Guided Wave Dispersion Curve
Guided waves propagate along the pipe axis and are restricted, guided by, and confined within the inner or outer diameter (boundary) of pipe. Since the wave is guided by the pipe boundary, the propagating wave should satisfy the boundary condition of pipe. This unique feature makes guided wave testing a very capable tool in long-range piping inspection from a single test location. The below figure is a dispersion curve showing the relationship between wave velocity and frequency in a 4.5-inch-OD and 0.337-inch-thick carbon steel pipeline. The pipeline figure illustrates three wave modes: torsional (T), longitudinal (L), and flexural (F). The velocity of the L- and F wave varies significantly with wave frequency and, therefore, these waves are dispersive. The velocity of the fundamental torsion wave (T(0,1) wave), however, is constant (that is, equal to the shear wave velocity of the pipe material) and therefore independent of frequency. The fundamental T wave is superior to the L- and F waves in application of long-range pipeline inspection because of the following reasons:
- The T wave is dispersion-free; therefore, any wave frequency can be used for long-range pipe inspection
- The T-wave does not interact with water or liquid due to its shear particle motion
- The T wave has a lower velocity than that of the L-wave so the T-wave has better sensitivity in defect detection at the same operating frequency
Even though a MsS probe can operate L-wave in non-dispersive region, it is recommended to use fundamental T-wave for pipeline inspection and monitoring due to the merits of operation.
If the guided wave testing system generates a single wave mode of the fundamental T-wave mode, a guided wave inspector can do a GWT inspection without any knowledge of the dispersion curve. Then, the guided wave testing becomes simply an ultrasonic testing inspection using the probe that generates ultrasonic waves of shear wave velocity in the material. This simple guided wave testing is mostly done by guided wave inspectors. An inspection report is made for facility owners and is easily understood. The report clearly shows defect location(s) and estimates of the defect(s) size.