In transit to your rig you will be flight levels or altitude, so standard setting procedures apply.
To effect a radar approach you will normally match your BARO to your radar altimeter and fly to a radar height, dependent on the height of the helideck.
All offshore approaches are flown the same manner. But you must remember to reset your baro back to local QNH, if you changed it of course, before you depart the rig.
hope this helps.