Bridge Removal Over Shallow Waters
The Montrose New Bridge , via which the A92 crossed the river South Esk on the East Coast of Scotland until last October, was in fact 73 years old and dying from a form of concrete deterioration known as Alkali Aggregate Reactivity.
The bridge to be removed consisted of two 60m side spans weighing 2,100 tons each and one 13m centre span weighing 300 tons. The plan to remove the sections, transport and place them ashore was complicated by 'The Scaup', a shallow bank in front of the southern side span which even local pilots had no experience of navigating as the water depth is too limited for any cargo vessel.
The contract for this difficult operation was awarded to Antwerp based Scaldis Salvage & Marine Contractors NV, which mobilised its multi-purpose, sea going heavy lift vessel Rambiz to do the job. The 85m LOA pontoon based craft maintains a shallow draft even when loaded, allowing it to operate in very shallow waters. Twin cranes reaching to a height of 90m deliver a lifting capacity of 3,300 tons.
Eight days before Rambiz appeared on site, an experienced Scaldis rigging team and four containers of the company's equipment arrived at Montrose.
Scaldis designed, provided for and assembled the entire rigging configuration, adding some 150 tons of additional equipment for the lifts, including a 68 ton spreader.
Rigging for the side spans was much more complicated than for the centre span. Some 64 slings were used, consisting of more than 2,000m of steel wire and 46 shackles and rollers, with the Safe Working Load varying from 85 to 500 tons.
Twelve sling protectors were installed at the bottom edge underneath the bridge and were manufactured specially for this project. They served not only to protect the slings but also to prevent them from cutting into the concrete.
The rigging configuration was built up in three parts, one part hanging in the Rambiz cranes and two lower parts pre-installed under each side span. Just before the actual lift off, the upper part was connected to the lower part of one side span.
The lift operations were planned at spring tide, with high water at its peak and tidal currents at their strongest. The ebb stream reached 9 knots.
Calculations imposed a six point mooring, for which Scaldis employed their own 9 ton Delta Flipper anchors.
Depth survey data, local water level and the vessel's draft were imported to the GPRS of Rambiz , enabling the Master to see his vessel moving on a chart, showing the actual clearance with the riverbed.
Moving and ballasting Rambiz over and off The Scaup was the most highly planned phase of operations. Within two hours the vessel was manoeuvred into lifting position, the rigging attached, ballast water pumped, the sidespan lifted and Rambiz then moved into deeper waters carrying 2,100 tons of reinforced concrete and 5,000 tons of ballast water.
The centre span was lifted and placed on 13 October, the south side span on 15 October and the north side span on 16 October, all to schedule and all enthusiastically observed by the local community. The operation was successfully completed without delay or damage.
MJ Information No: 20325
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