One of the most common and easiest inspection methods in the oil and gas industry is Ultrasonic testing or commonly known as UT. UT is a non-destructive testing technique based on the propagation of ultrasonic waves in the material tested. Usually, very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz, is transmitted into materials like carbon steel to detect material thickness or to find internal defect inside the material.
UT equipment consists of several parts that have different functions such as Pulser / receiver, transducer, and monitor screen. Pulser / receiver is an electronic equipment that can produce high-voltage electrical pulses. the transducer produces high-frequency ultrasonic energy. The ultrasonic energy is discharged and dispersed through or entered into the test material in a waveform. If there is a defect or discontinuity (such as crack) on the wave path, then some energy will be reflected back from the surface of the defect. The reflected signal wave is converted into an electrical signal and then displayed on the screen.
advantages of Ultrasonic testing:
- Sensitive to discontinuity on the material being tested
- have better or higher range for discontinuity detection than other NDT methods
- simple access from one side of material being tested
- High degree of accuracy in determining defect position,
- High degree of accuracy in determining estimated shape and size of the defect
- Simple equipment and easy to use, even for a new employee.
- Gives instant test results. No need to wait like on Radiographic testing
- can obtain a detailed description of test results
- can be used as a measurement of material thickness, like pipeline wall thickness, pressure vessel wall thickness or Tank wall thickness.
disadvantages of Ultrasonic testing:
- The surface of the material being tested shall be accessible for transmitting UT waves
- to become a reliable UT-Man, need further training and long working experience than other NDT methods
- Requires an intermediate medium (called a couplant) to transfer the sound energy in the test material
- it is difficult to be used for some material like irregular shaped, too small, too thin, or not homogeneous
- Wrought iron and rough-grained materials are very difficult to be inspected because the sound transmission will be low and there is a lot of noise
- usually difficult to detect a defect parallel with ultrasonic wave propagation direction
- Need a reference standard for tool calibration and characteristic analysis of the signal captured by the transducer