I've actually found a photo of the Ash now, so I'll put it in the Special Purposes gallery.
I think you're thinking of the IMP class vessels, designed by the late Technical Director of Harrisons(Clyde) Ian Burrows. They had a schottel forward and aft and were highly manouverable with little power and consequent small fuel bills. They were meant for the southern gas fields and proved themselves quite competent but it was difficult to persude oil companies who had conditioned themselves to more space, more power, more everything was the way to go. They all ended up as standby boats I think.
Stirling brigo far as i know she is still bobbing about the North sea somewhere was converted into a dive support vessel & renamed the Sea Mussel for a few years and then converted into an offshore supply boat owned by craigs & northstar.
After reading your posts ref the stirling vessels.I was taking the jack up rig Pride Montana out of Dammam port on Tuesday the 10th April i noticed the Al Mojil 41 lying alongside as we passed out of the harbour. I believe this was the Stirling Brig. She has had a lot of modifications done to her and looks as though she is used as dive/maintenence vessel with crane on after deck. I did not get a photo of her but as i am back and fore in this area a lot i will keep a lookout for her and try and get a photo for the SN
Stirling Brig - now Al Mojil 41. Stirling Ash - now Al Mojil 42. Stirling Sword - now Al Mojil 43. Stirling Eagle - now Al Mojil 44. Stirling Oak - now Offshore Supplier (after many years Seahorse Supplier in Cork). Stirling Rock - now Little Joe.
Stirling Cormorant and Stirling Skua are both still alive and kicking renamed after sale from Sterling as the Oil Puma and Oil Panther and working in West Africa where they have been since leaving Immingham in the mid 80's.
I saw the Oil Puma yesterday in fact.