Checking the thickness of a carbon steel pipe using ultrasonic testing (UT) techniques

Do you think it is possible to accurately check the thickness of a carbon steel pipe using ultrasonic testing (UT) techniques? An example of a pipe that was originally coated and has been in service for some time in a hostile environment is shown below.



It is suspected that the following types of pitting are present internally.



The coating thickness, which can also vary, will need compensating for at points 1, 2 and 3, as shown below.





Some factors that should be considered include:

• Lake-type pitting


If a spot-check grid system is used there is obviously a chance that the probe will not be placed over the pit. Line or area scans are more likely to detect this type of flaw.

• Cone-shaped pits

Often a total loss of all reflectors, which is similar to a loss of couplant. Evaluation of the remaining wall thickness may be difficult.

• Pipe-shaped pits

A loss of amplitude in the multiple reflected echoes is often the first indication.

From what has been shown, the simple thickness check has become more of a challenge. I also forgot to mention that you may be working at heights using rope access on a live plant with a wind chill bringing the temperature to below freezing, adding to the difficulty. The last survey showed that the wall thickness had only 1.5 mm of spare material before a decision as to whether to replace the pipe or to operate at reduced pressure was considered and the corrosion/erosion rate was 1 mm per year. The last survey was a year ago, resulting in the need for an accuracy of 0.5 mm or more. The part of the piping system that is more susceptible to degradation will need to be known in order to ensure that the minimum wall thickness is measured.

The oil & gas sector faces challenges such as this every day and has requested that PCN provides specific training and certification in this sector, which multiple Approved Training Organisations (ATOs) and Authorised Qualifying Bodies (AQBs) are now capable of providing, especially in the UK. A list of these can be found at: www.bindt.org/downloads/NDT_ATO.pdf (look for the UT column).

Accurate thickness checks are possible with proper training and understanding of the components but, without these, the chances are greatly reduced.

john.moody@bindt.org