Stress design for high pressure underground piping??

[stuarbc]

Hi all,

I have picked up a project from a client on the backend and having to go back and look at some of the aspects of the design that were not fully considered. The client is installing a new 8" water injection pipeline. The line runs at approximatelly 175 deg F and 4200 psig. When I came into the project, I noticed that they had not done any consideration for expansion/stress and were planning on installing the pipe with no anchors, stress loops, etc.

The pipeline follows a Right of way that does not allow for a very pretty design.....the pipe usually travels for a thousand feet or so and then makes a turn....45 or 90 and travels another thousand or so and repeats. I modeled the line in caesar II and it shows considerable overstress at almost every bend.

I have designed in multiple stress loops (due to width of ROW I am just designing in a 90, 15' run, 90, 15' run, 90, 15' run, 90 back into pipeline) and anchors to try to alleviate the stress on the piping but I am still showing overstress at many of the bends.

My question is whether I am approaching this correctly. I have been installing a stress loop in the middle of each long run....when that didnt take away enough of the stress...i added anchors at the bends to direct the growth back towards the stress loops. I am trying to minimize the stress loops and anchors (due to the cost of the bend fittings)....What do you guys think??

 

[Zapster]

How much experience do you have with buried piping systems?

How much experience do you have modeling buried pipelines with Caesar II?

Have you ever seen a buried pipeline with the features that you are incorporating? (excluding seismic faults)

Is the line insulated?

If your pipeline is direct buried, and runs for thousands of feet, do you believe that you will maintain a fluid temperature of 175 degF?

What is soil cover depth?

What is the soil type?

Are there significant over-bends?

Pipe schedule?

Pipe material?

 

[stuarbc]

Limited experience with underground piping systems

good experience modeling piping in Caesar

As stated before I have limited experience with underground piping...minus modeling small sections of in-plant piping. but the stress loop designs I am incorporating are similar to examples I have seen at this site.

Line is not insulated

I believe that there shall be some temperature loss but have been advised to assume negligable.

soil cover depth averages around 4-5 feet

sandy soil

no bends over 90


8" x60 sch100 carbon steel

 

[BigInch]

Unusual. Are you planning on burying the loops in the ROW?
Quite a high pressure line too.

I think you are not using pipeline bend radaii, right? Are you using fittings to make the bends?

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization

 

[stuarbc]

The current loops are both burried in the ROW and there are a few that come out of the ground in a very tight loop and go directly back in (something I thought was very strange)

The line is quite high pressure....and will be hydro'd around 5500 to 6000

They are using 5R hot bend fittings...I have recommended a larger radius bend but they have already been ordered....what is typical??

 

[BigInch]

Field bends of 1º/length of diameter, are typical on pipelines giving a much much larger radius. These do not concentrate the stresses either. And I don't believe underground loops are effective. After a number of thermal cycles, the ground could easily be packed up against the pipe on both sides, so they would lose their usefullness.
I've done a much larger diameter 30" hot crude oil line at 190ºF that went for 240 km without expansion loops. We limited the change in vertical alignment with field bent over and underbends to 7º. But it was at 2100 psig. I suspect a good portion of the expansion you have is pressure expansion. If you must bury the loops, I would tend to put them in concrete boxes to keep the soil from packing up around them.

I hope you are in a contolled area where you do not expect to have 3rd party damage from future construction activity, etc. Even though its water, its very high pressure to "bury and forget",if you know what I mean. Not something you'd want a backhoe to dig up some day.

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization

 

[LSThill]

COADE CAESAR II MAY HAVE THE FOLLOWING INCLUEDED IN SOFTWARE:

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

Design Guideline for Buried Steel Pipe

This guideline presents design provisions for use in evaluating the integrity of buried pipelines for a range of applied loads. Both new and existing welded buried pipe of carbon or alloy steel fabricated to ASTM or API material specifications and constructed in accordance with ASME B31 pressure piping codes are considered. The following load conditions are addressed: internal pressure, vertical earth loads, surface live loads, surface impact loads, buoyancy, thermal expansion, relative pipe-soil displacement, movement at pipe bends, mine subsidence, earthquake ground motion, effect of nearby blasting, fluid transients, and in-service relocation. The ASME B31 Guideline Committee currently is considering integrating the ALA-developed guidance into its standard.




L S THILL

 

[C2it]

Coade has included the ALA soil stiffness calculation method as an option in Caesar II version 5.2, which is out for beta testng at the moment.