You can tie up to an unmanned platform. No one will be around to tell you to leave. If you are asking about blue water semi sub platforms absolutely not. Never put yourself underneath a crane moving equipment on a platform, ship, etc. Also, all these installation have discharges for different equipment. If you pulled up under our ballast water discharge when we started pumping, you'd be sunk in seconds. Good practice is to stay a little distance away. All these offshore installations have swarms of huge resident barricuda under them.
Back in the day some of them in the ghetto had bouys for the supply boats to tie up to that we used with permission. Ram Powell was one I think from memory. It was also when you could mash yellowfin at them and be the only boat there.