A short history on some of this:
Long ago, OSHA had their General Industry ladder, stairway, and handrail standards. These were based on ANSI standards. The ANSI standards were updated and changed, the corresponding OSHA requirements were not.
About 20 years ago, OSHA published a proposed revised rule governing all of this. The proposed rule never was actually enacted, but, OSHA has stated that compliance with the proposed rule was a "de minimis" violation that won't be cited, etc.
Several years later, they republished a proposed revised rule. Once again, it never has actually been adopted.
The result is that right now, if you go to the OSHA website and look up the "OSHA Laws and Regulations", you get the "old" rule as currently in the CFR. But if you look up OSHA Publication 3124, you get the "new" rule, based on the proposed revisions, which conflicts with the old. So in effect, there are two different conflicting rules out there.
With that background, here is an interpretation issued a number of years ago on the issue:
With the latest revision of the "proposed" rules, a tank roof could either be a walking/working surface or could be treated like the roof on a building, depending on how often it was accessed.
MY interpretation is that if it's a walking/working surface, it should be enclosed by handrails (not just having handrails close to the ladder), but this seems to not be an issue with OSHA. Maybe they don't enforce that until somebody falls off.