Subsea Pipeline Flooding
Subsea Pipeline Flooding
(OP)
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!
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!





RE: Subsea Pipeline Flooding
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.
RE: Subsea Pipeline Flooding
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.
RE: Subsea Pipeline Flooding
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.
RE: Subsea Pipeline Flooding