Development of Structural Health Monitoring Technology for Pressure Containing Pipelines using Long Range Ultrasonic Testing and Acoustic Emission
TWI Industrial Member Report 1215-2026
By Dandan Liu
Industrial Need
The structural integrity management of pressure containing structures in process plant, particularly pipelines in refineries, is still an industrial challenge for several reasons. Firstly, pipeline infrastructure accounts for the largest number of assets. Secondly, 71% of reported accidents are attributed to pipelines or pipework. Thirdly, a pipeline’s deterioration is commonly caused by corrosion either external or internal with over 80% of pipeline operating costs being corrosion related. In order to mitigate risks, plant operators conduct specialised non-destructive evaluation (NDE) along the pipeline network to identify and repair possible critical locations. This NDE is usually mandated by reactive maintenance and can be expensive and location specific. Larger coverage inspections, such as pigging or in-line inspection (ILI) are also carried out as part of the integrity management plan. However, they are still costly and often do not provide sufficiently high resolution data for identification of potentially problematic areas. Both methods reveal the state of the asset at the time of inspection, but they cannot predict or provide early warning of critical failures. Consequently, continuous monitoring of structures and machinery is increasingly adopted as standard practice within the oil and gas industry. Nonetheless, the implementation of long-range technologies for early damage detection, particularly corrosion, remains an unresolved challenge.
The application of Acoustic Emission (AE) technology in pipeline structural integrity monitoring offers several significant advantages. Firstly, AE enables real-time monitoring of dynamic changes within the pipeline and provides early warnings when flaws or damage begin to develop through analysis of acquired signals. Secondly, AE sensors are typically installed on or near the surface of the pipeline without direct contact with the internal fluid medium, ensuring that the fluid remains uncontaminated and normal pipeline operations are unaffected during monitoring. Thirdly, AE is highly sensitive to minor changes within the pipeline and can cover large areas, allowing simultaneous monitoring of multiple sections. By using a suitable arrangement of sensors, comprehensive monitoring of the entire pipeline system can be achieved.
Finally, AE can detect the formation of various flaw types, such as cracks, corrosion, welding flaws, stress concentration areas and local deformation, by analysing characteristic parameters of AE signals. When combined with operating conditions, material properties, historical data and other relevant information, AE facilitates a comprehensive assessment of the pipeline’s overall structural health.
Long Range Ultrasonic Testing (LRUT) is a guided wave technology designed for elongated structures such as pipelines and rails, enabling long-distance inspection from a single accessible point. LRUT is regularly employed for screening pipeline sections for metal loss and associated corrosion issues. Unlike, AE which is a passive technique, LRUT is active and cannot only locate but also qualitatively assess flaws and areas of corrosion. AE and LRUT are therefore complementary methods.
In this project, LRUT was used to identify the location of corrosion on a 10 metre long pipe, followed by AE implementation for structural health monitoring under internal pressure and fatigue loading in the localised area.
Key Findings
- There is strong evidence that the rate of change in cumulative acoustic emission (AE) hits over time correlates with the corrosion rate of the carbon steel pipe.
- AE results are significantly influenced by background noises in pressure and fatigue resonance tests. The rejection of some low-amplitude and low-energy crack signals does not affect the identification of crack propagation processes, as higher energy AE signals remain detectable. Source localisation provides a highly effective method of filtering out noise in these tests.
- Corrosion patches are not always fatigue-critical locations and welds exhibited greater sensitivity to fatigue cracking in this study.
- Cracks were detected much earlier using the AE technique compared to strain gauges.
- Long Range Ultrasonic Testing (LRUT) accurately detected the corroded areas but was unable to detect the evolution of small cracks beneath corrosion pits.
Impact
A sensor array comprising multiple sensors can monitor extensive pipeline networks. Both sensor arrays and LRUT technologies are effective in detecting the presence and approximate locations of corrosion. LRUT can evaluate changes corrosion patch sizes, while the corrosion rate can be inferred from the cumulative rate of AE signals. Combining these technologies provides a comprehensive description of corrosion patches and subsequent cracking. LRUT offers the capability to size and locate welds and dormant corrosion locations, whereas AE primarily detects active corrosion or crack growth. However, cracks initiating beneath corrosion pits or welds may be missed by LRUT because their crack signatures are masked by the pit or weld in LRUT results.
AE technology also presents some limitations, notably high background noise level during field monitoring. This can be mitigated by source location or parameter-based filtering with thresholds optimised case by case. Despite these challenges, AE trials demonstrated that signals associated with corrosion and cracking differ in their parameters, enabling their use as an early warning indicator for crack initiation.
The key challenge of AE, however, lies in the number of sensors required to achieve adequate sensitivity across large pipelines. Therefore, a hybrid monitoring approach is considered. Firstly, LRUT can be employed to identify critical areas, followed by targeted AE for focused long-term monitoring of those zones.
AE could also be applied exclusively to high criticality or high stress regions. This strategy delivers both extended coverage and cost-effective operation.