Calmac generally award drydocking contracts to either Cammell Laird in Birkenhead or the Dales Marine Group.
The fleet has a set drydocking schedule and Dales will allocate a ship to whichever dock is most convenient/available, this is generally either Garvel in Greenock or Aberdeen as those are the two docks which have actual workshop facilities etc.
Inchgreen is long out of use and the up river docks owned by BAE are not available for commercial work.

Hi Bob-Inchgreen-Greenock-Ailsa Troon-Garvel-Greenock (Really busy)

Ardrossan to Aberdeen is long way to go for drydocking. Are there no drydocks left on the Clyde any more? (@ Jim McQ)

Good to see an oldie coming out of the fog. Is there a carcass in the foreground?