lateral buckle

[RandomAxe]

What is the minimum length of pipeline over which lateral buckling can occur? We have a repair to perform on a subsea pipeline that requires excavation of approximately 150m length of pipeline (trenched and buried). The pipeline will be reburied and dumped with rock but we may have scheduling issues and start up the pipeline again before burial and rockdumping - could lateral buckling occur over this length if its unburied in a trench for 150m? Any other issues that could occur? Thanks

 

[MFJewell]

Diameter of pipe, wall thickness, and density of fluid inside are needed to calculate an answer.

 

[LittleInch]

You also need temperature of fluid which is the key driver.

If the pipe is slender enough 150m is easily enough to buckle it if the temperature expansion is high enough.

This isn't something you can determine from forums such as this as there are many inputs in modeling this such as length of the other trenched and buried sections, coating and weight of concrete (if any).

If you start up and then rock dump, you could lock in stresses and strains that were not allowed for in the original design. you could easily get lift off from the trench bottom and then if the rock dump goes under it you've created a weak spot.

Too many variable unknown and you really need to get it analysed, probably by the original designers. Far too much risk here to just take a punt on it without doing some analytical work.

there are alternatives such as screw anchors, clump weights etc but if it needs to be buried to resist buckling then you need to respect that.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[chicopee]

I can not give you an answer, however, the answer should be available in one of my references titled "Theory and Design of Tubular Steel Structures"authored by Troitsky and sponsored by The James F. Lincoln Arc Welding Foundation. My copyright copy of 1982 has the following chapters: 9 for aboveground pipelines, 10 for underground pipelines and 11 for underwater piplines which will the relevant information sought out in your post. There could be updated copyrights of this book.

 

[BigInch]

UPLIFT BUCKLING (not lateral)

B31 code limits buckling stresses to 60% of stress at buckling load. You would have to keep temperature stresses well below those that would cause buckling, or not operate at all.

For calculating uplift buckling,

http://www-civ.eng.cam.ac.uk/geotech_new/people/bolton/mdb_pub/187.pdf

http://www-civ.eng.cam.ac.uk/geotech_new/people/bolton/mdb_pub/190.pdf

http://rules.dnvgl.com/docs/pdf/DNV/codes/docs/2007-10/RP-F110.pdf

Technology is stealing American jobs. Stop H1-Bs for robots.

 

[RandomAxe]

Thanks for the responses guys.

BigInch, how do you mean? If its not buried in that section how could it uplift? Or do you mean the reduced friction over the excavated length would cause an UHB in the buried section? I will have a read of your links.

It is an 8" carbon steel production pipeline, relatively stiff at 12.7mm wt, and operates around 60deg.

 

[LittleInch]

60C? 60F? what was the assumed lay temperature?

150m of 8" 12.7mm wt will be like a whip.

Agree if you're in a trench then lateral isn't that likely.

With the fixing of the pipe by burial / rock dump you've probably got around 200m of pipe either end of your 150 feeding expansion into your now unrestrained bit of pipe.

60C assuming a lay temp of 10c is on the margins, especially if you have no concrete or other weights holding it down.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BigInch]

Since this is a repair, it is covered in rock, or it would have buckled already. Removing rock from above it will leave some kind of lateral buckling support. Even if unburied and they start off laterally, they usually end up vertical anyway as there is no friction in the up direction.

Technology is stealing American jobs. Stop H1-Bs for robots.

 

[RandomAxe]

Thanks again. I have requested some design documentation from the original design, the repair is quite close to one end of the pipeline so thinking about it some more I'm not even sure this section will be anchored, which I'd imagine will increase the risk of a buckle (and/or walking).

 

[LittleInch]

You also need to find out if its the hot end or the cold end.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[RandomAxe]

It's nearer the hot end, so 60degC. I've ran my own quick calcs and I think it could be unrestrained in this section

 

[LittleInch]

Depends on the coating and how deep its buried, but about 200 - 300m of trenched and buried pipe is equivalent to an anchor.

Starting it up before you bury and rock dump sounds like a high risk strategy to me without some detailed analysis.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[dhengr]

RandomAxe:
60̊C is equal to about 140̊F, isn’t it? Then the expansion of the steel pipe due to a 140̊F temp. change will be approx. (.0000065 inches per inch per degree F)(1200")(140̊F) = 1.1"/100' of pipe length. Given your latest posts, it seems to me that the pipe is pretty much unrestrained at the hot end, near where you are opening the trench and making the repair. Then, the bigger question might be, can the piping and the equipment at the hot end take this kind of pipe movement? Is the terminal piping arrangement flexible enough to tolerate this longitudinal pipe movement? And, at this location, this kind of movement will occur every time there is a temp. excursion, unless you have a good solid anchorage point.

If the pipe line is truly fixed/anchored at some locations, the longitudinal stresses due to expansion/contraction caused by temp. changes can be very high, to the point of pipe yielding or local pipe buckling, due to local pipe details or stress raisers, and the like. Alternatively, the pipe was straight and 100' long prior to the temp rise; now it is 100' + 1 to 1.5" longer and it will buckle globally to accommodate this length change. What is the new arched shape which will allow this length change btwn. the anchorage points? The pipe is trying to relieve this compressive stress increase and one way to do that is a long sweeping buckled shape to allow the length change.