A Floating Dock during construction c.1938 at Haverton Hill. A Floating Dock would be taken out to sea. It would lower its base to allow ships to float over it and then rise up again lifting the ship with it. Repairs to the vessels could then take place as if they were at a dry dock. 
Picture Stockton Archive
A Pictorial History of Stockton-on-Tees
Four floating docks were built by Furness Shipbuilders according to the book by John M Evans Fifty Years of Furness.Contract no 139 Owners S.A.des Huileries du Congo Belge built 1928 fate unknown.Contract no 409 Owners Finnboda Varf, Stockholm, Sweden built 1946 fate unknown.Contract no 429 Owners Christian Salversen & Co Ltd Lieth fate unknown.Contract no 510 Owners Centrala Morska Importowa Poland still in service in 2001. I hope the above information helps.
I have now found the General Arrangement drawing for the 4000TLC Durban Floating dock and can confirm that the photo is of the Durban dock and not the Stockholm Dock as I had previously thought.
According to our company records the following Clark & Standfield designed floating docks were built by Furness Shipbuilders (the years given are within the design and build period): 1928: 600 TLC Floating Dock (Sectional Pontoon Type) for S.A. Huileries due Congo Belge – Location: Leopoldville, Congo 1936: 4000 TLC Floating Dock (Box Type) for South African Railways and Harbours – Location: Durban 1938: 9000 TLC Floating Dock for Finnboda Varf – Location: Stockholm 1947: 1000 TLC Floating Dock (Sectional Pontoon Type) for Chr. Salveson and Co – Location: South Georgia 1951: 8000 TLC Admiralty Floating Dock 58 1956: 74ft Long Extension to Sectional Box Type Floating Dock at Bergen for Bergen mek Verks 1962: 5500 TLC Floating Dock (Box Type) for Centromor, Poland
Thank you Peter Weatherall for confirming that I am not suffering some sort of “Old Timers” complaint. I remember working on piping installations on “Finnbod” and that was in 1947. The information supplied by David Westmore regarding the 1938 construction had me confused for a moment as I had not realised that Furness Shipyard had constructed two Floating Docks.
The floating dock built in 1938 was for the South African Railways and Harbour Board, the one built for Finnboda Varf Stockholm was built in 1947, it was I think of six sections and was self docking, each section could be unattached and dry-docked one by one. In 1948 a further section was built which increased its lifting capacity to 9000 tons and length to 538 feet.
The floating dock was designed by Clark & Standfield. It had a lifting capacity of 9000 tons and was 538 ft long. It was built in 1938 by the Furness Shipbuilding Co. Ltd at their Haverton Hill Yard for Finnboda Varf, Stockholm. The photograph shows the dock at her launch. More information can be obtained from the floating dock engineers, Clark & Standfield.