Gas leaking from a natural gas pipeline is an undesirable event. It represents lost energy, lost income for the natural gas industry, and a potential safety hazard. If leaking gas comes in contact with a lit match or a spark from a vehicle, an explosion or fire could result.
Gas pipelines leak or fail largely because of damage from excavation operations; other causes are corrosion, construction defects, material defects, and outside forces. Such forces include earthquakes, washouts, landslides, frost, lightning, ice, snow, and errors or intentional damage by humans. In August 2000, a large natural gas pipeline failed near Carlsbad, New Mexico, as a result of corrosion. Twelve people died in the explosion, which created a 25-meter- (86-foot-) long crater, shown at right.
The natural gas industry keeps a database on pipelines. Each pipe in the database is tracked, and when a pipe is found to have flaws of a certain number and size, a decision is made to replace it. Replacing these pipes is costly in terms of down time, so the industry is looking for better methods by which to obtain more reliable, more accurate data on flaw numbers and size, including the probability that cracks are about to form.
Currently, the pipe characterization technique used most by the natural gas industry is the magnetic flux leakage method. This method induces a magnetic field on the walls of the pipe. When a change occurs in the pipe wall geometry as a result of wall thinning, cracks, or pitting, magnetic flux leaks through these areas, and a sensor detects the leakage. The location and amount of leakage indicate the site and size of the flaw.
Venugopal K. Varma, a mechanical engineer in ORNLís Nuclear Science and Technology Division, hopes to integrate novel hardware and software components to improve a flaw detection method that uses electromagnetic acoustic transducer (EMAT) technology. An EMAT device, designed by Pennsylvania State University, would have no contact with the pipeline. It would electromagnetically induce an ultrasonic horizontal shear wave and measure the amount of sound energy reflected from and the amount transmitted through the pipe walls. An algorithm developed by ORNLís Steve Kercel would then recognize differences in patterns of reflected and transmitted sound waves and correlate those differences with changes in the pipe material caused by corrosion, circumferential and axial flaws, and pittings, for example. In this way, the location and size of each flaw (e.g., its length, width, and depth) could be determined.
This approach also uses wavelet analysis not only to detect flaws but also to reduce the amount of data being stored. This work is being conducted by Kercel and Raymond Tucker, both of ORNLís Engineering Science and Technology Division. Reduction of the data stored during flaw detection is necessary because the equipment for flaw detection must make continuous runs for hundreds of miles before the growing amount of data can be retrieved and analyzed. Although the refinement of this technology is preliminary, Varma believes it would surpass magnetic flux leakage in predicting the formation of flaws along and around pipeline walls.
ďI believe our technology will be able to detect small axial and circumferential cracks, metal loss, and corrosion all at once, with one single pass,Ē he says. ďIf we can prove it works, it should be of interest to the natural gas industry because it would provide more accurate information for its database.Ē This research is being funded through DOEís National Energy Technology Laboratory.
Thomas Thundat of ORNLís Life Sciences Division is developing a method to detect tiny leaks of gas from pipelines using microcantilever technology. A cantilever is analogous to a microscopic diving board that vibrates at a characteristic rate. If exposed to a pipeline that is not leaking gas, it will vibrate at a rate related to air currents. If it is placed in the vicinity of a gas leak, the frequency of its vibration will change. The shift in frequency may indicate the rush of gas through a pipe crack. The microcantilever sensor will be mounted on the EMAT, enabling the detection of both leaks and flaws in the same pass.
Personnel in ORNLís Quality Services Division are monitoring natural gas pipeline construction projects for the U.S. Department of Transportationís Office of Pipeline Safety. According to Barry Oland of ORNLís Engineering Science and Technology Division, ORNL is providing inspectors and technical personnel under subcontract to oversee the installation of gas pipelines running from state to state, to ensure that the installations comply with federal pipeline safety regulations.
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