Installation Temperature versus “Lock-in” Temperature

I've always argued that the temperature to use is the soil temperature at the bottom of the trench at time of burial as the pipe is in contact with it and assuming you backfill in a linear fashion, the pipe can expand or compress as it heats up or cools down and is then locked in once you've added backfill. This is often pretty close to ambient average air temp.

I've found that a number of people start off with temps quite low (say 10C) and not the actual ground temp at 1m at the time of burial. If it works then fine, but if you're failing the design then you need to modify it a bit.

IMO and according to the construction specs, the backfill should be compacted in layers and hence while it might take a while to finally "settle" this is only a small fraction and the weight on the pipe doesn't change and hence doesn't affect the friction on the pipe.

I agree with your 0 deg C before backfill in winter to reduce expansion stress / forces.



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Also: If you get a response it's polite to respond to it.

Thanks LittleInch.

An interesting argument seems to arise every time I broach this subject (see below). The longitudinal compressive stress formula (referenced above) does not account for any effect of hydrostatic testing on longitudinal thermal stresses.

Consider:

If the pipeline is truly “locked-in” or restrained immediately after backfill, and then filled with test water (at around 5 deg C) then the line would be subject to a longitudinal compressive stress. However, during the intervening hydrostatic testing, the pipe will be plastically deformed in the hoop direction (100% SMYS), which through the Poisson’s Effect would affect the longitudinal stress. The longitudinal thermal compressive stress is essentially a residual stress, and residual stresses can be relieved or at least re-distributed by plastic deformation.


I've also recently read a report with a reference to a definition for T1 (see below):

The term T1, the ambient temperature at the point of restraint requires clarification, as it is often the point of debate in the pipeline industry. In effect T1 represents the pipe temperature at which the pipe is fully constrained after backfill and the pipe has not yet incurred any longitudinal thermal stress. This temperature is not the ambient air temperature at the time of tie-in, nor is it the ground temperature at the time of installation (backfill). Although several values have been assumed by industry, most are approximately the average of the soil and ambient temperature at the time of backfill installation (~ -20oC).

Surely a sensitivity analysis should be undertaken since the actual "tie-in" temperature is debateable.

Ok, lets deconstruct this a bit

If the pipeline is truly “locked-in” or restrained immediately after backfill, and then filled with test water (at around 5 deg C) then the line would be subject to a longitudinal compressive stress. - yes assuming that the test water is below backfill temperature

However, during the intervening hydrostatic testing, the pipe will be plastically deformed in the hoop direction (100% SMYS), - WHY? You shouldn't normally be testing much beyond 90% SMYS IMHO, though I know some codes allow it, but at 100~% SMYS you're still not plastically deforming. The "M" stands for minimum and even then this is the point it goes from elastic to plastic. Any plastic deformation is very small.

which through the Poisson’s Effect would affect the longitudinal stress. - I agree

The longitudinal thermal compressive stress is essentially a residual stress, - Not normally no. Residual stresses normally come from elastic bending.

and residual stresses can be relieved or at least re-distributed by plastic deformation. - totally dependant on the direction of the stresses

As for the second para, - I've written some waffle in my time but that one is a master class

says it's not something (with no argument put forward), doesn't suggest any alternatives and then says choose something between the two. Pointless waste of space.

end of the day - choose a temperature with some level of argument as to why, get the client to agree and then go and od the calculation. Job done. Pipeline design is simply not that accurate to bother about whether the installation temp is 20,21, 22, 23 or 25 DegC.

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

First! SL <> longitudinal compression stress. SL = longitudinal stress. You don't know if the longitudinal stress is compressive or tensile until you give it a [&Delta;]T and a pressure.

If test water temperature is below lock-in temperature, the longitudinal stress will be tension and will remain tensile until the pipe temperature rises above lock-in temperature, no matter how much internal pressure is added. Pipe with only an internal pressure wants to contract longitudinally due to Poisson stress, so, if it is restrained it will always be in longitudinal tension until it is heated enough that longitudinal compressive stress added finally overcomes the initial tension stress. Only then can it chage to compression.

"during the intervening hydrostatic testing, the pipe will be plastically deformed in the hoop direction" No. Stress < Yield Stress remain within the Elastic deformation range. If you go over 100% SMYS, then plastic deformation begins to occur. That is usually limited to 1-2% of Yield Stress. And during hydrotesting you are probably not anywhere near thermal design temperature, so not much of the design thermal stresses will be redistributed if you do reach plasticity. If you did reach plasticity, then in this case residual stresses might include some of that longitudinal srress. Residual laods can be the result of either bending or axial stresses that go over yield stress.

At locations where tie-in welds are made, tie-in welding temperature is more important. The actual lock-in temperature might vary considerably from trench burial temperature. As tie-in welds are often made near where pipe leaves the ground near virtual anchors (where there is variable pipe restraint from 100% unrestrained to 100% restrained), these regions can be subject to considerably more thermal stresses.

In some soils, actual lock-in might never really occur at all, in others only after several thermal cycles have occured, in other soils only after a considerable time has passed, still on others it might be nearly immediate.


Reaction to change doesn't stop it

As always, thank you for the comments.