ABIR,
There are methods to establish vortex shedding frequencies. Most often they are applied to vertical stacks, flares, columns, and towers. It is also being applied for analysis of thermowells in fluid flow that could force the resonance of the component in flow stream (ASME Power Test Code 19.3 thermowells). The 20" pipe at ground level does not fit into the same geometry.
Your 20" pipe is either on structural pipe rack or at grade on concrete 'sleepers' for support. Anything within 5 diameters of the pipe will disturb the flow such that a simple vortex shedding analysis would be diffcult to apply. Adjacent pipes and structural steel that run parallel to the 20" pipe would break up the vortex pattern into complex turbulent flow. I would think that your static analysis is the determining wind failure mode. Vortex shedding frequencies in high winds would most likely be above the piping fundamental modal frequencies. If the project managers are not convinced without analysis of piping, then it could be more direct to just add 'turbulators' to the 20" pipe in the form of small steel tabs onto the bottom of pipe. These could be compared to the helical turbulators on flares and stacks
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R Yee
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