30" dia oil pipeline derating

[10815L]

Hi all,
we are in the process to derate the 30" API 5L B dia oil pipeline (158km), wall thickness is 80% gone due corrosion and this line was installed some 40 years back.
as per barlow's equation MAWP is coming 112 psi,my question is, can i further derate the line as operation manager requested to derate untill line is composite repaired.I don't have copy of B31.4 for reference.
thanks
10815L

 

[TD2K]

I don't think B31.4 is the appropriate reference, that's the design code for the pipeline when it was installed. I'd be looking at API 570 "piping inspection" to see if that's a better choice.

 

[pmover]

Yes, B31.4 is the correct reference.

"(c) If a piping system is derated to a lower operating
pressure in lieu of repair or replacement, the new maximum
steady state operating pressure shall be determined
in accordance with para. 451.7."

my 2006 version of B31.4 does not have paragraph 451.7, but I think the correct reference is 451.6 Pipeline Integrity Assessments and Repairs.

Paragraph 451.6 is too many pages to incorporate in this posting.

hope this helps.
-pmover

 

[zdas04]

None of the codes really control this. Usually the limiting factor is company policy.

I assume you solved Barlow's formula for pressure and put SMYS and the actual wall thickness (no future corrosion allowance) in to get the 112 psig (I get 122 psig using 20% of 0.375 in, but I assume that the wall thickness isn't exactly 20%). Most companies have a maximum ratio of calculated stress to SMYS, and the number is often around 20%, so if you put that number (say 20% of SMYS, which I'll assume is 42,000 psi) and a 2/64 corrosion allowance I get a new MAWP of 25 psig.

For this kind of problem the answer really doesn't matter (a wide range of answers are all pretty much supportable), the important thing is that you pick a method, document it, and follow it. If you use 100% of SMYS and no corrosion allowance you should put steps in place to never go above your 112 psig (ESD, PSV, pump kills, etc). Then if the pipe does fail you have an Engineering basis for having selected the wrong MAWP ("wrong" being defined as a pressure where the pipe failed anyway).

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[BigInch]

ASME B31G
Describes one method for determining the remaining strength of corroded pipelines.

https://law.resource.org/pub/us/cfr/ibr/002/asme.b31g.1991.pdf

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[10815L]

Hi David,
Here is correct information, piping class is API5L X52 and MAWP with out Corrosion allowance is 137 PSIG, MAWP method is given in API 579.
C/A 2/64 i can't understand it because menon does not discuss about it, can you please explain bit more.
Thanks
10815L

 

[racookpe1978]

How accurately do you know your existing corrosion condition and thus "max lost wall thickness"?

For example, it sounds like you've surveyed it pretty closely, or you would not be pulling it out of service, but how much margin do you "need" in today's enviro-regulatory-attitude to risk a break and lost oil? Is there a significant probability some part of the pipe might be pitted twice as deep as the "normal" parts you have measured?

As pointed out above, your failure point will be at the place where current corrosion is high enough, and new local stresses high enough, that otherwise sound metal around the initial failure also rupture.

 

[10815L]

Sir,
our inspection department used smart piging for thickness servey and they have give these values, the line is runing through desert area in north african country.
I want your input for fitness of this line because right now no plan to replace it. may be infutre if this line brust three / four points and lost tons of oil then may think to replace it.
Thanks
10815L

 

[BigInch]

Will you at least plese read B31G.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[ElSid1]

B21G latest edition is 2009. This link may help

http://www.novanumeric.com/samples.php?CalcName=B31G

 

[ElSid1]

Correction ... B31G latest edition is 2012. (Do not know how to edit a previous post) Last one I have access to is 2009.