Hydrotest pressure drop analysis 1

[nesamani1983]

Hi all, We have natural gas Pipeline laid in 1997. Pigging is done quarterly. API 5L X46 PIPE. Wall thickness= 4.36 mm.
We have done the hydrotest of 4'' 2.61 km long in service natural gas pipeline. The test pressure kept as 15 kg/cm2. But the pressure started dropping, to reach 10.8 kg/cm2.
I have the following doubts

1) The pipeline was connected with 03 flanges. Can it be the reason for the pressure drop?
2) How to identify the leak point, if any?
3) We are planning to pressurize the pipeline to 10.5 kg/cm2 & again observe for 24 hours. Will it pass, by any chance?
4) The pressure & temperature profile is attached here with. Kindly review and is it possible to calculate the hole size?
5) What is the way ahead to get the solution?

Thanks in advance.

 

[LittleInch]

1) What are 03 flanges when they're in town? Not a normal designation. But unlikely.
2) Look for wet patches along the route of the pipeline (seriously)
3) Doesn't seem likely and why back to 10.5 and not your test pressure of 15?
4) You can estimate the leak volume using bulk modulus and expansion of the pipe
5) Find the leak, cut it out and fix it.. But at 4" and 2.61km and only 4.4mm think probably easier to replace it or internally line it with a PE pipe.

It's beyond its probable design life and if it hasn't been maintained or subject to inspections or CP monitoring is basically too old and knackered to warrant spending another rupee on it. IMHO.

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

 

[StressGuy]

And, what's the reason for doing a hydrotest on this line now?

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[nesamani1983]

@Stressguy: Actually the pipeline was handling wet gas. Hence we wanted to do Integrity Assessment of this pipeline.

@littleinch: It's CP protected. In 2019, DA Direct Assessment done. Found no external corrosion in ECDA. 04 locations dig verification done with metal loss of 6% in ICDA. MAOP recommended was 49. Safe Pressure was 180 kg/cm2. But MOP is 5 kg/cm2.

How to identify the leak location? Till now, no wet patches observed in ROU.Any other methods?

 

[1503-44]

Fill it with gas and sniff test for methane.
Why was that not done before hydrotest.
Add mercaptan and nose test it.
Ultrasonic leak detector.
Infrared photography.
Increase the test pressure to 180. You will also find out how reliable your remaining wall thickness calculation is.


--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

That's almost certainly internal corrosion.

It's a fairly small leak so a pin hole / several drops per minute and in a 4" pipe very difficult to find a you can't send a pig down to find it.

Only other option is cut the line and segregate into two lengths then test each one then when you know which half do the same thing until your down to say 200m then just replace that 200m.

Or internally line it/ insert a PE pipe.

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

 

[LittleInch]

There are ultrasonic probes which can be used at intervals of about 100m I think which pick up the high frequency of a leak and can guide you to the location.

But always think of what the replacement cost is a otherwise you spend a lot of money fixing a 28 Yr old thin pipeline which could fail next year.

EDIT - Also search for "acoustic leak detection"

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

 

[TiCl4]

Any chance the water used to fill the pipeline was warmer than the underground temp? If you fill a pipe with warm water with no residual air and button it up, the pressure will drop dramatically as the liquid cools and contracts. Did you give it sufficient time to come to thermal equilibrium before sealing it for the hydrotest?

 

[LittleInch]

TiCl4 - You might have a good point but a pity it wasn't held for a full 24 hours as you would expect temperature to go up a little bit later in the day.

The pressure and temp rates kind of track each other and 1 C could be up to 3-4 bar difference. important when you're operating at such a low pressure test.

Brown is temp, Blue is pressure

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

 

[nesamani1983]

Thank you all for your response.