Onshore towers can have section diameters as
big as 1.5 to 5 meters ( 5 to 16. 4 feet) and thicknesses from 15 to 60 mm (0.59 to 2. 35 in.). The section
weights can reach 100 tons. Additionally, some of
these tower sections require conical bends, about a
4- to 6-degree angle differentiation from the bottom
of the section.
Tolonen said that even onshore towers are evolving in size. Some midsized towers are built with increasingly taller sections to minimize the number of
sections, which obviously increases the weights of
those individual tower components.
So how does this compare to the offshore towers?
Diameters for those wind tower sections are typically 3 to 7. 5 m ( 9. 8 to 24. 6 ft.), and the wall thicknesses
of the plate used to roll those “cans” are from 18 to
80 mm (0.70 to 3. 15 in.). The sections can weigh up
to 300 tons.
Another unique aspect of offshore towers is that
they have to be anchored in some way to the sea
floor. They require a base on which to sit, unlike
their onshore counterparts, and that foundation
must be able to withstand the fury of the sea. Therefore, the foundation has to be wider than the tower
sections that sit atop it.
The most common type of foundation is the
monopile, according to Tolonen. These are cylindrical in nature, like the tower sections, but larger
and thicker. A typical monopile section has a diameter between 5 and 8 m ( 16. 33 and 26. 25 ft.) and a
thickness between 40 and 140 mm (1.58 and 5. 51
in.). These sections can reach a weight of up to 800
tons. With the latest designs, the join between the
monopile and the transition pieces is conical and
the fabrication process, comprised of several conical
sections welded together, has become even more
To show that this type of fabrication was possible,
PEMA demonstrated an assembly line for helping
to align and fit up cylindrical and conical shells
into tower sections or foundations (see Figure 3).
PEMA personnel were able to fit up and tack weld
two shells in less than 30 minutes. Also, as part of its
presentation to fabricators, PEMA discussed its latest special machinery designed to bevel, weld, and
fit the flanges in large-diameter cylindrical sections.
Tolonen added that by 2020 extra-extra-large
monopiles might be required for the latest generation of wind turbines, which could be rated for as
much as 10 megawatts of power. These sections
conceivably could have a diameter of 12 m ( 39. 33
ft.) and weigh up to 1,800 tons each.
The other predominant type of foundation is a
jacket-style base, which the five wind turbines in
the Block Island Wind Farm sit on. These jackets
comprise support legs that are a lattice of pipe.
This interlaced structure is fabricated from several
thermal-cut pipes that are then welded together to
create the supporting structure.
U.S. fabricators are much more familiar with this
type of offshore foundation as it has been frequently
used in oil and gas applications. Even in this type of
fabrication, however, evolution is putting a strain on
fabricators’ capabilities. Tolonen said that pipe diameters are getting smaller, compared to monopile
foundations, yet wall thicknesses are increasing.
As far as the future, designers are looking at floating concepts or hybrid designs, where a tower might
not only have a wind turbine, but also a base that
would be able to convert wave activity to energy.
One fabricator mentioned a base design for shallower waters in which the foundation is a large,
wide shape, much like a bathtub, which is filled and
heavy enough to keep the tower erect.
Actually Fabricating the Sections
The task of producing these large tower sections
sounds daunting, but these fabricating shops have
not been without some technological help over the
years. For instance, DAVI introduced its first tower
rolling system in the late 1990s. Today the company
has more than 250 systems in operation around the
world, according to company officials.
DAVI used the seminar to unveil its latest development related to wind tower fabrication, an automated line with a patent-pending plate feeding
system that is designed to simplify the forming of
cones. The machine’s control software automatically calculates the movements and the position of the
system’s hydraulic guides, which allows the plate
to remain in the perfect geometric position as it is
being fed into the rollers. The precision feed also
ensures the correct rotation during the continuous
cone forming process.
Stefano Santoni, DAVI’s Wind Energy Business
Unit manager, said that the software is designed to
be user-friendly and calculates the guide positions
for each different cone dimension. This can be very
helpful for operators without a lot of plate rolling
“This unique and innovative plate feeding system
ensures that a consistent but limited specific pressure is applied on the plate to generate a continuous rotation for cone forming. At the same time, it
preserves the integrity of the plate bevel while the
high dynamic forces of the rolling process occur,”
“This system can help to meet 100 percent of the
tolerances required in conical rolls,” he added. The
system is designed to deliver conical angles up to 8
PEMA Welding Automation officials demonstrate how automated material handling equipment can be beneficial for
fitting up extremely large cylindrical and conical shells, such as those found in wind tower sections and foundations.
As far as the future,
designers are looking at
floating concepts or
hybrid designs, where
a tower might not only
have a wind turbine,
but also a base that
would be able to convert
wave activity to energy.