Seismic evaluation of buried pipelines

Good afternoon,

I am a student of Mechanical Engineering of the Universidad de Oriente (Venezuela) that this developing a thesis that consists on the evaluation of the mechanical design of the northeastern system of gas for the geologic crossings faults. I have already carried out analytically the calculations following the limits of ASCE, ALA and according to the guidelines of seismic verification for buried pipelines. However, I should carry out simulations in CAESAR II4.40 to validate the results. I were revising their web and I realized that they propose ways to carry out these evaluations. As could simulate the conditions for permanent deformation of the floor, bouyancy due to liquefaction, abrupt deformation of the floor (fault crossing) and propagation of seismic wave? How can I take these conditions to the program?, since it is the first time that I work with this. I would thank for any type of help they to lend me.
Cesar Moreno

c2 uses linear-elastic 3D beam elements. The loads are user defined, applied to the node points, and remain constant.

Deformation of the Floor: You would have to specify this as displacements of the (support) nodes.

Buoyancy due to Liquefaction: I suppose you could simulate this by altering the densities, but this would be a manual adjustment.

Abrupt Deformation of the floor: If you were running dynamics this could be done with a psuedo-static displacement. In statics you would have to displace a portion of the model to approximate this.

Seismic Wave Propagation: c2 can perform a (dynamic) response spectrum analysis, but I don't think this will be sufficient. C2 can also perform a time history analysis, which may do what you want, but the load definition will be tedious at best.

Thanks Richard
I know the elements behavior but I don´t know how to simulate it!
Does it exist some tutorial or manual that indicates me taking these conditions to CAESAR II?
It is the first time that I use it
how usually evaluate the seismic effects?

regards,
Cesar Moreno

Static seismic is performed using a uniform "G" load.

For an example fo a dynamic seismic analysis, review the NUREG9 example job file. The examples are briefly reviewed in the Applications Guide, which you can find on-line under the "Help" menu.

The buried pipe modeler in C2 will set the densities of all buried elements to zero. You cannot generate an acceptable mass model for these systems.

The free vibration approach to seismic evaluation does not apply to buried pipe.

thanks guys!

your help is very useful! one more question...
how can i change the espectra conditions of el centros´s earthquake?

regards,
Cesar Moreno

You cannot change El Centro. It is already documented and published. It is history. (Hah.)

Now, if you wish to analyze a system with a different response spectrum then just enter your own. Add your spectrum name and format in the list and then click on that data toolbar button at the top of the screen. At least that's how the current version does it.

Thank you guys
I will enter the conditions to the program. it has been very useful your help

"Alguein que nunca se ha equivocado, es alguien que nunca ha probado nada nuevo"

Regards,
Cesar Moreno

Hi Guys,

Thanks for your help but i have another question...
how can i modelate the restrain when separate the structure of the pipeline? like a rigid anchorage???

"Alguien que nunca se ha equivocado, es alguien que nunca ha probado nada nuevo"
Regards,
Cesar Moreno

Anyone???

Regards,
Cesar Moreno

I'm not sure what your question is. Are you asking "How do you model a structural restraint when you don't want to model the structure?"

I do not understand your question.

If you want to add a restraint to a system buried by the C2 modeler, add the restraint to the model AFTER it is buried (after the extra soil restraints are created).

Ok I try my best...

I want to know, how can i simulate the support condition in the ends of the section of the pipe when separate it of the pipeline?. I mean, i will analyse only a tract of the pipelines for fault crossing. What Kind of restriction would be placed in this case? What grades of freedom should be restricted?

Thanks again

"Alguien que nunca se ha equivocado, es alguien que nunca ha probado nada nuevo"
Regards,
Cesar Moreno

This isn't an easy one to explain in a textural format.

You need to be "really" careful here. You can't arbitrarily just stop the model because you'll ingore the influence of the components (modeled versus not modeled) on each other.

If you're approximating things with a "static seismic" analysis then maybe you can stop the model (after extending it into the area of "no interest" and crossing three restraints). However, if you're performing a dynamic analysis I would not recommend you stop the model. If you do, your natural frequencies will be (very) inaccurate and your results (very) suspect.

thanks Richard, but i have a few more questions.

Will a static seismic analysis an inexact approach?
How close is an analysis of this kind to the real values??
What capabilitys has the dynamic seismic analysis?

"Alguien que nunca se ha equivocado, es alguien que nunca ha probado nada nuevo"
Regards,
Cesar Moreno

It can be inaccurate. Approximating dynamic events in a static analysis provides results "close" to the actual behavior only if the response of the system is dominated by a single mode of vibration. If you have mulitple modes contributing the static analysis becomes more inaccurate.



I can't answer that one - it really depends on the system and how it responds to the load.



The c2 Dynamic Seismic Analysis is performed implementing the Response Spectrum Method. Check out the NUREG9 example job that ships with the software.

Once again, the free vibration approach to seismic evaluation does not apply to buried pipe. If there's enough room for the pipe to move about then it is not "buried".

I believe the only seismic load for buried pipe is the the displacement associated along the compression wave in the earth and the large displacement at a fault.

Static loads are useful for seismic analysis - see the new document B31E.

where can I get the new document B31E??

Many thanks and Regards!!
Cesar Moreno

It's available for purchase from ASME. I doubt libraries have it yet.
There is an earlier draft of this document found on the American Lifelines Alliance web site. Google "ALA B31E"