Pipe repair
Pipe repair
(OP)
Hi,
we had a large diameter steel gas-pipe that was installed in potentially unstable slope area. Our ILI geo pigging inspection data indicated that pipe may have deformed and experienced bending (that results in high stress close to yield) due to ground movement . We decided to mitigate the situation by cutting the pipe to remove the excessive stresses. After unpressuering the pipe and exposing it, we cut the pipe at the segment under concern to releif the excessive stress. Upon cutting the pipe we noticed that the differential movement at the cut section close to 180 mm occurred immediately. The question why such large magnitude-movement occurs..? Given this movement, can one back-calculate the stress existing in the pipe at that section before cutting.
Thanks for your thoughts
we had a large diameter steel gas-pipe that was installed in potentially unstable slope area. Our ILI geo pigging inspection data indicated that pipe may have deformed and experienced bending (that results in high stress close to yield) due to ground movement . We decided to mitigate the situation by cutting the pipe to remove the excessive stresses. After unpressuering the pipe and exposing it, we cut the pipe at the segment under concern to releif the excessive stress. Upon cutting the pipe we noticed that the differential movement at the cut section close to 180 mm occurred immediately. The question why such large magnitude-movement occurs..? Given this movement, can one back-calculate the stress existing in the pipe at that section before cutting.
Thanks for your thoughts





RE: Pipe repair
You don't say how or in which direction the pipe moved. Did it move to the side (primarily bending stress), or was that the movement of each half of pipe in the axial direction, ie 180mm farther apart (axial tension stress), or did it move in both directions?
You may be able to assume that the stresses now are minimal and figure out what forces are required to pull the pipe back together again. That can give you an idea of the stresses that existed prior to the cut. Some knowledge, or good guessing, of the pipe being subjected to elongation and lateral displacement must be had, or made. If such movement was due to axial stress over a pulled length of about 100m that would be at yield strength of an API 5L X60 pipe material. If you estimated that 180mm elongation occurred over 200m, then it would have been at roughly 1/2 yield stress.
If it was due to a combination of bending and elongation, it's a bit more complicated as estimate of beam lengths become involved, but theoretically you can deduce approximate stresses from the resolved tranverse shear and axial tension forces required to rejoin the segments.
RE: Pipe repair
Generally if the displacement is less than half the nominal diameter of the pipe then it is not noteworthy. This means that if your "large diameter" pipe is 400 DN or bigger then the displacement was trivial and a person should just realign the cut sections and reweld it. If the pipe is smaller then I would cut out a whole joint and replace it with a compound overbend.
As to calculating the stresses that resulted in the observed displacement, you would have to know the pipe's spring constant, which you don't know since that is not a parameter that is controlled in pipe fabrication. When we do field bends we find that nearly every joint of pipe requires a different amount of force to permanently deform. The force required is always more than SMYS, but there is so much safety factor in SMYS that the amount you have to go over is quite variable.
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Pipe repair
Spring constants of the soil or environment in which the pipe will be embedded are needed only to estimate the stresses and displacements that the pipe will reach, once under operational loading conditions. But if you already know the final positions of the pipe, you do not have to know the spring constant of anything. It's exactly like you can estimate how long it will take you to get from Houston to Dallas, if you know the speed limit, but if your watch said it took you four hours, you don't need to know the speed limit.
The differential stress story of pipe can be reconstructed with only,
1) The original position of the pipe (to estimate initial stress)
2) The final position of the pipe (to determine final stress)
3) The properties of the cross-section, section modulus, area of material and moment of inertia
4) Young's Modulus of elasticity of the pipe; steel is about 30,000,000 psi
Using beam analysis theory in reverse, i.e. instead of knowing load, lengths and properties and integrating those to determing shear and bending stresses, curvature and bending deflection, by knowing bending deflections and section properties, you can do each respective derivative in stead;the derivative of curvature is bending stress, the derivative of bending stress is shear and the derivative of shear is load. What more is there? Spline curves can be used to great advantage for this type of analysis, as beams naturally assume spline curves under bending stresses. Spline curves have a constantly varying curvature between known points of displacement. Once you have the equation for the spline curve, you can hire a mathematician to do the rest. Or just give him/her the two curves and say that the final curve is a spline and ask for the second derivative.
y = f(Displacement),
dy/dx = curvature,
d2y/dx2 = moment,
d3y/dx3 = bending stress,
d4y/dx4 = shear,
d5y/dx5 = load causing the displacement.
