The minimum tonnage on Master OSV is 1600 GRT and 3000 GT.
The maximum tonnage on Master OSV is 10,000 GRT and 10,000 GT.
Frankly, tonnage is not the issue. The USCG is much much more focused and restrictive on license tonnage than the rest of the world. It's ridiculously overdone. The U.K. (And the rest of the world) approach to CoC or license tonnage makes much more sense.
The issue is jumping from a license to rum a mud boat down in the bayou from one "port" to a handful of offshore rigs within the limited world view and bayou company culture and mentality, and then making the jump to any ship, anywhere, in any trade. if the USCG is going to allow this, why bother having a licensing system at all.