Nondestructive Ultrasonic Imaging Improves Pipeline Joint Coating Testing

By W. WEIBIN, PipeChina, Tianjin, China 

(P&GJ) — Long-distance oil and gas pipelines are commonly applied with three-layer polyethylene (3PE) anti-corrosion coatings, and the matching joint coating material is radiation-crosslinked polyethylene heat-shrinkable tape, which consists of a double-layer material of a substrate and a hot-melt adhesive layer.¹ Polyethylene joint coating is a key measure to ensure the long-term safe and reliable service of oil and gas pipelines, and its bonding quality is directly related to the corrosion protection effect of the pipelines.^2,3 When the bonding strength of the pipeline joint coating is insufficient, unbonded or weakly bonded areas may appear. At this time, if the outer surface of the pipeline is scratched or damaged under stress, it is likely to cause pipe perforation or large-area thinning, ultimately leading to serious pipeline leakage accidents.^4,5,6 

At present, the peel strength of anti-corrosion coating specified in the International Organization for Standardization (ISO) 21809-3 is generally used as an evaluation index for the bonding quality of pipeline joint coatings.⁷ However, this peel strength testing method has significant limitations, as it causes irreversible damage to the joint coating in the tested area, creating potential high-risk points for future pipeline failures. Furthermore, the results of local sampling inspections are highly dependent on the choice of sampling points and the testing operations, often leading to considerable deviations and failing to reflect the overall bonding quality of the joint coating. Other common testing methods, such as visual inspection and spark leakage testing, also have their limitations. Visual inspection provides only a superficial evaluation based on the surface condition, while spark leakage testing is restricted to identifying local defects like pinholes and sand holes.  

Therefore, there is an urgent need for non-destructive, accurate and efficient testing technologies. Recently, advancements in technology and the demand for efficient inspection have shifted scholarly focus toward non-destructive testing (NDT) of joint coating bonding quality.^8,9,10,11 Some researchers have conducted experimental studies on the bonding performance of heat-shrinkable tapes using ultrasonic testing with the water-immersion method, identifying time-domain and frequency-domain equivalent strength as two key indicators for quantitative bonding quality evaluation. Additionally, researchers have proposed new laser-ultrasonic testing technologies, investigating the propagation of laser-generated ultrasonic waves in the flat panel and curved panel of a composite.¹² Other scholars have developed microwave NDT techniques that assess joint coating structures and bonding quality through changes in the dielectric properties of the coating material. 

In summary, NDT technology effectively identifies defects in the bonding structures of joint coatings, with ultrasonic testing being the most widely utilized method for assessing bonding quality. Most other NDT techniques remain in the theoretical research phase, and few have been proposed for high-precision, quantitative analysis of the bonding quality of polyethylene joint coatings on oil and gas pipelines. Therefore, developing a high-precision, efficient and comprehensive ultrasonic NDT method for these coatings is crucial for ensuring construction quality, reducing future maintenance costs and guaranteeing the safe operation of pipelines. 

Principle of ultrasonic NDT. Ultrasonic NDT operates by measuring the distance between defects and the pipe wall through the time difference of pulse signals emitted by the probe as they travel to and from material defects. The size and orientation of defects are determined by analyzing the amplitude of the reflected echo signal and the probe’s emission position. Defects such as debonding, delamination and pores within the anti-corrosion coating or between the coating and the base steel pipe alter the sound pressure phase at the interface, enabling their detection and localization. This principle is grounded in wave theory. By integrating the acoustic parameters of both the anti-corrosion coating and the pipe material, a theoretical model for sound pressure phase change is established.  

Variations in acoustic impedance (Z = ρc, where ρ is medium density and c is sound velocity) among different media lead to changes in sound pressure phase. Consequently, parameters such as sound pressure phase and amplitude can be extracted and analyzed from the reflected echo signal. During testing, the actual detected phase is compared to the standard phase representing no defects, allowing for the calculation of the phase difference (Δφ). Utilizing signal processing algorithms, phase characteristics and local phase changes are analyzed, and combining this with amplitude attenuation of the reflected wave, the phase change is quantified using the formula Δφ = φ_tested - φ_standard. Based on the degree of phase change and amplitude attenuation rate, the type, size and location of anti-corrosion coating defects are determined, facilitating the assessment of the bonding quality of the joint coating and providing data support for the safe maintenance of the pipeline. 

Core Advantages and Promotion Prospects of NDT for Bonding Quality 

Working principle of ultrasonic NDT for the bonding quality of joint coatings. The pipeline joint coating comprises a heat-shrinkable tape layer and a pipeline layer. While ultrasonic NDT can be conducted using a probe placed outside the pipeline for simplicity, this direct testing method is often limited by the complex surface conditions encountered in practice. In some cases, direct contact testing is challenging, and the hard materials typically used for contact probes have significantly different acoustic impedance compared to heat-shrinkable tape, resulting in poor reflected waveform quality. In contrast, the water-immersion probe offers distinct advantages. Its acoustic impedance closely matches that of the anti-corrosion coating, allowing for more effective echo signal acquisition. By employing water-immersion coupling, the challenges posed by uneven surfaces on the testing probe are mitigated, leading to significantly improved experimental conditions. 

