Hi, You mean "yes, it is mandatory to have this on a Helideck" or "yes, S61 need this" ? What does this charging device used for? Is this for every time you need to start the helicopter? or it is only a device in case the Helicopter is out of battery? Is there any rules which I can refer to?
You have left out too many details for anyone to reply with anything useful.
Start with some definitions:
1. "Helicopter Charging System". Are you talking about a GPU (ground power unit)?
2. "each offshore drilling units". I know little about drilling rigs, but my guess is that requirements vary with the prevailing authority. For companies operating aircraft within U.S. airspace, then the FAA might be the ones setting the rules. It may also fall under the authority of the U.S. Coast Guard.
3. Sikorsky S-61. According to your link, that is a 50-year-old aircraft. My guess is yes, they probably would like to have a GPU on hand in such isolated locations.