Annular Plate Used

http://www.nistm.org/houstonSept2009/docs/2009_HoustonConf_Pres/Basic_API650.pdf?bcsi_scan_03deeaa357c9e78b=0&bcsi_scan_filename=Basic_API650.pdf

I don't know any design part. But, my opinion on this issue as follows..

If you are not interested in annular plate, then you have practical difficulty to make lap joint welding of bottom plates..You will face irregular edge plates during shell plate fit up on bottom plate..To avoid this you have to butt weld all bottom plates..then only you will get even bottom surface for shell plate fit up..to avoid butt welding of ALL bottom plates, annular plates are deployed on the periphery of bottom plates..so the butt welding can be restricted to annular plates (outer ring of bottom plates supposed to weld shell plates)