A new multi-million pound 700-tonne steel dock gate described as a ‘huge feat of engineering’ was officially opened in HM Naval Base Devonport, Plymouth on Monday 2 October.
After being towed across the North Sea from a factory in Holland, the caisson (or giant gate), was slotted in the dock or basin entrance, forming a watertight seal between the River Tamar and the dry dock or basin.
Commodore Ian Shipperley, Naval Base Commander, ceremonially opened the caisson at a ceremony attended by the makers, the MOD’s industrial partner Babcock and the naval base team who procured the caisson.
He said: “This is a really important day, marking the operation of the new caisson. It’s a great asset and is a tribute to the hard work of the team who worked throughout the caisson project process from the design, procurement and to today’s event. Their excellence has led to project being on time and to budget.
“The caisson is a huge feat of engineering both is size, advanced complexity and precision. The fit has to be down to the exact millimetre to prevent leaking. The idea is to maintain a certain depth in the basin, whatever the height of the tidal water outside in the river. Submarines and ships in the basin have to be floating, so a constant amount of water needs to under the hulls, even if the tide is low in the river.”
He added that the caisson was made by the world leaders Ravestein after a procurement process which was open and competitive, giving value for money to the taxpayer.
The naval base has 15 docks along four miles of waterfront with caissons controlling non-tidal basins containing submarines or ships.
The new caisson is made of steel with concrete ballast in the bottom to weigh it in position. Inside the caisson are water ballast tanks and two pump spaces. When the caisson is in the basin entrance the water is retained in the tanks to keep it in place. When a vessel is allowed through the entrance from either the river or the dock or basin, the water ballast is pumped out and the caisson is floated out of the way.
Once the vessel has entered or exited the basin the caisson is put back in position the valves opened to allow water back into the water ballast tanks it then sinks into the entrance to reform the watertight seal. Once closed, the top of the caisson forms a continuation of the footpath and road bridging the entrance.
The new caisson has advantages over the old versions – it is designed to remain in service for 25 years without the need for to be taken out of service for maintenance and servicing, boasts a new paint finishes and has a different operating system involving electric pumps to pump out the ballast water rather than being blown out by compressed air. The road across the top of the caisson has also been improved for vehicles and pedestrians with safety standards the same as motorways and road bridges.
This major project has required a close partnership between the contractors, the maintainer/operators (Babcock – Devonport), the design authority (Naval Base Commander’s team Devonport), Devonport Port authorities and Serco Marine.
