Span analysis

[JT100]

I am currently performing an allowable span analysis for a pair of piggybacked pipelines. I have been asked to use DNV 30.5. I am having a bit of trouble understanding the boundary conditions that are to be used, I understand that assuming fixed-fixed end conditions will underestimate the stiffness of the span supports, and it will therefore be conservative to use fixed-fixed as this overestimates the fixity of the span. I am unsure if this is true for calculating the natural frequencies of the spans- I have been given a formula to calculate the natural frequency and it contains constants that depend on the end conditions. Is it conservative to use the values for fixed-fixed end conditions?

 

[prex]

In the post title you mention an expansion analysis and in the body mechanical calculations, so there is a contradiction.
For mechanical supporting calculations (such determining the natural frequencies) the fixed-fixed condition is normally not conservative. For a thermal expansion calculation the contrary is normally true, though in both cases only an analysis of the specific pipe conditions may give a definitive answer.

prex

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[JT100]

So would a fixed-pin or pin-pin be considered conservative in calculating the natural frequency?

 

[BigInch]

yes

Fixed-Fixed is not conservative, neither is a continuous span, unless alternate spans are loaded at alternating frequencies, which may be the case with a non-perpendicular wave travel (and which is worse than single span alone).

Watch that the Von Karmen effect is well outside the span's natural frequency.

http://virtualpipeline.spaces.msn.com