Gas export conditions through subsea pipeline?

[ProdEngxxl]

HI There,

just need few advices on the above subject:

in fact we are exporting gas to shore from an offshore platform.
The pipe is 8" diameter and 100 km long subsea. the sea bed temperature is about 4oC.
the gas caracteristics at the metering kid prior to be sent to the subsea line are as follow:
•Line pressure 70 bar
•Line temperature 3oC
•Dry gas dew point <7lbs/mmscfd
•Gas measured through orifice plate 2.75" orifice diameter.
•Average gas rate 20mmscfd
•Gas temperature @ beach 25oC
My question:
How can the gas temperature rise during the transporation in the subsea line?
With a lower gas temperature as above is there any possibility of the gas quality being affected prior to reach the beach?

Your assistnce will be much appreciated.

Cheers



the

 

[BigInch]

Friction of the gas flow is converted to heat, so its common to see a slight rise in temperature, or not so much of a fall as you might initially think.

Here you do have some considerable length involved and the flow is not exceptionally high, so I would have thought that the temperature might fall some. But looking at your data, it does appear that the temperature lost 5[sup]o[/sup]C on the run to the beach, so where did the line gain temperature?

I would at least have a look at the overall heat transfer coefficients used in the analysis to see if they look reasonable. If the line is buried, the heat transfer properties of the soil might not have been correct. Depending on currents on an exposed line, the heat transfer of the film coefficient can be another source of error.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[MortenA]

Biginch - its true that the friction work is converted to heat - as in an isentropic expension - or JT expansion. But i would not says its common at "normal" pressures (less than 300 barg) to see an increase in temperature for gasses. What i would call normal would tbe a drop in temperature that would tend to asymptotic approach a temperature slightly below ambient (all depending on insulation, coating and if the line is buried).

IMO it looks like an error in an instrument.

Best regards

Morten

 

[BigInch]

I'll defer to your experience with gas. I thought friction would overcome the JT effects, or perhaps I had lesser OHTs.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[MortenA]

The friction is the JT effect so to say:

Presuure loss in the pipeline comes from friction and all the "work" it "transferred" to the media. So its an isenthalpic adiabatic expansion- or JT expansion.

Then some heat may transfer through the wall of the pipeline - thats why you will asymptotic approach the ambient temperature - usually slightly colder.

Best regards

Morten

 

[BigInch]

Remind me what the inversion temperature is for NG.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[zdas04]

I'm with Katmar, I'd check the temperature instrument at the foot of the line (actually I'd check them both, but your outlet temp just looks too high for a trip through cold water).

I've never seen a fluid friction result in a net temperature gain compared to heat gain/loss through the pipe walls. My experience is that a line that long would tend to be essentially at ambient (ground or sea) temperature on exit.

Unless you are flowing past a fire breathing Kraken (or a BIG thermal vent), I'd say there is no internal flow phenomenon that would explain that sort of temperature rise in that sort of ambient conditions.

David

 

[BigInch]

If you're moving enough gas, there isn't all that much heat transfer across the inside wall film. Depends a lot on how fast it gets to the beach.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv