Dec 2013
Gas Transmission
Manual Phased Array for small diameter offtake weld inspection.
Dec 2013
Apr 2014
National Grid Gas Transmission
Robert Bood,
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Network Innovation Allowance
Gas Transmission Networks
Last year, during grit blasting, a weld on a 2” offtake branch started leaking gas through a pinhole sized flaw. The subsequent investigation found that this was due to subsurface weld defects joining together to provide a gas path. The defects were not known about because small (around 2”) branch offtakes have weld geometries that prohibit inspection for subsurface defects via normally employed techniques such as radiography and manual ultrasonics. A small number of similar leaks have previously occurred and present a potential hazard to personnel. Also, without full weld inspection there is the potential for other defects to be present in these types of welds that could present integrity issues.

Manual Phased Array (MPA) is an advanced ultrasonic inspection technique that allows versatility in controlling the angle and focusing of the ultrasonic beam. In the hands of a skilled operator it has the capability to detect subsurface defects in components that are difficult or impossible to inspect using other techniques. MPA has not been used for these components in National Grid, and from discussions with inspection experts it does not seem to have been used for this elsewhere despite it having the potential to be successfully employed. This project will investigate if the technique can be used for inspection of welds on small offtake branches in the geometries typically used by National Grid.

The objectives of this project are to determine if MPA can be used to detect the presence of subsurface defects in the fillet and butt welds of small diameter branch offtakes, and if so, to develop the method into a fully specified practical technique.

Provide National Grid with detailed knowledge about the MPA technique and its applicability for use in the NTS in detecting defects in the butt and fillet welds of small branch offtakes.