Several tower crane manufacturers have now completed wind turbine erection trials using modified versions of their standard models. Extensive changes are often needed to the climbing crane principle to adapt the existing product and meet the specific needs of the wind power industry.
Many of these cranes are luffing jib designs. An exception is Liebherr’s 1000 EC-B 125 Litronic, a 125 tonne capacity flat top tower crane model that is a recent entrant to the market. A newcomer entering this capacity class with a new climbing crane designed for wind turbine installation is manufacturer Krøll in Denmark. The Krøll K1500L is in the same capacity class as the Liebherr but it is a luffing jib design.
The main design difference on the Krøll is the use of a steeply raised luffing jib that reduces the required overall tower height. It achieves this to the extent that a free standing version can be used instead of needing a tie-in support to the tower. This will allow erection of a typical 170 metre tall 5 MW class wind turbine. It is preferable for wind turbine manufacturers not to need a crane support facility built in to their towers unless absolutely necessary.
An alternative is the Kroll K1400-125 tonner on a standard M40 tower system. As a slightly modified saddle jib crane with the same load moment it would need 43 tower sections to reach the same under hook height as the specially designed K1500L, based on just 12 newly-designed M33 tower elements and the B33 bottom mast. It compares favourably with the Wolff 700B “custom” concept where the massive TV33 tower system had to be combined with the standard TV29 to create a free standing version.
To the tower
Krøll engineers found a solution that maintains the same outer dimensions of the crane tower from the base to the top. The 3.30 x 3.30 x 7.80 m M33 monoblock tower system is designed with road transportation dimensions in mind. It is also designed for fast climbing. As Jorn Lorentzen, head of the structural engineering department at Krøll, explains, the 7.80 m long sections are an optimum compromise between having few towers to climb and as few bolts as possible to be tightened, to speed up erection time. With two hydraulic rams integrated in the climbing frame the crane can be jacked at a rate of one tower section par hour.
A separate winch mounted on the erection beam above the climbing frame is used to lift the tower sections into place. During the climbing process the luffing jib boom balances the crane. Inserted tower sections do not have to be bolted to the crane upper before picking up the next tower section which saves time for each mast climb. As with lifting operations, climbing is possible in winds up to 15 metres per second.
Only the first four of the twelve M33 mast sections are re-inforced. They have the same outer dimension but are connected by eight screws instead of four per corner. According to Krøll this connection device with limited sized vertical bolt connections is more erection-friendly. Fewer heavy connecting devices, or slug bolts, have to be handled by riggers during the fast set up and climbing processes.
As it is not a combination with different systems the crane can directly climb from the start which means a relatively small and cheap assist crane with lower hook height is needed for installation. This under hook height is further reduced by the way the machinery deck is lifted using a mobile crane. The complete upper crane, without boom and counterweight, can be lifted by the mobile crane in one lift. Special lugs are fitted on the machinery deck and there is no need to add the high A-frame in a separate step.
For local transport on a wind farm site the A-frame can simply being folded backwards onto the machinery deck. Placing the winches inside instead on top of the platform makes this possible. In addition, the luffing jib rope can also remain reeved by just pulling back the pulley block. This further reduces crane installation time. For transport on public roads the A-frame can be removed to reduce the upper crane transport component weight from 82 to 55 tonnes. Also removing the hoisting winch further reduces the weight of the heaviest crane component to 38 tonnes.
On the smaller K760L wind turbine erection crane - a new model in fabrication at the time of writing - Krøll will place the luffing winch at the back of the A-frame. Even at standard road transport height the luffing cables can remain installed.
On the K1500L a resistor box is mounted at the rear of the side-mounted cabin platform. On remote turbine sites the crane is likely to run on power from a diesel generator so minimal power consumption is important – in this case 320 kVA. When lowering the hook energy needs to be dissipated in the resistor box.
Using luffing jib tower cranes for windmill erection eliminates time needed for crane climbing but it presents some inherent challenges in contrast to a saddle jib crane alternative. For most of the time the crane is working at steep boom angles which increases the risk of a gust of wind causing the crane to overboom. To counter this Krøll opted for a stiff A-frame with a spring-loaded buffer system touching the boom somewhat before minimum radius.
