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Development of a Prototype Magnetostrictive Sensor Torsional Wave Probe for Detection of Corrosion in Insulated Piping Filled with Liquid, 14-9228

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Principal Investigators
Hegeon Kwun
Sang-Young Kim

Inclusive Dates: 11/14/00 - 03/14/01

Background - Under a joint sponsorship by thirteen companies from the oil, gas, and petrochemical industries, SwRI has developed a magnetostrictive sensor (MsS®) technique and system for long-range guided wave inspection of piping for corrosion, particularly those pipes under insulation. The SwRI-patented technique involves launching a pulse of guided wave along the length of pipe and detecting signals reflected from defects at the position from which the original wave was launched. Using primarily longitudinal guided wave mode [more specifically L(0,2) mode], the developed system is capable of inspecting up to 24-inch-diameter pipes with a typical testing range of 200 feet (100 feet in each direction) from a single MsS location. Since mid-1999, efforts have been undertaken to transfer and commercialize the developed technique and system to various industries. The commercialization efforts in the oil and petrochemical industries, however, have been hampered because of the difficulty in inspecting liquid-filled pipes using the L(0,2) mode due to liquid-induced dispersion effects that produce extraneous signals that obscure the detection of defect signals. This difficulty can be overcome if, instead of using the L(0,2) mode, the torsional (T) mode is used for inspection. This mode, which is a shear wave, does not interact with liquid and, hence, suffers no degradation in inspection capability in the presence of liquid. To facilitate the commercialization of the MsS technology in the oil and petrochemical industries where the majority of piping carries a liquid product, development of a T mode MsS has become necessary and urgent.

Approach - To generate and detect T mode waves, the MsS requires a direct current (DC) bias magnetic field in a circumferential direction. The required bias magnetic field can be conveniently established by flowing a high DC along the lengthwise direction of the pipe. Because of a potential risk in generating sparks, however, this approach is not permissible in the oil and petrochemical industries. An alternate approach is using an array of MsS plate probe and magnet assemblies placed around the circumference of the pipe and operated in the shear horizontal mode (SH) in plate simultaneously. This approach requires curved MsS plate probes that conform to the pipe's outer diameter and DC bias magnetic circuits that apply magnetic fields along the circumferential direction of pipe. Although more complex than the DC electric current approach, this method could be implemented if the cost for the probe/magnet assemblies for various pipe sizes is economical. An economical T wave inspection may be achieved by combining data obtained by scanning around the pipe using a single probe and magnet assembly, instead of using an array. The objective of this quick-look research project is to investigate the feasibility of achieving T wave inspection using a single probe/magnet assembly.

Accomplishments - The feasibility of achieving T wave inspection by using a single probe and magnet assembly and combining the scanned data around pipe was investigated on 6- and 16-inch-diameter pipes. The scheme was not feasible because of the large amount of flexural wave modes generated as a natural consequence of asymmetric wave excitation around the pipe circumference and the inability to cancel these flexural waves by combining the scanned data. These flexural modes produced a high level of background noise and poor defect detectability. To operate in the T mode and suppress the flexural wave modes to an acceptable level, a relatively symmetric wave excitation around the circumference using an array of probe and magnet assemblies was necessary. A new sensor concept that does not require an array of probe and magnet assemblies and is economical and simple to use was subsequently developed and proven experimentally. An invention disclosure for the new sensor concept was submitted.

For more information, please contact Glenn M. Light, Ph.D.

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