Abstract
Manually applied ultrasonic (UT) weld flaw evaluation of size and shape requires well trained, experienced operators. Available automated ultrasonic (AUT) systems simplify evaluation by producing a real-time hard copy of the flaw parameters. The same instrumentation can also be applied for thickness mapping of vessels for corrosion surveys.
UT flaw evaluation also requires the operator to determine whether a flaw is critical (cracklike) or benign (rounded, irregular) since cracks generally must be removed and repairs made. Since operator flaw evaluation is prone to error, computerized flaw signature analysis methods applying measurements of UT signal rise time, fall time and pulse width are successfully being applied to sort cracks from benign flaws. The same computerized procedures are also applied manually for direct measurements of these time domain flaw parameters on the flaw detector CRT.
Accoustic emission testing (AET) was used experimentally for identifying the location of flaws in structures being stressed. Inconsistent initial results from these prior tests gave low confidence to the method. However, interest in AET was revived with improved instrumentation and understanding of the technique. From the good experience gained on AET of a large number of vessels, Exxon is making greater use of this technique for successfully locating cracks (SCC, fatigue, etc.) in metal pressure vessels. In addition to flaw detection, Exxon has had good success applying AET for locating leaks in underground piping and storage tank bottoms.
An experimental technique, currently under development, involves the use of impact analysis to locate corrosion in piping. The method is applied by local excitation (impacting) the pipe along its length; recording the resulting noise signal and analysis of this signature for frequencies characteristic of vibration modes related to the wall thickness near the point of impact. Others have used this technique on large structures (oil rigs, drill platforms, bridges, etc.) to locate cracked or missing members. Several promising laboratory tests have been conducted, however, additional effort is required to evaluate the effects of pipe size, insulation, support location, etc. on the capabilities of this test method.
