Gas pipeline (onshore) defect assessment to API 579 - allowable stress...?

[jaypatel23]

Hi all

Performing a FEA assessment of pipeline defects (girth weld metal loss) to API 579.
(p.s. what the defects actually are & whether a FEA was warranted etc. are all valid concerns but not the topic for this thread!)

Starting with API 579, Section 5 (general metal loss) leads you to Annex 2D of same code for FEA guidance. Need to check against the plastic collapse & local failure criteria - and only an elastic analysis is being performed. Both of these require the value of "S" allowable stress to be determined, which is where I need some help.

API 579 Annex 2D references ASME BPVC VIII-2 Section 5, which for "S" references Annex 3-A, which in turn references ASME BPVC II-D. Pipeline material is API 5L X60 which from what I can tell is not listed in the latter code (in Table 5-A). Nevertheless, Table 10-100 gives the criteria for allowable stresses listed in Table 5-A.It seems that "S" = min (SMYS / 1.5, SMTS / 2.4) although its not very clear (but "manually" checked on some cases in Table 5-A).

QUESTION 1 - IS THIS CORRECT TO DETERMINE "S"? IF SO, IS THERE A CLEARER SOURCE FOR THIS DEFINTION? IF NOT, HOW DOES ONE DETERMINE "S" FOR A GIVEN STEEL GRADE?

Going back to API 597, paragraph 2D.2.2 states that it should be based on the design code use for the component so for a pipeline it is could be ASME 31.8. But this code doesn't really have an "allowable stress"; there are design factors for hoop and combined loading however.
(Side note: B31.3 (for piping, not pipelines) does have a table for allowable stress values, which interestingly appear to be different to the criteria in ASME II-D Table 5-A...).
Generally, there is no further mention of using component design code in API 579 Annex 2D or ASME VIII-2... For example, the latter again references Annex 3-A for value of "S".

QUESTION 2 - CAN THE PIPELINE DESIGN CODE BE USED FOR "S"? IF SO, HOW?!


Note: I have access to the following codes (in case there are differences):
API 579 = 2016
ASME BPVC VIII-2 = 2019
ASME BPVC II-D = 2013

Appreciate your input on the above.
Have tried with no luck, but is there any guidance on pipeline FEA defect assessment people can share? Plenty of courses on process equipment etc.

Thanks.

 

[TGS4]

What was the original Code of Construction?

 

[jaypatel23]

ASME B31.8

 

[TGS4]

Then you would get your allowable stress from B31.8.

 

[LittleInch]

Have you looked at ASME B 31G?

Pipelines have higher yield stress pipes and generally allow higher factors of yield stress than piping or vessel codes.

I can't see why you can't use either the allowable combined stress of 0.9 x SMYS or if mainly in the hoop direction whatever Df that section of pipe is built to () it varies by location class).

for info I'm not sure about ASME VIII, but B31.3 uses the lower of 2/3 SMYS or 1/3 UTS. in higher grade steels the UTS doesn't increase as much and hence you get little benefit form using higher grade pipe steels in B31.3 compared tot he pipeline codes.

which is why you don't design pipelines to piping codes.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[jaypatel23]

Thanks all.
In this particular case, the loading is largely from internal pressure (no bending, low temperature etc.). So it would be more conservative to apply the DF for hoop stress (vs combined loading). But is that over-conservative...? Of course, all good if it passes with the former!

Still feel the guidance in API 579 on allowable stress derivation is unclear (at least for pipelines)...

 

[TGS4]

You are right that the 2016 Edition is a little vague on allowable stresses for pipelines. That will be corrected in the next Edition.