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Acceptance levels are not included in this standard, but can be applied depending on the scope of testing. The requirements are applicable to both manual and mechanised testing.

5.3.2.3 Ultrasonic guided waves Guided waves are low frequency ultrasonic waves that propagate along the length of a structure, guided by and confined in the geometric boundaries of the structure. Several modes of propagation can be used. Commercial equipment is available, working either by using piezoelectric elements or magnetostrictive sensors, and this has led to a strong increasing use on site especially for corrosion detection in pipes and piping.

Defect

Pipe

Transmitted Wave

Reflected Wave

Guided Wave Probe

Ultrasonic Wave propagates through tube wall

Figure 5.18 Ultrasonic guided waves propagation principle

Guided waves are sensitive to changes in cross section (increase or decrease) of the waveguide mainly by reflection. Other interaction modes are possible, such as mode conversion and diffraction, but they are not widely used in field applications. Guided waves are a screening technique and are generally not appropriate to characterise accurately the indications detected. The technique can classify the indications from their amplitude and when calibration can be carried out, the loss of cross section can be assessed. They are generally not appropriate to determine the residual thickness except in special cases. The position where the indication is detected can be assessed at ± 50 cm. An ISO standard prepared by IIW Sub Commission VC will be issued soon. Several Projects (MOSCO in France, OPCOM in Europe, Buried Pipe Integrity initiative in US) aiming to assess performances of guided waves inspection are being undertaken.

Figure 5.19 Testing loop built to assess performance of guided waves ( Reproduced courtesy: Institut de Soudure)

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