B31.X Code Implications of "Roping"

[KernOily]

Hi guys. Just wondering if any of you know what the B31 Code (31.4 in this case) implications are for roping a pipeline cross-country. This is the practice of laying the line down, either on grade or on supports, and forcing it to bend into very large radius bends to conform to the local topography, sort of like snaking a hose around your yard only it's done with steel pipe. This saves bends as well as trimmed or mitered buttweld fittings and thus saves welds.

I have an 8" XH CL900 X52 water pipeline and owner wants to minimize bends (either shop or field) and welds by roping the pipe. Any help, opinions, or experience would be very much appreciated.

Thank you!!
Pete

 

[LittleInch]

Errr, not quite sure what you mean by "implications", but see how this goes.

What you are referring to is elastic bending, i.e. stresses that result in less than the SMYS of the pipe.

The way it impacts the design is in the calculation for equivalent stress and calculation of longitudinal stress in 402.6.1, by use of the bending stress and section modulus.

Note that sometimes it is the +ve stress son the outside of the bend which results in max stress, sometimes it is the compressive stress on the inside of the bend and you need to calculate both.

Then if the stress passes the code check for your particular radius of bending then it's all good.

However it is quite easy to end up with thicker pipe due to a relatively low elastic radius and hence add costs this way.

In practice elastic bending in the vertical sense is relatively easy to undertake and measure to make sure you're not exceeding you calculated minimum bending radius. This minimum bending radius can be surprisingly high if your wall thickness is minimized for hoop stress alone. If you have a lot of crossings then don't forget to allow for the additional excavation costs required to gently slope your pipeline down and back up again. There's no such thing as a free lunch.

Side bends though can be much harder as you need to first build the pipe straight and then try to lay it in the trench where it is then hard up on one side of the trench, unless you've got a really gentle bend.

Personally I can't see the advantage of not using field bends - its the same number of welds as you use a single length of pipe and it makes construction a lot easier.

Also if you ever want or need to cut out a length to repair it, the residual stresses in the pipe will come back to pay you back big time.


a bend of up to say 5 or maybe even 10 degrees every couple of kms - I could understand that, but more than that and I wouldn't.

It is all dependant on location, angle and radius of the bends and quality of the construction.

Hope this helps.

LI



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