HYDROTEST ON SUBSEA PIPELINE

[NATIVES]

Pipeline is sitting at 10,000 feet water depth (approx. 4444 psi hydrostatic pressure). Pipeline needs to be hydrotested subsea. By that means, the testing equipment, pressure gauge, etc will be all located subsea at a water depath of 10000 feet. MAOP of the pipeline is 8000 psi, so the hydrostatic pressure should be 1.25 x MAOP = 10,000 psi.

The pressure gauge will be reading initial pressure as 4444 psi (same as hydrostatic pressure). Should the hydrostatic test pressure reading be 14444 psi (10000 + 4444 =14444 psi, the final guage reading) or just 10000 psi (by adding another 5556 psi (10000psi-4444 psi)?
My option is 14444 psi.
Thanks for your opinion.

 

[BigInch]

Natives,
Opinion doesn't have much to do with this.

Absolute pressure at 10,000 ft depth is

10,000 ft * 64 pcf /144 in2 + 15 psia = 4459 psia

If your pressure gage is at 10,000 foot depth and you pressure up the pipeline to 10,000 psig, it will read 10,000 psig ("g" does stand for gage, right.)

If your pressure gage is at the surface and you pressure up your pipeline to 1 psig, the pressure down in a pipe segment located at 10,000 ft depth will be 1 psig.

Maybe the attached spreadsheet will help, or possibly confuse you more. I hope it helps.

 

[NATIVES]

BIGINCH: Thanks for your response. I guess my question was what should be the test pressure reading on the pressure gauge when the "test pressure" is 10,000 psig? Should not it be 14444 psi since 4444 psi was the starting gauge reading ? Should not 4444 psig be treated as "0" psi for pressure test purposes?

Your statement : "If your pressure gage is at 10,000 foot depth and you pressure up the pipeline to 10,000 psig, it will read 10,000 psig ("g" does stand for gage, right.)" simply implies that we need to pressurize from initial 4444 psig to 10,000 psig to test the pipeline at 10,000 psig. I think, this way, the pipeline will be seeing stress of 5556 psi only, since initially 4444 psig was acting on pipeline externally as well as internally, effectively exterting no stress on the pipeline wall.
Thanks again for the attached spreadsheet. I will share it with my fellow workers.

 

[BigInch]

Gage pressure always reads the difference between inside absolute pressure and outside absolute pressure.

The pressure inside the pipeline will be 14459, outside 4459. The pipe will only feel stress caused by that same difference between inside pressure and outside pressure, 14459 psia - 4459 psia = 10,000 psi.