Two 750t crawler cranes have been used to build the “jacket” that will form the substation platform for the Borkum West II wind farm off the German coast.
The identically configured Liebherr LR 1750 cranes were used to position the 600t upper jacket section on top of the 900t lower section, each with a 42m main boom, 28m luffing jib and 31m derrick boom, along with 260t of suspended ballast.
The tandem lift took place at the ABC-Halbinsel offshore terminal in Bremerhaven, by crane and heavy load specialist Schmidbauer. Before it started, the firm carried out a detailed crane study that simulated all phases of the hoisting process.
Once the cranes had been moved into position on a bed of sand and load distribution mats, the load was fastened with the help of two auxiliary cranes. Four 400t shackles were needed, with the shackle pin alone weighing 75kg.
The cranes then lifted the upper jacket section out of its support framework, and then moved it to the position of the lower section, with two guides ensuring that the distance between the two crawler cranes remained the same at all times.
The load was held in place for two days while the top and bottom sections were welded together. Then a 450t telescopic crane with luffing jib was brought in to remove the shackles and ropes at a height of 50m.