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P-Delta analysis has been a near-mandatory requirement in structural design codes for many years due to the importance of its effects in design calculations. However, piping stress models have traditionally ignored P-Delta effects , possibly because since most older generation piping stress software programs are incapable of P-Delta analysis.
Although P-delta effects can have a significant effect on some plant piping layouts, P-Delta analysis with large displacements can be particularly important in analysis of buried and seabed pipelines where soil friction causes built-up compression forces that can make lateral or upheaval buckling a design concern. In the widely referenced paper, “About upheaval and lateral buckling of embedded pipelines”, author Dr. K. Peters emphasizes that rigorous analysis of upheaval and lateral buckling requires “second order solutions” (aka P-delta analysis), and he warns that “piping programs not able to produce second order solutions may not be used in solving upheaval or lateral buckling problems."
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Vertical displacements displayed below for weight case only (selfweight including fluid). Right-side window is weight case with consideration of P-delta with large displacements with max vertical displacement of -8.69". Left-side window are displacement results from the same weight case, but ignoring P-delta effects, with a calculated max vertical displacement of -41.5", which is what older generation legacy piping stress programs will report. With a long flexible pipeline like this, there is catenary behavior that needs to be accounted for in which axial load is being distributed to the anchors to help support the pipeline. P-delta with large displacements accounts for this real-world catenary behavior which made quite a difference in this long span example.
Vertical deflection results without P-delta large displacements Vertical Vertical deflections with consideration of P-delta large displacements
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Below is the Modal load case which is load sequenced to be based on the end state of the GR-PD load case which includes the effects of P-Delta with large displacements.
Consideration of P-delta effects with CSiPlant requires only a couple of mouse clicks and a small amount of additional analysis time for a typical model, so there’s no reason not to consider P-delta effects to ensure more realistic design calculations. You can create this the example above with your own piping stress program and compare with CSiPlant. Download a free fully-functional CSiPlant trial license by clicking the "Trial" link here https://www.csiamerica.com/products/csiplant
The CSiPlant model we used can be downloaded here P-Delta Large Displacement Catenary Example.cpdb
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