Plug the Closed Ends of a Pipe at Depth and Bring it to the Surface

[Guest_Alpha0]

Hi Guys - I have very simple question but I cannot get my head around.

Say if we cut pipeline into one section (12m) then close each cut end with temporary plugs
Water depth is 100m. Before the cut there is seawater inside the pipeline.
Now I understand there is no differentiated pressure between inside and outside of the cut section (pressure equalized).
Supposed we recover that cut section (plugged both ends) to surface. The outside pressure will be at 1 atm. Therefore the plugs have to be able to withstand at least 9 bar to avoid popping/bursting out by pressure inside.

Do I understand it correctly?

Appreciate your input here.

 

[Baluncore]

Welcome to PF.

Do I understand it correctly?

Reality is not quite that simple.
In theory, you can treat the water as being incompressible, so it's volume will not change when it comes to the surface in the sealed pipe. But the gas dissolved in the water is different. As the pressure is reduced it will come out of solution and so will apply pressure to the plugs. That is what happens to dissolved gas in the bloodstream of divers as they come to the surface.

If the plugs are able to withstand the pressure difference, the pressure inside the pipe will not be relieved and the gas will not come out of solution, well, not until the pipe begins to warm. As the internal water warms, the gas will come out of solution and the pressure will increase beyond the 9 bar, until something fails.

 

[Rive]

Do I understand it correctly?

Part yes. The pressure actually won't be that high. Within the usual range of 'pipe' and 'plug' the pressure will cause slight mechanical deformation, and that's just enough to lower the pressure.
I would dare not give an estimate, though.

 

[Guest_Alpha0]

Guys - Thanks a lot for the responses. I think I got an idea to work around the problem.
Thank you!

 

[Baluncore]

Consider putting a large bubble or pocket of air in the pipe at 9 bar before it is sealed. The pipe will then have no stress in the walls. As you bring the pipe to the surface the external hydrostatic pressure is reduced by 9 bar. The internal pressure remains at 9 bar, so the pipe is going to expand in diameter and lengthen slightly as it surfaces with the increasing differential pressure causing tension in the wall. The internal air bubble will largely maintain the internal pressure as the pipe increases slightly in volume. The amount the pipe expands due to differential pressure will also be a function of wall thickness, thin walled pipe will change more.

If there is no big air bubble in the pipe, then once on the surface where the pipe and water warm up, I expect that the water will expand faster and force out the end plugs, or split the pipe longitudinally. You would need to compare the thermal expansion of the water volume with the thermal expansion of the pipe material. An internal air bubble would prevent that expansion failure due to warming.