Tower cranes and wind energy
By Laura Hatton09 May 2014
According to the World Wind Energy Association (WWEA) in 2012 more wind turbines than ever before were erected with some € 60 billion invested worldwide. China and the USA were the largest investors, each country representing about 30 % of the 44,799 MW new wind power, followed by Germany adding 5 %. Growth decreased, however, in the first half of 2013 and a further decline is forecast. Turbine construction companies have to struggle with a shrinking market leading already to a decrease in wind turbine prices and putting pressure on installation costs, specially on crane and transportation expenses. Three trends are clearly visible.
Larger turbines are in favour to raise productivity. Now 2 and 3 MW units are common, with towers from 60 to 125 metres. Over the last 25 years the cost of electricity from wind turbines has dropped by more than 80 %. A major portion of the installation cost is setting the tower, nacelle and rotor. Contractors are demanding lifting equipment that does not significantly add to the installation cost despite the greater height and capacity needed.
Turbines are getting taller so that they produce power even on low-wind days, a trend seen in Germany and elsewhere. Increasing hub height from 70 to 125 m can increase turbine output by up to 59 %. Another boost, from 90 to 160 m, means 45 % more output. Experts already see 200 m hub height becoming a target within five years, which would dramatically change the required crane capacity. Steel towers above 100 m approach a cost-prohibitive limit. Hybrid towers of concrete and steel or, for example, laminated timber panels, also influences the crane choice.
Installation sites are increasingly in remote woodland and mountains where rough terrain and environmental preservation mean limited transport access and cramped erection space. Most turbines are installed using specialized truck or crawler cranes designed for maximum capacity at long boom combinations and minimum radius, with short set up and de-rigging times.
Bring on the towers
Considering the success of tower cranes in the development of the high rise construction industry it makes sense to examine how far this crane type could be a better choice for the impending turbine erection challenges. The two main tower crane types have different benefits and limits.
With conventional truck and crawler cranes in this application there is a focus on fast set up time and limited rigging space so a highly mobile tower crane with self rigging features could be the choice. Limited under hook height and capacity restricts their application to about 2.5 MW turbines on 70 to 100 m towers. The design benefit of being split into a small number of transport units means short rigging time but also a major restriction in the weight and size of the individual crane components that need to cope with the site access.
As an example, when the Grove GTK1100, forerunner of this crane breed, is moved in four truck loads, the heaviest trailer weights 140.5 tonnes. As developed in 2006 it is a tied-in 76.5 m vertical telescoping mast on an 18 x 18 m outrigger base supporting a modified upper from a 450 tonne all terrain crane. Capacity is 55 tonnes at 18 m radius and 107 m hook height.
To accommodate larger turbines the Megalift1500 extension kit for the Grove allows the top slewing superstructure to be rigged with 56 tonnes of counterweight self-erected using a special spreader installed at the rear of the tower mounted jib crane. The resulting increase to 600 tonne-metres maximum load moment raises the lifting capacity at 18 m radius to 78.5 tonnes.
Advantages of the GTK design are: fast set up time of six hours; the ability to transport the crane in 4 to 6 truck loads 3m wide and 4 m high, instead of more than 20 units for a conventional crane; and quick and safe movement on a wind farm site in folded condition. Disadvantages of the GTK design are: lack of access to the winch and engine during operation; the need for 40 minutes of calm wind conditions to extend the vertical tower before it can be tied in using the folding-bar pendants with the outrigger ends providing the stiffness to stand wind speeds of 130 km/h.
The highest rates of turbine installation are achieved in the north eastern provinces of China. For standard 1.5 to 2.5 MW turbines Chinese crane manufactures developed specially adapted lifting equipment as alternatives to 400 tonne crawler cranes and 600 tonne truck cranes. Harsh weather demands operation down to minus 35 degrees centigrade and relocation over rough terrain and access roads on steep slopes.
Zhong Sheng Construction Machinery (ZS), based in Nanjing, developed its ZSTL series of fast rigging bottom slewing jib cranes on a hydraulically driven wheeled chassis. For short distances the crane can move itself rigged, without counterweight when the outriggers are folded to the chassis, the telescoping mast is retracted and jib folded. In the direction of travel 30 % slopes can be handled by the 5.5 m wide rough terrain undercarriage. For transport by road, outside the wind farm, the diesel-hydraulic crane is split into undercarriage, telescoping mast and jib sections.
