Horizontal Directional Drill Internal Pressure Design Factor

[demax182]

I'm new to this and am hoping somebody can help me with this problem. I was asked to calculate the internal design pressure of a crude oil pipeline that will be horizontally directionally drilled under a major highway. They've asked for calculations for two scenarios, pipeline that is buried 45' deep and pipeline buried 100' deep. I've looked through the 49CFR195 and ASME B31.4, but cannot find any guidance on what design factor to use in directional drills. As of now, I'm using the conservative figures of 0.5 & 0.4 for the 45' & 100' depths respectively. However, I really have no basis on how I determined those factors. Are there any rules of thumb on determining design factors for drilled pipeline? Thank you in advance.

 

[LittleInch]

deamx182,

As you've discovered, there are no written down design factors (AFAIK) to deal with horizontally drilled pipe. The common use is to use 0.6 Df as an acknowledgement that once you drill a pipe, you cannot reapir it and hence you need to add a bit more "meat" to avoid issues in later life from internal or external corrosion. You can also assess your pipe in terms of combined stress given that the pipeline is fully restrained but in addition to the normal stresses, also has a permanent bending stress depending on your tightest drill curvature (usually 600D or greater). This may show that you need a certain thickness pipe to be below the maximum stress allowance. Depth of drill is irrelevant.

Whilst thicker pipe may seem a good idea, it also becomes stiffer and hence harder to pull through a radiused bend so there is alsways a trade off between a thin pipe easier to pull in than a thicker one which might increase pull forces as it takes more effort to pull it against the curved walls of the drill.

My motto: Learn something new every day

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

 

[demax182]

Thank you!

 

[rconner]

Just a cautionary note -- I believe I attended a paper presentation nearly twenty years ago by a gentleman named Hugh O'Donnell (who I think may now be deceased), wherein Hugh related an interesting case study. In a lengthy and quite deep HDD pull of rather large diameter (30"?) steel pipe, the pipe emerged with an interested crowd of dignitaries gathered on the far end rather unceremoniously (basically it was flat as a flitter). As it was decided the pipe was apparently creased on some old wood piles on one side of the crossing as it traversed, they decided to do it again (buying new pipe, re-welding, drilling, pulling etc.) After all of this the pipeline emerged once again inverted, and by at least this time I suspect (as any others might who are aware of all that goes in to such major crossings) with considerable angst developed amongst the parties involved.
It was eventually realized that the pipe had been buckled, by a combination of differential external to internal pressure and what else happens (deformation/making more susceptible to buckling and stress) to a welded pipeline like steel, hdpe, or fpvc when it is bent and pulled in an arc of HDD. I guess it might even be argued the external pressure of a fluidized annulus of drilling mud and soil cuttings et al might even be a little heavier that pure fresh or salt water. At least according to O'Donnell, the depth is thus not necessarily irrelevant, at least in some cases.

 

[LittleInch]

That's a fair comment - because the OP was talking about DF, I assumed he was using the main pipe as his HDD pipe. If you're putting in a thinner carrier pipe or just a thin pipe full stop (D/t > 80) or a PE pipe, you will need to consider collapse pressures, but 30 odd meters shouldn't be critical.

My motto: Learn something new every day

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