Literature Review on Dual Sensing Applications for SHM of Pipelines
By D Liu & S Hurrell
Background
Structural integrity management of pressure containing structures in process plants, particularly pipelines in refineries, is still an industrial challenge for several reasons:
1. Pipeline infrastructure accounts for the largest number of assets.
2. 71% of reported accidents are attributed to pipelines or pipe-works.
3. Pipeline’s deterioration is commonly caused by external or internal corrosion with over 80% of pipeline operating costs being corrosion-related.
In order to mitigate risks, plant operators conduct specialized non-destructive testing (NDT) along the pipeline network so as to identify and retrofit critical locations. This NDT is expensive, location specific and usually mandated by reactive maintenance. 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 expensive and often provide low resolution when identifying potentially problematic areas. Both methods reveal the state of assets at the time of inspection, but they cannot predict or provide early warning to critical failures. For these reasons, continuous monitoring of structures and machinery is becoming normal in the oil & gas industry. The implementation of a long-range technology capable of providing early damage detection, especially from corrosion, is still unresolved.
TWI has long range acoustic emission (AE) and guided wave ultrasonic (GWU) testing technologies. It is of interest to TWI to qualify the AE technology for early damage detection in pressure pipelines using conventional lead zirconate titanate (PZT) sensors. Although detecting active corrosion with a 10 meter sensor spacing is not feasible, the detection of the pit-to-crack transition and crack propagation stages may be possible. This could provide an early warning of damage and at the same time identify problematic segments of pipe for further investigation.
GWU testing is regularly employed for screening pipeline sections for metal loss and hence corrosion problems. It is an active technique, as opposed to AE that is a passive technique, which is able to locate and size qualitatively areas of corrosion. AE and GWU are therefore complementary.
This project will use GWU inspection practices for identifying corrosion areas on a 10 meter long pipe. Pipe samples will be pressurised and structural health monitoring applied using AE and GWU. GWU and AE transducers are normally discrete however, in this study, guided wave (GW) transducers will be investigated as sensors for AE testing. Further, digital methods of combining the data from both technologies will be sought.
Key Findings
- Using AE tests for detecting corrosion and other environmental damage mechanisms is more difficult than detecting initial fracture or fatigue crack growth.
- Similarities in ultrasonic wave propagation in both AE testing and GWU testing, suggests the methods could be combined using one transducer/sensor.
Impact
With regard to the project’s objectives, the literature search has shown:
1. No published information is available that quantifies AE’s detectability and GWU sizing accuracy of the pit-to-crack transition.
2. Digitalisation techniques for combining AE and GWU have been investigated by TWI in previous collaborative projects.
3. Previous work by TWI has validated AE and GWU test results using Phased Array Ultrasonic Testing methods to visualise the extent of corrosion and measure corrosion pit depth.
4. For the optimum implementation of AE testing in corrosion monitoring, the literature search has revealed the problems in signal interpretation, and offered some solutions that will be trialled in the experimental work. For the implementation of GWU testing, TWI, having developed the Teletest®, has more than 20years in best practice.