Transmission, Distribution, & Refining
Ultrasonic Measurement of Plastic Strain in Pipelines


The goal is to develop nondestructive ultrasonic measurement methodologies to characterize natural gas pipelines suffering exterior mechanical damage, as a means of determining the likelihood of eventual failure. This will help to maintain the integrity of the nation’s natural gas transmission infrastructure.

There are a variety of nondestructive methods for locating areas of a pipeline that have suffered mechanical damage (i.e., bends, dents or dents with gouges), either by a third party or from natural events (e.g., landslide). When such damage is located however, the pipe must be exposed and inspected to determine if the extent of the damage requires replacement. Currently, there is no reliable, nondestructive method for determining if the damage is sufficient to materially affect operational safety.

The ultrasonic methods being developed under this project utilize electromagnetic acoustic transducers (EMATS) for measuring residual stress and plastic strain distributions in natural gas pipelines. Anomalies in these characteristics within localized areas of pipelines that result from third-party mechanical damage are potential causes of subsequent pipeline failure. Such delayed failure can occur because gouges and dents provide an initiation site for crack formation. Localized changes in curvature and wall thinning due to the damage can lead to cracking if the line is pressurized for higher service pressures or during hydrostatic testing. If these methods can be employed to relate ultrasonic data to mechanical properties, and if these mechanical property data can be related to safe operating ranges, a means for distinguishing between critical and non-critical damage could be developed. Such methods could then be proven for use with an in-line inspection platform.

The characterization of stress states and the relationship between these mechanical properties and ultrasonic measurements increases in complexity as the nature of the damage moves from simple bends, to dents, to a dent with a gouge. 

Test rig for pipeline inspection

Performers: Pacific Northwest National Laboratory (PNNL) – project management and research products

Richland, WA 99352

Project Impact:
Of particular importance to the Nation’s natural gas infrastructure is the accurate prediction of the lifetime of damaged pipelines due to outside force. In order to accurately predict the remaining life it is essential to accurately determine the stress and strain in the damaged region. Currently there is a significant technological gap inhibiting accurate diagnostics and prognostics for pipeline life assessment.

The potential impact of this technology, if developed to the point of accurate and dependable evaluation of damage to pipelines from outside force, is that it could greatly enhance the overall safety and reliability of natural gas delivery using the pipeline system. The types of damage (dents and bends) for which this technology is being developed often go undetected by traditional inspection techniques due to re-rounding of the pipe from internal pressure. This results in a greater potential for pipe failure. If this type of damage is found and properly evaluated it could reduce the amount of lost product from leaks at such sites, reduce potential for environmental impact from gas leak emissions and increase the safety and reliability of the pipeline infrastructure by reducing the overall likelihood of pipeline failure.

During the first phase of the project, researchers established a relationship between the change in ultrasonic response (shear wave birefringence) and plastic strain with excellent agreement between laboratory test measurements and theoretical predictions. This was accomplished by:

  • Performing measurements on a pipeline in a laboratory setting while pressurized with water to establish a relationship between ultrasonic response and stress and strain in a biaxial stress state.
  • Performing measurements on plastically deformed and ruptured tubes and compared results with theoretical predictions of strain.
  • Designing and building a simple test rig to enable data to be acquired inside a pipeline while moving.

The first phase of this project involved establishing the relationship between residual strain and the change in ultrasonic response (shear wave birefringence) in laboratory samples under a uniaxial load. Initial measurements on samples in both axial and biaxial states showed excellent correlation between shear birefringence measurements and finite element modeling (FEM) models. In the second phase these measurements were compared with theoretical predictions. The excellent agreement between experimental measurements and theoretical predictions for the uniaxial deformation provides motivation and insight for applying this methodology to the more complicated stress and strain gradients expected in dented regions. In addition, a simple test rig that will allow the collection of data while moving through a pipeline has been built.

During Fiscal Year 2004 the main task was the completion of design work toward participation in the NETL-sponsored demonstration testing of advanced pipeline inspection sensors conducted outside Columbus, Ohio, September 13-17, 2004. During preparation for and participation in this demonstration, researchers:

  • Designed and fabricated a simple cart to center the residual stress/strain sensors in the inside of the pipe.
  • Designed and obtained a 24-inch diameter test pipe with calibrated dents for evaluation and calibration of system capabilities.
  • Performed Finite Element modeling of dents in calibrated pipe.
  • Performed measurements on dented pipe with sensors on cart.
  • Completed analysis of data collected during demonstration testing and provided results and feedback for inclusion in the demonstration final report.
  • Presented project results at 9th SPIE Annual International Symposium on Nondestructive Evaluation for Health Monitoring and Diagnostics and the Annual Review of progress in Quantitative NDE meeting.

