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- since 1996 -
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Technical Discussions
UT Man
UT Man
08:23 Apr-19-2009
UT signal question

area of suspected pitting


Normal backwall echo




I have scanned a dry gas lift line 8" diameter, carbon steel. It is 30 years old. The pipe was found to be good internal condition with clear and strong backwall echos, however at the 3 o'clock position I completly lost the BWE and had a multifacted signal roll along the timebase between 3.5mm-7mm with a full screen height peak at 5mm. It looked awfully like internal pitting to me due to the multifaceted signal and the fact it was rolling. As mentioned there completed backwall loss. It was approx 10mm x 10mm in size. The boss is now having us check it with shear waves, and looking to radiograph it next week. Any other input on what this could be would be greatly appreciated. The engineers dont agree it is internal wall loss or ptting due to the service of the line and location of area. My job now depends on calling this right!

I have attached images of the screen.

Epoch XT, 0 degree 5 Mhz dual crystal 10mm probe, sensitivity 2nd BWE to FSH

Thanks very much

    
 
 Reply 
 
Chris Chartier
NDT Inspector, Welding Inspector, Advanced NDT, DR, Pipeline Inspector, Welding Inspector, Pipeline Integrity SME
NDT Group Inc., Canada, Joined Oct 2008, 15

Chris Chartier

NDT Inspector, Welding Inspector, Advanced NDT, DR, Pipeline Inspector, Welding Inspector, Pipeline Integrity SME
NDT Group Inc.,
Canada,
Joined Oct 2008
15
15:53 Apr-19-2009
Re: UT signal question
In Reply to UT Man at 08:23 Apr-19-2009 (Opening).

Paul,

Just because there is no internal metal-loss mechanism present doesn't mean it is impossible to have internal metal-loss. Manufacturing metal-loss from the steel and/or pipe mill is possible and I have seen it isolated and clustered with depths ranging from 20% to 85%. These features were present in natural gas pipelines with no apparent internal corrosion mechanism. They can be better characterized by AUT, RT and in extreme cases, cut it out.

    
 
 Reply 
 
Udo Schlengermann
Consultant, -
Standards Consulting, Germany, Joined Nov 1998, 177

Udo Schlengermann

Consultant, -
Standards Consulting,
Germany,
Joined Nov 1998
177
18:22 Apr-19-2009
Re: UT signal question
In Reply to Chris Chartier at 15:53 Apr-19-2009 .

Hello,
Because the pipe was in service for 30 years there is reasonable suspicion for gas attack from inside. Sour gas e.g. causes hydrogen to penetrate into the metal and collect at internal impurities in steel. At non-metallic inclusion layers resulting from the steel rolling process the steel sheet delaminates internally. There is no corrosion visible on the pipe internal surface, but internally the steel may be delaminated in steps like terraces. Because of the trapped gas the layers cannot be penetrated by ultrasound anymore (loss of BW echo). This damage of course weakens the pipe wall without any loss of material. If additionally pitting corrosion from inside reaches such an internal delamination suddenly the remaining wall thickness is only a small part of the original one and the pipe may burst.
Best regards

    
 
 Reply 
 
S.V.Swamy
Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex , India, Joined Feb 2001, 787

S.V.Swamy

Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex ,
India,
Joined Feb 2001
787
18:37 Apr-19-2009
Re: UT signal question
In Reply to UT Man at 08:23 Apr-19-2009 (Opening).

A borescopic inspection should confirm whether it is pitting (depression) or not. I would have said it looks more like clustered porosity. But a radiographic test would probably give some definite idea.

    
 
 Reply 
 
Ryan Burns
Ryan Burns
02:24 Apr-20-2009
Re: UT signal question
In Reply to S.V.Swamy at 18:37 Apr-19-2009 .

If you can truly walk the signal from nominal WT through to the minimum thickness without intermittent signals from the backwall, I would agree that you have pitting. However, I would also have changed to a smaller diameter transducer to ensure that what I am seeing is true wall loss and not HIC, MIC, H(L)THA, or something else weird. What is your material? What is the temperature of the line? Could it be ERW pipe? IF you are on the seam, I have seen indications of non fusion that are a little tricky to map out. Looks like you are dealing with Sch 100 though, which should be seamless. Wish I had a scope and the access to help you out. Like Chris mentioned, run a C scan with rastered SW to see what you have. Good luck.

Ryan.

    
 
 Reply 
 

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