Most of our pipelines are large steam lines run cross country. We have problems with ground subsidence which causes displacements between pipe anchors.

How should we be managing and designing for this problem? Should we be adding some extra displacement to the thermal range Ignoring it and calling it cold forming (ie just let the pipe bend, which is what has been done for the last 50 years with no failures)

These ground strain's are non cyclic and causes self limiting stress.

We use B31.1 code for the design and the pipes are grade B carbon steel.

Greetings Kevin,

I am a B31.3 sort of guy, practicing in the area of Petrochem etc. According to ASME B31.3 319.2.1 Displacement Strain (c) Externally Imposed diplacements..... "in connected equipment. MOVEMENTS DUE TO EARTH SETTLEMENT, SINCE IT IS A SINGLE CYCLIC EFFECT, WILL NOT SIGNIFICANTLY INFLUENCE FATIUGE LIFE. A DISPLACEMENT STRESS RANGE GREATER THAN THAT PERMITTED BY PARA. 302.3.5(D) MAY BE ALLOWABLE IF CONSIDERATION IS GIVEN TO AVOIDANCE OF EXCEESIVE LOCALIZED STRAIN AND END REACTIONS." and para 302.3.5(d)is the allowable displacement stress range SA = f(1.25Sc + .25Sh) or SA = f(1.25(Sc+Sh)-Sl)

These are big allowable stress numbers and we say you can go higher. How much you ask? Well it's up to you to determine.

I would like to comment on your post as far as the use of "self limiting". I would not necessarily consider settlement to always be self limiting it depend entirely on the soils properties, the footer design etc. I would also suggest that a way to consider the type of stress a loading imposes is to ask oneself "If the load continues to the point of failure, what type of failure would occur?" in other words a fatiuge crack, or a collapse? In this case it very well might be collapse.

I hope this helps your consideration on this matter, I would also suggest that your years of experience should also be considered.

[ May 23, 2001: Message edited by: John C. Luf ]

[ May 23, 2001: Message edited by: John C. Luf ]

John Thanks very much

The B31.3 cluase is very useful. Just how much extra stress/strain is the big question now. Since we have not had any failures experiance does not give any upper limit.