The major difference between helicopters and airplanes is during instrument operations the airplane lands at airports equipped with all sorts of aids to flight and use standard procedures. Helicopters on the other hand do just the opposite. A couple of bright lights, some dim deck lights, and a windsock really do not compare to rabbits, lead in lights, runway lights, vasi's, and all the rest of the gear that awaits an airplane at the end of an IFR flight.
I totally agree with the statement above, and will add further, that some of the deck designs are quite frankly appalling. I am sometimes surprised there arent more accidents/incidents. Whilst there are certain criteria involved in the design of a deck and its maintenance, I am sure most of us have landed on decks where this is "stretched" to say the least. Until the oil companies start taking this seriously - nothing will change. How many times have we seen lights not working, cranes/ships/various obstructions blocking overshoots, and all manner or clutter in the supposed clear area under the deck creating extra turbulence? Would this be acceptable on other forms of public transport flights? It all boils down to cost, and the customer must be prepared to pay up first, unfortunately they wont until accidents happen. We have had to collisions with rig structures in the last few months alone, one with 19 fatalities. There are many improvements needed, and my company has made many suggestions for improvement, but it always falls on deaf ears. Where there is an incident though, the customer is the first to call for heads to roll.
Can you imagine calibrating all of those ILS approaches offshore on a regular basis? A logistical nightmare, especially when the equipment would be rig based, and prone to slight misalignments (rigs are metal and subject to movement and expansion/contraction) A small movement at the rig would cause a big discrepancy over the length of the approach path?? Don't know but just throwing ideas in here now. Aircraft doing calibration flights regularly adding to the airspace traffic.
Also - how would the min RVR be calculated and assessed/reported?