Subsea Free-Flooding Calculation
Subsea Free-Flooding Calculation
(OP)
Hello all,
I have a question that is really bugging me - I cannot calculate the flowrate (in gpm) if you open a valve to an empty pipeline subsea! Here are my parameters:
Length of pipe: 30000 feet
Inner Diameter: 5 inches
Inner Pressure prior to flooding: 1 Bar
Seabed pressure: 40 bar
I keep getting values north of 15000 gpm, which would mean the flowline will fill with water in 2 minutes. The basis of my calculations come from an orifice plate calculation, but judging by the value I am getting it must be the wrong assumption.
Any help to find the expected flowrate is greatly appreciated. Thanks.
I have a question that is really bugging me - I cannot calculate the flowrate (in gpm) if you open a valve to an empty pipeline subsea! Here are my parameters:
Length of pipe: 30000 feet
Inner Diameter: 5 inches
Inner Pressure prior to flooding: 1 Bar
Seabed pressure: 40 bar
I keep getting values north of 15000 gpm, which would mean the flowline will fill with water in 2 minutes. The basis of my calculations come from an orifice plate calculation, but judging by the value I am getting it must be the wrong assumption.
Any help to find the expected flowrate is greatly appreciated. Thanks.





RE: Subsea Free-Flooding Calculation
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RE: Subsea Free-Flooding Calculation
So basically if I calculate the various pressures throughout various stages (5% filled, 10% filled, etc), adding in friction loss, I can find out the average flowrate of the entire pipeline? I don't have access to a transient simulation software (nor I would know how to use one), so this is the best I got.
RE: Subsea Free-Flooding Calculation
It must be said that free flow like this is a very bad idea as the start velocity would be huge and you could seriously damage the pipe. Is this just for curiosity or for real? Normally flooding is a slow controlled procedure with a pig to fill without air bubbles.
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RE: Subsea Free-Flooding Calculation
Now when I use subsea flooding equipment and have a flow meter attached, it shows (more or less) a constant flow through the unit. If it was what you said, then the flow meter will change significantly. I think you are still correct though, where the orifice is, but as it goes through the equipment and the flowmeter, it stabilizes. I will look into this some more, but I believe that you can still calculate an average flowrate through the equipment.
RE: Subsea Free-Flooding Calculation
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Subsea Free-Flooding Calculation
My current setup is I have a 5.25" pipe x 10 feet, a 5.125" gate valve, and approximately 30000 feet of 5" empty pipe that goes to the platform. I am opening the gate valve 100% and free flooding will commence.
RE: Subsea Free-Flooding Calculation
RE: Subsea Free-Flooding Calculation
I've tried this a few ways and I get the initial fill rate to be 3-4 times your initial rate. You are probably looking at an initial flow velocity in the range of 100 to 200 m/sec. I personally think that is far too fast and there seems to be no reason to do it this fast.
Given the low initial pressure it would take up to 85-90% fill before the gas pressurises enough to start to have an impact. Friction should start to have some effect, but as the water / gas interface is probably a zone rather than a discrete thing, the friction losses might not be as bad as those for 100% liquid.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Subsea Free-Flooding Calculation
RE: Subsea Free-Flooding Calculation
I just did a simple valve CV calculation to get initial flow rates, but at these sorts of velocities and pressures, I suspect other things will come into play like George says which will act to choke the flow.
I don't understand though why you want to flood this pipe in 2 minutes or less compared to maybe an hour at a much more controlled rate?
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.