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An island race

Floating 57,000t concrete submarines 20km across one of the busiest shipping lanes in the world was always going to be a challenge.

Ballasted so that only 300mm showed above the water level, the tunnel elements drew 8.5m. But most of the Baltic on the route was less than 8m deep. The only possibility was to follow the dredged shipping channels until they intersected the 10m deep trench dredged out for the tunnel, swing the units round and tow them up the trench into position.

'But the trench is only 3m each side wider than the element,' points out 0TC production manager Dave Bolton. 'Given the range of weather conditions we were expecting there was no way we could rely entirely on tugs to position the elements. A much more accurate system was needed.'

He goes on: 'We reckoned that by fitting multiple winches on to the element, working off fixed anchors and taking advantage of satellite navigation technology we could both swing the elements and position them within 500mm.

'The real problem was deciding which fixed anchors to use.'

Driving piles as required was one answer. Current speeds and variable seabed conditions pointed towards a more sophisticated solution, the so- called 'anchor islands'.

0TC ordered eight of these islands for the contract. They were basically, miniature, three-legged, jack-up barges. These could be moved from position to position with little fuss and operate into the shallowest water. Once their legs were down they could provide the anchorage needed for the complex manoeuvring in the final stages of float out.

Buoyancy control on the elements is mainly down to five tanks; three which take up all but 20m of the central motorway bore plus one at each end of the outer rail bore. These supplement the carefully calculated areas of first stage ballast concrete poured before float-out.

Normally two elements would be strapped together before flooding. Winches on the bunds would move them to the lower basin, then hold them in position as the water was lowered back to sea level.

All being well the elements would be separated, winched to opposite sides of the basin and moored up, while towing and mooring bollards were fitted. The anchor island would be positioned, the floating outer gate towed away, and the first of the elements would begin its ponderous move out of the basin. As it emerged the two sections of the catamaran pontoon would be floated into position at each end.

'This pontoon was a new idea to me,' Bolton admits. 'But we needed to place the element very accurately with 10m to 12m of water above. The catamaran promised precise control of the lowering process, and it also carried all the positioning winches.'

Moored up to dolphins outside the basin, the element would be fitted with its access shaft and command/control and survey towers with the aid of a floating crane. Then it was a matter of waiting for a weather window.

Tugs played a secondary role during the turn and final positioning. Their job was to move the eight islands from place to place as the turn progressed and the element began its final journey. Islands would jack- up, the wires from some of the pontoon winches would be connected, the element would move slowly forward. As it moved, islands left behind would jack-down and be towed past the element into new positions, ready to take the strain again.

Acting together, the winches and the satellite navigation system ease the element into its target position before the eight hour lowering operation began. Once a diver with a video camera had confirmed lowering accuracy, the winch on a small pontoon moored to the previous element would ease the barely-buoyant mass forward until the two halves of the guide frame were mated.

Finally, with backfilling well advanced, the temporary prestressing bars could be released and ballast concreting completed. Ballast tanks and bulkheads, like the guide frame, could then be dismantled and returned to the casting yard for re-use.

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