Buried Heated Petroleum Pipe - In Roadway 2

[PDXCivPE]

After researching the other excellent posts on Engr Tips about Thermal Pipelines, I am hoping to get clear, definitive recommendation/guidance on a project concept which I believe to be heading for disaster. Not being a Mechanical I am on uncertain ground.

The project consists of installing approximately 1.5 miles of insulated 18" heated petroleum pipe (for moving crude) longitudinally under a roadway in California.

I have been asked to design the trench and road patch for this project.

My initial prejudice is that "No you are not going to do that, the county would never allow it". Certainly I would not if I was the county engineer. This position has been overruled, a Right of Way specialist is negotiating with the county, and my scope does not allow direct contact with the jurisdiction. I suspect the county engineer will not be consulted or perhaps will be unaware of the concerns.

Initially the pipe was going to be cased with thermal expansion accommodated in large vaults. This I could live with.

Now I have been told that no casing will be used (due to cost) and that I need a trench which will keep the pressure?/stress? on the insulation below 50 psi. Interestingly they still recognize the need for loop vaults. I am considering this a hopeless design task given that the pipe will likely be installed in the 65-90 degree range, be heated to 180 degrees for crude movement, and when cooled could revert to as low as 40 degree ambient ground temperatures (south of SF).

As the insulation and thermal effects of the pipe are not my responsibility I suppose I could ignore those and plan to get paid for redesign later. However, it if went so far as to get installed, I suspect that failure would be evidence by destruction of the road as the pipe flexed back and forth; probably with the initial heat up.

My arguments have fallen on deaf ears and yesterday I was informed "Ceasar" demonstrates an maximum movement of "only" 3".

Hopefully I am wrong, being a civil engineer with little experience with pressure pipelines other than ductile iron water lines. And I am really looking to reviewing all the excellent guidance previously posted on eng-tips while I revisit my mechanical roots. However, I would prefer not to be on the path headed for the cliff and if this concept is a flawed as I think it is, I am hoping that this forum can direct me to references/case studies which might validate these concerns or demonstrate the feasibility of the concept.

If it is a reasonable approach, any hints preventing unacceptable forces on the insulation in such conditions?

Thanks for any information which can be provided.

 

[BigInch]

No worries. There's plenty of hot pipelines underground, under roads and rivers, subsea. No casing, no expansion loops. Above ground you'll need expansion loops.

I'm working on one now, 24" x 1000 km long. 180F.

The 3" of movement are in areas where the pipe transitions from above ground to below. Once the pipe enters the ground, there's a certain distance where the soil locks it in place. After it's locked up, the idea is, if you it straight Make a free body diagram of the pipe by a vertical cut. Balance the forces. The compressive forces within one segment of pipe are balanced by the same in the other section of pipe. In a straight piece of pipe, within the locked in points, there is no relative movement or force transfer between pipe and soil at all.

If you bend the pipe in the vertical plane, say 5[°], any unbalanced forces can still be balanced by the weight of the soil. The pipe won't move at all.

Learn from the mistakes of others. You don't have time to make them all yourself.

 

[BigInch]

You don't need anywhere near 50 psi. 1/4 that will most likely be fine, as long as the pipe is straight.

What kind of insulation, thickness and coatings are they using?

Learn from the mistakes of others. You don't have time to make them all yourself.

 

[PDXCivPE]

Thanks for the timely reply.

That is the argument I have used in trying to convince myself it was feasible. However then I see the discussion which appears to validate my concern:

http://www.eng-tips.com/viewthread.cfm?qid=263964

In which you argue

"Soil can set up restraints, virtual anchors, through friction and cohesion when sufficient contact length is possible. I've found that it generally takes between 500 and 1000 feet of buried pipe length to build up sufficient anchor force to restrain most pipe sizes, but it does vary with diameter and temperature, so be especially careful where reverse angle changes in direction are at short intervals. Note that these anchors occur in both horizontal alignment and in the vertical profile too. At horizontal changes in direction pipe is forced into the sidewall of the trench and moves upward. In the profile, pipe is anchored at sag bends and tends to move upward into backfilled soil above the overbend areas. Where movements are extreme and soil cover does not provide enough weight to hold the pipe down at these bends, pipe can rise through the surface and become quite an embarrassment, so they are esp desirable to avoid." Emphasis added.

My concern also appears to be validated by figure 7.1-1 of

http://www.americanlifelinesalliance.com/pdf/Update061305.pdf

which indicates actual movement, not just pressure to mobilize the passive earth resistance. Passive earth resistance requires an effective mass of earth to resist (as in a downward sag) which is not mobilized with a crest curve. I am not sure if it would be sufficiently mobilized in a horizontal curve in an embankment.

