RSTRENG MYTHOLOGY FOR PIPELINE DEFECT ASSESSMENT 1

[09091960]

Hi All,

Following are the summary of two defects( Named as 7&7A) found in
one of our pipelines. As these two defects are 300mm apart under the defect interaction rule we can consider as one defect.(Named as 7 com.)

defect 7 ( Assessed defect length of 2.060Meter with an effective defect length of 1.340 Meter)
defect 7A ( Assessed defect length of 1.6 Meter with an effective defect length of 0.850 Meters)

If we consider pipe line design factor of 1.39, both defects need to be repaired under the RSTRENG mythology. Further as per the attached RSTRENG calculations show that when two defects combined together, effective length only pointing out to area where the defect 7 is. Combined defect effective length is smaller than if we consider the two defects separately. Appreciate if any one could explain this situation.

 

[09091960]

Please find the attached combined defect calculations (Defect 7 com)

 

[cloa]

LOL- ;-)
You mean using the the RSTRENG method not mythology.

 

[09091960]

Thanks for pointing it out. I mean "RSTRENG methodology"

 

[brimmer]

I'll look at this when I get a bit more time, but you say the defects are 300mm apart and considered interacting? What interaction rule are you using? These defects if that far apart should not be interacting. Typically use 6t for interaction rule.

 

[brimmer]

I noticed in the calculations the effective length start and stop are the same in both (which should be the case), but in one the max pit depth is 3.11mm, and in the other it is 2.91mm, which explains a difference, is there a data entry error? Confirm your max pit depth.

 

[09091960]

Hi Brimmer,

Please ignore the grid 2 reading where it shows all the depth with 20% added to the original reading.Therefore 2.91mm is the correct reading taken from the field. As you have seen I have attached above in my mail defect 7 and the combined defect (7 Com). Herewith I will attach the 7a defect which you see in the 7 Com starting from the 2360mm mark to 3950 mm. As mentioned above
defect 7 & defect 7a both has failed but if you combined the two defect with the gap only one defect has failed. Further when you are carrying out the repair do you consider the repair to the effective area only given in the softwere or else do we have to consider the total length of the defect what you find it in the pipeline. In some defects I have noticed that effective area doesn't seem to be correct when you consider the area which is ignored from the calculation. Have you come across similar situations?

 

[brimmer]

I usually consider the repair area the effective failure area, if this area is repaired, the rest won't fail. This is at a minimum, it is you or company preference what you repair. The rest of the area I repair if I have a sleeve that fits over everything in stock available, I use it. If not, I repair the effective failure area only with the sleeve I have available. Sometimes corrosion is extensive over a joint, so I would only repair failed area(s).
I have to go back to interaction, these 2 corrosion areas are not interacting, I am not understanding why you consider these interacting when there is a 30cm gap between them? Can you look at the KAPA manual and what it uses for interaction?
The program will pick out the worst area, which it has done (the same area in both sheets). The program is also seeing these areas are not interacting. Keep these areas in separate calculations, burst pressure did not get worse combined because it is not worse combines, these are 2 different areas that would burst independently of each other and not interact, the program is seeing this.
7a was not attached, but I will copy grid and run through tomorrow when I get back to my office and write you then as well.
Other question, what is considered failure in your case? Burst pressure below 10177 kPa?

 

[09091960]

Hi brimmer,

Correct.Safety factor 1.39 gives you 10177.

Herewith I will attach you diffect for your reference.
Another noticable issue in this years dig up program is that significat difference in diffect lengths compair to pgging data.
We used the combine (CDP/AFD) pigging tools in our latest pigging run.
CDP - Corrosion detection tool
AFD - Axial flow direction tool

 

[09091960]

Hi Brimmer,

I'm having difficulty of uploading the file.

 

[brimmer]

AFD - axial flaw detection. Sounds like you are running a rosen pig. Question - does your pipeline have long narrow axial corrosion? If not, a normal MFL pig would do better. You will have varying degrees of success correlating the corrosion data. It really pays to have an inspection company out there who understands how to correlate ILI data. One thing to watch is grouping/clustering of the data by the ILI vendor, are you comparing individual features to individual features? Groups to groups? etc. Also, perhaps the ILI vendor has not reported corrosion <10% in depth, but the extra lengths you are seeing are minor superficial corrosion? In any event, if you have detailed corrosion measurements and photos, you can give this data to the ILI vendor and have them explain or re-grade the log.
KAPA is only picking out the worst area in the calculations, I believe that is all it will do, not show two separate areas. Remember, corrosion less than 10% is not a threat and considered normal wall in B31G and modified, RStreng anything less than 18% I believe.

 

[09091960]

In 2008 test has been carried out by running a CDP tool followed by AFD tool to only a slected sections of the pipe line. Results shows that significant increased in ERF>1 values when analysis carried out by the pigging company by using AFD/CDP combined method and it was the case in the field data as well. We have completed assessments of all the defects ERF> 1 within the years 2008-2009 period and currently we are down to the defects in the range of ERF values 0.98. As you have questioned many defects which was falled in to the catergory ERF>1 are long defects in the axial direction. Currently what we got is defects that are in the range of 200mm to 500mm long in the axial direction and the odd one with over 1M long.

 

[archeratslt]

Very interesting discussion, Thanks to all,

Dear 09091960,


Do you mind to share the output (KP, depht, length, WT, ...) from IP with us? I am working on something and need it badly.

Thanks in advance,

 

[amaple2010]

Great job! 09091960

Thank you so much for sharing!