Assessment of internal corrosion defect 1

[09091960]

Hi All,

Based on the ILI data, we are planning to excavate internal corrosion defect in our pipe line. Once excavated what sort of NDT methods can be used to find out the length, bredth and wall loss of this internal corrosion defect?

 

[rneill]

There are a variety of different tools available depending on what type of anomaly it is you are trying to verify. examples include:

- Hand Held Low Resolution Yoke: looks for cracks, surface breaking indications, etc.
- A variety of hand held UT probes and tools are available that are good for evaluating either metal loss (general or pits) or cracks.
- Automated UT is available and consists of a motorized arm that travels vertically and horizontally on rails to map out the anomalies.
- Laser mapping can be used for external corrosion and is more accurate than traditional methods such as pit gauges and depth micrometers.
- Eddy current lift off measurement arrays can be used.

and of course, you could always cut out the section of pipe and then open it up to visually examine and evaluate internal corrosion defects.

 

[unclesyd]

What is the size of the reported defect in what size pipe?

Does the report say anything about the wall loss?

 

[09091960]

Size of the pipe is 6". Reported defect size is 75mm x 50 mm
with a wall loss of 19%.
are planning to have SLOFEC inspection method.Have any one with passed experiance with this system? How accurate for the size of the defect which I'm face with?

 

[deco0404]

This may sound like stating the obvious, and perhaps I am reading this incorrectly, but as you are removing this section why do you need to do NDT on it? As it is a small section of 6" pipe you could just put it on your desk and measure with a ruler! .....but you already seem to know what size the affected area is.........

If you know exactly where the corrosion is, x-ray is also a good way of determining length & breadth........

To be honest though, this is quite a sizeable area of 6" pipe, and I would be just as concerned with what is causing the corrosion, rather than sizing it!

 

[rustbuster]

Slofec is more of a screening tool, I would go with AUT or Phased Array for acurate sizing.

Good luck

 

[rneill]

Once you've field verified the anomaly dimensions, you might want to give some thought to what method you'll use to evaluate the remaining strength of the pipe. While ASME B31G is quick and easy, it might result in fairly conservative results for such a large area of metal loss (as might the RSTRENG-85 method).

I'd consider taking accurate river bottom profile measurements of the anomaly and evaluating using the RSTRENG EFFECTIVE method which takes into account this profile rather than just applying the deepest spot to the entire length.

More info on RSTRENG EFFECTIVE can be found in the paper "Continued Validation of RSTRENG", PR 218-9304, Pipeline Research Committee, AGA (authors, Kiefner, Vieth, and Roytman)

 

[09091960]

Well we normally use RSTRENG TO assess external corrossion defect. My concern is to get the accurate river bottom profile for the internal corrosion.
From the sample we got it from the internal surface of the pipe (black powder) mineralogy of scale deposit shows
Sulpher - Dominant
Magnetite (Fe3O4) - Sub dominant
Siderite (FeCO3) - Accessory
Mackinwaite (FeS.9) - Trace

will these composition aid the corrosion process?

 

[09091960]

Operating condition for the subject gas pipeline

MAOP- 7.322 MPa
Dia meter - 8"
Wall thickness = 4.78 mm
Material - API5LX42
Design factor- 0.72

 

[brimmer]

Since the corrosion is internal, the only way you are going to be able to get detailed measurements from the outside is with UT. The SLOFEC method will find the corrosion area, but won't give you detailed profile measurements you will need to run RSTRENG. Use automated UT scanner, or use manual UT, if manual someone will have to draw a grid on the outside of the pipe and search for the deepest area of corrosion in each grid square, somewhat time consuming. Also, if manual UT make sure they have a smaller diameter probe, don't use a typical half inch probe, as it is too large to get enough detail, use a pencil probe or small diameter probe. You have to specify, otherwise they typical NDT "company" or "guy" will not care but give you garbage results. By the way, 19% in depth you are right on the limit of even being able to use RSTRENG as the feature is not deep, based on your parameters you don't even fail B31G, you pass with a safety factor of 1.65, and RSTRENG near 2.

 

[09091960]

Thanks everyone for your input to this topic. Special thanks to brimmer for nailing it.

 

[KlausTruelsen]

I would like to second what brimmer says and recommend the ASME FFS-1 (API 579) standards as a way to evaluate the corrosion dammage. This standard will provide a less conservative approach as well as provide a solid framework for the assessment.

Also my experience with metal loss corrosion is that automated UT scans are the way to go. does hovever requirer an experienced NDE "guy".

Best of luck.