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Problems And Answers
This DPC roundup of port and harbour maintenance projects
and products takes us from Europe to Australia and on to the
Americas - we kick off in Scotland
Rosyth has been through difficult times over the past decade.
The Babcock-owned yard on the north bank of the Firth of Forth
has suffered from tightening government purse-strings —
but a Royal Navy contract to build two huge aircraft carriers
by 2016 has transformed its prospects for the next few years.
It will also transform the historic Number 1 Dock, because
current capacity is insufficient for the two 65,000-tonne
284m-long, 74m-wide, 56m-high carriers — to be named
HMS Queen Elizabeth and HMS Prince of Wales.
Originally built at the beginning of the 20th century, the
dock was extended in the 1960s and is long enough to accommodate
the vessels, but advances in naval architecture mean that
the dock's cross-section is unsuitable for modern warship
building. And contractor BAM Nuttall has won an infrastructure
upgrade contract worth £35M ($51M) to reconfigure the
dock and to carry out work on the adjacent non-tidal basin
by October 2010.
Scope
Had naval architects perpetuated the historic V-shaped warship
design then it would have possible to retain the huge slabs
of granite, called 'altars', that bench out from the sides
of the dock and allowed ships to be propped upright while
work continued. But now they have to be cut back to the same
width as the top tier (coping level).
The gated dock entrance is also being widened to accommodate
the ships, as is the sliding gate — from the current
38.1m to 42.1m — that separates the non-tidal main basin
from the Firth of Forth.
The team will also install the 120m-span, 1,000-tonne capacity
Goliath gantry crane that will straddle the dock, plus foundations
for the `skidding' mechanisms that will allow shipwrights
to slide the prefabricated sections of each aircraft carrier
into position. Shipyards around the country are building different
sections of the carriers, which will be welded together at
Rosyth.
Making it happen
A total of 87 reinforced concrete 1,200mm-diameter bored piles
will be positioned 3m into the underlying sandstone rock on
the eastern side of the dock as foundations for the crane,
while a further 295 raking piles (1-in-4) are being driven
as much as 7m into a rock socket on the western side.
Even the concrete pads on the dock floor are significant
pieces of engineering — they must be able to withstand
the crushing force of the 65,000-tonne ships. These heavily
reinforced plinths are cast in two sections with the first
recessed 300mm into the dock floor. A 2m-thick slab is cast
on top of them before the skidding mechanism's fixed.
As this is written, demolition and removal of the stepped
granite altars is under way, with teams using a wall saw to
slice through the rear of the 3-tonne Norwegian granite blocks
on the line of the existing face, enabling the top row of
altar stones to be removed. An excavator with a rock wheel
attachment will then cut through the remaining step before
the whole process is repeated.
When the team's removed them, the dock floor will be about
9m wider — but the stepped altars are also structurally
important. In order to retain integrity, BAM Nuttall's drilling
specialist sister company BAM Ritchies is installing 150 rock
anchors up to 21m into the sides of the dock that will be
then grouted into position and tensioned with a 100- tonne
stressing load.
Finally...
The dock can be split into two through an intermediate gate,
enabling one side to be used as a drydock while sections of
the warships are floated in on the other side. The main gate
to the dock is to be removed, refurbished and reused as the
new gate for the direct entrance — and that alone has
required temporary works (to allow dewatering of the dock
mouth and direct entrance) that are themselves massive feats
of engineering.
Work on the dock's entrance — and the direct entrance
to the main basin — has meant building a huge cofferdam
around them. "There are five 18.5m- diameter circular
cells," project manager Dougie Grant told DPC, "and
five arcs that link each cell."
Built using 700 tonnes of 500mm wide Arcelor steel sheet
piles, the cofferdam cells were placed from a working platform
floated into the main basin and backfilled using a 6E-grade
granular material — about 8,000 tonnes in each. The
link arcs were placed in a similar way.
They're of a size that had not been attempted for many years,
but vital to the project's successful outcome.
Article courtesy of Dredging and Port Construction -
march 2009
Further Info
Peter Bishop - Head of Public Relations & Corporate Communication
BAM Nuttall Limited
St James House, Knoll Road, Camberley,
Surrey GU15 3XW
Tel: 01276 63484
Fax: 01276 66060
peter.bishop@bamnuttall.co.uk
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