Hello everyone,

I have to assess the effect o f lowering the pipeline section. The matter is that the pipeline was constructed and some its section was lowered. There is site survey report of initial and lowered pipeline section indicating all required information of elevations. The major part of the pipeline was unexposed while lowering operation. There are the following options of doing assessment in Caesar(if there is any other, please advise):

1 Convert the pipeline to obtain soil springs and then apply the displacement at required nodes through Cnode fixing all degrees of freedom; assign DY with certain negative value.

2 Assign the displacement before conversion to obtain soil springs. One thing: lets say originally I made my input for displacement vector in field of node XXX2 and while conversion process displacement vector passes from original field to preceeding field of node XXX1 but the displacement magnitude is still assigned to node XXX2. I suppose Caesar does this to avoid having a conflict of numerical boundary conditions.

Experimenting with the mentioned options I get much higher stresses following option #2. However I think that the results of option 2 are incorrect due to unrealistically high stresses. The other concern is that I should restrain unexposed portions of pipeline while displacing required part.

I don't however tend to fully trust to option 1 as well.
Can anyone please advise me if my approach is correct and how this assessment could be done in the best manner?

I have already done the assessment as per the API RP 1117 practice. I wanna have an additional type of analysis.

Regards
Zhambul Igizbayev

I am unsure of your goal here. It sounds like an existing, buried line will be re-buried at a lower elevation.

With CAESAR II, you model the system as if is was not buried and then you "bury it" with a separate processor which adds soil restraints and removes weight from the buried section(s). Note, too, that all restraints defined in the initial model (pre-burial) in the sections to be buried will be removed. It is up to you to enter these additional buried-pipe restraints in the "buried model" instead.

CAESAR II should not change any existing node numbers, it should only add more. Your comment regarding XXX1 & XXX2 does not sound right to me.

Zhambul,

Are you saying that the existing line was merely displaced into a new configuration before it was re-buried?

CAESAR does not put an "output" file into the buried model editor and re-analyze a pre-stressed condition.

If I understand you correctly, then what CAESAR has analyzed is the original configuration with a displacement/force applied towards a small section of the line that's resisted by the soil springs.

If this is what you truly want, then you need 2~2.5 CAESAR files.

File 1: An unburied CAESAR file that estimates the stresses involved with manually stretching the line into its new configuration. You're going to have to manually "bury" the line, not with springs, but with displacements.

File 2: An unburied CAESAR file that indicates the original configuration before any manual displacements were applied to the line.

File 2.5: The buried version of File 2.

The stresses from File 1 and File 2.5 would then have to be manually combined in order to achieve the actual stresses for a manual "come-along" piping configuration.

I will note that modern piping construction generally does not permit adjustments to the pipe like such, unless it's for very long distances, in which case the come-along installation is minor considering the scale of the operation.

Did I understand you correctly?

Gents,

Please, see the attached picture of the situation I want to describe.

The thing is that I have to lower pipeline section without cutting due to future obstacles pipeline will see within some time.

I need to understand the stress magnitude caused by such movement in operation.

Need to add that the pipeline section has already been lowered and now I got to assess if it is still applicable for service.

Double lined yellow line is a new position of pipeline.

Also need to mention that I have 58 deg bend at IP 8. This bend is in plane view not in profile!!! So, the movement in operation was done with exposure of the bend which is not covered by API RP 1117 scope.

Red line pipeline is initial position of pipeline before lowering in operation.

Step 1, Model 1 (and its buried equivalent).
Model the piping in its deformed shape. Bury it and analyze it as though it was originally built in the final configuration. The length of your model should be 3 to 4 x L.

This shouldn't fail, but if it does, abort. If it doesn't work without being pre-stressed, it won't work being post-stressed.

Step 2, Model 2.
Take Model 1's buried pipe and save it as a separate file.
Make a note of all the vertical coordinates of the pipe being dropped.
Delete all soil-springs from the piping being dropped.
Replace vertical components of piping being dropped to match what is currently exists.
Input the displacements to the pipe that would force it into the new configuration.

Run this as a sustained event.

If Models 1 and 2 result in low stresses individually, then their combined stresses will also likely be low to moderate.

If they are moderate to high but don't exceed allowable, and you believe that they can exceed allowable if combined, then you need to output the stresses and combine them according to the piping code in question to obtain your answer.

Michael,

Thank you for your consultation. I will follow your steps and let you know as soon as I have results.
This time I have been given a different task.
There is another issue with this line. This is a potential of buckling. As you can see there is an abrupt drop in elevation at IP8 point, this is due to lowering operation. The thing is that 18" future pipeline on the picture is going to be constructed over lowered pipeline without its shutting down. The line is in risk since will see less cover and then less loaded with soil.

It is worth to mention that the profile is in different scale in the horizontal direction so the picture is misleading.
Horizontal scale 1:1000
Vertical scale 1:100
Operating temperature 90degC
Design pressure 132 barg
Pipe API 5L X60
Sour service

Going to use OTC 6335 equations.

Regards
Zhambul Igizbayev

use "PIPLIN" for large deformations.
http://www.ssdinc.com/services.htm