pipeline allowed deflection

[soil1999]

Hi everyone,
A buried cased transmittion steel pipeline with dimater of around 1 ft and internatl pressure of around 2100 Kpa experience maximum deflection of 1 in along a 25 m length due to surface loading. How can one know that this deflection is acceptable in terms of servicability/integrity and strength. Is there a way to calcualte strees induced in this pipelies from the imposed deflection. Thanks

 

[BigInch]

There are common beam formulas to determine the maximum deflection at the centerline, or at any other point. Just reverse them and determine what load and resultant stress is responsible for the measured deflection, such as
[Δ] = wL^4/384/E/I
could be solved for w and the maximum moment would have been = wL^2/8 and bending stress [σ]= Mc/I

 

[Jmoore1]

On top of what BigInch said, as the distributed weight on top of the pipe (if assumed constant) can be used to find the bending stress (you would want to check your max bending moment and any stress concentrations along the line, reducers, valves etc...)...additionally you would want to check the shear stress across the wall of the pipe, and any thermal and internal pressures.

Your best bet would be to apply an external load inside of Ceasar and have ceasar calculate the total stress on the line, if your above allowable stress of the line then you need to do something (remove the load or support the pipe)

 

[Jmoore1]

you would also have supporting distributed loads from the soil....you would need soil samples above/below the pipe to determine why the deflection is happening, if they were uniform distributed loads the pipe wouldn't deflect so something is happening underground (your pipe has probably compacted the soil in some areas and not other)...although you probably could get a reasonable estimate of stress in the method above using ceasar but your looking at a more complicated solution as you will have a distributed load above from soil/pipe and below from the soil....

HydroVAC down to the pipe at the largest deflection and place a strain gauge?

 

[BigInch]

Jmore, Its a cased line, so its doubtful there is a need to consider soil load on the pipe itself, which is why I suggested using only a simple beam formula.

Soil1999, There are probably some spacers somewhere inside that casing, as 25 meters is quite a long span to go without using them. Without spacers the deflection is likely to be enough to contact and possibly short out the pipe to the casing and lose your CP.

 

[rconner]

The combination of the modifier "cased" and then a stated causative agent to the deflection (I assume of the carrier pipe) "due to surface loading", is a little confusing. Are you saying you know the casing was straight to begin with, and the casing along with carrier piping were both somehow bowed by some sort of traffic loading above (I ask this as sometimes casings are not installed perfectly straight to begin with)?

 

[Jmoore1]

Ah good call big inch, agreed, whether the pipe is being bent by the casing or not, biginch's method should give you relatively close stress's..

 

[waterpipe]

Agree with rconner that it's confusing stating the cased and deflection under surface loading.

Soil1999, do you mean the ring deflection (pipe ovality) or longitudinal stress (deflection). BigInnch has taken it as the second one.

 

[stanier]

I assume the posting is in regard to ring deflection. In Australian Standard AS 2566 Buried Flexible Pipelines the maximum allowable deflection for a welded unlined steel pipeline was set at 5%. The work by committee was based on AWWA, ATV and standards.

Notably this standard is conservative and was developed for the water transmission industry.

Are there not maximum deflection criteria is other pipeline standards?

 

[BigInch]

For cased pipe, you wouldn't normally be too concerned with ring deflection as the casing is taking that soil load and has no hoop stress at all. The only deflection you would need to worry about is longitudinal at the centerline of a span between the casing/pipe spacers that are the farthest apart. The casing is attached around the pipe only with a rubber end seal, so no casing force is actually ever applied to the carrier pipe either.

 

[stanier]

I took it that the carrier pipe had been defelcted by 1". In this case would not API579 cover the defect?

 

[BigInch]

I took it as being just a simple sag deflection caused by pipe and content weight in a long span between spacers, or in the absence of spacers, the span between ends of casing, where the casing was sealed by a welded steel plate, or passing through some kind of reducer welded to the outside of the carrier pipe and to the casing. Usually a flexible boot is used for the casing to carrier end seals, but rigid seals are ... shall we say... not totally unknown. I didn't take it as "dent" damage, just a sag bend deflection.