Extension of Nonlinear Harmonics to Determining Delayed Failures in Damaged Pipelines, 18-9449

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Principal Investigators
G. Graham Chell
Alfred E. Crouch

Inclusive Dates:  01/01/04 - Current

Background - Delayed failures occur in gas and liquid transmission pipelines when a pipeline is mechanically damaged (e.g., gouged by digging equipment) and cracks initiate that propagate over time to cause pipe rupture. Delayed failures are a fairly small fraction of total pipeline incidents but can have tragic consequences, resulting in loss of life. The Institute has led the development of nonlinear harmonics (NLH) as a potential inline inspection method for detecting damage in pipes. The NLH method consists of using an alternating magnetic field impressed into a thin layer on the undamaged inner surface of a pipe to sense the strain anomaly produced by damage on the outside. Recently the NLH method demonstrated a capability for detecting gouges in pressurized pipes and ranking their severity against burst pressure, but the major concern of the pipeline industry and regulatory authorities is delayed failures.

Approach - The approach adopted in the current research is to demonstrate through use of cracked laboratory specimens that NLH can detect changes in strain anomalies produced by propagating cracks, allowing the remaining life of a damaged pipe to be assessed. The crack-free back surfaces of tested specimens are periodically scanned using NLH sensors to detect strain anomalies produced by growing cracks. By comparing NLH signals measured at different times, the increase in the likelihood of delayed failure (i.e., reduction in remaining life) will be assessed.

Accomplishments - It has been demonstrated that NLH sensors can detect strains and strain changes under complex stressing conditions, indicating the ability of NLH sensors to detect strain anomalies. Preliminary data from NLH sensor scans indicate the sensors are capable of detecting cracks (see illustration), but more data are needed to assess the detailed sensitivity of the NLH sensors to crack sizes and crack growth rates.

This illustration shows that NLH sensors can detect cracks in a laboratory specimen. These measurements, made on the crack-free face of the specimen after 1,000, 10,000 and 26,000 fatigue cycles (going from left to right in the illustration), reveal a changing pattern caused by the growth of cracks, which are indicated by dashed lines because they are hidden from view on the other face.

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