Keeping in mind that tanks inherently are flexible to some extent, they still need a degree of rigidity especially in the s-t-b area. This presents a set of conditions that are adressed in API-650 and must be considered closely. another thing to consider is the stresses applied to the shell-to-bottom (s-t-b-) connection when large diameter nozzles are installed close to the bottom, but not close enough to warrant tombstone style repads. when the thicker plates are used then they are to be butt-welded so that there is not a large thickness change where the plates pass under the shell plate. These annular ring plates perform a number of functions, one being as an aid in load distribution of the shell weight to the foundation, another is being a suitable thickness to weld a double beveled shell plate to. The thicker plates used in the annular ring area are pimarily for larger diameter tanks that have thicker shell course on the bottom shell course ring.
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