Underhook height between 90 and 110 m and 80 to 160 tonne capacity can be achieved. Similar design features are incorporated in the QLY series of hydraulic wheeled tower cranes manufactured by New Dafang Group. Specialized in shipyard transporters and rubber tyred machines for bridge segments, the company uses its experienced of hydraulic independent suspension and steering systems for the crane undercarriage. The 4.5 m wide carrier can be adjusted +- 300 mm for longitudinal slopes up to 25 % and transverse slopes up to 7 % with the retracted three section telescopic mast upright. The QLY9096 developed to install 2.5 MW wind turbines has conventional H pattern outriggers. The updated QLY9096A has its multi-steering and hydraulically driven wheels directly under the swinging X pattern outriggers. The more compact chassis allows turning on narrower access roads. At a height of 96 m capacity is 90 tonnes and, in modified configuration at 115 m, capacity is 65 tonnes.
The QLY1560, for 2 MW turbines, can be moved on site at 4 km/h between pads with its telescopic mast folded back to reduce the point of gravity to master 30 % slopes. Like the mast system the jib is made up of a three section telescopic rope-moved system. No more room than the size of the crane base is needed to install itself. A complete 100 m hub windmill installation can be realized on a 30 x 40 m spot.
From Japan a patented design study carried out by Mitsu Engineering & Shipbuilding was made for another fast rigging mobile tower crane concept for raising 100 m class turbines. A multiple section telescopic mast is carried on an all terrain crane carrier. On site the mast is completed using hydraulically operated bracings delivered in a second truck load. The mast is lifted upright close to the bottom wind turbine tower section installed by an assist crane. A bracing surrounds the foot section of the tower where remote controlled roller chain arms form a ring-shaped tie-in support adjustable for the different outside diameter of wind turbine towers. On top of the telescopic mast a jib crane, transported by the third transport unit, is installed by the assist crane. It is tied back to the mast foot by a pair of winch-operated suspension guy lines. Together with the bracings installed to the windmill tower when the crane is jacked, these suspension lines provide stability for the slim crane mast.
Economical and versatile crane concepts reach their limits as you pass the 140 m hub height point. Giant crawler cranes with 200 m hoist height available today are expensive to transport because of their oversized components. Long erection corridors are needed for their boom installation. At and above the 650 tonne crawler crane class, as exemplified by the Terex 3800 Superlift, the mobilisation and set up costs rise exponentially to the crane size. Even for this crane to lift 78 tonnes to 146 m hub height a 150 m long corridor must be provided to rig the boom. The superstructure has A-frame quick connection and winches weighing 68 tonnes, which leads to a heavy special transport requirement, sometimes causing permit problems in the remote countryside with small access roads.
Further stripping the superstructure is possible but adds to erection time on site. Bigger mobile cranes also cause higher ground pressure, which also may cause extra site preparation work adding to the crane costs. In addition, long boom combinations are more sensitive to wind conditions leading to more out of service days and the lifting capacity is decreasing substantially. Little wonder that as higher windmill towers are rising, alternative construction methods are considered. Capacity and the free standing ability of European-designed climbing tower cranes could see wind turbine erection become a new market for this crane type.
