Topic: Application cranes to solve challenges in construction
Bridge building project
Newer and bigger offshore wind turbine construction sites will place unheard-of demands on turbine construction. The new generation of crane around-the leg (CAL) is targeting a broad mix of offshore and onshore construction firms, shipyards and shipping firms—businesses from differing arenas going into offshore wind turbine installation.
Contractors will need specialized turbine installation cranes with lifting capacities over 300t, to install the turbines’ monopile foundations, and long slender booms for lifting 200t nacelles 100m above sea level, which is twice the height of average offshore crane booms designed for oil drilling. There are only a handful of dedicated turbine installation vessels. Many of these are jack-up vessels.
Jack-up vessels originate in offshore drilling. They motor to the site, lower legs to the seabed to enhance platform stability, and then raise the long boom stowed on deck. Because the jack-up’s legs touch the ground construction on a jack-up is more comparable to being on land, as it makes the ship independent of the waves so the crane operators only have to worry about the wind. Despite these advantages, manufacturers have sought to eliminate the need to stow the crane boom on the deck instead of cargo, by moving the crane from the deck to around the leg of the vessel, creating the CAL.
Positioning the crane on the leg frees up valuable deck space. Its advantage over a stand-alone-crane is that it rotates around one leg of a jack-up vessel, and that due to this position only relatively little deck space is covered by the crane. It’s the most efficient to have one ship which is capable to take the parts from the shore and install them right away instead of having separate ships, one which is doing the transport and one doing the installation. The calculation in the end for fewer ships is more economical.
Also contractors want to use CAL vessels to increase the speed of construction, seeing that downtime due to adverse weather brings heavy costs. This jack-up system gives the possibility to work with a broader weather window, because if vessel in a jacked-up condition, it doesn’t matter how high the waves are.
The point of overturning stability of the jack-up during crane operations is one of the seriously considered design criteria. If the leg(s) are overloaded this could result in a “punch-through” effect and severe listing of the platform, because the support of one leg is failing.
Neck-to-neck competition has pushed firms to come up with innovative features to simplify what is currently a hazardous operation in the budding industry of wind turbine installation. The features designed have included split booms and boom parking systems, intended to capitalise on the deck-space saving format and allow vessels to stow more turbines. Liebherr’s contribution with the CAL 64000 – 1500 Litronic is the optional twin boom which may provide a more space efficient way of parking the boom. Typical offshore cranes usually take up nearly half the deck, thereby preventing large turbine loads. The twin boom design had been especially developed for cranes on jack-up vessels where users often claimed the ‘problem’ of having to park the boom laterally over the deck. Instead of a single lattice boom of a truss work that can only be stowed by folding diagonally to one or the other side of the leg longitudinally opposite of it, the twin boom is divided into two smaller booms, linked by massive steel ropes, that sit on either side of the lateral leg. A split boom could be fitted over the forward leg but this operation can’t be done with the legs fully retracted in floating condition. Thus the crane can’t be used during harbour conditions.
These bulky booms are not only difficult to stow, but difficult to maneuver during parking. With Neuenfelder Maschinenfabrik’s OC900030 crane, it has innovated a unique boom parking system that compensates for human error by automatically folding the crane the last meter. Imagine if you are parking the crane and you see the tip of the boom 100m from a little hold in the boom, you’ll have problems hitting it. You can cause damage if you do something wrong, or you may need someone to assist you. If you do it automatically you don’t have to worry about it.
In the past some contractors were wary of buying dedicated turbine installation vessels, forseeing stagnation in wind turbine construction, but these CAL cranes are designed to be multi-purpose. As a back-up use of these vessels, in the offshore oil and gas market there will be a need for installation and decommissioning work. Especially in the Southern part of the North Sea, a large number of jacket type production platforms have been installed, that will need to be decommissioned at some time.
Bridge building projects
The number of scale of bridge building projects has been on the rise in recent years. Since the Brooklyn Bridge became the first bridge to use a steel suspension cable design, suspension bridges have been the main technology in bridge building architecture. The increasing size of prefabricated elements used in the construction of suspension and cable-stayed bridges has led to a demand for bigger cranes with greater lifting capacity, on projects already challenging enough due to the very nature of their construction.
Most suspension bridges are made of two main supporting piles. The suspension cables are joined in the middle of the bridge. The span of the bridge generates different heights of the bridge piles so as the span gets bigger, the piles grow proportionally. It's now becoming possible to build bridges with longer spans than ever, so consequently the lifting requirements of cranes are greater.
Prefabricated bridge elements and systems offer significant advantages in terms of construction time, safety, environmental impact and cost. However while prefabrication is solution to one problem, it throws up a whole new challenge for crane operators and contractors, particularly on projects where weather conditions may only allow brief periods of construction.
The tendering process for these kinds of projects is lengthy, costly and can cause construction time to be squeezed. And high performing lifting equipment can help keep construction times to a minimum. For the most recent Potain bridge projects, the cranes were designed to withstand wind speeds up to 250km/h. This has a strong impact on tower crane mast structure, the number of anchorages to the bridge and the climbing sequences. In some cases we had to consider the "vortex" effects of the tower crane in the wind shadow of the bridge pile, due to turbulent storm wind. It's critical to evaluate these types of risks and design the cranes accordingly. Only major tower crane manufacturers are able to do this.
The recently completed bridge to Russky Island is now the world's longest cable-stayed bridge and commanded the use of the world's biggest cranes. This project was realized with two hammerhead cranes, the MD1100 and MDT368. During the build, cranes lifted a variety of equipment and construction materials. Prefabricated elements on this project weighed up to 30t. Among the most challenging lifts undertaken by the MD 1100 was placing metallic blocks to connect cable stays to the top of the pylon. Each 22t block was lifted more than 325m at a radius of 36m. The MD1100 is a heavy lift tower crane that can pick up loads of up to 19t out to a radius as far as 60m. Both cranes were climbed to the top of the bridge pile. The positioning of the cranes on this type of bridges was of capital importance, as the dismantling of the cranes at the end of the job site was strongly restricted by the stay cables of the finished bridge. For this particular project the MD1100 was positioned in a way that it was able to dismantle the MDT368 at the end of the project.
The issue with the construction of pylons is height and designing hoist rope winches which can carry the sufficient amount of hoist rope for the height. For example, Denmark Kroll have made cranes for buildings of 800m in height and designed winch with big wire rope capacity for many years.
As the cranes reach a maximum final height of 149m, the hoist speed is very important. For this kind of project, manufacturers have optional mechanisms with a maximum hoist speed of 228 m/min so the crane can work as fast as possible. And for projects where the cranes have to work at even higher heights, manufacturers offer mechanisms with drums with capacity for more hoisting rope up to 1280m, so the crane could work at a height of 320m with double reeving. Without such drum capacity, the crane could not work this high, or could only work with simple reeving, which means that could only lift half of its maximum capacity.
There is a visible trend to use high performance tower cranes for big infrastructural projects. The erection and dismantling time for a tower cranes is similar to lattice boom crawler cranes, but the ground preparation is much easier for a tower crane. Another growing interest to use tower cranes is for wind mill erection and especially in remote, difficult to access, high wind areas. To catch higher wind speeds, the wind mills itself and the generators get bigger and heavier. For those reasons, plus in general terms, expected that the number of high performance cranes in the range above 600m should double in the next five years.