The results achieved have shown a direct empirical and quantitative correlation between the speed of ultrasonic waves and the degree of stress and strain in metal. In addition they have demonstrated the feasibility for ultrasonic shear wave birefringence to accurately determine the severity of damage in dented pipelines. These results show that the non-contact ultrasonic methods can be utilized in real time on the inside of pipelines to map the severity of dents. These results are very encouraging and show that ultrasonic measurements have the potential to accurately asses some forms of damage in dented pipelines. The ultrasonic measurements are sensitive to the degree of stress and strain in the specimens and can be applied to bent sections as well as dented regions. In addition, the EMAT sensors have shown potential for being configured for use on PIGs and inspection robots.

During the phase of work which included FY 2005 and the 1st quarter of FY 2006, several advancements were made towards characterizing mechanical damage in pipelines damaged by dents. The project advanced the data analysis and understanding of the interrelationship between the ultrasonic measurements and the residual stress and strain in the pipelines (see figure below). The correlation between the ultrasonic measurements and the finite element prediction of the residual stress are very good.

The ultrasonic measurements near a plain dent and the corresponding residual stress in a 24” diameter pipe.

The cart utilized in the field test in FY 04 has been updated and advanced significantly. The cart is capable of scanning the inside of a pipe to detect and characterize mechanical damage from denting and bending of the pipe. The EMAT sensor can be scanned axially and circumferentially to map out the contours of the plastic deformation. A preliminary graph from the field test in January of 2006 (see figure below) shows that dent locations and length can easily be seen in from the decreases in the amplitude of the signal and the corresponding mechanical damage can be assessed from the birefringence.

Preliminary results from the January 2006 field test showing distinct indications of dents and the corresponding thickness independent ultrasonic measurement indicating the degree of mechanical damage. The amplitude of the ultrasonic signal and the ultrasonic measurement are plotted as a function of distance along the pipe.

The motorized cart can be utilized for field inspection and for testing various speed and data acquisition parameters for integration with appropriate robotics platforms.

During Fiscal Year 2006 the main task was the completion of modified design work toward participation in the NETL – DOT sponsored demonstration testing of advanced pipeline inspection sensors conducted outside Columbus, Ohio, January 9-12, 2006. During preparation for and participation in this demonstration, researchers:

  • Modified cart to carry the residual stress/strain sensors in the inside of the pipe.
  • Performed measurements on dented pipe with sensors on cart.
  • Completed analysis of data collected during demonstration testing and provided results and feedback for inclusion in the demonstration final report.

During the 2006 demonstration testing Measurements were performed on two 24 inch diameter pipes containing dents and dents with gouges. All dents were successfully detected and estimates of the size were provided. The ultrasonic strain measurement correctly ranked 7 out of the 9 reporting locations for 100% detectability and 77% accuracy on ranking severity.

The sensor was a non contact electromagnetic acoustic transducer (EMAT) that was scanned along the axis of the pipe at several distances from the dents placed at top dead center. The resulting data clearly showed a deviation at each reporting location with a dent and no deviation where there is no dent. The ultrasonic shear wave birefringence is independent of thickness which is critical for characterizing mechanical properties due to deformation because a simple thickness measurement is NOT an accurate assessment of strain.

The inspection speed was as fast as 5 inches per second and the electronics can operate as fast as 4 or 5 feet per second (~3 MPH). The measurements were performed in a 24 inch pipe and are amenable to pipes as small as 4 inches in diameter. The technology proved to be very sensitive to mechanical damage due to dents and is also ideal for application where pipelines are bent due to subsidence or other earth movement. This technology is ready for incorporation onto robotics platforms and for field testing and subsequent commercialization for specific applications.

Current Status and Remaining Tasks:
All activity under this effort has been completed. Final results are incorporated into the 2006 Demonstration final report and the associated appendix.

Project Start: August 13, 2001
Project End: December 31, 2005

DOE Contribution: $1,093,000
Performer Contribution: $0

Contact Information:
NETL – Richard Baker ( or 304-285-4714)
PNNL – Paul D. Panetta, Ph.D. ( or 509-372-6107)

Additional Information:
Final Project Report - July, 2006: Pipeline Inspection Technologies Demonstration Report [PDF-6.26MB]

Final Report Appendix [PDF-4.99MB]

Technical Project Report - August, 2005: Characterization of Plastic Strain in Pipelines [PDF-908KB]

Progress Report [PDF-159KB] - August, 2004

Status Assessment [PDF-115KB]

DOE Fossil Energy Techline: National Labs to Strengthen Natural Gas Pipelines' Integrity, Reliability

Pipeline Inspection Technologies - Demonstration Final Report [PDF-1530KB]