Mars, thank you, been a while since I read the AC and I was reading an old one that I had saved, anyway the new text is...
(3) Elevated PPR Heliports. At PPR rooftop or elevated facilities, the TLOF may be a minimum of two times the maximum dimension (length or width) of the undercarriage of the design helicopter if the height of the TLOF surface above the surrounding area is no greater than 30 inches
(76 cm), and there is a solid surrounding area equal to the rotor diameter (RD) able to support 20 lbs/ft 2 (98 kg/m2) live load. The center of the load bearing area of the TLOF should be the center of the FATO.
My understanding is that all that is need for a helideck for e.g. a B206-B3 (the A/C that at one point was considered the standard offshore workhorse) would be 11.5ft (EMERGENCY FLOWAT GEAR w/ AAI FLOATSTEP Length) x 2 = 23ft. In the GoM they round it up to 24ft for simplicity. As the heliports are elevated there should be no obstructions in the FATO (Final Approach and Takeoff Area) so all they then do is put a safety net around the TLOF (Touchdown and Liftoff Area).
However API also has it own guidelines for helideck design, "API RECOMMENDED PRACTICE 2L" and requires the diagonal distance for a helideck to be the width of rotor of the largest helicopter to be used on it. So a diagonal distance for a 24ft/24ft deck would be 33ft which is still based on a B206B3.
You choose which guideline to use as I have had people reference both of them.
We should be using 1D or at the very least 0.83D. Sadly I fear that this will not happen as it would be cost prohibitive to increase the side of helidecks in a struggling GoM market.
Calling for clearance to land greatly depends on the customer. Most of the time it is only required on drilling rigs, however the Shell's and BP's do require several calls to maned platforms and rigs.