Pressure Vessel Study - Why is not showing up?

Hi,

I would like to run a pressure vessel study, I have created individual load case studies (static analysis solid bodies) but now can not select the pressure vessel study to combine them and linearize the stress.  Does anyone know what the criteria is for using the pressure vessel study?  I am just modeling a flange part but don't really know why its not coming up.

thanks,

ed

SolidworksSimulation