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Heavy duty

Jetties - A new nuclear submarine jetty in Scotland has to be capable of withstanding apocalyptic events. Andrew Mylius reports from Faslane.

Invisibility and surprise are the submarine's major tactical strengths. During months spent prowling the ocean depths, few, even among the crew, know where a boat might be headed, or when it will return to port. And that makes building a new jetty in the middle of an operational submarine base an unusual challenge.

'We have to be able to accommodate a home-bound sub within 48 hours. We will be working around the subs.

We are programming work so we can change what we are doing at a few hours' notice, ' says Amec design manager Bob Hanna.

Amec Design & Project Services is design and build contractor for a vast £130M oating jetty at Faslane, part of HM Naval Base Clyde, on the west coast of Scotland. The jetty has been designed to fit the needs of the Navy's new 9,800t Astute class hunter-killer submarines. Captain Peter Merriman, Astute programme director for client Defence Estates, has been procuring the three new submarines and the jetty that will serve them.

Infrastructure built for military applications is heavily biased towards functionality over economy, and has to be battle hardened - the jetty will have huge reserves of redundancy.

'You have to have the capability to carry on operating even after seismic events, ship collision, fire, nuclear explosion or military attack, ' says Amec project manager Alistair Parr.

'We're designing for Biblical events - for a one in 10,000 year earthquake, for example.

And you don't want cliff-edge failure if that scenario unfolds.

We are demonstrating margins of 30% beyond what's strictly necessary.' The decision to procure a oating rather than a xed structure was driven by seismic performance requirements, Merriman's desire for easy mooring, and by the question of how to deliver a major project without disrupting the operation of the base.

'Piles at each corner keep the jetty in position, but it rises and falls with the tide, and there's a degree of lateral play as well.

That helps with the seismic aspect, ' says Parr. The structure will be largely isolated from any ground tremors by water.

'And with a floating design you don't need to worry about the changing tide, ' explains Merriman. 'There's a 3m tidal range on the Gare Loch [on which Faslane is located]. With a xed structure the boat's moving up and down against it and you are constantly changing mooring warps - you're either paying out rope or taking in slack. With this, the boat and jetty move together.' Opting for a oating structure also enables on-site works to be limited to the installation of four vast tubular steel piles to which the jetty will be anchored, and a 160m long approach viaduct crossing the foreshore and shallows. This will consist of piled piers and precast concrete deck units with an insitu concrete topping.

The jetty itself and a 60m long, 400t steel link-span bridge connecting it with the approach viaduct, are being fabricated off site and towed to the base.

Contracts for piling and fabrication of the linkspan are out to tender. Hanna expects a foreign contractor to win the job of installing the large diameter piles for the floating structure - 'size-wise, they're right on the edge of what's possible with British-owned marine piling kit'. But he would like to find a local rm to build the linkspan 'because it's easier to control delivery'.

Amec is building the 200m long, 11m deep and 28m wide jetty in a commercial dry dock at Inchgreen, Greenock, 15km away on the south side of the Clyde estuary. Inchgreen only became available late on, however, and the jetty was designed to be built in a smaller dry dock at Hunterston.

The jetty's reinforced concrete hull is composed of 12 watertight cells running its full length, but at each end 'there are 10m long, 800mm thick cantilevers, which were originally designed because of the limited length of what can be built at Hunterston', says Hanna. They were going to be added after the cellular structure had been cast and oated out of the dock. With the change of construction yard it would have been simple to extend the cells over the jetty's full 200m. But there is a construction advantage in keeping the cantilevers that will become apparent later in the programme, Hanna explains.

Holes in the cantilevers will be married with the jetty's anchor piles, which will be installed to an accuracy of +/-200mm. 'By leaving construction of the cantilevers until last, when the piling is complete, we'll be able to accommodate any variation in the tolerance of the piles.' Designing the jetty with a cellular internal structure ensures it will stay afloat even if one cell is punctured. It also segregates services, safeguarding against re or explosion in one service setting off a chain of failures.

Structural robustness comes from 500mm thick external walls. Internal walls are 550mm thick at the lowest level, reducing to 350mm thick at the top level. The jetty's deck is a massive 600mm thick over its mid section and a still hefty 400mm at each end. 'There will be a heavy steel 'forepeak', used for mooring the submarines projecting from the jetty, at its seaward end and the linkspan landing on it at the other, exerting large bending forces.

Variations in deck and wall thicknesses account for that, ' says Parr.

Post tensioning would have been an efficient way of producing a very strong structure, but questions about durability of the stressing tendons over the jetty's 50-year design life meant it was swiftly ruled out. 'And what happens if a tendon is severed in an attack?' Parr asks.

As much as 450kg of steel per cubic metre of reinforced concrete is being used, consisting of double layers of T32 bars at 125mm centres, says Amec project manager for the Inchgreen yard Simon Hough.

As well as providing huge strength, the rebar will help prevent cracking - 'one of the chief challenges with a oating structure', Hough adds. 'It's designed to be watertight, with a maximum 0.2mm crack width limit. Cracks should be self-healing.' Concrete is a C40/50 mix with 50%?70% of cement replaced with ground granulated blast furnace slag to limit heat of hydration. A very wet, airentrained mix is being used to ensure good penetration of the reinforcement and improve durability. Air entrainment introduces microscopic air bubbles to the mix, which help the concrete withstand chlorides and extreme temperatures.

Rebar will be protected from attack by an embedded electrical cathodic system.

PROJECT SAFETY

AFR (2006): 4.86. This seems high as every accident is reported, however trivial.

Safety innovations: Passive fall protection; joint worker/management committee to improve performance; cash reward for every accident-free 28 days; screening of operatives involved in high-risk work.

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