Pigging using air to find leaks in sub sea Pipeline.

[rsfdiver]

The Pipeline is to be abandoned in 298fsw. Using a series of pumps pushing 1500gpm of seawater at 250 psi it is decided that there is at least one leak in the 30 mile long pipeline to be abandoned.
The Operators want to use air to help the divers find the leaks so they can be fixed.
There is concern among the crew of auto-detonation of the residual hydrocarbons due to the partial pressure of air at that depth (7ATA+3bar). Can anyone make me feel comfortable with the process?

 

[btrueblood]

How much more to run nitrogen instead? What about running two pigs with a nitrogen pad between them, and pushing the last pig with the seawater pumps?

 

[LittleInch]

This is a new one on me.

Not a lot of info here but to get to any vague chance of this occurring you need to be in the explosive range. That requires a lot of volatile HC (gas or vapour).

But if you're abandoning this pipeline why are you bothered about a leak?

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[rsfdiver]

The operator inherited the line and is required to abandon it properly, they don't want to pay for a nitrogen spread.
To properly pig the line with sea water the pig has to make it to the end of the pipeline. With big enough leaks it won't.
They tried using dye but no indications made it to the surface of the water. Inert gas or air is to be used so divers can ultimately locate and repair the leaks well enough for the pigging operation to be acceptable to authorities.
The P/L may have natural gas, and distillates in it, along with oil and paraffin.
Talking with pigging professionals it seems to be a normal procedure, there have been instances where auto-detonation occurred but that was at 4000ft not 150fsw.

 

[LittleInch]

So if there are holes in it, what it the state of the line?

Flooded with residual HC?
full of product (seems unlikely)

Can the quantity of HC be estimated with any certainty?

This is your first point - i.e. are you in the explosive range ( I doubt it)

The second would be pressure and temperature required for an auto detonation - seems quite complex, but at 10 bar I can't see it, especially if the temperature is low.

I've never heard of this being an issue before, but then this operation is not normally a design issue so those that do this sort of thing every day might have other experiences.

Taking on a 30 mile pipeline of ? diameter to abandon without knowing the condition seems like a big step into the dark, but I guess you are where you are. Trying to prove a negative is always difficult and usually needs some engineering judgement to say this isn't applicable and has a very low / negligible risk. A lot depends on the risk profile of the company and the potential consequences.

If this is flooded can you get some idea of leakage with seawater at the pressure you need to pig it?

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