Pipeline Stress Analysis 6

[ONENGINEER]

I am a soil engineer but recently involved in stress calculations for gas pipelines. Could someone introduces to me a classical method of calculation? Going through some calculations by others has not been conclusive to me. In particular:

What factor of sefaty is used in oil&gas projects to calculate maximum allowable stress. Of course as in other disciplines, there may not be a unique f.o.s but still looking for a so-called industry standard.

Is the code b31.8 the only code used in o&g design work? If not what is/are the others?

I have read in another Eng-Tip posting that "max bending stress= maximum allowable stress-0.3*hoop stress-thermal stresses" What about the longtudinal stresses, which would also exist if hoop stress existed. Where does the coefficient 0.3 come from. What is the range of this coefficient used by varios designers in the US practice? I guess shear and tortional stresses could also come to this equation but if so what numerical coefficient shoiuld be associated with them.

I know the above questions are primitive ones for the knowledgeable experts on this site but the responses would be a starting point for me.

 

[daily123214]

Regarding pipe stresses, I have seen people on the internet say some plastic deformation is good since it redistributes the stresses along the pipe.

How does this happen exactly?

 

[1503-44]

Stress results when a material resists force. If the material yields, it becomes weaker and it cannot resist the amount of force that initiated yielding. Movement is allowed. With movement in progress, further stress increase is not possible. Movement continues until a new equilibrium point is established. That always results in a lower energy state. Stress is reduced.

Say that a length of pipe goes over yield stress at a point of high bending moment at the center of a region where soil restraint is not total fixed in the lateral direction. Let's say that what made that happen were some end bending moments developed due to some expansion force wanting to move the ends of the pipe together a bit, but a slight misalignment eccentricity between pipes created bending moments at each end. Those end bending moments stopped the pipe displacement, by virtue of the stiffness of the pipe between them prohibiting further rotation of those ends. At least until the increase of bending moment at the ends also raised the bending moment everywhere along that pipe segment by the same amount. That increase in bending moment everywhere was enough to raise the previously small bending moment at the center of the pipe segment enough to reach yield. If the pipe did not resist the increase in end moments, the end moments would not increase, the pipe would simply bend more. In order to create stress, there must be some resistance to the applied force. The pipe resists, until it can't any longer and yields. With free movement now allowed, further resistance is impossible. Once the center point yields, the ends are allowed to rotate again, as the stiffness of the now yielded pipe between is insufficient to resist that rotation by transferring shear force and reverse bending to the ends. As the pipe deflects laterally with the yielded point at the center, it cannot even resist the initial end moments and now those reduce too and that reduces stress everywhere else. Centerline is no longer yielding. Movement stops. Lower stresses everywhere.

Yielding is OK, as long as it does not lead to progressive failure. As long as movement stops within the operating limits and excesdive loads are not transferred to other components, no harm done. Once forces are removed, or reduced, no, or only lower stresses remain. Hydrotesting to yield, makes the pipe nice and round.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[LittleInch]

I've been involved recently with an HDD where they were concerned about thermal movement/stress. They did some tests and found that it took 6 months for the drilling mud to become like the surrounding soil in terms of friction and compressive strength.

So it is time dependant. However the pipe will follow the edge of the drilled hole dependant on the pull forces and the bend radius. Don't forget the pipe is probably buoyant inside the drilled hole.


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

 

[ONENGINEER]

Interesting to know the mud is so-called solidified in 6 months, which could suggest that some pipeline sections that were buoyant at the time of pullback would no longer be so after ~ 6 months. Were the tests part of a published R&D program that I can read?
Maybe the thermal stress manifests at the two vertical bend locations, which one could assume as fixed points (?). Any elaborations on their concern?

 

[LittleInch]

I wasn't close enough to know if they were published or not.

The issue seemed to be that if the pipeline went into operation before the 6 months, the pipeline behaved more like an unrestrained pipeline and created larger displacement of the end points. Leave it 6 months and you could model the HDD bit as fully restrained and smaller movements due to thermal contraction.

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

 

[ONENGINEER]

Thanks for highlighting this subject. In cases that the HDD is connected to the main pipeline network, the thermal induced displacement should show itself as concentrated stresses at the entry and exit point connections with the start of operation. Being buyout or loosely restrained, they could check stress calculations to see if the HDD tangents within the bore could bend and kill the thermal stresses in the first 6 months. Just curious I have not done such a design myself.

 

[1503-44]

HDD bends are very long radius. Effectively pretty straight pipe when compared to a standard over or underbend. Axial displacement at the ends should be accommodated with increased flexibility at the tie in to the main line.

Overbends are the worst places where thermal displacement of a hot buried pipeline become problematic and may require additional burial and bend angle limits per joint of 6 to 8° in some cases to keep potential lift up under control.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[ONENGINEER]

from previous post: .... Overbends are the worst places where thermal displacement of a hot buried pipeline become problematic... What is an overbend, please?
My understanding was that an overbend is the smooth-curved continuation of the pipeline outside the bore lifted by cranes or over a ramp to enable placement of the pipeline inside the ground, which implies that the pipeline is at ambient temperature.

 

[1503-44]

Over/under bends bends are changes in vertical bend angles necessary to conform to the vertical profile of the pipe trench, typically made by a bending machine. Overbends are made "going over" the top of a hill, underbends at the bottom of a revine or valley. When traversing undulating terrain, pipeline longitudinal expansion is often forced upwards from the underbends at low points of the trench towards overbends made at the higher elevations. The expansion displacements converge at the high points and tend to lift the pipe off the trench floor. In extreme cases the pipeline may suddenly buckle upwards and "pop up" out of the ground.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[ONENGINEER]

1- Reading the last tow posts from 1503-44, I am not sure if I got the point what the overbend terminology means. Is this an alternative terminology for any convex section of a pipeline within the vertical profile of the alignment? And is so the term concave for underbend?

 

[1503-44]

Convex and concave are terms used to describe curvature of surfaces, especially in optics. I've not seen those terms ever used to describe earth's surfaces nor pipe configurations of any kind.

Overbend. A bend conformin to a rise, as in going over a bump or small ridge, swale, or hill top.
Underbend. A bend conforming to a depression, or pipe bent into a "U" configuration.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[ONENGINEER]

Thanks 1305-44

 

[1503-44]

Sag bend. A vertical elastic bend made by gravity action alone, ie. no bending machine. Not an ideal bend as all elastic bends leave residual bending stress in the pipe, but from time to time, they have their uses.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.