Directional Drilling Question 1

[looksatstars]

We directional drilled a 3" HDPE dr17 sewer forcemain, 200 metres in sandy conditions. Driller bored the pilot hole and reamed back with a 10" reamer and used mud in the retrieval.
But when the forcemain came back it was oblong shaped. it doesnt look like the pipe failed but definately stressed.
The driller said he did nothing wrong, the engineer said that pipe shouldnt have failed as it had a safety factor of 8.
So im left with contacting the manufacturer. Just wondering if anyone has any input.
Thanks

 

[rconner]

You did not mention if you had yet contacted the manufacturer, if so what did the manufacturer say, nor exactly what the resulting dimensions (inside and outside, on both the major and minor axes, so to speak) of the “oblong” shape you observed. However, I have seen sort of oblong-shaped hdpe pipe before, even before installation!

Nevertheless, without knowing any more about the severity of what you are talking about nor your specific application this is however possibly an indication that the pipe has been “stressed“, and also while not necessarily the case this could conceivably be the result of a strongly pulled and/or curved path (as by bending of a long welded or fused tube or pipe) in HDD installation. Notice e.g. one type of behavior explained in generic terms per the website at

http://www.tubebendingconcepts.com/...ss/effects_on_the_tube_structure/default.html

, “as a result of the tube's structural deformation, bent tubes will have a certain amount of out-of-roundness detectable to the eye. This out-of-roundness, in some of the more generous applications, can be minimized to some degree, but not eliminated.”

While ovalled pipes obviously have some lesser flow area than circular, also offer lesser external “buckling resistance”, and this could be an indication of other problems, this of course doesn’t necessarily mean you will have any more substantial problems in your specific installation. While not necessarily advertised, some hdpe manufacturers do acknowledge and forewarn to some extent of such behaviors in detailed literature (see e.g. page 11-13 of the document

http://www.isco-pipe.com/pdf/directdrill.pdf

).

Another type of perhaps non-obvious, over-stress problem that has occurred in some pulls of hdpe is a type of “necking”-down (i.e. constriction), and sometimes even severely so, of the pipe. Interestingly, this severe over-stress problem has been found to occur somewhere up in a buried pull, and thus may not even be necessarily detected e.g. by just an acceptance pressure test of the pipeline! In addition to such supposed safeguards as “break-a-way” swivels etc., some specifications cognizant of such behaviors are now additionally requiring properly run mandrel testing etc. of installed plastic pipes, presumably to guard against acceptance of over-stressed/deflected pipe conditions.

 

[looksatstars]

Thanks for the input.

Well we did a second pull of about 100m (with smae materials and equipment) and the drillers said the pull came thru no problem, but the pipe again was oblong!!

The shape of the HDPE pipe on the spool is 3.25" x3.5" so the pipe is not completely round but pretty close.
The shape after the pull is 5" x 2".
The manufacturer said it is not common for the pipe to stress that far out of shape. So far no one is to blame, everyone did their job satisfactory.


I am going to get a pipe video done (ie put a sewer camera down the spooled side end) and see when the deformation happens. If the deformation happens right away, I think it will be safe to assume it is from the soil pressure (which is the contractors explanation).

 

[rconner]

Interesting. Wow, while I'm not going to guess exactly what has happened to this pipe, the 2”x5” finished shape of course appears to be like a 3-1/2” O.D. (IPS-sized?) circular pipe somehow deformed or deflected say an incredible [(3.5-2)/3.5]100% or ~42+ percent of the intended diameter! Even if there were no other concerns of over-stressing etc., it would appear, if you’re looking at a basically elliptical inside shape, that at least your flow area has been reduced perhaps near 27% (and of course even more than that at the inner protrusions of every joining weld “bead”).

I’m guessing also that with a “minor” axis inner diameter for the pipe (I suspect of perhaps roughly 3.063-1-1/2” or 1.563”, and less than that at any fused joints in th eaffeted area), it would appear there might also well be an eventual battle getting any sort of inspection instrument (or some pigs etc.), or at least those intended for 3” or larger pipes, through same, if that were ever needed for any reason in the future. [In this regard, I believe some folks in some areas have in some cases after the fact felt there was a need for cleaning or inspection instruments (e.g. for removing protruding inner beads and/or looking at fusion/weld quality in various fashions etc. in at least some buried/installed hdpe pipelines).

 

[shirsch]

I would also be worried about how the deformation will effect your connection to addtional fittings. I don't think the manufacturer makes joints or fittings for a 5" x 2" oblong pipe.