In case you don't know what a caisson is, it is the mobile "gate" that dock use to close off docks so the water inside can then be pumped out allowing work to take place.I refuse to respond to anymore of you ill formed comments on this topic, as far as I'm concerned it's case closed.
BTW there are 14, count them 14 drydocks in Devonport Naval,base not 15 Drydock 13 does not exist.
Essentially it is like the ballast tank of a submarine and can be filled with water to sink it within two slots at the dock entrance. It can be built in sections depending on the size of the dock concerned.
The process would be started with the dock open the vessel having work would enter the dock, the caisson moved to the dock entrance and flooded down blocking the entrance. The water inside would then be pumped out allowing work to commence.
At the end of work, sluices in the dock would open flooding it and allowing the vessel to be refloated. The water in the caisson (ballast) would then be pumped out raising the caisson so it can be floated away.
If you look at photos you can sometimes see the caissons moored to the side of the turning basin.
If you think sticking a caisson in front of a dock has increased the size by 50 feet (or more) good on yer!
As to Dock 13 it did exist but is no longer used. Simply put there is no longer a need for all the docks, however if you look at aerial photos you can see where the old ones used to be although some have been built over. You will find that the diagrams of the port do not show all the old docks. Surprisingly enough docks built for old sailing corvettes, frigates or two or three deckers are no longer needed.