The MSA, in all of the standards, for ARA is 1500', until on final approach, descend to 1200' when 4.5nm away from the rig. You then descend to the level segment 2nm away from the rig...
It also states that if you observe and obstacle within 4nm of the rig, raise the MDA 200' to 550' (min)
The 1500' is a minimum per the criteria, min segment altitudes are a design, they are not automatic. If there is another rig within 11 nm, the MSA is raised. This MSA assumes 500' ROC (req obstacle clearance), so it plans for obstacles 1000' high...if you have floating platforms taller than this, (I know, virtually impossible, but as soon as I state it, an internet ref will show up!) the design would raise the MSA. Another option would be to issue a NOTAM for the mobile platform with dates, and temporary MSA.
The IFR procedures code in the location of the waypoints with min segment altitudes as a navigation aide. Use the weather radar for spotting uncharted obstacles, there is enough clutter already on the screen..the crew still needs to aviate.
http://www.caa.co.uk/docs/33/2010001.pdf
Here is an accident report compilation for 2000, note page 25 (FITWO/CFIT)
http://ihst.rotor.com/portals/54/ind...try%202000.pdf
According to Eurocontrol, since 1976, there have been 7 fatal accidents, with a loss of 94 lives, and 34 non-fatal accidents while on ARA type approach.
Safety Council.
http://easa.europa.eu/communications...20Brochure.pdf