Probe separation distance (PSD) applied to an industrial girth weld application

Calum Hoyle
TWI, Wales 

Girth weld inspections play a key role in the maintenance and management of various different types of pipe that are mainly used within the oil & gas industry. The most modern inspection techniques use a combination of full matrix capture (FMC) and total focusing method (TFM) to produce accurate and high resolution images of indications within a girth weld. Through the exploitation of multiple modes, being the pulse echo response from both individual probes as well as the pitch catch response, when firing on one probe and receiving on the second. This combination of modes gives full coverage of the weld within the imaging area, giving an image of the weld cap, root, near and back wall, as well as any defects that might occur within the weld. This is achieved through fully calibrating the set-up ultrasonically. Part of the calibration set-up required is the distance between the first elements of each probe. This paper highlights the necessity of having a correct probe separation distance for the girth weld scenario. It shows through experimental results how changes in the probe separation distance (PSD) can affect the image that is produced. Different types of pipe, as well as different calibration samples, with different thicknesses and different known defects will be tested to show how an incorrect PSD can change the appearance of a defect as well as the known geometry of the testpiece.