The crane’s boom has to be stored in a safe out of service position while keeping in mind the risk of collision with the rising wind turbine tower. Krøll chose a rigid square section boom with wind sails on the outer boom sections to allow the 70.1 m boom to be parked at 12.5 m radius when weathervaning.
Working at minimum radius with a raised boom increases the risk of collision with the turbine components as they are lifted. Kroll designed a boom tip section to give more freedom between hook and load. In addition, the crane can have the proven AH Industries tagline system where two additional winches mounted under the boom foot will guide sensitive loads like the windmill blades using controlled tension.
For the base a specially designed 16 x 16 m cross base was designed with height-adjustable pyramids at each corner. These cones can be adjusted to compensate for as much as a 2 % slope. Separate 150 tonne cylinders at each corner can help to level the crane base. The pyramids bear on 7 x 2.4 m load distribution mats to keep the average corner pressure to around 20 tonnes per square metre. To speed up crane replacement in a wind park, Krøll suggests using two crane bases. That way no time is wasted when dismantling the upper crane at one site and reinstalling it ready for climbing at the next.
Concrete foundation anchors may be used as an alternative to the standard cross base. The first three K760Ls will have this. Also helping to speed up installation there is a power cable reeving system at the base of the tower. It means that when the crane is jacked up or down, the power cable follows automatically.
For hoisting Krøll chose constant four fall reeving for all loads up to 125 tonnes. At that maximum the load is lifted by the 154 kW hoist winch at 6 m/min and the unloaded hook can be lowered by 22 m /min. Compared with an ordinary crawler crane of similar capacity, the Krøll needs about half the time to lift a turbine gondola to 179 m hub height. The Liebherr 1000 EC-B, with 110 kW hoisting winch works in 6 fall configuration to lift 125 tonne loads and changes over to 4 fall operation for lifts up to 100 tonnes.
To minimise the number of lifts for crane installation process Krøll suggests using a 500 tonne capacity telescopic crane as the assist crane for the K1500L. With that the upper crane is made up of the complete machinery deck, including pre-assembled winches, A-frame, slewing ring support (total 82 tonnes) and the complete 70.1 m boom (another 34 tonnes). Only the crane driver cabin platform and counterweight blocks have to be added in further individual lifts.
The first K1500L has been delivered to TotalWind. It chose the luffer, which weighs 580 tonnes, as the most economic alternative to a 1200 tonne capacity class crawler crane weighting more than three times as much in rigged condition. The K1500L passed Initial field testing at a wind turbine site in Poland. Dismantling, moving and re-erecting it on the next wind turbine site took four days, including the climbing procedure.
The three smaller Kroll K760L versions being built offer a lifting capacity of 95 tonnes to a free standing under hook height of 152 m, again based on the M33 tower system. In January 2016 these will be used to erect one of Southeast Asia’s largest wind parks - 70 units of the V110-1.8MW Vestas turbine in Thailand. Energy Solution Management Co.Ltd. ordered the cranes.
Krøll is responsible for the electric crane drive, the tower and the climbing system while sister company Favelle Favco will deliver the crane upper, including the boom.
Bigger and taller
As turbines get taller and bigger and the locations are ever more remote with greater space and site access restrictions, the wind industry is acknowledging the potential of specialized tower cranes. Transport costs for the installation of large mobile cranes above 1,000 tonnes capacity also underline the benefit of a lightweight tower crane alternative. As a crane manufacturer with a long history of developing custom-built cranes Krøll could set new standards.
Some of the features incorporated in these custom-designed wind turbine erection cranes may become standard on the general construction luffing jib models. One example is the hydraulic tool for pulling and inserting pins. Components are all less than 3.30 m wide and are designed to reduce rope reeving work during crane rigging procedure. Tower crane manufacturers willing to take the challenge and compete against mobile crane concepts for wind turbine installation will also gain experience that can be transferred to benefit the general high rise construction crane market.