vesselguy,
Sorry, not 50ft, but just for your info as some of the same principles apply - on my present project we shipped in many tanks around the 40ft diameter, and one at 45ft diameter. Most of the journey was by sea, but at the fabrication yard and at site, transporters were used (operating two abreast for these larger tanks).
The tanks had 4 padeyes welded through the top ring for lifting, and two lifting beams positioned below the cranes beam to provide vertical lifting at each point. Most tanks had platforms with up to 8 support legs to the tank wall which probably provided additional lateral stability of the tank top above that provided by the ring. The largest tank mentioned above had a cone roof as well.
To overcome concerns regarding tank floor bending, an internal ring was welded to the floor and cables tensioned from the ring back up to lugs welded to the internal wall. This system worked Ok - the tanks arrived at site with no more problems than what they left the fabrication yard with. We had also specified butt weld floors specifically because of the lifting and handling that the tanks had to endure.
For transport we had the fabricator provide stiffened sections around the base to attach chains for road transport and hold-downs for sea transport, with additional reinforced lugs welded around the circumference, generally halfway up the shell, but calculated to be in sufficient number to withstand the generalised sea loadings.
Regards,
John