Q: 16" buried pipeline hydrotest 1

[zengx]

Good day all,

We are hydrotesting a 16" CS buried pipeline with a crude oil service.
This will be done by 2 stages:
stage 1: 2 hrs strength test at nearly 1126 psi.
stage 2: 24 hrs tightness test at nearly 1070 psi.

At the end of stage 2, when we are reviewing the pressure and temperature charts, there was a 150 psi drop against a 3 degrees C drop only.

I couldn't find an acceptance criteria for such a case, but normally we estimate it. Like if there's 20 or 30 psi drop; we accept that. But 150 it quite a pressure drop.

Appreciating your inputs.

 

[BigInch]

Buried, fully longitudinally restrained, pipelines can have pressure changes of 75 to 120 psi per 1[°]C
Off hand I'd say you did well.

What would you be doing, if you knew that you could not fail?

 

[zdas04]

I would say that you did better than OK. If the bulk of the pipeline had really dropped 3C, the pressure drop would have been nearly 3 times as large as you saw. The temperature in a buried line is anything by homogeneous so that temp drop was certainly local to the instrument.

It has been a really long time since I did a 24 hour test (they were often required in the 70's, but by 1990 they had become rare), but the last one I did caused me to develop acceptance criteria as part of all of my test procedures. Typically I define "success" as any test that ends up with pressure at the test point higher than MAWP (the procedures do not allow adding water after the test starts, but you can bleed pressure off if necessary). It is pretty arbitrary, but I've never had an auditor take exception to it.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

"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]

I've got a spreadsheet that will keep up with theoretical vs actual pressure vs temp for a closed-in hydrotest segment including elevation profile in my trick bag, but it hasn't made it to market. It calculates the theoretical pressure at any point, given the test pressure reading and any number of temperatures along the pipe; the more you enter, the better the result. It predicts closed-in pressure at any point along the pipe, or alternatively the leaked volume if actual perssure is lower than theoretical. Do you suppose it's worth marketing?

What would you be doing, if you knew that you could not fail?

 

[zengx]

BigInch and zdas04: Thanks for your inputs; I was about to doubt my concepts. Appreciate it.

BigInch: I believe it's worth.

Mohammed Diab.
Inspection Supervisor - Saudi Aramco.

 

[zdas04]

I do the same calcs in MathCad.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

"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.

 

[racookpe1978]

BigInch:

Do you have a thermal lag rate (varying temperature (of the soil -> of the pipe & fluid)) vs soil depth, soil temperature for that spreadsheet?

Seems that, over a long period of 4-12 hours of the test, as the air temperature, sun's exposure, and soil temperature are going to vary, the internal pipe temperature is going to vary as well. But the pipe temp's are going to be slower, since the deeper you go, the less the soil temp will change.

So, whatever the temperature of the fluid is as it first goes in, over a period of time, it will (almost) assume the soil temperature around it, but will always lag the soil temperature, which is lagging the top few inches of soil temperature and air temperature.

My hydrostatic tests have always been in steam, oil, and water pipes out of the ground, so I don't have any experience to compare with your oil field/oil pipeline knowledge, but if 0.5 to 1 degree matters in correcting for temperature in a long pipe.... Topside, outside of the ground in power plants, the thermal lag is 100% in that "little bit of metal" around a large mass of water, so it was only pressure changes we monitored.

 

[BigInch]

I can add the thermal lag from soil insulation if you want, but 1 m underground it really isn't much. If possible, it is better to let the temperatures stabilize, meaning water temperature getting close to ground temperature and of course, to allow some time to settle and vent the air out of the pipe. With long pipeline segments, that can take the most patience. Beginning the test only after a stable pressures has been reached is important. What affects pipeline tests most temperature wise is if there are any still exposed portion of pipe in the trench and the test manifold heating up during the day. Once stabilized, I've found that the underground temperature doesn't change but for a very small amount, 1 to 2 deg C is common. The manifold could easily be 5C, or more on a hot day.

What would you be doing, if you knew that you could not fail?

 

[zengx]

BigInch: It's exactly like what you say...

After filling the water, we are giving 24 hrs for thermal stabilization between the water and the pipe.

There are no vents nor drains installed. Rather, for venting, we are running a foam pig forward ahead of the water being filled, and for draining, we are running the pig backwards.

There are 2 small portions of the pipe, at the start and at the end of the pipe section, above-ground to allow the installation of 2 temporary test headers for launching and receiving the pig.

Indeed, the manifold and pressure gauges are installed above ground, and are being affected by a higher temperature specially during the day. While the probe of the temperature recorder is dipped in the soil trying to read the temperature of the largest segment of pipe which is buried.
Thus, the pressure of the buried section cannot be actually measured. And we don't have a record of the above-ground temperature.

It would've been easier to re-test, but when this problem reached me, I found them already de-watered and the system is under positive pressure lay-up. Which leaves re-testing as an impractical solution at this stage, specially that it's a 30 kilometer pipeline section.

Mohammed Diab.
Inspection Supervisor - Saudi Aramco.

 

[zdas04]

The pig during filling helps very little. The water is saturated with air at atmospheric pressure and surface temperature. When you cool the water, and change the pressure (maybe you are going over one of the 200 m dunes that that field is famous for, pressure at the bottom is pretty high and when the pig goes over the top pressure gets pretty low) some amount of air is going to evolve out of solution. These air pockets can cause more trouble getting a good test than temperature problems.

The 24 hour soak is a good thing, but it is useful both for equilibrating temperature as well as for allowing the dissolved gases that will evolve out to do so. If you don't vent the gases off, the soak probably did more harm than good.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

"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]

Air temp is getting up into the low 40s in Sharjah in the afternoon, but I imagine that 1 m underground it's probably still only 24-26. Dry air temperatures in strong sun can change as much as 20-25C between day and night. Humid air maybe 10-15C. Soil 1 meter below, 1/10th of those.

I also like to have large capacity pumps and to fill the section as quickly as possible to cleanly push the air ahead of the water and without any pigs, given my choice. They can give you trouble in hilly terrain, but I suspect you have a flat profile and there are no pig run-away problems, so a pig might be working to your advantage in keeping water on one side and air on the other.

It is important to meter water pumped in and equate that to pipe volume of the test segment to get an idea of how much air might still be in there. Air in the line is the most problematic, since volume change of air pockets with both temperature and pressure, can make for very erratic readings. In any case Mohd you have apparently made a successful test. IMO the 24 hour stabilization time was the most important factor. No worries.

What diameter and length of test segment was involved?

What would you be doing, if you knew that you could not fail?

 

[zdas04]

He said 16" and 30km. Not a small test (around 250 m^3 if you figure ID^2 in inches coincidentally equals bbl/1000 ft).

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

"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.

 

[zdas04]

Make that 4000 m^3, it is late and I messed up.

David

 

[BigInch]

So he did.
So you did.

What would you be doing, if you knew that you could not fail?