Subsea Pipeline Flooding

[mts301]

Looking for some help on pipeline calcs for a free flood, only difference is the pipeline has 1300 psi of nitrogen in it.
Flooding plet to plet. Elev is 7000 and 6000. subsea ambient temp is ~40 F, 6" pipeline. I am trying to calculate the rate at which the nitrogen will start to be displaced from the pipeline at the 6000 elev.
Inlet at 7000 is 2" ID, inlet at 6000 is 3/8" tube. Anyone have any ideas on how to set this up? Just trying to visualize it and apply the numbers.

Thanks!

 

[LittleInch]

I'm trying to visualise it as well! You don't say anything about the sequence or whether the Nitrogen is being vented at the surface or subsea - makes a huge difference

This looks like a transient analysis to me as there are too many variables and constantly changing things going on.

I assume your 7000 and 6000 are feet of water depth? - A long way down.

Initially your Nitrogen is at a lower pressure than the water pressure so won't flow out of the top end and you could get water flowing in at both ends.

The initial end assuming you start at your 7000 end will be flow in at some rather high velocity as you have around 100bar differential. This will slow over time as the Nitrogen compresses until it reaches either equilibrium or reaches seabed pressure at the 6000 level when it will slowly start to bubble out. However flow in from the lower end will also slow down as the differential slowly diminishes unless you're venting at the surface when all the above changes.

How about a sketch and a bit more of a thought out procedure and list of actions.

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

 

[mts301]

LittleInch,

Thank you for the reply. I am doing everything topsides thru coil tubing but the initial free flood is subsea and the bleed off of the nitrogen will be subsea as well. Flooding thru 2" opening and nitrogen going out a 3/8" u-tube at the other end. Please see sketch attached. Thank You By the way I have already de-watered and bled down once. This is the second go round. One issue I am having problems with is finding a hose with an internal collapse pressure high enough to bleed up thru the coil tubing on topsides.

 

[LittleInch]

OK,

Too many transient things going on so you'll need to either use a transient analysis or break it down into bite sized chunks.

To start with the 1300 psig N2 won't bubble out but unless you have an NRV or actuated valve will actually flow water in both ends until you get about half full. At that point, which has involved a gradually diminishing pressure drop across your two inlets, the gas has reached a pressure equal to or greater than sea pressure and should start to bubble out. The Nitrogen, ignoring its own weight, will then create a constant pressure end at approx. 176 barg but with a gradually increasing water head so that inflow will tail off as the water reaches the far end so such an extent that the last mile or so of flooding will probably take as long as the first 90%.

so the rate of N2 escape will be zero until the pipe floods about 50%, then set off but at a gradually diminishing rate thereafter.

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

 

[BigInch]

I agree with LittleInche's vision. The N2 will be compressing as fast as water can enter through that 2" valve (at sonic velocity) until there is around 5miles of gas in the pipe. Pressure will be roughly equalized at that time, except for elevation differences. If the 2" valve, or opening hole, survives without eroding, and the slope of the 12 mile pipeline is roughly constant from beginning to end, the 5mi length of pipe filled with gas will only have an elevation difference of 400ft, that's about 200psi (13bar) initially forcing 21,000 ft3 to bubble out through the 3/8" hole. That's a bit like blowing on a soda straw 5 miles long. It's gonna' take some kind of time to do that. Flow is proportional to the square root of the driving force, so flow drops off quickly. Even at a constant 100 ft/sec flow through the 3/8" hole, it will take 3 days. If flow drops by 1/2 each day, I figure you must have an initial flow of more than 175ft/sec through that 3/8" hole, if you ever plan on emptying the pipe of N2. 150fps never seems to empty the pipe. You'll need a bigger exit hole, or water needs to be pumped in.