I’m not sure how much of her activity on the Clyde was secret and how much was just so run of the mill as not to be the subject of note. She was rather large to be hidden!
AFD4 she was towed from Devonport to the Clyde in Aug/Sept 1941 and remained there until 1948, first under RN ownership and then that of Metal Industries from 1946 to assist with ship-breaking. Her lifting capacity was some 32,000 tons still making her amongst the largest AFD in WW2.
But she was one of 4 floating dry docks in the Clyde during WW2. There were also the following:-
AFD7, the Portland Dock, with a lift capacity of 2,200 tons. She was towed north from Portland between Nov 1940 and April 1941 when she arrived on the Clyde, where she remained until Sept 1947.
AFD15, lift capacity 540 tons. A new build in 1941 and based at Stornaway, she was transferred to the Clyde in Oct 1943 where she remained until sold in 1946.
I’m not sure where exactly either of these two might have been located in the Clyde. AFD 7 possibly spent time at Rothesay until AFD20 arrived.
AFD20, lift capacity 2,750 tons. Another new build in 1943, she was based ‘Doon the Watter’ at Port Bannatyne, Kames Bay, near Rothesay from Feb 1943 until Nov 1944 when she left under tow for Manus in the Admiralty Islands in the Pacific.
As for AFD4’s involvement with HMS Roberts, it was a normal post completion docking for a ship leaving the builder’s yard, with minimal work involved rather than wholesale completion of a newly launched ship. Roberts had been launched in Feb 1941 and AFD 4 only arrived on the Clyde in Sept. AIUI Roberts left the John Brown yard on 6th Oct for her acceptance trials. She was docked on the 13th Oct and finally commissioned on the 27th before spending the rest of 1941 working up, before deploying to Alexandria via the Cape in Jan/Feb 1942.
Some photos here, although you have probably seen them before.
Floating Drydock Medway Dock AFD4 1912 Swan Hunter Wigham Richardson Wallsend
www.tynebuiltships.co.uk
As you are aware, the Clyde was a busy place in WW2. In particular with it being a loading point for the WS troop convoys to the Middle & Far East between 1941 and 1943, it saw many large passenger cargo liners that would have needed repair and maintenance work. AFD4 would have been ideal for these regular dockings.