Modifications to No.1 Dock at Babcock’s Rosyth dockyard in preparation for the final assembly and outfitting of the massive new Queen Elizabeth (QE) class aircraft carriers have now been successfully completed.  The achievement marks a significant milestone in the substantial programme of civil engineering works being undertaken at Rosyth to allow entry of the mega-blocks, assembly and then departure of the completed 65,000 tonne, 280 metre long, 74 metre wide, 56 metre high vessels.

Works to modify the dock began in March 2008.  The dock (originally built in 1916 and extended in the 1960s) was long enough to accommodate the new carriers, but work was required to cut back the huge granite steps (known as altars) benching out from the sides of the dock (giving it a V-shaped profile) to accommodate the modern warship flat-bottomed and slab-sided designs, and to widen the gated entrance.

Following the construction of a massive five cell, 50,000 tonne cofferdam (a temporary structure designed to create a dry work environment) the dock was de-watered in October 2008, and work was then undertaken to remove the intermediate caisson cills and jambs; install 165 rock anchors through the dock walls; build the new propped gate cills and jambs; demolish the original entrance to the dock and construct a new, wider, entrance; strengthen the under floor culverts; and reconstruct the foundations for the new gate.  With the removal of the ‘altar steps’ the dock floor is some nine metres wider.  Two intermediate gate positions, recessed into the walls, have been located to suit the build strategy for the huge carriers and the modified floating caisson gate has been installed.

The primary skidding system, which is used to support and move the mega-blocks in the dock once the water has been removed, has also been constructed.  This is connected to the dock floor and is designed to transmit the large imposed loads from the vessel safely down through the underlying rock strata.  The system consists of five rows of heavily reinforced concrete runways measuring 2 metres wide by 1.4 metres in height, amounting to a total of 1,150 m of runway containing over 7,750 tonnes of concrete.  In the intermediate gate positions the runways are constructed using pre-cast concrete in the form of blocks that can be lifted in and out as required to allow the intermediate dock gate to be installed and removed during the exacting build sequence.  The primary skidding system carries the upper docking and skidding system comprising stools, jacks and rams which are required to support and manoeuvre the sections of vessel to a high degree of accuracy during assembly.

Works to No.1 Dock were finally completed at the end of August when the final blocks of the ship support system were set in place and the new dock filling and emptying systems installed.  Commenting on the achievement, Babcock project director Sean Donaldson said: “Completion of No.1 Dock is a major milestone in the preparations for the carrier, and comes just as the first of the component units for the carrier sponsons arrive here in Rosyth.  The completion of the works within the planned timescale and budget, despite significant technical challenges, is testament to the efforts of the team involved, which comprised Babcock/Halcrow and BAM Nuttall staff.”

In addition to the works to No.1 Dock, work is continuing to modify the direct entrance which provides access into the basin.  Rosyth has the largest non-tidal basin for ship repair in the UK, separated from the Firth of Forth by an existing 38.1 metre wide entrance, featuring a sliding gate to hold the water back.  Work is now underway to widen the entrance by four metres, to 42.1 metres.  So far 52 secant piles have been constructed to a depth of 25 metres, and dredging for the basin cofferdam completed.  The massive inner and outer coffer dams are being progressively prefabricated and installed on site and the under water operations to raise the level of the wing walls above water level and install the massive sea bed struts is taking shape.  Construction of the permanent foundations will involve people working over 10 metres below the sea bed (some 25 metres below sea level).  The widened entrance is due to be completed in summer 2010.

The programme of modifications at Rosyth to accommodate the new QE class aircraft carriers is being undertaken by Babcock on behalf of the Aircraft Carrier Alliance, which includes the MoD, Babcock, BVT Surface Fleet, Thales UK, and BAE Systems.

 

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