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Nitrogen Mothballed Gas Pipelines

07/23/2011 2:27 AM

We are installing new offshore gas gathering and transmission pipelines and have to preserve the integrity of the lines for five years before commisioning natural gas through the lines. Our preferred option for the mothballing is nitrogen packing of the lines above atmospheric pressure. Can someone share ideas and thoughts on how we can top the nitrogen in the line to maintain the pressure in the line since leaks may be inevitable. The lines terminate at the blind flange of risers at the platform ends

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#1

Re: Nitrogen Mothballed Gas Pipelines

07/23/2011 3:01 AM

If this is offshore, and any pipelines are submerged in seawater, then the nitrogen pressure must be maintained greater than the seawater pressure at the deepest point. How about using compressed nitrogen cylinders with pressure regulators set accordingly?

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#2
In reply to #1

Re: Nitrogen Mothballed Gas Pipelines

07/23/2011 3:18 AM

Yes Tornado, the pipelines wil be submerged in seawater and you are right about the pressure. Any help with a schematic P& Id for the topping with compressed gas cylinders? Thanks

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#3

Re: Nitrogen Mothballed Gas Pipelines

07/23/2011 3:42 AM

For freshwater, 1 psi ≈ 2.31 feet. For seawater, I forget the density, but say 1 psi ≈ 2.24 feet. If the maximum pipeline depth is x feet, then the seawater pressure is x/2.24. Add maybe 15% to this for your nitrogen downstream regulator setting.

I don't have good drawing facilities on this computer, but a typical nitrogen cylinder regulator has two gauges; the upstream one shows remaining cylinder pressure, the downstream one shows the pressure in the pipeline, which would be set at 1.15 x/2.24 in this example.

By periodic inspection or telemetry, if the cylinder pressure falls close to the desired pipeline pressure, it is time to change to a fresh cylinder. With luck, this will never or seldom happen.

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#4
In reply to #3

Re: Nitrogen Mothballed Gas Pipelines

07/23/2011 5:04 AM

Thank you Tornado. More comments are still welomed

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#5
In reply to #3

Re: Nitrogen Mothballed Gas Pipelines

07/23/2011 6:14 AM

Shouldn't that be "x times 2.24"?

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#6
In reply to #5

Re: Nitrogen Mothballed Gas Pipelines

07/23/2011 11:36 PM

1 psi per 2.24 feet multiplied by depth in feet gives the units of psi. So Tornado did his algebra correctly.

Cheers !!

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#7
In reply to #6

Re: Nitrogen Mothballed Gas Pipelines

07/23/2011 11:59 PM

Second time not so goodly, but I must admit too I missed it, just from assuming it said what I thought it would. Cheers!!

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#8
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Re: Nitrogen Mothballed Gas Pipelines

07/24/2011 6:42 AM

My bad - sorry to Tornado.

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#9

Re: Nitrogen Mothballed Gas Pipelines

07/24/2011 6:52 AM

One would hope that as these lines are to be used for NG in 5 years time, there will be no leaks at this present time, as I guess the pipelines are steel and have been "bombed" to spec?

I have seen off-shore, a few years ago, at the blind flange a simple manifold with pressure gauge and simple pressure controller to "top up" the pressure inside the flowline down to the SCSSV. I guess it can also work manually too.

You might want to work out the volume of the flowline and how much gas will will need to complete this project, which end to you intend to install the equipment? Hopefully onshore!

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#10
In reply to #9

Re: Nitrogen Mothballed Gas Pipelines

07/24/2011 8:10 AM

The whole pipeline system is offshore. From one offshore platform to another and also from platform to subsea temporary PLRs until the gas gathering and compression platform is installed.

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#11

Re: Nitrogen Mothballed Gas Pipelines

07/24/2011 2:57 PM

Tornado's answer is bang on and Im sure he meant a system simular to this where you can connect an endless no. of cylinders.

A few simple mods to this type of system would work for you.

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#14
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Re: Nitrogen Mothballed Gas Pipelines

07/25/2011 2:35 AM

Yes, this picture shows what I meant.

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#12

Re: Nitrogen Mothballed Gas Pipelines

07/24/2011 3:14 PM

I'm curious to know who you work for and who owns the pipeline you are building. And where it is located.

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#13
In reply to #12

Re: Nitrogen Mothballed Gas Pipelines

07/25/2011 2:30 AM

normally for conservation with Nitogen you use a over pressure of 50 milli bar.

I suppose the pipe is empty, otherwise why do you want to fill it with Nitrogen ?

So the sea water pressur is only on the outside of the pipe, and you do not need that high pressure as stated above. For the rest the suggestions above are perfect.

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#15

Re: Nitrogen Mothballed Gas Pipelines

07/25/2011 8:21 AM

Bear in mind that the differential pressure at depth will be negligible and the differential pressure where the pipes come onto land will be around the pressure at depth. So the best leaks will be on land.

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#16
In reply to #15

Re: Nitrogen Mothballed Gas Pipelines

07/25/2011 10:17 AM

If the pipe doesn't leak, and both ends are above water level, the pressure inside the pipe will be atmospheric, plus the weight of the air below sea level.

The water has no effect on pressure inside the pipe.

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#17
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Re: Nitrogen Mothballed Gas Pipelines

07/26/2011 1:04 AM

Assuming any reversal of pressure on the (old?) seals doesn't result in leaking

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#18
In reply to #16

Re: Nitrogen Mothballed Gas Pipelines

07/26/2011 1:45 AM

True, but PW is speaking of the differential pressure between the gas (air or nitrogen) inside the pipe vs. the water outside. The internal gas could be pressurized anywhere from near vacuum on upwards, until the pipe ruptures.

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#19

Re: Nitrogen Mothballed Gas Pipelines

05/27/2017 7:33 AM

hi mates i have a similar problem. my company just built a 775km onshore pipeline which is suppose to be lying idle for 5yrs including some vessels and tanks and i have been task to work on a preservation method for it. please any idea how to go about it? i want to mothball it with nitrogen but still cant get the codes and calculations for it please will be very happy if you have any suggestions

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34point5 (2); Brave Sir Robin (1); brich (1); ddaniel (3); Henk klep (1); JohnDG (2); lyn (2); PWSlack (1); Rescue (1); samuel anang (1); Tornado (4)

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