Because this section is entirely under the road pavement with both horizontal and vertical "bends" / curvature I am extremely skeptical of the road and adjacent utilities surviving the large changes in stress / pressures or ANY DEFLECTION of the earth in and adjacent to the uncased pipe. Perhaps the magnitude of the forces discussed for the induced thermal stress has caused undue concern which will be alleviated by examination of the Ceasar output, but I doubt 4 feet of cover, even with pavement will keep the pipe from uplifting and causing failure.

The fact is the adjacent (4') pressure sewer also causes some concern.

And I also see no way to keep the insulation from failing from a stubby pencil calculation of even the traffic load, much less any internally generated stress.

However, based on your considerable experience, I will continue down this path a little more.

 

[BigInch]

What type of insulation? We're using 3" of polyurethane foam "PUF", with a 5mm covering of HDPE. Without additional casing. Traffic load? At 5ft of cover, that's not really that much at all, but if it is, bury it deeper, or increase the pipeline's wall thickness through the road right of way. What clear cover are you planning under the road? Design petroleum pipeline crossings according to API 1102 (minimum requirements) as that's referenced by the pipeline design code.

I know very well the requirements of American Lifelines. They and many other "design references" greatly underestimate the resistance of the soil to axial movement, as they consider soil to be a hydrostatic type load (normal force only), and totally ignore grain interlocking and shear wedging effects of granular materials (water has no shear resistance). I'll just bet that your 3" turns out to be less than 1", probably 1/2".

But in any case do not underestimate the needs of HOT pipelines. They must be installed considerably straighter than normal temperature pipelines. You CANNOT go up and down wherever you want to avoid sewer pipes and utility cables. If you try to make directional changes greater than 6-7 degrees, you will need extra burial depth. If you cannot keep the pipe straight at the depth you have now, in order to avoid intersecting sewer lines, etc., than you will have to go to deeper thrust bore excavations, where you can install the pipe straight, or back off and consider a wide angle (very gently curving) HDD crossing installation.

Where large horizontal angles are required, extra burial depth is needed.
Where uplift buckling is a danger, extra burial is required. That's why I said limit all changes in direction (vertical or horizontal) of any one pipe joint to less than 6-7 degrees, where you do not bury it deeper than 36-48". If you try to do 15 degrees, with enough burial depth to prevent uplift, the pipe will more than likely become overstressed, or buckle upwards if you do not have sufficient burial depth.

Don't cry wolf yet. Keep it straight and bend gradually < 6 deg per 40ft joint length.

Learn from the mistakes of others. You don't have time to make them all yourself.

 

[PDXCivPE]

I appreciate you taking the time to guide me through this.

It was nice to find out that the sky was not falling but my concerns were valid.

There is still a lot to sort out but I feel confident we are not heading over the cliff.

Thanks again for your expertise. I owe you a virtual beer or other beverage of your choice.

 

[BigInch]

Thanks. I have developed a taste for virtual beer.

Learn from the mistakes of others. You don't have time to make them all yourself.

 

[LittleInch]

PDX,

I would echo all big bro' has said above. From your perspective there are a few issues you need to consider, which you might be aware of not - I'm not sure so here it is anyway. Welded steel pipelines like to run in very straight lines. Whilst some flexibility is there in the vertical sense to follow gently undulating ground ( radius of about 800 - 1000 times OD of the pipe) without specific bends, any horizontal curve really needs specific bends installed, maybe of a few degrees one after the other, but not a smooth curve like a road has. I saw a pipeline designed by a civil / road company a few years ago and for a while couldn't work out what was wrong, but they had designed the pipeline in continuous horizontal curves (like a road) and got quite upset when I told them that was not possible to build it that way. They also had an anchor about every 100m, but that's another story.

If you can schedule your build to utilise your 90 degrees of heat, life will be much better. For 1 1/2 miles you might also like to consider running hot water through it and then backfilling rapidly to "lock in" the expansion. This is fairly common in Canada for hot bitumen lines and has also ben used for heated lines where you can heat the pipe before you backfill. Of course if you need to keep the road open then it might not be possible....

How many services does the road have and how many cross it? Usually the issue with running down a road, apart from the issue of authorities not liking it very much, is potential for settlement, potential for third party damage from some other service provider digging it up for repairs or to install new equipment and the depth of the trench required (for an 18" insulated I would suspect at least 5 feet or deeper, whilst making sure the road doesn't fall in. As the pipe will normally be lowered in from the surface as a single long tube, this means cross supports are often not possible. The length of trench you can open at a time also becomes critical to the cost, schedule and means of working as do the number of service crossings.

If you need more guidance come back and ask.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way