As I understood it SCS was originally reopened to support the construction of Sullom Voe Terminal with good old Dan Dair HS748s and Loganair Bandits around 1978. Ops to the rigs were expressly forbidden at that stage hence the 748's into ABZ and the Dash-7's into Unst.
I left there in '83 at the end of construction and offshore ops from SCS were still verboten then so I've no idea when it changed.
Ah 748, 20tonne twin dog whistle. Some of the DD crews got them in in horrendous conditions. I have no idea what the 748's limits were but it looked impressive to me.
Good luck to whoever gets the contract. From my experience in the North Sea, aircraft maintenance must be horrendous in the prevailing conditions.