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Steady as she goes

Defence contractor Babcock Rosyth Defence has come up with innovative civil engineering solutions for a submarine support cradle at Rosyth dockyard.

The seaweed-encrusted submarine on the raised concrete slab looks tiny inside Rosyth's cavernous 80 year old Number 3 Dock, originally built for Britain's massive First World War battleships.

Close up, however, the vessel towers above the dock floor. It is enormous. Equally surprising is the concrete slab underneath. What seemed unspectacular from a distance turns out to be a highly sophisticated £2.5M 'floating' cradle.

The submarine is sitting on a matrix of huge steel blocks fixed to four massive concrete planks supported in turn by hundreds of rubber pads. It is the world's first earthquake-proof submarine docking cradle and engineers are testing its structural integrity using the 4,600t retired hunter/killer submarine HMS Swiftsure.

Full scale earthquake simulations for the structure are impossible, so dockyard owner Babcock Rosyth's consultant Babtie has carried out extensive calculations which prove that the structure could withstand an earthquake measuring five on the Richter scale.

The cradle is part of a £40M seismic upgrade of Rosyth Royal Dockyard's nuclear submarine refitting facilities and has been designed by Babtie using state of the art earthquake technology developed in the US and Japan.

Rosyth's naval dockyard was established in 1912 to build battleships during Britain's First World War militarisation. Though its massive docks are hugely robust - they are founded directly on sound sandstone bedrock and have 7m thick concrete floors - no thought was given to earthquakes in their original design.

In 1994, when Babcock Rosyth Defence's nuclear licence for the dockyard came up for review, the nuclear safety authorities - the Ministry of Defence Central Plant Control Authority and the Nuclear Industry Inspectorate - started probing the dock's seismic safety case.

That led to the progressive upgrade of the Number 2 and Number 3 drydocks starting in 1995.

'The brief was to provide enhanced facilities that would make the risk of a nuclear incident as low as reasonably practicable,' says Babcock Rosyth Defence chief civil engineer Mike Murray. Built by contractor Nuttall, the new submarine docking cradle formed part of this programme.

Its main requirement is to support submarines ranging from 4,600t hunter/killers to 7,800t Polarises while they are refitted and refuelled. Submarines must be kept vertical when fuel rods are removed from reactor cores. Any movement of a submarine during this work could make the reactor unstable, and in the worst case, could result in a reactor core melt down.

Submarines are traditionally supported in the dock on a system of blocks fixed to the dock floor. This system resists the vertical loads imposed by the weight of the vessel but only gives nominal horizontal resistance.

As a result a conventional cradle would be unable to resist horizontal loads imposed by a powerful earthquake and there is a risk that a submarine could be overturned by the massive lateral forces. Horizontal loading induced by a Richter scale five earthquake would be more than 40,000kN.

Seismic forces are among the extreme loads considered as part of the safety case for Britain's nuclear industry, which designs on the basis that an earthquake measuring five on the Richter scale will take place once in 10,000 years.

The quake is considered as a series of accelerations and frequencies with the peak horizontal acceleration taken as 1g. Adjusted for extremely good site conditions - Rosyth is built directly on to the sandstone bedrock - this has been reduced to 0.25g and modified for the traditional docking cradle's natural frequency of around 8Hz.

Two other cradle designs were considered. One was an extremely robust structure that could resist the very high loads. The other idea was to reduce the loads on the cradle and vessel by isolating the cradle from the dock with rubber bearings. Both systems were costed at similar amounts but the base isolation cradle was chosen.

Crucially, the 'floating' cradle system removes the risk that a submarine will move in the cradle during an earthquake. This would still have been an issue with a more robust cradle with side walls to support the submarine.

By resting the cradle on rubber bearings that dampen the horizontal forces the submarine/cradle system is isolated from the effect of an earthquake on the dock. Reducing the loads also made it possible to design a less bulky structure that will allow work to be carried out underneath the boat.

'Though it retains the original block and capper arrangement, a feature of the new cradle is that blocks can be removed from any position to allow work to be carried out,' says Baird.

The cradle will support submarines on the steel blocks fitted with Douglas Fir softwood cappers profiled to the shape of the vessel hull. These are secured to four 29m high, 9m long and 500mm deep modular concrete rafts each sitting on 120 rubber bearings costing a total of £500,000. These are bolted to concrete plinths cast on to the dock floor (see box).

Supplied by Andre Rubber of Burton on Trent, the 400mm square bearings are made from a special rubber compound designed for stiffness and damping. The bearings contain 12 layers of rubber reinforced with 3mm thick steel shims. The Malaysian Rubber Producers Association helped in the design of the bearings. A typical load on each plinth is about 250t.

Although base isolation is becoming commonplace in buildings in the US and Japan, where there is a lot of seismic activity, it is a completely new system for supporting nuclear submarines.

'Number 3 dock at Rosyth is now the safest nuclear submarine refuelling and refitting dock available in the UK,' Murray claims proudly.

HEAVYWEIGHT GATE: Babcock's earthquake proof submarine cradle is part of a four year seismic upgrade in which the most noticeable project has been a giant new concrete dock gate to replace the old steel entrance.

This involved constructing a huge inverted door frame inside Number 3 Dock, comprising two massive jambs on either dock wall and a giant sill across the dock floor connecting the two sides.

The dock gate is an enormous 6,000t floating concrete caisson which won the small project category of the 1996 British Construction Industry Awards. The caisson butts up to the new concrete frame to create an earthquake resistant dock gate. A rubber lip seal on the leading edge of the caisson is forced against the jambs by the hydrostatic pressure of the sea.

When in the closed position the caisson sits on a gravel bed which provides the base seal. It is floated out to a temporary berth when the dock is open.

Because construction of the sill and jambs required partial demolition of the dock structure while the dock was in use, planning of the 300m3 pours was complex.

The pours also presented a challenge for the contractor. 'Because of the massive volumes we had to develop a system for minimising the temperature difference between the centre and the surface of the concrete,' says James Scobie, area manager for main contractor Nuttall. 'This was achieved by delivering cooling water to the core through a network of steel pipes. Thermocouples monitored the temperature.'

The concrete caisson gate replaces the original 15m high sliding steel gate which did not provide adequate seismic resistance. The steel gate opened by sliding sideways into a narrow chamber in the dock wall. The chambers have also been strengthened with steel props as part of the upgrade.

'About 70 steel props each weighing about 12t were required,' explains Babcock Rosyth deputy project manager McBride. 'That totals 800t of steel.'

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