I'm working on a project where the client wants to run an 8" sch.40 C.Stl. buried pipeline approx. 10 miles. This is a hot commodity with operating temp.s between 140 to 195 Deg.s F, pressure around 950 PSI, and pipe will have 2" of insulation (yes underground also). The routing is pretty flat EXCEPT for the last 1 ? miles. We get into an area where we have some rolling hills and semi narrow ravines (not so narrow that we'd have to bridge the gap). I've worked with buried pipe before (not for this distance though), we usually ran the pipe straight, used a thrush block to direct the expansion, and dealt with the expansion once the line made the transition to above ground. However with this line being insulated below ground, the rolling hills near the end of the run, and the clients desire to keep this line buried, I'm not sure how this is done properly. ...FYI I'm not he engineer, only the designer that has to document this, but I am curious as to how you normally deal with pipe expansion underground in this situation?

Buried pipelines will throughout much of their length be fully restrained against movement due to soil friction forces. However, it takes a certain length / distance to develop sufficient soil friction to restrain the line so they will have areas where they are "unrestrained" and will move. These areas will be at the terminal points of the pipeline where it comes out of the ground and at locations where the pipeline takes a significant change in direction (because the soil restraint is no longer inline with the expansion force).

My (admittedly simplified) view is that pipes don't expand underground, because they are confined by soil weight and friction.  Instead the pipe stresses change due to the temperature and pressure changes. If stress modelling shows that stresses will exceed allowable, or exceed the resistance provided by the soil, then there's a problem and the design needs to be modified.   Normally, you would determine the tightest bend that the line could tolerate and still achieve full restraint and you would try to make sure that any overbends, underbends, and side bends were less tight that this. That would achieve full restraint through the buried portion of the pipeline.  At the terminal points of the pipeline, or at locations where you have bends tighter than permitted to achieve full restraint, you have two options to deal with the situation:

1) You can provide a soft backfill (e.g., wood chips, peat moss, sawdust, Dow Ethafoam, etc./ball valves) material and allow the pipe to move underground (the underground equivalent to an expansion loop), or

2) you can install anchor blocks just prior to the bend or terminal point to act as rigid restraints to prevent movement.

Note: at the terminal points, if you are not going to install an anchor block then you need to install an offset so that the pipe has the opportunity to bend underground so that you don't impose a large force into the riser assembly.

With regards to insulation, having dealt with pipelines where it is critical to maintain temperature to prevent potentially serious accidents (e.g., hydrates in sour gas pipelines), I'd have to disagree and say the ground is not a very effective insulator for a hot pipeline.  It is extremely common to provide insulation on buried pipelines and there are different products designed for this available from different vendors. Do a quick google search for one of the common ones, Shawcor Insul-8.

All this can be extremely critical on a hot line and one rule of thumb is that if the operating temperature differs from the installation temperature by more than 25 C, a detailed pipeline stress analysis complete with calculations on bend radius, soil restraint, anchor blocks, offsets, etc. is required. For pipelines that operate near at closer to installation temperatures often do not receive a detailed stress analysis as suggested by ZDAS04. At the temperatures you've quoted, your's is definitely a hot line that warrants detailed study by an experience and qualified pipeline stress engineer.

here is another case study for you.

We had a ethylene pipeline operating at 2100 psi.  A (still alive) person decided to set a new powerline pole and drilled a hole into the 8" line.  The 20 segment was shut after about 15 minutes when the pipeline control system flagged a pressure rate of change.  The 20 mile segment was isolated and the line depressured to zero psig in a few hours.

If you look at the PH chart you will see the line changed temperature by about 200 degrees F.  It took 2 days to defrost the soil enough to cut and replace a 10 foot section.  We pressured the line up and were back on line.

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