One of the longstanding problems facing the global oil & gas industry has been how to efficiently detect corrosion under insulation (CUI) in pipework. Over the years, a plethora of non-destructive testing (NDT) solutions have been developed to solve this problem with varying degrees of success. One technology that stands out however, is Guided Wave Ultrasonic Testing (GWUT).
Guided Wave Ultrasonic Testing works by generating mechanical stress waves that propagate along an elongated structure while guided by its boundaries. The generated waves can travel a long distance with little loss in energy, thus making them efficient.
How It Works Exactly
A guided wave test involves attaching an array of low frequency transducers around the circumference of the pipe to be tested. The transducers generate an axially symmetric wave that propagates along the pipe in both the forward and backward directions of the transducer array. The equipment operates in a pulse-echo configuration where the array of transducers is used for both the excitation and detection of the signals.
When the wave gets to a location on the pipe where there is a change of cross-section or a change in local stiffness, an echo is generated. Based on the arrival time of the echoes, and the predicted speed of the wave mode at a particular frequency, the distance of a feature in relation to the position of the transducer array can be accurately calculated.
GWUT uses a system of distance amplitude curves (DAC) to correct for attenuation and amplitude drops when estimating the cross-section change (CSC) from a reflection at a certain distance.
Guided Wave vs Conventional Ultrasonic Testing
Guided Wave Ultrasonic Testing is markedly different from conventional Ultrasonic Testing (UT), in that it uses very low ultrasonic frequencies compared to those used in conventional UT which are typically between 10~100kHz.
In contrast to conventional Ultrasonic Testing, where only the area underneath or in the direct vicinity of the transducer is inspected, GWUT allows the entire pipe wall to be screened from a single transducer position within the diagnostic range of the test. This feature makes GWUT an ideal tool for screening long lengths of pipes for defects and locating them for prove-up inspection.
Advantages of GWUT include:
• Large sections of piping can be inspected rapidly and safely often with no scaffolding;
• Minimal removal of insulation is required for Corrosion Under Insulation (CUI) inspections;
• Detection of internal or external metal loss;
• Localized damage can be pinpointed and characterized as to length and depth;
• GUL is an excellent tool for cased crossings and unpiggable pipes;
• Point of contact corrosion can be found without lifting pipes;
• Data collection is fully automated ;
• GWUT can be used to inspect buried and road crossing pipes;
Today, GWUT is widely used to inspect and screen many engineering structures, and it has found great relevance in the oil and gas industry, where it is increasingly being used worldwide to test in-service pipework for corrosion.
Adoption of GWUT technology is on the rise in Africa, particularly in the Nigerian oil and gas industry where it has been found to be an effective and economically-viable non-destructive pipeline testing method.
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