leak test of underwater pipeline 1

[Tess72]

Case of a 40 years old oil pipeline crossing a canal in parallel with other hydrocarbon pipelines. Following a leakage detection, the oil pipeline was repaired by posing internal sleeve of thermosettling resin. the subsequent 48h hydrotest of the pipeline did not show any leakage, even if temperature drop and consequently pressure drop were registered during the night. After 2 months a leakage was detected again in the canal. Now the owner is commissioning a pneumatic leak test procedure on the portion of pipeline crossing the canal, not relying on the previous hydrostatic test (mostly because after one hour of a pigging with nitrogen of the line bubbles were observed on the surface of the canal), to confirm the leakage from that portion or from one of the parallel pipelines.
In my opinion the execution of a pneumatic test would require the navigation to be prohibited in the canal and the parallel pipelines to be emptied. I would suggest to carry out a second hydrotest in the portion of pipeline crossing the canal.
Is my assumption correct? would you carry out the pneumatic test? Any idea on how to confirm/exclude the leakage from the specific pipeline in a more safe way?
thank you in advance

 

[LittleInch]

Can you get access to one side or the other to insert a pig, whether crawler ot blwo though and do an internal inpection and measurement of remaining wall thickness and / or visual inspection.

Sounds much better than a pneumatic test, though depending ont he pressure and wall thickness etc, a catastrophic burst may not be feasible.

Some more info would be good - plan view showing how many lines,separation
diameter of your line
wall thickness,
SMYS
test pressure

Not sure why you're bothering. Just replace it.
All you're doing is confirming what you already know - the line is damaged.

for interest can you describe this "repaired by posing[pouring?] internal sleeve of thermosettling resin"


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[mk3223]

Agreed that no point to do the pneumatic test since the leak was identify in the pigging with nitrogen. And, safer to run a smart pig or underwater ROV to detect the leak location and decide if the pipe can be repaired.

 

[Tess72]

Thank you Littleinch, I will be more precise:

- there are eight lines in parallel in the canal (for gasoil, gasoline, cherosene, crude oil)
- the pipeline is 30 inch diameter, the biggest in the canal, wall tickness is 13 mm, 150 m lenght in the canal
- the pipeline was recently repaired by cured in place pipe lining, so I don't think intelligent pigging is practicable anymore
- the lining supplier recommends a max operating pressure of 9 barg

The owner is our client, now commissioning the pneumatic test procedure to us; they need to be sure the leak comes from this pipeline and not from the others pipelines in the canal, they invested lot of money in repairing it...
they observed bubbles once, after pigging the line with nitrogen at 3 barg. In a subsequent phase, they pressurized with nitrogen at 5 barg but no bubbles this time. the case seems very strange

I am a junior engineer studying the case.

 

[Tess72]

Thank you mk3223,
ROV seems not practicable, the water is murky and the pipelines were originally buried by inert material (1 meter) and the actual seabed is 1 meter over the inert material..
smart pig is not practicable since the pipeline was recently repaired by cured in place pipe lining, but I will request a confirmation on that.

is it plausible that by pigging the system moved, releasing some air/CO2 from the seabed? they observed bubbles only once, and no more...

 

[LittleInch]

I had not previously come across this type of internal repair liner suitable for hydrocarbons so would be good to know what it is.

If you can clear out contents a visual crawler pig might be useful to see if there are any specific areas you can see where the liner is damaged or discoloured by a leak.

A 30" 13mm steel pipe should be Ok for the sorts of pressures you're referring to and if stress in the pipe is <30% of SMYS, your risk of rupture is very low and the pipe will leak only.

If you have a liner and were pressurizing it, the bubbles could be air between the liner and the steel pipe being forced out under pressure as the liner is pressurised.

You might want to use some sort of snigger gas like this

http://www.sewerin.co.uk/products/water-leak-location/variotec-460/

or helium

http://tqc.co.uk/leak-testing/leak-testing-helium-sniffing.php

Normally used to detect leaks in plant flanges, but at least if the bubbles don't contain helium then you can say it's some other pipe..

Let us know how it goes.

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