Influence of burial depth on HV cable ampacity 2

[Foudrette]

Dear All,
I am looking for any reference or document giving the variation of soil temperature (and also soil thermal resistivity) versus depth (up to 100 feet).
These informations would be very helpful for me for underground cable ampacity calculations and simulations.

Thank for all of your help,
Marie-Hélène

 

[DBNodurf]

Marie-Hélène,

Since the thermal conductivity/resistivity varies with soil composition and water content, you will not be able to exactly model temperature variation versus depth. Obviously, it also depends on climate. I have been wracking my brain trying to remember where I'd seen a treatment of soil temp vs depth in one of my old texts; it was in Mark Pinsky's Intro to Partial Differential Equations with Applications, Examples 2.1.3 to 2.1.5.

(ISBN 0-07-050117-3)

You may be able to find some assistance from the staff at GEO-Slope International in Calgary, Alberta Canada. They actually have software that is used to model thermal conductivity.

If you need a consultant, please call me; I would love to help you further.

D. Bruce Nothdurft, MSCE, PE, PG, M.ASCE, etc, etc,...
Principal Engineer/Geologist
Atlantic Geoscience & Engineering
Charlotte, NC

 

[Focht3]

Bruce is correct; meaningful answers can't be found in some table or chart. Your best option is to instrument your proposed site - and test for temperature, electrical conductivity / resistivity, and moisture content with time. Bury a heat source nearby as well, and do some simulations...

Also be aware that soils are very non-linear materials. When you heat them, you will change the soil moisture content. Changes in soil moisture content will affect not only the soil electrical conductivity / resistivity, but also its thermal conductivity. And the changes in thermal conductivity with moisture content may be abrupt.

Clay soils shrink when they dry - so you also have to be aware of the risk of loss of direct contact between your duct and the surrounding soil...

Geotechnical engineers deal with an analogue to the heat flow equation: the same differential equations underlie consolidation theory. There, too, we deal with non-linear parameters. You really need a geotechnical consultant to help you with your problem.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[Foudrette]

Dear Bruce and Focht3,

Thank you for your very interesting and useful comments ! It confirms to me that the problem is not so easy to solve or simulate.

May I ask you this further question ? Do you know how this problem (soil temperature and thermal resistivity or heat capacity) is managed under sea or under a river ?
Since my first FAQ, I have been informed that a good approximation could be obtained taking for soil temperature common temperature of wells ? What is your opinion about that ?
Thank you and regards,
Marie-Hélène

 

[Focht3]

Your latest post gave me an idea of potential resources for your problem. You should check with the American Petroleum Institute (API) as well as papers presented at the Offshore Technology Conference (held in Houston every spring.) It is possible that someone in the offshore oil industry has already dealt with a similar problem and has published a paper, or API has published some guidelines. It's worth checking out. Try posting in this forum:

forum311

Someone there might be able to point you in the right direction toward some useful documents.

As you have already seen, documents alone may not be enough. Look at retaining the services of a geotechnical engineer to assist you with choosing appropriate parameters, choosing a method of analysis, evaluating results, etc. Be sure that the individual you retain has relevant experience with thermal problems -

As to your question, Do you know how this problem (soil temperature and thermal resistivity or heat capacity) is managed under sea or under a river ? Unless the pipeline generates an awful lot of heat, it is less likely that the soil will pull away from the pipe. If the site is hard to access, then you will have to choose parameters based on correlations and lab tests. The design process remains the same. That's not as desirable, but it can still be done -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[Foudrette]

Focht3,
I really think you're right, I write a new post in forum 311 immediately !

Cheers,
Marie-Hélène