if they developed a Rotordyne for North Sea operations,
It's not the cost that is so much of the problem, it's the size. At 90 feet rotor diameter and 33,000 lbs AUW there are severe problems coping with anything that size offshore. Not only would you have insufficient clearance for the rotors but most helidecks are not designed for that weight.
During the days of the Chinook only certain decks could be used and some only a certain way; something that can be done in a Chinook because of its tolerance to cross and tailwinds.
Oil companies have gone off big movers; offshore trials with the EH 101 Heliliner did not come to anything, the beancounters wont risk it.