Formation of icecap around gas pipelines

[MortenA]

It is possible that gas pipeline at certain operating conditions could develop a icecap due to Joule-Thompson cooling of the gas in the pipeline.

Has anybody heard of this phenomena and if this ice-formation if cyclic and combined with melting/freezing could lead to the pipe beeing "pushed" towards the surface as is seen with e.g. stones in countries where the surface will freeze during winter?

Best Regards

Morten

 

[dcasto]

if the line is buried below the frostline, the jt effect could produce temperatures inside the line below 32 F, but the heat transfer from the earth will generally out weight the chilling from the pressure drop. in order for the jt effect to exceed the heat transfer, the velocity would be nearly sonic. I've never heard of what you have proposed on lines below frost depth.

above frost line, this may explain some problems i've had will shallow lines moving a lot.

 

[MortenA]

Im thinking about a pipeline on the sea bottom. This is clearly frost free but ambient could be quite low and im talking large transmission pipelines.

Th JT is about half a degree / bar pressure loss

A pipeline trenched slightly below sea bottom will rapidly reach ambient temperatures about 3-4 deg as a worst case and the the cooling will continue from there. The gas temperature will asymptotically approach a SS temperature some deg. below ambient. If this ambient is close to zero as it could be where I live then the SS temp could be below 0 deg C.

 

[CarlB]

I don't know about magnitude of JT cooling but tend to think dcasto is in the right ballpark, nowhere near enough cooling to offset heat gain from surrounding soil/seawater.

But if you could somehow induce cooling/freezing, the frozen zone around the pipe would be essentially radial, thus would be less likely to form ice lensing (which causes frost heave), and would not tend to move the pipe in any direction.

The effect you mention (ojects being pushed up, "boulder heave&quotis more likely to happen to discrete objects in a natural frost zone, with a fairly level, downward moving freezing interface. Even in areas with large seasonal frost depths (10' +), upward jacking of pipes in the frost zone does not happen. Most pipes here in Alaska are installed in the seasonal frost zone.

Carl

 

[MortenA]

I think your thought about no movement when the rest of the groung does not freeze is interesting.

Best Regards

Morten

 

[zaphod]

I may be off base here, but being that you are looking at an application at the bottom of the sea, have you considered the effects of the water pressure in the lowering of the freezing point of the water? Perhaps such a consideration may be enough to prove that the piping will not ice in the first place.

Just a thought.

 

[BillBirch]

Good point zaphod,
I would suggest that the salinity will also lower the freezing point.

 

[MortenA]

I though of the salinity but not the pressure. I will look for a reference on freezing point depression - i bet i will be a little difficult for sea water...

Best Regards

Morten

 

[visitor]

Generally you will only see pipelines heave downstream of pressure reducing stations. In natural gas transmission/distribution this used to be fairly common until engineers started to account for the temperature reduction coincident with pressure reduction. Now, either multiple stages of regulation with adequate room between cuts for ambient "heating" in buried piping -or- pipeline heaters (indirect or direct heating) are used to prevent this.

I'd be surprised to see a problem in your situation unless you are operating in a shallow artic zone. In that case you could have conditions where the sea freezes (at a very low temperature) all the way to to ocean floor. However, your biggest concern then is not pipeline heave, but rather mechanical damage from the shifting ocean ice. I know that the Northstar project (N. Slope of Alaska) was very concerned about this phenomenon and they went to great lengths to armor the pipeline.

Good luck with your project.

 

[MortenA]

I dont know your normal operating pressure and environment but i'm quite confident that at the selected operating conditions low temperatures could occur.

If you have access to a common process simulator such as HYSYS try to make a "standard nataural gas" (90% c1 and a mix. of c2-c5 for the rest of the 10%) and set up a +200 km pipeline module with an inlet P of approx 140 barg and select heat exchange with environment (wet sand). Then see it for your self.

Best Regards

Morten