This method combines ultrasonic phased array technology, multi-probe setup, C-scan technique and large-area automated high-speed scanning. It utilizes a water-coupled nondestructive tester with dual-crystal probes arranged in a double-row configuration to assess the bonding quality of the heat-shrinkable tape on the pipeline joint coating. The probe emits a pulse signal onto the surface of the anti-corrosion coating. When the pulse signal excited by the probe passes through the 3PE anti-corrosion coating, the generated reflected and transmitted waves cause changes in the sound pressure phase.  

These changes are accompanied by variations in the intensity of the received ultrasonic signal, which are used to analyze the bonding quality of the joint coating. For instance, the pulse signal (T₀) emitted by the probe reaches the sample surface after traveling a certain distance, producing a surface echo (R₀) and a transmitted wave (T₁). When T₁ interacts with the outer surface of the anti-corrosion coating and the adhesive layer, as well as the inner surface of the adhesive layer, it generates echo signals (R₁ and R₂). The signal may also undergo multiple reflections within the pipeline layer, resulting in weak multiple bottom-surface echo signals (R₃₁ and R₃₂), which hold little research value. Consequently, the analysis primarily focuses on echo signals R₁ and R₂ to evaluate the bonding performance at the joint coating interface (FIG. 1).

FIG. 1. Theoretical model of testing with water-immersion probe.

The multi-probe phased array technology, supporting a frequency range of 0.3 megahertz (MHz)–24 MHz, is combined with large-area automated high-speed scanning to generate C-scan images. Experimental research establishes a correlation model between ultrasonic imaging of joint coating quality and bonding force, creating a parametric relationship between coating quality and testing features. This model links the strength of the received ultrasonic signal to various zones of peel strength, allowing the bonding quality of the pipeline joint coating to be visualized as a color-coded nephogram.  

In practical applications, probes are arranged in a specific pattern on the scanner of the ultrasonic testing system according to the width of the pipeline heat-shrinkable tape. The scanner conducts a thorough inspection of the entire tape while scanning circumferentially around the pipeline, with the results automatically displayed in image format. FIG. 2 illustrates an example of the ultrasonic NDT image of a standard test plate for the joint coating structure. In the sound field image, the horizontal axis represents the scanning distance, while the vertical axis indicates the size of the area covered by the sound beam, with signal amplitude represented in different colors. By analyzing the amplitude changes in the echo signal, the quality and location of defects can be identified.

FIG. 2. An example of the ultrasonic NDT image of a standard test plate for the joint coating structure.

This equipment has a wide range of applications, including routine inspections of pipeline anti-corrosion coating bonding quality, NDT of joint coatings during construction, spot checks during operation, inspections after peeling or repairs and traceability analysis of anti-corrosion coating quality during accident investigations. 

Application of ultrasonic NDT for the bonding quality of joint coatings. From 2024 to 2025, this equipment was deployed for onsite application tests at a natural gas pipeline construction site. A total of 135 joint coatings were inspected, with more than 20 identified for peeling verification based on inspection results. FIG. 3 displays the onsite inspection equipment, while FIG. 4 shows the comparison with peeling verification results.

FIG. 3a and 3b. The onsite testing equipment.

FIG. 4. Image acquisition and peeling verification of non-bonded area 1# and defective area 2#.

The verification results indicate that this NDT equipment accurately detects the bonding quality of joint coatings, providing precise positioning on pipelines without damaging the heat-shrinkable tape. Both coverage and accuracy rates for joint coating inspection reached 100%. The system is also capable of automatic scanning, increasing inspection efficiency by more than 200% compared to traditional methods, with daily inspection mileage exceeding 10 kilometers and inspection costs reduced by 40%. Applying this equipment on an in-service natural gas pipeline can shorten the construction period by 20%. 

Takeaway. Ultrasonic imaging NDT technology for assessing the bonding quality of polyethylene joint coatings on oil and gas pipelines enables routine inspections without compromising the heat-shrinkable tape’s integrity. It is effective for inspecting anti-corrosion coatings on in-service and under-construction pipelines, as well as for checking joint coatings after spot checks, peeling and repairs. Onsite application tests have confirmed 100% coverage and accuracy rates in joint coating inspections. This technology effectively ensures the safety of pipeline workers and nearby communities, demonstrating strong potential for widespread application. 

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About the Author 

WANG WEIBIN​ is a senior engineer for PipeChina, and a researcher for long-term engagement in pipeline/tank integrity management, inspection and evaluation, and new energy strategic emerging businesses​. He has led or participated in more than 14 scientific research projects at various levels, including the National Key Research and Development Program of China and the National Natural Science Foundation of China. Weibin has a solid theoretical foundation and rich practical experience in pipeline technology research and development, new technology research, promotion and application.​ Weibin earned a PhD in materials processing engineering from Beijing University of Technology.