Anchorage of pipelines

[B.L.Smith]

Dear Sirs,

I've seen anchor of pipeline before it goes aboveground at above and underground interface. This anchorage is usual at aboveground Line Break Valve(LBV) stations and crossing of pipeline with river that the pipeline pass through a bridge. Can you explain the reason of this anchorage?

Best Regards,

 

[LittleInch]

When pipelines are buried for a reasonable distance (~ 100m) they are described as fully restrained, i.e. regardless of any force on them in an axial direction caused by pressure or thermal expansion or contraction, the pipe itself does not move.

However where pipes come out from the ground the can move or provide a force onto other equipment.

Often this is not actually a problem so long as the design of the pipe above ground allows for tis movement ( a few mm), but in some cases either the designer of the system can't cope with this or the small movement actually causes an issue.

Anchoring pipelines can cause more difficulties than not anchoring it as the expansion still needs to go somewhere, but this is just how it is. Pipes above ground have much less resistance to movement and hence long runs normally require expansion loops or bellows to prevent buckling and large forces being developed.

Waa this just a general question or was there something specific in mind?

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

 

[bimr]

In operation, the pressure and temperature of the fluid induces stresses in the pipeline. On one hand the internal pressure, which is normally higher than the atmospheric pressure, creates both “hoop” and longitudinal stress in the pipeline. This will lead to a tendency for the pipe to straighten at bends (the Bourdon effect). This movement can be compounded by the temperature of the fluid which will cause thermal expansion (or contraction) of the pipe. Should the soil not provide enough longitudinal restraint by friction, the pipe will tend to move along its axis.

Comprehensive analysis of the restraint, movement and the resulting stresses within the pipeline is required to ensure that pipeline stresses will be within acceptable limits. Analysis often shows high levels of movement at bends and at the ends of a pipeline, where the pipeline comes above ground. Movement can be controlled by additional soil loads or the incorporation of anchors. Alternatively, expansion loops or bends can be incorporated to allow movement without unacceptable stresses.

Movement can also result in upheaval buckling. This can occur at an overbend or a vertical imperfection in the bottom profile of a trench and can result in the pipeline coming out of the ground and possibly pipeline buckling. If the pipeline upstream and down stream of the overbend or imperfection is locked in position and expansion of the pipeline occurs from these fixed points then the pipeline relies upon the soil overburden to keep it in place. If this overburden is insufficient then the pipeline could move vertically. Furthermore, the more it moves vertically then the lower the soil overburden becomes, hence allowing even greater movement.

The greater the burial depth, the greater the restraint on a pipe will be both in the axial and radial directions. This is, however, only true up to a point where the soil load does not increase anymore with soil cover.

 

[rconner]

Thanks, bimr(and good pic). I will attempt a little elaboration however on your third sentence, "This will lead to a tendency for the pipe to straighten at bends (the Bourdon effect)."
While it is arguable how much there is to be worried concerning same, I think there may be in general be some misunderstanding of the "Bourdon" effect. Actually, as I envision it Bourdon effect can be in effect a tendency towards lengthening of a pipeline caused by effects of pressure thrust in e.g. the approaches to at least a non-externally blocked or anchored major thrust focus like a bulkhead or closed valve etc. on the end of a line. [I guess however that lengthening in the case of a near full circle/circular "Bourdon tube"-based pressure gauge e.g. does result in a larger circle, in effect/indeed "straightening" that curvature some, and additionally toggling a lever at the end attached to the gauge needle to multiply the effect.]
I saw L.C. Peng has in the past referred to Bourdon effect as "pressure elongation", and in the case it appeared at least of a bulkhead or closed valve on the end of a line case he represented same with some rigor in strain magnitude as "e1" = PD(1-2µ​)/(4tE).
While in the real world of quite stiff (welded or flanged steel piping most on these lists are familiar with) I believe the Bourdon and at least somewhat off-setting and also pressure-related "Poisson" contraction effects may indeed be measured in only "mm's" (as mentioned by LittleInch), I should note this is not necessarily true of e.g. polymeric-type pipes, that have quite low short-term elastic moduli to begin with, and that relatively very low short-term modulus in fashion non-obvious to some may even be effectively reduced several times less in some cases when loads are applied in the longer-term. Greatly increased strain is due to the pressure elongation strain as related by Peng above being inversely proportional to the effective Young's modulus.
Out of curiosity, I looked at a case of a 30" O.D. (little more than 24"ID) DR11 hdpe pipe that had one free end and the other anchored 100 m (of course 328 ft) away, but no meaningful anchorage soil or otherwise e.g. like on rollers or frictionless supports between that and a free end (say a closed mechanical joint valve etc). I then pressurized this model to 200 psi to be held, and applied Peng's relationship. While anyone else who wants to try their own exercise can apply their own or more accurate numbers, I assumed E = 100,000 psi with µ = 0.4 for hdpe at some duration pressurization and got a resulting tensile strain of 0.0011, and thus an aggregate stretching movement of the free end then of 0.11 meters over an aggregate 100 meter length (i.e. 110 mm or about 4-3/8"). I then did the same thing for a 24" OD steel pipe 6.35 mm (0.25") thick the same length and got a Bourdon tensile of only 0.0001 and a movement of the free steel end of only 6.4 mm (apparently verifying the statement of LittleInch), nearly 20 times less than the plastic! [I guess I should probably also mention thermal movements can be much more, and if the pipe is heating up as some do after installation, the additional thermal growth of the plastic can be even much greater than this, and also much exaggerated relative to the metal, in that the coefficient of thermal expansion may also be 15-20 times that of the steel!] Such movements are perhaps things that may need to be considered in at least some situations e.g. at changes in direction and some inevitable transvrse connections to pipelines.
If one really wants to understand Bourdon effect, get hold of a cylindrical, tube balloon (like used to tie balloon "poodles etc), blow it up with air pressure, and observe closely what happens.
All have a good weekend.

 

[B.L.Smith]

Thanks for ur replies. Bmir, I have two questions. How can I calculate the anchor force at interface of the above and underground? Does this anchor design for upward force or it should be design for axial force?