bgoat,
Horizontal axis tanks can be constructed to:
API-620
ASME VIII (with or without stamp)
UL142
(API-650 only pertains to vertical axis, flat bottomed tanks)
Of the three standards, UL-142 is the "minimal" way to go. (In my opinion, suitable only for atmospheric storage of petroleum products)
The process chemical industry uses ASME-VIII a lot....
You should also know that there is a commonly used (but ancient) methodology to analyze welded saddles on horizontal axis tanks. It is called the "Zick method"
MJC