Abstract
The work presented in this paper shows that Fiber Bragg Grating (FBG) optical fiber sensors can potentially be used as receivers in a long-range guided wave torsional-mode pipe inspection system. Benefits over the conventional pulse-echo method arise due to reduced total travel distance of the ultrasonic guided wave reflections, since reflections from defects and structural features do not need to propagate a full round trip back to the transmitting collar. This is especially important in pipe configurations with high attenuation, such as coated and buried pipelines. The use of FBGs as receivers instead of conventional piezoelectric or magnetostrictive elements also significantly reduces cabling, since multiple FBG receivers can be placed along a single optical fiber which has a diameter on the order of only around 100 μm. The basic approach and sample results are presented in the paper. Additionally, a brief overview of some topics in ultrasonic guided waves is presented as a background to understand the inspection problem presented here.
Highlights
Guided wave ultrasound is increasingly being used to solve inspection problems in a wide variety of applications
The combined group 1 and group 2 signals both show good flaw detection capabilities. Both Fiber Bragg Grating (FBG) sensor groups detect the flaws at 7 feet and 10 feet from the collar location, even though multiple sensors in group 2 are on the far side of the pipe from the flaw to which they are nearest
This confirms that the potential to miss a flaw based on its relative location to a single sensor is greatly diminished using this approach versus placing a single fiber sensor at a single axial location
Summary
Guided wave ultrasound is increasingly being used to solve inspection problems in a wide variety of applications This is because guided waves can be used to inspect over long distances from a single probe position, offer complete volumetric coverage, and can inspect hidden/inaccessible structures, such as those under water, coatings, insulation, and soil. Before discussing the merger of ultrasonic guided waves with fiber optic technology, the major topic in this paper, a brief review of some of our first accomplishments on various topics in ultrasonic guided waves is presented. This illustrates the background leading to the topic of this paper. Some selected developments over the last few decades include the following
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