In-service Pipeline Repair of Sever Damage - Pressure Reduction Consideration

[auzie5]

Background:

EPRG and National Grid Transco (formerly British Gas) recommend pressure reduction to at least 85% of that at the time of the damage. When visual examination during excavation reveals the damage to be more than superficial or when ILI results categorize the damage as severe or extreme, National Grid Transco additionally recommends that the pressure be reduced to the lesser of 85% of that at the time of damage or to a value corresponding to a hoop stress of 30% SMYS. The 30% SMYS guideline is reportedly based on full-scale test results that show a rupture failure of part-wall or through-wall defect is unlikely at this stress level. Thus, the 30% SMYS guideline is based on considering the consequences of a failure.

Question:

Is the pressure value corresponding to a hoop stress of 30% SMYS based on nominal wall thickness? Or is it based on the discrete wall thickness at defect location?


Bonus Question:

Does anyone know where to find a copy of the following documents? I realize the first one may be an internal standard for the British Gas Co but if anyone out there is with them I would like to reach out to ask if they would be willing to share.

1. The National Grid Transco (formerly British Gas) document BGC/PS/P11 gives procedures to be adopted in the event that damage is detected in any of its transmission pipelines

2. The procedures have been described by Pallan (Pallan, W., 1988, “Transmission Pipeline Repair,” The Pipeline Journal, 102.)

 

[BigInch]

See page 10

http://www.penspen.com/wp-content/uploads/2014/09/time-dependent-steel.pdf

 

[auzie5]

Hi BigInch,

Yes that's the document I am referencing which lead to my confusion. Were you including it so that others can interpret it themselves or have I overlooked something in the document that clarifies if the hoop stress should be based on nominal versus discrete wall thickness at the defect location?

I suspect they mean nominal since they say the 30% SMYS guideline is reportedly based on full-scale test results that show a rupture failure of part-wall or through-wall defect. Discrete wall thickness of a through-wall defect would be 0mm…

I’d appreciate hearing your opinion on what value to use BigInch.

Thanks for the comment!

 

[auzie5]

Correction below (typo - unfinished sentence in my last post):

I suspect they mean nominal since they say the 30% SMYS guideline is reportedly based on full-scale test results that show a rupture failure of part-wall or through-wall defect is unlikely at this stress level. Discrete wall thickness of a through-wall defect would be 0mm…

 

[LittleInch]

Auzie,

Of course it's nominal. If it was anything else you would end up with a pressure of 5% of the original.

I probably have that spec somewhere, but there's no way I can upload to a website.

I looked up the 30% issue some time ago and there are lots of data which seems to show that for more modern steels - a lot of the original stud was done in the early 1970s - it was actually more like 45% before a hole would develop into a rupture.

It's pretty scary looking at a gouge in front of you with 70 barg on the other side but the cold reality is that if it was going to go pop it probably would have.....

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

 

[auzie5]

Thanks LI. I agree. I think I was just looking for the forum to reinforce my own opinion (I was especially hoping for a response from yourself, BI, zdas04, or any of the other great contributors).

Now consider this very interesting real life example (follow link to video - below).

Burst Test in a Controlled Environment:

https://www.youtube.com/watch?v=GIqCE3yLJ4U

The N-Vision data logger is great to have in the screen shot since you can clearly track increasing operating pressure until failure.

Test Results:
Failure occurs at ~2,787psi.

Thought experiment (Part A):

Assume:

We found this leak in service at 2,700 psi.

The actual failure (rupture) of the line will take place at some unknown higher pressure (or possibly will experience a “time dependent failure” if left at 2,700psi).

Let’s be conservative and assume the line is on the brink of failure at 2,700psi.

According to the empirical evidence developed by Battelle and British Gas, we might decide to lower the discrete operating pressure at the leak location to no more than 80% of what it was when the leak was discovered (i.e. 2,700psi is a pressure that we know did not rupture the line so a safe operating pressure to investigate and repair the leak would be 80% of that).

Therefore, a safe pressure for inspection and repair would be
• 2,700psi x 0.8 = 2,160 psi

From the video, 2,160psi occurs at the ~1:08 minute mark in the video (as can be seen on the N-Vision data logger).

Compare the leak characteristics at ~1:08 to the rupture that occurs at the ~2:30 minute mark (at ~2,787psi).

Visually and audibly, the decision to lower discrete operating pressure at the leak location to no more than 80% of what it was when the leak was discovered appears to be valid.

And according to the empirical evidence developed by Battelle and British Gas, 2,160psi should be below the “threshold for time dependent behavior” and therefore the leak should not experience a “time dependent failure” at 2,160psi if left at that pressure indefinitely (neglecting any other component loads and setting erosion effects aside).



So here are the thought experiment questions for you:

Grade and WT for this test segment are unknown but you knew it was operating at 2,700psi without rupturing.

