Hydrotesting 100 foot long offshore pipeline at onshore shop

[NATIVES]

A 100 foot long pipe is going to be installed at a water depth of 8000 feet. External hydrostatic pressure is 3550 psi. Pipeline MAOP is 5,000 psi and hence hydrotest pressure is 6,250 psi. Pipeline is going to be hydrotested onshore prior to installation.
1. Some of us think pipeline should be tested onshore at a pressure of 6,250 psi.
2. Some think it should be tested at 6250 - 3550 = 2700 psi only. There is no need to test it at higher than 2700 psi. Logic is that if the pipeline would have been tested offshore after installation at a pressure of 6250 psi, the net pressure the pipe wouldhave seen is 6250 - 3550 = 2700 psi. So, pipeline should be tested at 2700 psi onshore.

who is correct?

If the option #2 is correct, what happens if the external hydrostatic pressure is also 6250 psi? No need to hydrotest the pipeline at all?

 

[cloa]

Don't double post- kill your offshore structures forum post as its not a offshore structure- its piping.

 

[BigInch]

NONE OF YOU ARE RIGHT.
That makes me feel more than a little bit disappointed, as this isn't much more than K-12 physics.

Since MAOP units are usually referenced to atmospheric pressure, I will assume that you meant 5000 psiG (referenced to atmosphere) pressure capacity, 5000+14.7 = 5015 psia internal pressure (when referenced to anywhere).

Now, if you put the pipeline under a uniform 8000 ft seawater, external pressure is 3555 psi + 14.7 psia = 3570 psiA.

MAOP, the maximum pressure difference between internal and external pressure, according to API RP 1111, is 5015 psiA - 3570 psia = 1444 psi
MAOP is limited to 80% of hydrostatic pressure, BTW, fortunately much greater than 1444 psi.

Testing the pipe to 1.25 x MAOP requires a pressure differential between internal and external pressure of 1.25 x 1444 = 1806 psi

When you test it at 8000 feet under the sea, the internal test pressure therefore must be at least 3570 psia + 1806 psi = 5376 psia
If your pressure gauge is measuring the difference between internal and external pressure immediately across the pipe wall, it will read 1806 psig (referenced to -8000 ft of seawater). If you measure the pressure with a gauge referenced to surface pressure, it will read 5376 - 14.7 = 5361 psig (referenced to atmosphere at 0 ft MSL)

If you do test it at the surface, the hydrotest pressure need only be 1806 psiG.
Note: If a pipeline, you MUST hydrotest it after installation, NOT before.


"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[NATIVES]

Big Inch: First all, I have not forgotten K-12 physics. 14.7 psi is too small number to play with in this calculation. So I ignored this number to make things simple.
I have a few questions, though.

The well is flowing into the pipeline with a pressure of 5000 psi at a water depth of 8000 psi. Why should not the pipeline be tested sub sea at a pressure of 6250 psi (5000 x 1.25)?
In reality, the pipe wall will see 5000 psi internally when the well is flowing regardless of water depth outside.

Quote: "MAOP, the maximum pressure difference between internal and external pressure, according to API RP 1111, is 5015 psiA - 3570 psia = 1444 psi"
What will happen to this calculation if internal and external pressure are the same? Will RP1111 give MAOP of zero? No need to hydrotest sub sea or onshore?

 

[BigInch]

B31.8 ss A841.3.2 (b) Internal and external pressure. Refers to API RP 1111.
B31.8 ss A842.2.2 a) Hoop Stress. For pipelines and risers the tensile hoop stress due to the difference between internal and external pressures shall not exceed the value given below.
Sh = (Pi-Pe) * D/2000/t
or,
Sh = (Pi-Pe) * (D-t)/2000

Maximum differential pressure is the case. Your pipe will never see a 5000 psi max differential pressure. Consider an onshore low pressure pipeline, Inlet pressure required is 100 psig, outlet pressure required is 50 psig. MOP = 100 psig. Internal pressure is 114.7 psia, yet the pressure design condition is 114.7 psia - 14.7 psia = 100 psi. If that pipe was on the International Space Station, the design pressure would be 114.7 psi. If that pipe was installed 33 ft below water level, the design pressure would be 0.

If maximum internal and external pressure are equal, internal design pressure case = 0

Test pressure, see A847.2
The hoop stress shall be checked to see that it is within allowable limits considering both the internal and external pressures when determining the maximum hydrostatic test pressure.

Your controlling wall thickness design case is probably going to be local buckling and crushing from external pressure.


"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek