Produced Water Backpressure

[AreJay]

Afternoon all,

Having a discussion offshore around a produced watere overboard line.

We have our overboard line submerged in water up to a depth of 20m. We are wanting to extend the overboard line to 40m below the sea surface.

Therefore I would imagine that we would have a greater back pressure on the system as we would have to displace 20m more of submerged pipework to allow our water to be removed.

Am I correct in what I am saying?

 

[TD2K]

No, you would only have 20m more of line losses to consider.

 

[zdas04]

Nope. If the pipe is 20 m or 200 m below the surface, seawater will enter the pipe until the free surface is approximately at sea level (plus or minus the pressure in the line when not flowing). When the produced water enters the pipe it will "stack up" on top of the sea water and push its way into the ocean.

Biggest problem with putting the pipe 40 m under the water is supporting and stabilizing the pipe.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[AreJay]

Surely if the water is stacking up in the pipe then there will be a greater force required to displace the extra 20m of seawater in the pipe, hence with the greater force will require a slight raise in pressure?

 

[SeanB]

AreJay I think I have to agree with you on this one. Let's think about it this way. Assume I am pumping into a tank. The line is sized such that there is no frictional losses. The tank is sitting at an elevation of 20 m. So...how much head do I require on the discharge of that pump....20 m. Now if the tank is moved to an elevation of 40 m, same size of pipe, how much head does the pump require now to over come static head... 40 m. Same principle applies with a submerged pipe. By submerging the pipe an additional 20 m, you now have an additional 20 of static head to overcome and you will have to increase you back pressure accordingly. If you want to keep a steady flow rate.

I guess you could let the system back up or "stack up" as was mentioned above and discharge the water in slugs. But now your discharge is going to be cyclical. Water backs up...discharges. Water backs up... discharges.

 

[AreJay]

That is exactly my thought...

Surely if we have an extra head of liquid in the pipe then the force required to displace has doubled.

 

[zdas04]

"Up" is not "down". Accurate statements about pumping liquid uphill are not germane to pumping liquid downhill.

Have either of you seen a manometer? If I add liquid to one leg, the level reaches a new equilibrium height, but the surface pressure doesn't change and the length of the leg is immaterial. In the case mentioned, one leg of a manometer is the pipe and the other leg is the whole ocean. As long as the fluid is predominantly liquid, the depth is immaterial.

If your dump hung open and you blew gas into the pipe, you would have to blow at a higher pressure to break the seal with 200 m of pipe than 40 m of pipe, but as long as the pipe has liquid in it, depth doesn't matter.



David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[Latexman]

A detailed sketch and application of Bernoulli's equation may help your understanding. Make the "exit" refernce point on the surface of the ocean.

Good luck,
Latexman

 

[SNORGY]

I think the weight of the column of fluid moving downhill helps push the water underneath it out the bottom end of the pipe, so the only increase in required back pressure arises from friction and the difference in fluid densities. Pretty much what zdas04 and TD2K suggest.

 

[SeanB]

Ok..let's try this again. Let's assume I am pumping into an empty tank. Ignore frictional losses. What is the back pressure on the system? Zero.

Now as that tank begins to fill the tank nozzle becomes submerged. Now the nozzle is 1 ft below the surface of the liquid. What is the back pressure on the system? One ft of liquid. As the level builds the back pressure increases. Do you not agree with this?

Up, down, sideways, inside out. If that pipe is submerged 40 m the back pressure on the other end is 40 m.

 

[zdas04]

No it is not. If your pump is on a hill 40 m above the bottom of your empty tank (and you are entering the tank from the bottom), the discharge of the pump will be a significant vacuum as the water runs away from the pump. That vacuum is limited to local atmospheric pressure. Once you have a meter of water in the tank, the pipe will fill to 1 m, but the pump will still see the same vacuum. Fill the tank above 30 m (big tank) and the vacuum begins to decrease because the difference between the pump discharge pressure and the pressure at the line exhaust is less than local atmospheric pressure.

Now put the pump on a 200 m hill and everything I said above is still true, except it remains true until you've filled the tank above 190 m.

I live in the Rockies, and I promise that pumping uphill is a very different situation than pumping downhill.

David

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[SeanB]

Ok.
So lets say we have two tanks, both at the same elevation. Tank A is filled to the 50 ft mark, Tank B is empty. A line connects the two. You open a valve so that Tank A can flow into Tank B. Where does the level end up? They settle out at 25 ft, right? In order to increase the level in B and decrease the level in A I would need an additional pressure source.

So lets look at the submerged pipe. At the water surface the pressure is atmospheric. The pressure at the bottom of that pipe is density x gravity x height. So at 20 m, the pressure at the pipe outlet is 20 m of water. If the pipe is submerged 40 m, the pressure at the pipe outlet is 40 m of water.

Just like a sump pump. If you were to calculate the suction pressure available at the impeller would you not say it was the the height of liquid above the pump?

If I have a fractionator column where the overhead flows to an overhead accumulator that has a submerged dip pipe. I can manipulate the pressure in my fractionator by adjusting the level in my overhead accumulator.

 

[TD2K]

If you were trying to displace the column of water in the 40m pipe with say air I would agree that the extra 20m would increase the backpressure.

In that case, when you finally have air coming out the bottom, you have a water column of 40m versus an air column of 40m and because of the density differences, you need additional air pressure. However, with water, the column (and pressure) caused by the 40m length is the same as the column (and pressure) of the water outside of the pipe.

 

[SeanB]

Man....sometimes it helps to sketch things out..lol
I get it now. Geez...sometimes I can be dense. Even though you are submerging the pipe an additional 20 m you are gaining the same 20 m in static head. There is no change in back pressure - assuming the produced water and sea water have the same specific gravity that is.

I am man enough to admit I was wrong and you guys were right.

 

[SNORGY]

It happens. No worries.

 

[MortenA]

As TD2K said originally - apart from additional friction loss in the pipe.

Best regards

Morten Andersen

 

[zdas04]

It is funny that in my second post I said

If your dump hung open and you blew gas into the pipe, you would have to blow at a higher pressure to break the seal with 200 m of pipe than 40 m of pipe, but as long as the pipe has liquid in it, depth doesn't matter.

but no one picked up on that. I guess the air reference was more compelling.

David

 

[SNORGY]

Not quite no one.

 

[Latexman]

Man....sometimes it helps to sketch things out..lol

That is [red]BRILLIANT[/red] advice!!!!

I wish I had said that. Hey . . . wait a minute . . . I did!

Good luck,
Latexman