Design integrated benefits
- Tower cranes can be installed inside or close to the turbine foundation by a relatively small auxiliary crane reducing dramatically installation space requirements to less than 40 % in comparison to standard crawler and lattice boom cranes
- Compared to conventional cranes transport logistics are simplified because, in nearly all cases, standard trailers can be used and the crane components can be easy split to components under 30 tonnes
- In contrast to mobile cranes the lifting capacity is only marginally influenced by the increase in working height, even with the extra weight of the rope. Lifting height is mainly only restricted by winch capacity
- Generally tower cranes can work in wind up to 20 metres per second while mobile cranes have to stop work at least at 9 metres per second
- Compared to mobile cranes tower cranes generally achieve higher lifting speeds
- For the same hook height tower cranes show less deflection of the tower and boom system than mobile cranes for their long boom combination. Heavy loads, therefore, can be placed more easily and precisely
- Already installed foundation anchors can be reused by even smaller tower cranes for maintenance work at existing wind turbines
- Modern European tower cranes are driven by frequency regulated drives. Hence a suitable power supply by power pack must be provided in the remote location of windmill construction sites
- Where the cranes have to be installed on the windmill foundation to avoid special ground preparation work suitable foundation anchors must be integrated into the wind turbine foundation. If, in addition, due to the requested height, the tower crane has to be anchored to the windmill tower, customised solutions have to be found, which further influence the turbine tower design
- Depending on the tower crane installation solution climbing procedure during the turbine tower construction period may lead to interruptions of the construction progress
- The tower crane has to be dismantled nearly completely when being relocated to the next site making this crane type more effective for isolated wind turbines than wind farms. A high risk of accidents, however, with mobile cranes moving partly rigged from one site to the next one, means that from a safety and insurance point of view, also for mobile cranes, complete de-rigging will become more popular
- Tower cranes arrive on site split down in smaller parts than large crawler cranes. They also have to climb to reach their under hook height so overall installation time is generally one day longer than mobile cranes. Tower crane manufacturers, however, are working hard to speed up erection while mobile crane builders have to split their cranes into smaller units due to poor infrastructure in remote areas and the increasing difficulty of getting transport permits for heavy loads
Pros and cons
With respect to the two basic types of top slewing climbing tower cranes, recent trial installations of wind turbine towers in Germany made clear several design-integrated potentials benefits and challenges. First, when using a crane of saddle jib design, depending on the cabin location, the crane driver sits at eye level with the nacelle simplifying this delicate operation as far as his view is not obstructed by the massive trolley and hook block.
A very precise level lifting operation is possible thanks to the horizontal movement of the trolley. The tower of a saddle jib crane must be higher than that of the luffing crane to reach the required hub height, making the saddle jib concept more expensive, since it requires either tie-ins or stronger tower sections.
When using a saddle jib crane after the last blade is installed the rotor star has to be moved in synchronisation with the climbing down tower crane to let the counter jib pass the blades before the tower crane can turn at a greater radius when it is lowered below the nacelle. In cases of a short gondola the wind turbine is simply turned in its direction to let the tower crane climb down.
If using a luffing jib crane design, the raised jib reduces the required tower height so that cost intensive bracing of the crane at the turbine tower may be not necessary. Where crane climbing is delayed construction work can be kept going when choosing a luffing jib crane thanks to the extra lifting height provided by the raised jib. Generally, in comparison to saddle jib cranes, for luffing jib cranes the load moment can be raised dramatically when being operated at minimum radius as is requested for windmill construction.
With luffing jib cranes, however, precise horizontal load movement relies on a sophisticated steering system and the experience of the crane driver for the delicate nacelle and blade installation under the working area with no eye contact. Generally, tower crane cabins offer a relatively poor view for work above the cabin compared with mobile cranes. A camera at the end of the jib is essential.
A long jib adds to the under hook height but, at minimum radius, small outreach changes are more delicate than if using a short jib. It should be considered whether extra reeving of the luffing winch could add to the desired precise movement with limited outreach changes during windmill construction. In the author’s opinion it may be advisable to add hydraulic buffers which could move the jib outwards when strong wind pressure from the front resists lowering of the jib by the luffing winch alone.
Luffing jib cranes have a design-integrated higher corner pressure on the crane foundation. To obtain a comparable under hook height with a saddle jib crane, the luffing jib alternative generally generates 30 to 50 % more foundation pressure. Where the tower crane uses the foundation of the wind turbine the corner pressures of the tower crane are negligent compared to pressure of the wind turbine.
Luffing jib cranes have a larger minimum working radius when a long jib is used to reduce the requested tower height. Hence the crane foundation must be realised in a wider distance to the turbine tower. As a benefit this distance will allow the tower crane to free slew even after the turbine is finished adding to the safe operation in high wind regions.
