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HDPE Transitions
2

HDPE Transitions

HDPE Transitions

(OP)
I'm laying out the gathering system for a new field development, the client likes SDR-7 HDPE and I've modeled the trunks, laterals, and individual well flow lines and have settled on 10-inch SDR-7 on all the trunks, 6-inch SDR-7 on the laterals, and the well flow-lines are 4-inch SDR-7.  

The pressure drops and velocities all make good sense, but now I'm looking at how to tie the pipes together.  My first look said that I didn't want a bunch of weight stacked above a plastic pipe so I was going to transition to steel a joint either side the tie-in risers.  As I read about this stuff I'm thinking that maybe I'm being excessively conservative.  If I use a 10X6 reducing tee in SDR-7, transition to steel and immediately swedge down to 4-inch S-40 steel for 3 ft, and then set about 700 lbs of valves, fittings, piping, and flanges on top of it will I break the 10-inch?  If it is a problem, I can come off with the tee on the horizontal and 90 up with a thrust block under the 90, but I wanted to avoid the potential water trap.

David

RE: HDPE Transitions

(OP)
No, this is kind of a fishbone system.  The backbone is 10 inch SDR-7.  Some number (a couple of dozen) of 6-inch laterals will come off the backbone.  A couple of hundred individual well flow lines (4-inch SDR-7) will run to the laterals.

I'm trying to determine the "best" way to connect the flow lines to the laterals and the laterals to the trunks.

David

RE: HDPE Transitions

(OP)
Sketches are hard here, the layout looks something like:

                 WWW->WWWWWLWWW<-WWW
WWW->WWWLWWW<-WWWWW        L
        L                  L
        L                  LWWW<-WWW
        L                  L
        L                  L
TTTTTTTTTTTTTTTTT->TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT Comp
                           L
                           L
                   WWW->WWWLWWW<-WWW        

The "W" are 4-inch SDR-7 well ties, the "L" are 6-inch laterals, and the "T" are 10-inch trunks.

David

RE: HDPE Transitions

(OP)
Looking at ISCO's specs on HDPE, I'm finding yield points in the neighborhood of 1600 psi in compression (3200 psi in tension).  The solid plastic area of a 10 X 6 reducing tee is 16.9 in^2 on the 6-inch leg.  In my simple mind it looks like I could stack something like 27,000 lbf of stuff on top of this tee before I approach the yield point in compression on the 6-inch so my 700 lbf of valves and fittings should be fine (41 psi compressive force).

Is it save to assume that the 10X6 reducing tee has been reinforced to allow the 10-inch leg to resist buckling when the branch is a max load?

David

RE: HDPE Transitions

I wouldn't assume you could carry that compressive load, due to the geometry of the tee.  I think you might get some local buckling in the T's wall.

I would put some flexibility into the tie-ins and the laterals as they approach the laterals and trunklines, respectively, to avoid large axial loads from dumping into the connection.  I think large axial loads from the lateral would tend to displace the 10" first, rather than buckle the T.  There will probably be some thermal movements and I think allowing for those movements might be good, as it will aleviate any punching and bending into the 10", so I
don't think I would use thrust blocks either.

Actually I'd just do it like this and not look back.


use valve pads or sand-cement bags under the valves and their fittings.

http://virtualpipeline.spaces.msn.com

RE: HDPE Transitions

(OP)
I found a local guy with experience with HDPE and his concern was "cold creep".  He said my force calculations were fine, but a steady load on HDPE pipe walls will tend to creep with time and with a constant 40 psi of force the pipe would most likely collapse into itself within a few years.

I've decided to transition to steel about 1/2 joint either side of the tee for the valve set and then all the problems just vanish and I can build my valve sets the way they need to be.

Thanks for looking into this.

David

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