coating, assess the severity of these defects, and estimate their size; based on the inspection results, it can then provide well-founded recommendations for maintenance and rehabilitation. The measurement process is simple and highly accurate, with minimal susceptibility to external interference and virtually no dependence on terrain conditions. However, the method requires walking along the pipeline for the entire survey, cannot indicate the effectiveness of cathodic protection or detect coating delamination, and is subject to certain errors arising from environmental factors such as stray currents and the resistivity of surface soils.

3. Pearson Test

The principle of the Pearson leak detection method is to apply an alternating current signal to the pipeline; this signal leaks into the soil at any defects in the pipeline’s anti-corrosion coating. Consequently, the farther one moves from the defect, the lower the current density becomes, resulting in an alternating voltage gradient on the ground surface directly above the defect. During the detection process, two testers position themselves 3 to 6 meters apart, wearing shoes with metal studs or holding probes, and transmit the measured voltage signals back to a receiving unit. After filtering and amplification, the detection results are obtained.

               The Pearson test method is currently the most widely used testing technique in China, and its advantages are:
1. It has relatively mature application experience, fast detection speed, and the ability to detect corrosion coating defects and metallic objects along the pipeline.
2. It can identify the size of damage points and even detect minute leaks, with excellent performance feedback in the inspection and operational maintenance of long-distance pipelines.
               The limitations of the Pearson correlation test are:
1. The entire testing process requires walking;
2. The extent of the damage cannot be specified;
3. Requires high operator skill;

               4. Standard Pipe/Soil Potential Testing Method

The principle of the standard pipe/soil (P/S) potential measurement method is to use a multimeter to measure the potential between the grounded Cu/CuSO4 reference electrode and a specific ICI monitoring point on the pipeline surface. By analyzing the relationship between potential and distance, the potential distribution can be characterized, allowing differentiation between the current potential and historical potentials. The measured cathodic protection potential can then be used to determine whether the external coating of the pipeline is effectively protected.

Currently, the standard method for measuring pipeline protection potentials on the ground is the standard pipe-to-earth potential test. Its advantages include no need for trenching, ease of data acquisition in the field, and rapid inspection speeds (10–50 km per day). Typically, a test stake is installed approximately every 1 km; however, this method can only provide a general assessment of the coating integrity along that pipeline section, without enabling detailed evaluation of coating defects or precise identification of their locations and distribution. Consequently, this method is not suitable for pipelines without cathodic protection or for pipelines lacking test stakes.
  

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Source: Oil Pipeline