People really do not understand the GOM. 24'x24' is the smallest heliport approved for use by PHI. Aside from that, there are just so many structures to land on and many of them are normally unmanned. If the platform is a 24'x24' then it is probably unmanned most of the time. Therefore no one would be there to "give permission" to land. Similarly, a small jack up rig would not normally have a dedicated person manning a radio. So no, it is not usual to secure permission before landing on a rig or platform.
Having said that, it is unfathomable that even an inexperienced GOM pilot would try to land on a 24'x24' with a rig derrick hanging over it when there was a perfectly good, useable helideck nearby. What was he thinking??