I have to perform stress analysis for a 42” crude oil pipeline, which is laying across underground for approximately 90km. For your information, the routing along the 90km span contains a lot of change of directions.

1) I would like to enlighten me about the extension of the analysis .I believe that the stress analysis by Caesar should be limited only in the launching and receiving area. I.e. from the battery limit point, where the pipeline comes above the ground until its first virtual anchor due to soil.
I was driven to this decision based on the fact that almost the between area of the routing has close to zero displacements, since is fully restrained by the soil.
Furthermore, it is a very long pipe and it is impossible and time consuming for me to include all the 90km routing in a single Caesar calculation.

2) Also what about using field constructed 30D bend (as per B.31.4) in every change of direction all along the routing? This type of bend minimizes the excessive stresses and consequently there is no reason for analyzing the whole routing? What type of bending it is commonly used in underground pipeline cases?

Please provide your comment about the above subject.

P.S. I didn’t provide the whole data for the line and the soil, since I would like to illuminate me about the philosophy of the analysis in general and not for the specific case.

I anticipate your reply

Please anyone?

One ideea, you need to split the entire pipeline in segments, basically for each changed in direction and calculated the virtual anchor point( the lenght of the pipe which is restricted by soil, check Peng data or other books). You don't need to modelled all pipeline , only the part which is changed in direction and modelled until the point where the pipe basically is stop.

Some of your assumption are not correct.

Is not an easy job and definitely is not for a beginners.

I think with 30D you will never got fine meshing with CAESAR buried pipe modeller it will consider it as straight pipe. CAESAR buried pipe modeller only calculates expansion stress which you will get less with long straight line but you need to check various stress at road crossing as per "ASCE guidelines for buried steel pipeline".

Thank you Marian

I will consider your answer, for the first question.
Which of the above assumptions do you think are wrong?

Has anyone idea about the second question,type of bends which are used?

Well I believe that I have done a lot of reading around the theory of buried lines analysis,but I am still in dark as far as conserns the practical aspects of design.

Can anyone piping expert upload for me an example of a buried pipeline stress calculation (Ceasar file), in order to figure out the philosophy?

Any answer/idea is welcome.

Regards

I guess the major problems are the thermal load(expansion) and partially longitudinal expansion under internal pressure for the pipelines with long lengths.

It is difficult to stop the thermal expansion. In case the thermal load is very small you may have more options to consider anchor locations which may include the virtual anchors. Your pipeline is very long. In case you are using kind of anchors you may be ablo to split the model into some small models under the thermal loads.

Or you need to use your judgement by considering mid-line directional angle change locations (should be sufficient angle change at each location and you need two directional changes in a line to be able to split the model) and use the location in the middle as anchor to split the model. There will be some discrepancies at those locations and check the analysis can give you this flexiblity by watching the stresses at the anchor locations considered.

If you cannot run a complete model everything will be based on the assumptions, and you need to check your assumptions are adequate in the results of the analysis.

I have similar job at the moment and we needed to comprimise on the simplifications on the model side without changing the concept to be able to model and to run the analysis within Caesar II restrictions. Sometimes you need to ask for the straight lines between directional angle change locations. if you follow the surface you are going to end up with enourmous model that Caesar II cannot handle.

Hope it helps.

Ibrahim Demir

Dear Balckout,

At stations you need to do upto virtual anchor length.

At horizontal bends you should do both sides of bend upto the vitual anchor length.

If your change direction doesnot develop or has vritual anchor length then you had to include the complete routing upto staight length which develops virtual anchor.

At staitions if you had a horizontal bend before the virtual anchor length then you include the bend in your model.

Check LC Pneg paper it clearly defines moving portion and restrained portion.

As a thumb rule all moving portion shall be analysed. Al restained portion doesnot require analysis.

In your layout you had to define the virtual anchor locations. Any moving portion (horizontal / vertical bends) in between the two virtual anchor locations need to be analysed.
Any straight length in between two virtual anchor points does not require analysis.
Pipeline bends are the overstressed points so require careful consideration.

Thank you all for answering!

Now what about the types of bend which used in underground piping?

According to B.31.4, we have the option to choice between two types of bends:


• The bends made from pipe as per B.31.4 Par. 406.2.1
Minimum Radius of
Nominal Pipe Size Bend in Pipe Diameters
NPS 12 and smaller 18D
14 21
16 24
18 27
NPS 20 and larger 30

• And the factory made bends.

For the factory made bends, Is that correct, that the bend radius should be between 5D to 4D?

Does anyone know, when we are using the one type of bend and when the other?

The long radius bends for underground pipelines are for process (fluid flow) or for stress reasons?

Is it a common practice, in case of a stress failure of the pipelines, to be used the maximum bend radius, in order to minimize the stresses around the unrestrained areas?

Regards

Many principal reason of "The long radius bends for underground pipelines" are for maintenance propose named pigging.

Bends commonly used in pipeline construction: 1)Induction/hot bend (3D-5D) is "hot formed" in the factory and stress normalised after the process; is normally used when space is restricted and at scraper trap stations; 2) Cold bend with radius as indicated by asme B.31.4 Par. 406.2.1, is a field bend using bending machine and mandrel-this is cost effective and economical of producing bend readily from straight pipe available in the construction site; the min. bend radius of asme b31.4 is to limit the stress, thickness and wrinkling of pipewall after cold-bend is performed on pipe. 3) Elastic bend - the natural elastic property of pipe (hence called "natural bend") is used to achieve the change in direction of pipe; most common used in cases where there is very large area available in the right-of-way to change direction of pipe route (vertical or horizontal) and in HDD method; the minimum elastic bend radius is calculated by formula where cold allowable stress is the limiting factor (read pipeline rules of thumb handbook by gulf publishing). In all these type of bends (esp the hot bends), you have to check if your pipeline shall be subject to pig-scapping for maintenance or in slurry-fluid service (therefore long radius of bends are used in pipeline). Notably intelligent pigs are long and must pass these bends. Practice is to check the min required bend radius of intelligent pig from manufacturer.

sorry, it should read "pig-scrapping" (not pig-scapping).