RE: Pipe repair
"Hire a mathematician"? Isn't that another way of saying "you really have no way of knowing"?
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Pipe repair
If you know the initial and final position, length doesn't enter into the equation for bending stress because you only need the length to estimate a curvature by Δy/(L/2). If you have the actual position of the pipe for 3 linearly adjacent points, 2 intervals of some Δx, you can calculate the curvature directly at the central point, after calculating the relative lateral deflection to the adjacent two points.
To do the same with axial stress you'd need to know the elongations between given intervals of length. Calculating Δx / L. That might be difficult, unless you knew the original and final positions of two or more girth welds. Axial stress would equal Δx/L * E. A rather severe relative axial displacement of 1" elogation between girth welds 40ft apart = 1"/40'/12"/' x E = 0.002083 in/in x 30,000,000 = 62,500 psi tension. If it was at operating temperature, you would have to adjust for the expansion or contraction stress at that temperature as well.
RE: Pipe repair
Thank you very much for your thoughtful replies. Unfortunately we did not have intial profile of the pipe since the pipe was installed long time ago so we wanted to make sure of the out-of-straightness observed from geopigging when the pipe was exposed was not from initial installation but it was due to pipe movement under ground-movement. actually the Pipe Diameter was 700 mm and differential movement (around 180 mm) was vertical (in the direction of the suspected dominant displacement of the pipe segment)and there was some lateral movement of around there 100 mm). But given the movement in the vertical and lateral direction, I guess at least we can conclude upon observing these movements that the pipe had significant elastic bending stresses ( rather than elongation stresses since the movement did not occur in the axial direction).. is that correct..?
.Given the fact that pipe had no pressure and exposed for around 20 m, I don't think this movement is due to pipe own weight because the pipe segments are stiff enough to hold themselves after cutting
BigInch you are saying "The movement occurred because you released the internal restraining stress. You turned it into a Free Body. Shear, tension and bending stresses holding the metal together, all of a sudden were cut off and made zero"
Where these internal stresses coming from if the pipe has no pressure : Own weigh of the pipe or the boundary conditions at the end of the exposed segments ( around several meters far from the cut section). I thought that (given the differential movement is vertical) that they all due to bending loading that existed in the pipe before cutting..do you agree with me..?
Thanks alot for your thoughts
RE: Pipe repair
The vertical and horizontal directional displacements 90degrees apart can be seen as resolved components of one total bending moment in a principal plane. Much the same as the hypotenuse is equal to the square root of the sum of the squares of its component sides. If you had 30mm deflection in the horizontal direction and 40mm in the vertical, then total displacement in the principal plane of bending moment was 50mm. (30^2+40^2)^0.5 = 50mm. The bending stress is related to curvature, so it is important to estimate the span over which that 50mm of bending deflection occurred. Once you have that, then you can estimate the curvature and arrive at the bending stress that caused that curvature.
RE: Pipe repair
If the pipe was not under bending loading, it should not have experienced these differential movements. ..?
Given the section modulus ( steel pipe diameter of 700 mm and wall thickness of 5 m , exposed span of 20 m), this bending should not be due to pipe own weight. It is rather due to pipe movement (which we believe caused by ground displacement). I am not sure if you second this..?
RE: Pipe repair
RE: Pipe repair
RE: Pipe repair
Are you serious? I'm amazed anyone managed to build it, but that is a seriously thin walled pipe.
Is the variance vertical or horizontal when you cut it?
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Pipe repair
RE: Pipe repair
A cantilever might be used to represent each of the cut pipe ends.
You now have to pull both cantilevers back together and rotate each facing end with an applied moment so that the two ends meet straight on, i.e. without any miter angle. The moment needed to make the rotation will be the bending moment present just before the cut was made. The lateral force necessary to move the two ends together in the lateral, or vertical direction will be the shear at the cut point. If you have to stretch each end to get them to meet, the amount of force required to do so will be the axial tension force.