RE: Offset OR Expansion Loop requirement for buried Pipelines

From: <Tony>
Date: Sun Nov 11 2007 - 16:02:00 EST

Re: Intentional Offsets in Buried Pipe

Some rules of thumb we have seen that might be helpful are included below:

  1. When the buried pipe total length is not long enough to develop lateral bearing lengths in between discontinuities then regular design principles may carefully applied to restrain the pipe and control stresses. In this case, intentional offsets might be employed.
  2. When the buried pipe total length is many times the virtual anchor length, then individual sections of the system are often analyzed separately, and in these local instances, restraint, or geometry changes might be needed where interaction occurs within lengths on the order of the pipe lateral bearing length. Minimum overburden, overbend specifications, minimum and maximum bend radii allowed, and various soil property effects are individually evaluated, and then where components which produce a discontinuity stress are within a virtual anchor length of each other, their interaction may need to be evaluated. The straight buried pipe, fully restrained equation in B31.4 (419.6.4(b)) is suitable for most long, straight buried pipe at least one virtual anchor length away from discontinuities.
  3. CAESAR models are sufficient for many buried pipe systems. A sensitivity analyses should be conducted so that variations in assumed soil properties do not control the solution. The American Lifelines Alliance document: http://www.americanlifelinesalliance.org/pdf/Update061305.pdf <http://www.americanlifelinesalliance.org/pdf/Update061305.pdf> should be consulted for soil stiffnesses and compared to the CAESAR soil stiffnesses used. High yield-to-tensile strength materials, local buckling, cyclic pressures, and subsea conditions should be approached carefully.
  4. There are a number of consultants that have considerable experience designing, analyzing and installing buried pipe. Most will perform design reviews and offer second opinions at a very reasonable price. They should be used liberally before they all retire.

Regards,
Tony

From: PipingDesign@yahoogroups.com [mailto:PipingDesign@yahoogroups.com] On Behalf Of Balakrishnan
Sent: Sunday, November 11, 2007 5:53 AM
To: PipingDesign@yahoogroups.com
Subject: Re: [PipingDesign] Offset OR Expansion Loop requirement for buried Pipelines

Friends,

Mr. Mohsen is suggesting usage of Sleeves or reducing the fluid temperature.

I fear the discussion is not going around the Subject Topic. This discussion was regarding the design of Metallic Buried Pipelines for Liquid or Gas Transportation as per ASME B31.4 and 31.8. And the question was whether there is any need of giving intentional offsets or loops in the Pipeline Routing or not?

Recently I tried a stress analysis model with CAESAR-II. The same route with and without offests did not give much difference. There was a difference of hardly 4% in the maximum stress level. I agree that there are apprehensions over using CAESAR-II for buried models. In my model, the one without any offset gave lesser stress. This happened because there were many bends in the model with so many offests or loops.
(Normally allowable stress values at bend locations are very low as per stress formula given in B31.4 and B31.8).

Though the stress models without expansion loops / offsets are giving less Code stress / percentage stress levels, I am not supporting the view of laying down pipes in STRAIGHT lines across many kilometers. This view is based on the experiences with upheaval buckling & snaking. These 2 problems (upheaval buckling & snaking) associated with Buried high temperature Pipelines suggests that The soil stiffness is NOT sufficient enough to make the Pipe FULLY restrained.

Regards,

Balakrishnan

mohsen sabzalian <sabzalian2004@yahoo.ca <mailto:sabzalian2004%40yahoo.ca> > wrote: HI Emmanuel
As you are perfectly aware we are using underground pipelines for ease of routing and keeping the temeprature rather unchanged therefore tensile and compressive stresses due to changes of temperature and pressure is a little strange.
For any reason if one is to use hot fluid through the underground pipes he can do any thing to reduce stresses!!!!!!!!!! because the assumption is that the pipe does not move otherwise it is not called underground pipelines.
For solving the problems based on my experience one should use sleeves or reducing the temperature of fluid before entering it through the pipe.
regards
Mohsen

Emmanuel Tompeter <tompeteremman@yahoo.com <mailto:tompeteremman%40yahoo.com> > wrote: Dear Balakrishnan,

Thanks for so much for bringing an interested topic like this for discussion.

Before i contribute please explain more in details follow.

1.Transition point between the fully restrained and the partially restrained portion.

2. Which portion of the pipeline is said to be Partially Restrained portion & Fully restrained portion? ie Above ground portion and Underground. and why ?

3. you mention two major stress that act on a buried pipeline. what about longitudinal tensile stress result from combine effect of temperature and presure change?

4.longitudinal compressive stress result from combine effect of temperature and presure change?

I'll be grateful if you through more light on the above issue so that, we can discuss.

Thanks,

Tompeter,Emmanuel

> Dear Pipeline Engineers,
>
> For a buried pipeline which is Continuously in
> Fully restrained condition, the only stresses (in
> addition to hoop stress) acting on the Pipeline
> under Soil / Sand are:
>
> 1) Thermal Stresses due to Difference between
> Operating & Instalaltion Temperatures.
> 2) Poisson effect (gamma X Hoop stress)
>
> But this portion of Pipeline cannot move because
> of the soil / pipe interaction and the resulting
> resistive forces. Before going into my problem,
> please note the Design parameters which are given
> below:
>
> Service: High Pressure Gas
> Pipeline Size: 14"
> Design Pressure: 130 Bar g
> Design Temperature: 110 deg C
> (Operating Temperature: 90 to 95 Deg C ONLY)
>
> Materail of Construction: Duplex Stainless Steel.
>
> Now let us check the Virtual Anchor Length (L):
>
> We have an Above ground portion and Underground
> portion of Pipeline.There is a transition point
> between the fully restrained and the partially
> restrained portion. For the given Desin parameters
> and Soil properties we can calculate the Virtual
> Anchor Point which is the transition point between
> Partially Restrained portion & Fully restrained
> portion.
>
> Now, coming back to my Design issues:
>
> I am planning to bury the entire 30 Kilometer
> Pipeline with 20 to 30 meter above ground
> installations at either end for Launcher / Receiver
> facilities. Previously (in simillar case) we used to
> give intentional offsets / expansion loops on the
> buried route. This is to ensure that the maximum
> straight length (say L meters ) is always less than
> twice the Virtual Anchor Length (2L) and hence
> throughout the Pipeline length, it is behaving as
> Partially restrained pipe. This approach will ensure
> that the pipe is free to move in both directions
> with respect to the mid point of 2 x L.
>
> But, our client is questioning this approach.
> According to him there is no need of Stress analysis
> of the fully restrained portion of Pipeline. He is
> assuming that fully restrained pipe has ZERO
> mobility, even due to temperature difference. Also
> he is asking me to route the pipe straight
> throughout, unless there is no geographical
> requirement of offesting.
>
> Our stand is that theoreticall this concept of
> FULLY restrained portion may look fine. But we
> normally come across issues like upheaval buckling
> (vertical movement) and Snaking (Horizontal
> movement) of pipe inside buried area. That means
> Pipe has always a tendency to move within the
> trench. So we conclude that there is a need of
> frequent offests for a high temperature metallic
> buried Pipeline.
>
> I would like to get some feedbacak on this
> requirement of Offests on buried routing of metallic
> pipelines (desin Temp 110 deg C) from your
> experiences & industrial practices.
>
> Regards,
>
> Balakrishnan
>
>
>
>
>
>
> "We make our world significant
>
> by the courage of our questions
>
> and the depth of our answers"
>
> -- Carl Sagan
>
>
>
> ---------------------------------
> Flying to Bangalore or Bhopal? Search for tickets
> here.
>
> [Non-text portions of this message have been
> removed]
>
>


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[Non-text portions of this message have been removed] Received on Sun Nov 11 16:02:00 2007

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