Induction bend in muskeg area

[sam62]

Hi all,

I read in a document that no induction bend will be used in muskeg area without providing more details on why. Can someone shed some light on this please? Thanks.

 

[SNORGY]

sam62:

That very likely would have been taken out of context and may or may not necessarily be true depending on the properties of other surrounding soil, wall thickness, pipe material used, service fluid, installation temperature, hot temperature, restraint locations, and other things.

If you are installing pipe into muskeg your virtual anchor lengths will be relatively long, and any bends in the system will be relied upon to absorb correspondingly higher thermal growth. It might well be that such bends, in muskeg, will see higher stress levels than they would see in better soil conditions.

To me (a generalist), this is really a stress / flexibility analysis issue. Others in this forum who have spent most of their careers in pipelines might be able to comment on the validity of the statement in general from a more informed perspective.

Regards,

SNORGY.

 

[ColonelSanders83]

I would be very interested to know what document said this and the basis for which this direction has been given.

A question properly stated is a problem half solved.

Always remember, free advice is worth exactly what you pay for it!

http://www.ap-dynamics.ab.ca/

 

[BigInch]

Bend movement will be very much unrestricted and with very long virtual anchors, displacement is likely to be great. The muskeg on the outward side will be compressed and the space left on the inside of the bend will be filled with water. The muskeg will no longer be muskeg, leaving more like a compressed peat on one side and a potential creek at next snowmelt on the other.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[SNORGY]

What Biginch said is also true. This effect will contribute towards undesired downward settlement of the bend as well. But this will be true at any bend, induction or otherwise.

Regards,

SNORGY.

 

[sam62]

All: Thank you very much for your input. My knowledge in stress analysis being close to basic, your replies have allowed me get some understanding on this.

ColonelSanders83: The document in question is a spec from a reputable engineering company but without more details. It was just like a blank statement.

Sam62

 

[BigInch]

I think "induction" got in there by accident. The only real difference would be the sharper turn that you could do with induction in comparison to a cold bend, perhaps increasing the load concentration on the muskeg. But as I implied already, why make a bend in muskeg. Most of the time you could probably avoid doing that if you tried.

Water surrounding the pipe (with no buoyancy control) would tend to float the pipe as well.

Other than those considerations, nothing comes to mind that would make induction bends any more unique than cold bends.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[SNORGY]

Once again, BigInch has beaten me to it in the above post.

After some thought, it occurred to me that more gradual bends will end up with the soil's lateral bearing resistance spread out over a larger area (length), tending to displace less and flex less. Induction bends are made in a hot condition in order to achieve a tighter bend radius without unacceptable cross-sectional deformation and strain hardening. It could, therefore, be that the avoidance of "induction bends" was really intended to mean an avoidance of "small radius bends" (e.g., 10-D as opposed to 20-D), perhaps with a view towards minimizing the stresses on a component that might already be locally stressed due to wall thinning, strain hardened, or ovalized. As stated, however, it is misleading to specifically draw attention to "induction" bends because, given the choice for a given bend radius, I would prefer induction bending over cold bending.

As for buoyancy, I probably tend to see smaller diameter pipe than BigInch, so "sinking" in muskeg is usually more of a concern than "floating".

Regards,

SNORGY.

 

[BigInch]

Generally 6" (except for the lightest wt) and less will sink.

Also in general, you should avoid any induction bend, simply due to the extra costs involved with the bend, transport and fitup complications. Out on a pipeline R^oW you're usually going to have enough space to make a cold bend, so I don't really understand why you wouldn't have that near muskeg areas and would have to use a hot bend there. Its not like such an area is going to be filled with a lot of fixed obstacles you have to avoid with tight turn hot bends. I've only used them on offshore platforms when connecting the riser around a manway, stairs, locating an ESD valve and hooking up into a launcher on the lower deck piping. Sometimes a standard 5R won't fit up and you simply must have a special bend made. Out on the R^oW, its hard to see why. If anybody has any outright guesses they would like to propose in order to try to get to the bottom of this muskeg issue (so to speak), I promise I won't laugh if you publish.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[SNORGY]

BigInch...

As a Canadian, I assume that most Americans (for example) do not see a lot of muskeg (well...they might call it "quicksand" down there) unless they are in Alaska. In such instance, virtually all of your comments with the associated connection to geographic remoteness apply.

In the CSA Z662 Code (I haven't read the B31.8 Code but I suspect that most of what's in Z662 probably came from it), there is some nebulous criteria for cold bending of 12"-D pipe that limits you to 1.5 degrees change in direction per diameter of pipe length. Right or wrong, a lot of folks apply this for all pipe sizes (I am not one of those folks), and are disillusioned with the conclusion that they need a 60-D minimum bend radius for cold bends. I think what is more common industry practice on both sides of the border is to limit outer fibre elongation to somewhare around 2% for cold bends, from which something around a 20-D cold bend is generally accepted. In the absence of something more definitive from CSA, right or wrong, there is sometimes also a propensity to apply ASME B31.3 criteria of 5% towards the outer fibre elongation permitted for impact-tested materials without post-bend heat treatment; that would limit one to about a 10-D bend.

Personally, I would not cold-bend to as tight as 10-D without subsequent inspection for ovality, wrinkling and strain hardening at the extrados, none of which I would be immediately satisfied with without such testing.

To me, what it comes down to is whether or not you can fit a 20-D bend in the ROW. If you can, it's completely unnecessary to call for induction bends in anything other than prefabricated risers.

Which is pretty much what you have been saying all along.

Regards,

SNORGY.

 

[BigInch]

I'm sure a quick glance through B31.4&8 would be worth your time. You could probably come to know it as the Old Testament, when laying alongside the CSA. Nothing wrong with .3, but I do think its still just a touch too conservative to use as a fall back for normal temperature cross country pipeline work.

Having worked for Northern Natural Gas on some large diameter lines touching Canada up in northern Minnesota area, I have stumbled into muskeg more than a few times, as well as some other slippery, slidey stuff on occasion.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus