Bigger turbines require smarter construction

US: It was 2005 when Mortenson Construction installed its first turbine larger than 2MW, a 2.5MW Clipper, and since then a third of all turbines installed by the company have been 2MW or greater. For the last two years, almost half of all the wind projects it has built have required the larger turbines.

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One of the primary reasons that the US wind industry is shifting to the installation of larger, higher megawatt turbines is simply that they produce more energy. Wind projects can therefore operate with fewer turbines, reducing infrastructure and environmental impact, and bringing down the owner's costs. So the move to these larger machines is likely to continue.

From a construction standpoint, however, the erection of larger turbines entails specialised equipment and training, as well as, in some cases, additional scheduling and labour considerations. From the Repower 2.05MW to the Vestas 3MW, each one of the larger turbines presents its own particular challenges.

Just one example is the Mitsubishi 2.4MW turbine, which requires on-site assembly of the nacelles. There are three major components of the nacelle that must be assembled on the project site rather than in the factory. The assembly can either take place on the ground or in the air, and the process requires multiple shifts. This means more trained staff on the ground and sometimes one dedicated crane for the task.
As well as the variations in each model, the landscape can make a difference.

The issues involved when installing turbines on rocky and mountainous terrain in British Columbia are quite different from those encountered on the sandy regions of the Texas Gulf Coast. "The construction challenges of working with large turbines are primarily logistical," says Kenny Oxford, senior superintendent for Mortenson Construction.

"A well-coordinated effort is crucial for on-time project delivery. Detailed pre-project planning ensures the necessary crews, cranes and other equipment are on the ground during turbine erection to meet tight schedule demands."

The 600-tonne class

High-capacity lifting equipment is needed to erect many of the larger turbine types. Whereas 400-tonne crawler cranes are used to install turbines smaller than 2MW, many of the larger turbines require 600-tonne cranes. These cranes are heavier and some have a back mast, which adds capacity, provides a counterweight and allows the pendants to move up at a greater angle, reducing bow in the boom.
These larger cranes are, however, slower. Assembly can take longer and the presence of a back mast requires more time to manoeuvre around obstructions.

For example, the Manitowoc M18000, a 600-tonne lattice crawler crane, might require 30 semitrailer loads to bring it to site, plus more than 30 crew hours to assemble. Its 440-tonne counterpart would require fewer than 20 semitrailer loads and around 14 crew hours to assemble.  
Reinforcement or expansion of existing roads is often necessary to accommodate the heavier loads and the wider crane tracks. Crane walking paths to facilitate travel become a critical component of a project.

The complex boom configurations of many of the larger cranes are sensitive to grade changes, so it is not uncommon to mat the entire walking path for the larger cranes, even in good soil conditions, using the standard oak hardwood crane mats. This leads to planning in and paying for one or two additional forklift trucks, plus operators.
It is not just logistics and availability of the 600-tonne cranes that present new challenges for the construction team.

There is also an increase in the overall time required for turbine erection. Heavier turbine components and slower line speeds result in "picks" that can take up to three times longer. So, while a 400-tonne crane can take a 1.5MW turbine from the ground to the top of the tower in 15 minutes, it may take up to 45 minutes to lift a single component of a larger turbine. This can slow the installation process of erecting the larger turbine by two or three crew-hours, which must be accounted for at the planning stage in order to maintain schedule goals.

Another factor that must be considered at the planning stage is crane configuration, including line speed, which cannot be changed once the crane is assembled. This means that if more than one turbine type is to be used on a project, the crane must be configured appropriately for each type, or a second crane brought in. Inadequate planning for equipment needs for wind power projects can result in schedule setbacks and have a huge financial impact.

Complicating the shift towards larger turbines is the reality that there are fewer crawler cranes of the 600-tonne class available than of the 400-tonne class, and fewer crane operators who have been trained for this specialised work. As the economy improves and the number of wind power projects increases, the 600-tonne cranes could become a scarce resource.

A tailored service

The push for larger, more productive and more efficient turbines will continue to drive manufacturers and, as new turbine types are released and selected by developers, contractors will have to find the most effective process for turbine erection.

This means tailoring the process during the pre-construction stage according to the unique combination of terrain, climate, schedule and turbine types involved in the project.

With this shift to larger turbines now becoming established in the US industry, contractors will be called on to demonstrate their expertise in engineering, procurement, construction and balance of plant. They will have to ensure careful pre-planning and maintain an open dialogue throughout the construction process.   

Tim Maag is vice-president and general manager, renewable energy groups, Mortenson Construction

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