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Walking on thin ice

On the move

'The crux of the project lies in getting all the parts of the new station from ship to shore. That's the major risk, ' explains Tuplin.

BAS has chartered a supply ship to transport the 16,000m 3 volume of Halley VI in at-pack form. During summer Antarctica's sea ice recedes allowing ships close to the shore ? but close is a relative thing. There can be anywhere from 100m to 800m of sea ice to cross before the relatively solid mass of the Brunt Ice Shelf itself is reached.

Sea ice thickness can range from 1m to 2m.

This sea ice has forced BAS to put a 9.5t weight limit on sledges used to unload the supply vessel.

Sledges themselves weigh in at a hefty 3.5t, meaning that components for Halley VI can only have a maximum weight of 6t.

'We've been forced to pare things down and search for weight savings wherever we can, ' says Michael Wright, project manager for Halley VI designer Faber Maunsell. 'There's been a balancing act between making components as large and integrated as possible for speed of construction, yet making them light enough to transport.' The space frames for each pod will be the heaviest individual units, fully fabricated from 60mm diameter tubular steel before shipping.

Frames will be mounted on lightweight temporary skis for the trip from ship to construction site.

'Each leg weighs just over 2t, ' says Hammerton.

'We'd like to have attached legs and permanent skis [measuring 3m long by 400mm wide] to the space frames in Cape Town, but the sea ice makes that impossible.' Halley VI will be built at the site of the existing Halley V station, which will provide accommodation to the construction crew. When completed in 2010, it will be slid inland to its new location. This will be the first true test of the station's long-range mobility.

Morrison and BAS carried out hauling trials last year. Each of the standard pods will weigh 80t and the large living module 140t, says Hammerton. 'We did trials to 150t to test the pulling power needed and the bearing capacity of the snow.' Conservatively, this is 50kN/m 2, though 'we can improve that by churning up the surface, compacting it and allowing it to freeze overnight.

'Trials showed we need 10t-11t line pull to move a unit once it's been freed from the ice.' The skis freeze to the surface, Hammerton explains. Pairs of bulldozers will be used to haul each pod, with an additional pair helping to 'break sticktion'.

Jacking and moving the pods will impose signicant bending forces on the space frame and the connections between frame and legs. 'At times it's possible there will be only two skis in contact with the surface, so the space frame has to be very stiff. There will be maximum 35mm distortion in each corner, ' says Wright.

To prevent embrittlement in the very low temperatures Faber Maunsell has specied ductile low carbon steel for the space frame, with a Sharpie Value at -56C of 27 Joules. 'That enables the structure to cope with tensile loads.

It's a grade of steel that is fairly common in Russia, for example, but not at all common in the UK. We've sourced it internationally, ' says Wright.

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