On behalf of Enercon, Belgian crane service provider Aertssen removed the 72-metric-ton, 58-meter-long rotor blades from an Enercon E-126 wind turbine at a height of approximately 136 meters for maintenance. However, what may sound like a routine job was actually a tricky challenge, and it was only the team’s extensive experience and two CC 3800 lattice boom crawler cranes that enabled them to successfully carry out the corresponding lifts.
“Before anything else, we had to decide which crane to use for the job,” said Aertssen Project Manager Peter Bertels.
There were two options available: either a large crane that would be able to take care of the lifts by itself or two, more compact, cranes that would tackle the lift in tandem. Ultimately, the team decided on using its two CC 3800 cranes, as they were more cost-effective, would be powerful enough when used together, and had the required reliability and precision characteristics for the lifts. However, it was first necessary to make sure that the ground bearing capacities at the work site in Estinnes would be sufficient for the two crawler cranes, as they had originally been calculated back when the wind turbine had first been erected, for the use of a single crane only.
A total of 52 truck runs across a distance of about 130 kilometers were needed in order to get the two cranes from the Aertssen branch in Stabroek to the work site. Once there, a five-member Aertssen team set up the cranes within a week. The team started by setting up the first crane, which was placed a bit farther away from the wind turbine, on the corresponding access road, and then proceeded to set up the second one, which was closer to the tower. For the configuration, the team decided on the main boom variant with a 141-meter LSL and 12-meter LF4 arrangement with an offset of 20 degrees. The Superlift mast had a length of 36 meters, and the crane was set up with 165 metric tons of counterweight and 50 metric tons of Superlift counterweight.
Once all these steps were completed, the two cranes and the Aertssen team were ready for the technically ambitious lifts: The 72-metric-ton rotor blades not only had to be taken from a height of 136 meters and safely lowered to the ground, but they also had to remain perfectly balanced while being lowered. To achieve this, the operator of the CC 3800 that was closer to the tower first guided a sling, which was hanging from a spreader beam with four-part reeving, around the tip of the rotor blade. The operator of the other crane then repeated this same procedure. When “lassoing” the rotor blades with the slings, the two crane operators depended on the instructions of Enercon employees who were also on-site for the job.
“The reason is that even though we’d installed cameras on the spreader beams, our crane operators were operating remotely, and were therefore unable to determine whether the slings were at the right height,” Bertels said. “It was kind of like lassoing a tiny animal.”
Once the two slings had “lassoed” the rotor blade, the cranes swung them to the intended rigging points, at which point it was time for the tricky part of the job: First off, the cranes needed to rotate parallel to each other in order to get the blade out of the rotor. Once this happened, crane 1 had a gross load of 46 metric tons on its hook, while crane 2 had a load of 36 metric tons. After this, the operators were able to set down the blade at a radius of about 22 meters. However, the blade had to be kept in a perfectly horizontal position at all times in order to prevent it from sliding off the slings, which is why digital spirit levels with wireless transmission capabilities were installed on the blades in advance to let the operators know, at all times, that the blades were level.
It was only necessary to make two brief corrections throughout all three lifts, which totaled a distance of about 400 meters. One week later, after the required maintenance was complete, the rotor blades were reinstalled on the wind turbine.
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