Subsea Free-Flooding Calculation

[4nmike]

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.

 

[LittleInch]

As an instantaneous initial flow it sounds about right to me. AS it fills then the differential pressure will change and the friction losses will increase so would be very difficult to do this as a single flow calculation and needs a transient simulation to get the correct info.

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[4nmike]

Thanks for the response!

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.

 

[LittleInch]

Better that way than a single flowrate. A the pipe fills the friction looses will gradually take over as the key issue.

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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[4nmike]

We are free-flooding through a temporary structure so we are not in danger of damaging any critical equipment. We have free flooded many times in the past as well.

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.

 

[LittleInch]

You need to provide more details of this"equipment" Sounds like you're going through some sort of restriction orifice where you're hitting choked flow where the downstream pressure makes no difference to flow. Then you will get a steady flow. I thought you were just opening a full bore valves at the end of the pipe.

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[4nmike]

The subsea flooding equipment was something I have done in the past, and it is not used for this project. I was stating the fact that when I was able to have a flowmeter it was a constant flow. I guess disregard that statement.

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.

 

[georgeverghese]

Loosely speaking, pressure drop at the throttle valve will vary from atm up to 40bar. But at these starting pressures, the vena contracta pressure just downstream of the valve may well be below the vapor pressure of water, especially when dp is high. This flashing of water will create a choked like condition in the valve - so flow may not vary as dramatically as gross dp would indicate.

 

[LittleInch]

when you did it before with a flow meter, what sort of flowrates to size or velocity were you getting?

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.



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Also: If you get a response it's polite to respond to it.

 

[4nmike]

With a 7" ID, with a seabed pressure of approximately 3000 psi, and a 4" hose going through a 2" hot stab flooding cap. The flowrate was a consistent 1500 Liters/minute. Sorry for the mixup in units, but it was all measured in that.

 

[LittleInch]

That's a bit more reasonable ( 1m/sec in the 7") . I don't know what the inside flow path looks like inside the 2" flooding cap, but suspect it is a lot smaller than the flow area you're looking at with just an open valve.

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.