The Bregenz Festival’s production of Verdi’s opera A Masked Ball was staged in the open air on the banks of Lake Constance in Austria. It ran for two seasons but has now given way to a new production of La Bohème. In the latter months of 2000 the old stage set, pictured above, had to be dismantled. This meant a 25m-high book and a huge skeleton (whose skull alone weighed 3.6t) had to be removed.

The stage is anchored to the bottom of the lake and consists of a concrete core measuring 30m by 34m. Within it are dressing rooms and the orchestra pit. For the conversion work, about 500t of steel and 300t of timber and other materials had to be dismantled, moved and re-assembled. The chosen tool for the lifting work was a tower crane mounted on a pontoon.

The floating crane needed a working radius of 42m, a hook height over the pontoon of 28m, a tip load of 1,150kg and a maximum load capacity of 6t. The pontoon supporting the crane is 15m by 13.5m. Maximum permitted pontoon gradient was 3° and its oscillation frequency was between 0.3 and 1.5 cycles per second. Festival organiser, Bregenzer Festspiele GmbH, selected a Liebherr 71K self-erector.

The crane will remain on site for another year. It is now helping to erect the set for next season’s festival offering, and afterwards will be used to dismantle it.

Having the crane mounted on a pontoon presents particular challenges. Wave motion and listing of the pontoon impinges on lifting capacity and reduces the life expectancy of the slewing ring and slewing gear.

Liebherr explains that its 71K has got around these problems with electronic equipment such as a programmable logic controller, continuously variable drives controlled by frequency converter and an electronic load moment limiter. With these it is possible to amend the electronic load curve of the crane in line with the stability of the pontoon to keep available the required tip load. The maximum load range is moved inward, closer to the tower.

The advantage of a self-erector is that when the wind rises, coming in off the lake, the jib can be swiftly folded away and the tower telescoped down. This saves significant wear and tear on the slewing ring and slewing gear, Liebherr says.