In Voerde, on the banks of the river Rhine in Germany, there are four lignite (brown coal) power stations capable of producing 2,157 MW of electricity. That is enough power to serve three million households, which gives an idea of their significance.
The political decision in Germany to phase out nuclear power generation following the Fukushima disaster in Japan, and the increase in renewable energy, makes medium-sized coal power plants essential as a flexible reserve to compensate for weather conditions, i.e. when the wind suddenly drops and wind turbines cannot produce electricity.
Two 350 MW blocks (West I and West II) were built in 1972 and 1973 and are operated by Steag. Another two blocks, Voerde A and Voerde B, were added in 1982 and 1985 with 710 and 747 MW capacity.
The old plants also have to continue production because of the time it takes for public acceptance of where to site a new power station and to get the permits to go ahead. In addition, renovation work is often done on cramped sites. All four Voerde plants are lined up between the Rhine and a busy road, over which pass the main power lines from all the plants. All four are close together and the two blocks in the middle are only separated by a small courtyard. Conveyer lines further reduce crane installation, setup and prefabrication space.
In 2010 a heat exchanger, consisting of rotor sections and a 43 tonne shaft, for the West I plant was replaced using a conventional 500 tonne capacity lattice boom truck crane. The next boiler house in the line, West II, is surrounded by numerous obstacles making any crane installation a real challenge.
Alstom Power Energy Recovery GmbH, supplier of the new heat exchanger, used experience gained on the 2010 job. It planned just four weeks for all necessary lifts, from opening up the roof of the preheater (LUVO) building until closing it again. At first glance the short crane utilisation time suggests a mobile crane application. There was, however, no access within a 120 metre radius. A big rig standing at long radius was out as there was no space for boom installation.
An initial feasibility study suggested using a 30 x 20 m courtyard between the ash silos and the oxides of nitrogen removal plant and the boiler house. A 700 tonne capacity telescopic boom crane with fly jib would be set up there, skidding components of the heat exchanger over the roof of the LUVO building before lowering them on strand jacks. This plan was ruled out by insufficient load bearing capacity of the roof structure.
The entire project to provide another decade of operation at the plant by replacing the heat exchanger was in doubt due to a lack of erection space. Shutting down the plant if the old heat exchanger broke down was also considered. Economic operation of the remaining other blocks was under threat.
Faced with such extreme consequences of not finding a technical solution, the Steag project team discussed another potential solution with local crane rental specialist Franz Bracht. Years of experience at the complex Voerde power station site and recent developments in large luffing jib climbing tower cranes led to Franz Bracht's suggested solution. It was to install in the 20 x 30 m courtyard a tower crane that could reach a free standing over the 70 m-tall boiler house to cover the 54 m high LUVO roof opening at 40 m radius.
A number of restrictions had to be overcome. The tower crane had to be installed on an extremely narrow site, using a telescopic boom crane, at minimum height and climb up to a free standing height of about 70 m to reach over the nearby boiler house. The mobile assist crane could not being rigged with any kind of jib extension to increase its under hook height.
Conveyer lines crossing through at about 26 m height left no space to lift in the required tower crane boom in one piece. The boom had to be installed in sections added from a second gap formed by an access road where another telescopic boom crane had to be placed.
The tower crane had to be a luffer to raise itself in the gap surrounded by nearby buildings between 54 and 70 m tall. The tail radius had to be limited to 11 m during the climbing process due to a conveyer line tower intruding on the erection space. All crane parts had to be delivered in the middle of the cramped plant, under conveyer and pipe lines that restricted headroom to 4 m. Narrow streets and a tight corner restricted vehicles to 20 m long and 3.50 m wide.
Erection and dismantling of the tower crane had to be fast to minimise occupation of the yard normally used by trucks to access the ash silos. The tower crane base had to fit into ground where many pipes ran for the plant, which had to be kept live as the other blocks were still in service during the renovation of West ll.
With the crane in a yard between 70 and 80 m-tall buildings, higher wind speed conditions had to be considered for a safe and fast climbing operation. The boom had to be stored in a steep, 75 degree, position during the climbing phase to avoid collision with surrounding buildings when weathervaning.
In the first planning stage it was discussed to lift in place the rotor in segments up to 46 tonnes at 40 m radius. Meeting this requirement was the Wilbert WT2405L e.tronic, a popular heavy lift luffing jib tower crane on recent new power plant projects in Europe. Franz Bracht approached Wilbert to develop a crane concept for the Voerde plant.
