Anchor Blocks 2

[clooney]

Are we always required to place an ANCHOR BLOCK at Above Ground (AG; hereinafter)to Underground (UG; hereinafter) transitions? I am assuming in order to completely isolate the movements of AG piping from UG piping, I would always have to recommend an Anchor Block. However; from what I have seen done in the field, not all AG/UG interfaces have anchor blocks. WHY NOT?
To get a definitive answer on displacements at AG/UG interface, I suppose I can model the UG piping and AG piping in CAESAR II based on provided soil data from Geotech people. And I have done this before. But often times, as a result of bad planning, clients come up with a several mile long buried pipeline plan profile (with inaccurate soil data) which terminates at an AG valve station or a metering station and they expect answers within days! because the contractor has leased his equipment and is waiting for the "go ahead" to start digging so he can get paid. Well, that is how it works in the middle east. Projects are infested with Bad Management, Bad MC's (or no MC at all), Clueless contractors, ..., etc.
Anyhow, when I have to come up with answers on such a short notice, do I really have to take the whole buried portion into consideration? OR is it O.K. to simplify the problem by studying the sags and rises of the profile and significant changes in direction (horiz.& vert.), and only take into account say a few hundred yards of the buried line between the AG terminal point and the point in UG where I feel the pipe is relatively restrained due to the shape of its profile (interacting with the soil)? Remember most of these lines are large bore Carbon Steel pipes and have design temperatures of around 180°F max. & installation temperatures of around 70°F unless it is summer time (considering solar rad. effects, installation temp can reach up to around 170°F). These lines are usually interconnecting gas transmission lines between different plants which are miles apart.
And what bothers me the most, is that what actually is built at the construction site is not necessarily what the "Approved for Construction" plans show. At least that has been my experience with most clients and contractors overseas. It makes me feel that perhaps it wasn't worth investing that many engineering hours trying to come up with somewhat realistic design loads for the Anchor Blocks. But let's not get side tracked, I want to concentrate on the technical aspects of making a good engineering decision, all things considered.

How do you guys approach this task? please, share your views, guide me,...,
Thanks guys

Farzad

 

[bvi]

Look in the appendices of ASME B31.1. There is an appendix that describes how to evaluate thermal expansion of buried piping. If you do not have an anchor block, you will have what is called a virtual anchor some distance into the soil. That is were there is sufficient frictional forces developed by the soil, that the piping thermal expansion is completely restrained. Between that point and where the piping exits the ground, there would be some movement of the piping due to thermal expansion. It is somewhat more complex than this, considering changes in direction, but this should give you a general idea as to the approach.

 

[DrillerNic]

clooney: in answer to your question, not always, if the pipe isn't over stressed. As you seem to be concerned with pipelines, ASME B31.4 and ASME B31.8 have sections on the expansion, flexibilty and restraints of pipelines; the spec break between pipeline and piping is usually down stream of the 'swan' neck, so these codes would be applicable

 

[clooney]

Thanks for the references you pointed out. I will have to do some reading on my own. So if I understand "bvi" correctly, "virtual anchors" have to be determined based on personal engineering judgement. I know CAESAR II buried pipe modeler has an "iterative process" of establishing the virtual anchors, I'll have to look into that too.

Thanks again for your replies

 

[bvi]

You can calculate where the virtual anchors are. The calculation of virtual anchor location includes consideration of force required to restrain the thermal expansion, and the resistance of the soil from friction and, at changes of direction, stiffness of the soil. Don't use the CAESAR calc blindly, last I looked at it I didn't think it did it quite right automatically. Review the B31.1 appendix that it is supposed to be based on.

 

[clooney]

Thanks again, bvi. I'll be careful with CAESAR. But from what I remember reading in CAESAR manuals a few years back, thermal expansion induced displacements of buried pipes are controlled by the Spring model simulating soil resistance (based on shape, friction, soil active & passive resistance, etc.) which sounds close to what you were saying.
And whether the soil interaction is passive or active depends on the initial soil spring value and the tendency of the pipe to expand in length. Once the soil is displaced beyond a certain limit, it will exhibit "plastic" behavior. This is where CAESAR calculations become iterative in order to determine soil spring values and pipe movements underground. I have to go over it again myself.

Farzad

 

[mmartens]

I design under ground pipelines and ANCHOR BLOCK are bad. You can model an anchor block as a line stop by using equlivent areas rule and the horz. pipe spring constants. However, the better soluation and more cost effect is let the pipe move by adding a jog in the line with two elbows.

Michael