Samenvatting

Het aantonen of corrosie gedetecteerd kan worden in de omtrek richting van pijpleidingen. Er wordt gebruik gemaakt van zeer dispersieve akoestische plaatgolven genaamd Lamb waves. Als transducers worden er Elektro-Magnetische Akoestische Transducers gebruikt.

Corrosion inside pipelines caused by salt water is a major problem for maintenance in the oil and gas industry. Corrosion reduces the wall thickness of the pipe. There is a need to determine the severity of corrosion inside a section of pipeline to decide if it needs replacement. A Non-Destructive Testing (NDT) technique using ultrasonic guided waves is one of the ways to accomplish this.
Guided waves in plates are called Lamb waves. These waves are guided by the pipe wall and can travel along the circumference. This allows a measuring unit consisting of a source and receiver on top of the pipe to detect corrosion at the bottom of the pipe without requiring direct access to it. The speed of the wave is dependent of its frequency and wall thickness of the medium. By sending a short sine burst and comparing the arrival times of this wavelet the wall thickness can be measured.
TNO wants to investigate the use Electro Magnetic Acoustic Transducers (EMATs) as the source and receiver of Lamb waves for sizing corrosion in pipes. EMATs do not require mechanical contact with the pipe wall to excite or detect Lamb waves. This is an advantage over the widely used piezoelectric transducers. The goal is to make a wall thickness profile of a section of pipe with defects.
The distance between the source and receiver EMAT is important to consider. Because of the geometry of the pipe circumference and different wave modes with different speeds, the wavelets might overlap at the receiver. Using a Finite Difference (FD) simulation a range of acceptable source receiver distances is identified.
Measurements have been carried out on three pipes: one with artificial defects (TNO pipe) and two with real corrosion spots (Shell and Dow pipe). Using the TNO pipe a measuring method is established. Measurements have been done with different centre frequencies of the wavelet. A centre frequency of 180 kHz was chosen as the optimal centre frequency. Measurements comparing sections of the pipe with mill scale and without mill scale reveal that magnetostriction inside the mill scale increases the overall amplitude of the signal. It especially increases the amplitude of the A0 mode.
A section of the TNO pipe has been measured and the results are compared to known defect locations. The defect locations are able to be identified from the measurements. A wall thickness profile has not been made but an approximation of the shape of the wall thickness profile has been acquired.
The approximated wall thickness profile of the Shell pipe has been compared to the wall thickness profile acquired from previous measurements. The general shape corresponds and the primary defect location is identifiable. The measurements on the Dow pipe underestimate the relative depth of the pitting corrosion. This might be caused by the small size of the pitting.