Assume that 2,700psi puts this test segment on the brink of failure (seems conservative).

By lowering the discrete operating pressure at the leak location to no more than 80% of what it was when the leak was discovered (i.e. lower the pressure to 2,160psi) it is predicted that the line is now safe to inspection and repair.


Question 1: Do you believe the test segment is in fact below the threshold for “time dependent behavior” and therefore the leak should not experience a “time dependent failure” at 2,160psi if left at that pressure indefinitely?


Question 2: Would you go have your lunch with an umbrella under that leak if you knew nothing about the grade and WT of the pipe? (Keep in mind the person tasked with the repair just might).


Empirical evidence developed by Battelle and British Gas suggests this would be safe (strickly from a material performance point of view, not from a safe work practice point of view). And the video appears to support their recommendation.

Note that the Battelle and British Gas results are based on material performance independent of knowledge of the material properties.

(write down all your answers before continuing…)

Remember, we do not know the nominal or discrete wall thickness at the defect location so we cannot calculate a pressure reduction that leads to a hoop stress equal to 30% SMYS.



Thought experiment (Part B)

The fact is that that burst test was completed using NPS12 x 0.219”mmWT X42 pipe with arc burns placed all over it.

The predicted ultimate bust pressure using minimum tensile strength is calculated as:
• P=2(60,000)(.219)/12.75 = 2,062psi

The pipe failed at ~2,787psi or ~726psi (~35%) higher than predicted ultimate burst pressure (and it had arc burns all over it).

If we had discovered the leak at 2,700psi and instead decided to proceed with a pressure reduction that leads to a hoop stress equal to 30% SMYS based on a hoop stress calculation using nominal WT we get:

P=[2(42,000)(.219)/12.75]*0.3 = 433psi

So for this example, a pressure reduction that leads to a hoop stress equal to 30% SMYS based on a hoop stress calculation using nominal is clearly far more conservative than a pressure reduction to no more than 80% of the operating pressure when the leak was found (i.e. 2,160psi versus 433psi respectively).

But again, from that video, you would never know the grade or wall thickness or the fact that it is covered in arc burns so you would not be able to calculate what pressure results in 30% SMYS…


So here again are the thought experiment questions for you:


Question 1: Do you believe we are in fact below the threshold for “time dependent behavior” and therefore the leak should not experience a “time dependent failure” at 2,160psi if left at that pressure indefinitely?


Question 2: Would you go have your lunch with an umbrella under that leak if you knew the grade and WT of the pipe was X42 x 0.219mmWT (i.e. you knew that you were having lunch beside a pipe operating at above predicted ultimate bust pressure)? (Keep in mind the person tasked with the repair just might).


Question 3: How about knowing it is covered with arc burns? They didn’t fail at 2,700psi either so in theory, if we are in fact below the threshold for “time dependent behavior” at 2,106psi the other arc burns on the pipe should not experience a “time dependent failure” either (related to ductile crack growth...unless you have brittle hard spots?). Have you changed your lunch plans now? Would you send a worker in to repair this leak at this pressure?


Empirical evidence developed by Battelle and British Gas suggests this would be safe. And the video appears to support their recommendation.
Note that the Battelle and British Gas results are based on material performance independent of knowledge of the material properties.

 

[BigInch]

All the 85% reductions in pressure pre-assumes that the pipeline was operating at or below actual MAOP when the leak occurred.
I don't stay near any potentially high pressure leak for any longer than to figure out it's leaking.

Most incidents actually occur at pressures well below MAOP anyway, think about that one.

Assuming that it was operating legally (below MAOP to begin with), I suppose that if it's leaking, then the pressure inside would probably be 85% or less of the pressure at which the leak initially occurred anyway.

 

[auzie5]

BigInch,

I don't know why I was fixed on trying to apply those common pressure reduction practices to a burst test scenario. That was a terrible way for me to frame the problem which lead me down a very confusing road. Thanks for reminding me that the pressure reduction rules only apply to legal pipelines (i.e designed to <= 72% SMYS).

But spinning my wheels all day on this one inadvertently led me to some deeper appreciation for in-service repairs. I'll follow up with a post of some excellent literature in addition to the article we referenced earlier. All the research lines up with what we're doing day-to-day but now I have a deeper understanding of why it works out for us...and what margin of safety we're dealing with...

Line designed to 72% SMYS and operates somewhat lower (let's say 60% SMYS). Then we find a defect. Lowering by 20% again would give us 48% SMYS. Or if we're really worried we can opt for 30% SMYS. Defects in ductile steel line pipe tend not to rupture at 30% SMYS so when in doubt...

I can see clearly again...thank you. Time to keep moving forward (because hindsight can be embarrassing...).

Thanks for the help as well LI.

 

[BigInch]

Exploring limitations is good exercise.