Last summer Belgian lifting contractor Sarens was contracted to place 306 concrete blocks to shore up the sea defences in the Spanish port of Llanes. Each block weighed 60t.
The first option for such a project is to mount a crawler crane on a barge and use a second barge to transport the blocks. But the Atlantic tides and the size of the harbour made this approach impractical and risky. Instead, Sarens chose to position a crane on the quay and transport the blocks over land. The crane was 10.3m wide; the quay was 8.107m wide.
The first challenge was lifting gross loads of 75t out to a maximum radius of 50m. A lattice boom Liebherr LR 1750 had a strong enough chart to place the blocks at 38m radius, but a few calculations based on the Archimedes principle found that the force of buoyancy acting on a block was sufficient to submerge them at 38m and move them out to 50m. This meant, however, that the crane had to work from four different positions.
To solve the problem of the quay being narrower than the crane, a 3m-high steel frame was constructed to support the crane. Investigations into the stability of the crane showed that extra counterweight was required, to give a total of 320t.
To relocate the crane to the four necessary working positions, this structure was mounted on Sarens’ skidding system, which it calls Sarskid. This system, with Teflon plates, was used to move the 990t total load (crane, counterweight and steel frame) over a distance of 200m.
All the concrete blocks were placed using a controlling GPS system mounted on top of the boom of the crane. The crane operator followed the movements of the blocks on an extra display screen installed in his cabin and each was placed exactly according to the design drawings.
Sarens made a separate risk analysis for every element of the project – the skidding system, the extra counterweight, the exploitation of the Archimedes principle of buoyancy, the construction of steel structure, the fastening of the crane on the support structure, the fastening of the steel structure on the quay during lifting, the supporting jacks underneath the steel structure, etc. However, it also made a global risk analysis for the total concept. As well as analysing every possible worst case scenario for each part of the concept, its influence on the other parts was also explored.
The Euro 1.5m project took eight weeks and was completed on 30 June 2003 just before the start of the tourist season. The client was the Gobierno del Principado de Asturias.