If a luffing crane is placed as close as possible to the turbine tower to make use of the steep increase of load moment at small working radius, the crane boom has to be parked in a steep position and it cannot slew free with its machinery deck as soon as the windmill tower gets higher than the crane tower. A longer wind-sensitive construction phase must, therefore, be considered in comparison to the saddle jib solution where the crane can weather vane free over the tower under construction before the blades are installed. In the author’s opinion luffing jib cranes may be more suitable to be placed outside the windmill foundation on free standing towers while saddle jib cranes are to be preferred when bracing the crane tower to the wind turbine tower. The new Wolff 700 B custom luffing jib crane offers unique installation flexibility. It can be safely parked for wind speeds up to 130 km/h with its slew brakes on. Climbed down one to two elements it can be parked safely for wind speed up to 170 to 180 km/h.
Tower crane manufacturers could create very specialized climbing cranes optimising the wind turbine installation procedure. For economic reasons, however, every attempt is taken to adapting existing crane models to customer requirements, which, in turn, lead to technical compromises that have to be accepted. First, Liebherr upgraded its 630 EC-H 40 Litronic standard tower crane in co-operation with Max Bögl. It was a customised 630 EC-H 70 Litronic heavy lift crane suitable for a 70 tonne hook load at 171 m under hook height to raise 3 MW turbines at 140 m hub height.
In this case the requested 151 m under hook height only required one tie-in support to anchor the crane at the turbine’s hybrid tower. Up to 98.1 m under hook height the crane is operated free standing on a combination of 3.40 x 3.4 m 1000 HC and 2.45 x 2.45 m 500 HC tower system. In addition to the special tie-in tower section, which diminished the need for an anchor frame, many further measures were taken to speed up crane installation.
On the other hand the customer Max Bögl, who operates already five units, liked to keep the option to transfer the cranes into a standard tower crane version if requested. To make use of the standard 110 kW drive unit a special trolley was designed with one hook block allowing a 6 line fall operation. In contrast to a conventional double trolley arrangement, limited working radius can be reduced and the hook block is small enough to be lowered into the turbine tower opening. The 30.9 m short jib had to be reinforced for the higher lifting capacity but it could be easy changed for the standard jib version were the cranes to be used some day for conventional construction work.
In the way that wind turbines are rapidly getting bigger and bigger, the lifting capacity of tower cranes must also grow and installation needs to get even quicker to compete with development in the mobile crane sector. A year ago Liebherr-Werk Biberach, therefore, presented its 1000 EC-B 125 Litronic suitable for 5 MW class wind turbines. To reduce engineering and manufacturing expense as much as possible, components are identical to the as well new 1000 EC-H 25/50 Litronic. The requested increase in lifting capacity from 50 to 125 tonnes, however, and the fast rigging devices led to a rigid flat top design with a compact counter jib and a 31.5 to 36.5 m short jib as well as a new fast climbing system for the 1000HC tower system.
In response to the new EN 14439 safety requirements during climbing, a sophisticated electronic control permits crane movements outside those necessary for the climbing operation. For each step of the climbing process the load moment of the crane is reduced individually. Up to 17 m/s a safe climbing operation is possible. For the first time Liebherr uses one long hydraulic ram to jack the upper crane for a complete 5.8 m long tower section in one stroke. When the maximum free standing capacity of 108.30 m is reached, the crane has only to be braced once to reach 175 m under hook height.
Again for tie in support, a special tie-in tower section without time-consuming collar installation is used, with just two struts, in contrast to the normal three, reducing the installation time by about 75 %. In contrast to the 630 EC-H 70 the hook block allows a change between 6 fall and 4 fall operation within 5 minutes by means of two pins. The crane components are designed to allow basic crane installation with a 200 tonne capacity telescopic boom crane. Jib parts can be transported inside tower sections to reduce the number of truckloads for transport.
Including climbing to 160 m under hook height, crane installation is claimed to take about 40 hours. On customer request Liebherr has built a flexible stationary 18 x 18 m undercarriage that allows different angles for the outriggers, giving a 15 to 20 m wide crane base. With this special undercarriage it is, for example, possible to place two outriggers on the circular foundation wall of a wind turbine tower while the other two legs are placed outside the turbine foundation on 6 x 2.5 m outrigger pads. The 1000 EC-B 125 can reach a free standing height of 108 m supported by the new undercarriage as well as on foundation frame.