One WT2405L from the Wilbert rental fleet was the basis of a new version designated WT1305L e.tronic. It could handle the necessary 28 tonne load at 48 m radius and delivery was possible in three months. The machinery platform was shortened to 11 m and the A-frame height was reduced by 4 m to allow installation using a Liebherr LTM 1400 wheeled mobile crane with 60 m telescopic boom. The 400 tonner was placed inside the narrow yard together with a 70 tonne capacity LTM 1070 for assistance work during the basic crane set up.
The Wilbert had a 5 m runner added to the 48 m main boom to increase maximum radius for the roof dismantling and refitting work. Under the restricted rigging conditions this boom extension could be installed more easily than a longer main boom. In addition, the second hook offered a fast 4 tonne single line capacity for small loads to be quickly placed on the power plant roof.
The 20 m A-frame was split into head and base sections that were reconnected using a nut and bolt connection. The first two base sections of the boom had to be rigged in a single load over a conveyer line, held in place by the tower crane itself on special rigging ropes connecting the A-frame head with the boom foot. The rest of the boom was then added in the air, section by section, with the assistance of a 200 tonne capacity LTM 1200 set up in another building gap.
For safety reasons all boom sections that had to be added in the air were fitted by Wilbert with integrated additional access platforms. Reeving the luffing ropes was made easier by an onboard assist winch. A monoblock tower system without a change in dimension had to be used to climb up the whole way, free standing, to 80 m. The 3.30 x 3.30 m tower and 3.00 m wide boom made access easier on the narrow roads at the plant. In addition, only a monoblock tower system would accommodate the fast rigging time.
Installation of the base crane and climbing cage under the restricted site conditions took seven days. Another five days was needed for climbing the crane to its full height. A lack of storage space meant that all crane parts had to be delivered to the erection site just in time prior to their installation. A pre assembly area in the plant was set up where crane components were stored and prepared for rigging before being transported to the site by low loaders.
In addition to the tower crane foundation challenges, the complex underground installations also presented problems for the mobile cranes. For the tower crane ten concrete piles were driven into the ground to support a 7.30 x 11.60 m concrete foundation in which the first tower section anchors were placed. For the LTM 1400 concrete piles also had to be installed under the outrigger pad positions supported by steel beams bridging the essential cooling water pipeline of the power station plant. This foundation work had to be started well ahead of the lifting project at a stage when even the required tower crane capacity was still under discussion.
Günter Kronewitter, Wilbert sales manager, says the Wilbert design concept to use the same tower system for the whole modular design "Heavy-Lifter" crane range was recognised to be a welcome tool providing the necessary flexibility during the project development stage. All foundations will be left in the ground to enable tower crane installation up to the 2,400 tonne-metre class for possible future works. The anchors for the Wilbert TV33 tower system were covered over after the job was finished so that the space could be used again as an access road to the ash silos.
With the foundation work close to the River Rhine, the pendular counter ballast design of the Wilbert heavy lifter was be a real benefit. The maximum force generated by the 80 m free standing Wilbert 1305L e.tronic was about 2,000 kNm lower than the ground bearing force of the Wolff 900B with fixed ballast. The lifting capacity at 48 m radius, however, is 5.5 tonnes higher.
Going to plan
The lifts were carried out to the full satisfaction of the customer. Wolfgang Schulthoff, deputy head of Voerde Power Plant Steag GmbH, says that the tower crane handled the loads precisely and the whole job was finished well ahead of time. That extra time made it possible to replace two sound absorbers within the reach of the crane in the shut down time of block West II.
Heavy lift jobs using mobile cranes are usually guided by radio control. In this case, however, the crane driver had a direct a view of the load for most of the lift. In addition, out of service time due to high wind speeds was reduced. The tower crane can work at up to 20 m/sec while mobile cranes generally have to stop work at 9.8 m/sec.
As Schulthoff concludes, the choice of a heavy lift tower crane for just a dozen lifts was not a cheap solution but, with the site restrictions, it was the only one suitable. It also demonstrated benefits inherent in this crane type over large mobile cranes. Similar jobs will soon come up at power stations in Germany, under even harsher conditions. Franz Bracht already has another industrial plant renovation job in the pipeline where only the close co-operation between a custom made tower crane concept and a well engineered mobile truck crane assistance plan can overcome site restrictions.