Wind turbines grow larger and more powerful year by year, and the increasing dimensions along with the harsh environments where the turbines are installed create engineering challenges for large flange connections that join wind turbine towers to their foundations. To prevent bolts loosening – an issue that incurs significant repair costs and downtime – the connecting flanges must be flat to within millimeters.
Capable of two-millimeter accuracy, the “Goliath” milling tool to be launched this fall by Danish machining tool expert CNC Onsite, can mill flanges of tower bases, monopiles, and transition pieces up to 10 meters in diameter.
A critical mechanical joint within the wind turbine structure, large flanges are technically complex to mount reliably, requiring pairs of matching surfaces fixed in place with bolts. Traditionally, after the manufacturer has welded the flange into the structure – a process that can warp the flange – the surfaces have been corrected by hand using heat treatment to produce similar skewness. Goliath achieves fine tolerances as part of the production process post welding by milling, grinding, and grooving the flanges to create faces with the required structural fit.
Precision flange milling
“We constantly develop our manufacturing to ensure we can deliver the right products with the right tolerances that the industry requests. With larger flanges, integrating Goliath into our manufacturing process will allow us to continue to do just that,” said Klaus Munck Ramussen, senior vice president for Bladt Industries, a steel contractor specializing in large-scale steel structures. “We are proud to support the development of the industry and, by securing the tolerances, we contribute to securing the integrity of these large structures and thereby the production of green power for many years,” he added.
With offshore requirements guiding the expansion of turbine power and sizes, the wind energy sector is integrating suppliers’ technology into their manufacturing processes. CNC Onsite is a longtime supplier of foundations and transition pieces, and its recent work is illustrating this expansion trend.
CNC Onsite applied its experience operating flange-milling machines to Goliath, making it stiffer, stronger and more precise than the previous machines.
“Obtaining a global flatness of a couple of millimeters on a four to five meter diameter flange can be challenging enough, but obtaining the same result on today’s eight-plus meter flanges is simply not possible with previous methods,” explained Søren Kellenberger, CNC Onsite sales director.
“Once it is up and running in the autumn, we would not be surprised if we can obtain even better tolerances,” said Kellenberger.
Flatness and fatigue can affect bolts
Achieving the best possible fit between the wind turbine tower flange and its base during the manufacturing reduces the requirement for routine retightening and associated downtime, and potentially leading to longer service life for the wind turbine.
“Because incorrect bolt tension is so problematic, the industry has long aspired to maintenance-free bolted connections that require two criteria: sufficiently flat flanges and correctly tightened bolts,” explains Kellenberger explained. “Today, reliable bolt-tightening methods are available to manufacturers. If the flanges are skewed, however, the bolts can still fatigue, even break, potentially leading to a complete collapse of the turbine,” he added.
With Goliath, a solution that delivers precise flange flatness is available now, and together with precise bolt tensioning it will help to secure the integrity of the structures and therefore reduce maintenance costs.
An offshore wind turbine standing idle can cost more than €8,000 euros / day, with transport and labor costs adding to the total. Maintenance over the lifetime of a wind turbine can represent up to a quarter of all costs.
Flange-milling solutions for all turbine towers
In addition to Goliath, CNC Onsite offers a range of in-house flange-facing tools that cover diameters from 1.8 to 10 meters.
“Working automatically using pre-programmed parameters. Goliath, which can work both horizontally and vertically, can cope with features on large flanges such as double-tilted flanges and requirements for parallelism of the flange surfaces,” Kellenberger said.
Designed to work quickly without compromising the high-precision work, the tool carries out some steps simultaneously to save time.
“The Goliath machine can manage the many new complex flange designs, making it probably the most advanced and precise flange milling tool on the market,” Kellenberger added.