With IFR procedures, you are not at elevations where there would be an obstacles other than other aircraft, until you are on final, and within .5nm to .3nm of the rig. This allows the crew to spend the workload on flying the helo, rather than trying to determine offset distances from the rig using a weather radar.
As with any instrument procedure, radar is still actively used to avoid obstacles.
There are many instances on approach to the rig, where, especially at night, the crew becomes disoriented during the offsets, a search on the internet picks up quite a few...but I will hazard this one.
http://www.aaib.gov.uk/cms_resources...9%20G-REDU.pdf