A luffer first
Wilbert was the first crane company to perform a test turbine installation with a luffing jib climbing crane in 2012. To raise an Enercon E-101 with 133 m hub height in an ecologically sensitive forest area, a standard version of the heavy lift WT2405L model was rigged free standing on a stationary portal mounted with 91.89 m tower and 60 m jib. With a lifting capacity of 96 tonnes, even the 87 tonne heavy base concrete ring of the wind turbine could be handled easily by the tower crane alone.
The crane was based outside the windmill foundation on the standard 12 x 12 m Wilbert portal used for power station projects. Its rigging took a complete day. Hence, for future projects, a new multifunctional 17 x 17 m cross type undercarriage where four massive outriggers are simply connected by hydraulic pins to a central adapter was designed to save time.
At the corners containerised crane mats distribute the corner forces to the soil. In feasibility studies Wilbert’s engineering department combined this new crane base with a combination of the 6 x 6 m Wilbert tower system and the 3.30 x 3.30 m monoblock tower system to obtain a lifting capacity of 128 tonnes at 192.2m free standing under hook height of the WT2405L. With respect to the long assembly process for the 6.60 x 6.60 m tower system, which has to be split for transport, on the outrigger ends of the crane base anchoring devices are intended to be installed for a strand jack guided tower system, similar to the well known guying arrangement for cooling tower cranes. Therewith the free standing capacity of the transport-friendly 3.30 x 3.30 m monoblock tower system could be raised significantly.
For its first windmill installation project Wolffkran, together with customer Max Bögl, chose an anchored solution in the windmill foundation realised by a custom-built base section. In addition, in contrast to Wilbert, the upper crane has been extensively modified. Based on the transport- and erection-friendly 700B luffing jib crane series, Wolffkran reduced the counter jib radius from 8.9 m of the standard version to 7.7 m by using steel ballast instead of concrete.
Based on the 900B series, instead of the standard triangular lightweight jib, a stronger 50 m jib was attached to the machinery deck, allowing 80 tonnes to be lifted at 10 m radius in 4 fall operation and 95 tonnes on 5 falls, instead of the 50 tonne maximum provided by the standard 700B version. Even the 88 tonne base section of the turbine tower can be placed by the tower crane alone. A combination of the 3.30 x 3.30 m base tower section with the new transport optimised Wolff TV29 2.90 x 2.90 m tower system rises free standing under hook height to 162 m. That is suitable for installing wind turbines with 143 m hubs.
For turbines up to 200 m hub height the TV29 tower system would require just two tie-in supports. Using this tower system the crane could climb directly from ground level after being assembled by a 250 tonne telescopic boom crane. The compact Wolff 700B custom represents an economical way to benefit from the extra lifting height of the luffing jib principle combined with the Wolff typical extraordinary transport- and rigging-friendly crane design. For lifting capacities above 120 tonnes Wolff is already considering a custom design upgrade of its standard 1250B, again with a square heavy duty boom in contrast to the standard triangular light weight one.
Taking the challenge to speed up crane erection and to dramatically raise the maximum capacity to provide a custom modification of the standard series tower crane for wind turbine construction can open up new markets outside the wind energy industry as well. Liebherr, for example, said it could imagine the 1000 EC-B 125 with 160 kW hoisting winch being well adapted for heavy industrial projects, power plant construction sites and bridge pylon construction. Wolff is already bidding for high rise steel sites which traditionally were served by the powerful Favelle Favco luffers. In addition, other features like the increased free standing capacity and fast climbing systems could become welcome features of the standard climbing crane series.
If wind turbines are getting even higher it should be considered for the crane to climb the wind turbine tower reducing notably the size and number of crane parts that have to be moved in. Of course, such a concept is relying on a close relationship in basic engineering studies between the crane builder and wind turbine manufacturer.
In the USA Barnhart proposed a lifting frame carrying a double boom luffing jib crane with can be installed around the windmill tower base section by a small truck crane. As the wind turbine construction progresses the special climbing crane is moved upwards by four strand jacks. To reduce side loading of the windmill tower a movable counterweight arrangement has to be operated in correspondence with the jib movement.