Engineering Insights
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VectorPly Corp. (Phenix City, Ala.). He does say that the composite
tower's wall thickness is comparable to that of a steel tower sized
to meet the foating platform's loads. Hettick adds that the coupled
model output also identifed frequency harmonics to avoid and
other safety factors. Tese prompted Ershigs' selection of E-glass
laminates with considerable axial reinforcement for the VolturnUS
tower and larger towers to come.
Although the 1:8-scale tower was made in one section, a mono-
lithic design isn't feasible for the proposed 280-f/86m tall tower
that will support 6-MW foating turbines, says Hettick. For it, Er-
shigs will produce "can" sections similar to those used to assemble a
steel tower. Built up from large infused curved panels arranged, ac-
cording to the patent, around a rotatable mandrel, they'll be joined
to form a cylindrical section via a proprietary winding process. Fin-
ished cans will be bolted together close to the launch site, avoiding
truck transport issues that would be raised by larger assemblies. "It's
not a radical concept, it is a proven technology," says Hettick. "Te
industry is comfortable with bolting steel sections together, and it
will work for composites."
Te 1:8-scale prototype's tower supports a 20-kW turbine with
a 30-f/9.2m diameter rotor. Broader at the base and more tapered
than a land-based tower, it is connected to its hull in a "similar man-
ner to the way land-based steel towers are connected to concrete
foundations," says Dagher.
A UMaine patent also is pending on the VolturnUS hull, which
comprises a three-legged arrangement of hollow, semisubmersed
vertical and horizontal concrete "columns" (see drawing, p. 46).
Each leg is anchored via mooring line to the seabed. Outftted with
several Webcams and more than 50 sensors that collect wave and
wind data as well as tower stress and strain readings, the prototype
already has witnessed a wide range of winter storms, says Dagher,
and has demonstrated excellent stability and small accelerations
even in turbulent seas. Hull and tower performance data have been
within 5 percent of the coupled model's predictions, he adds, which
validates the design. "Te data we've collected has allowed us to un-
derstand the platform's, and the tower's, performance in extreme
conditions, because its smaller size means that the waves are pro-
portionally larger in comparison," says Dagher. "It is a very efective
way to de-risk the full-scale platform design." Consistent with the
project's goal of producing electricity at a competitive rate, he con-
tends, "a composite tower supported by a foating concrete founda-
tion actually made the project more economically viable."
For these reasons, the much larger hulls for commercial-scale
turbines will be of similar design. Ershigs, in fact, has already fab-
ricated half-scale samples of the 6-MW turbine tower modular
sections, which have been tested extensively in the UMaine ASCE
laboratory for both ultimate strength and fatigue performance: "We
joined sections together by bolting, as they will be on the platform,
then simulated a 60-year service life," says Dagher. "Te good news
is that at the end of the fatigue test, the residual strength still ex-
ceeded the initial design strength!"
Fabrication of full-scale, commercial foating turbines is planned
to begin in 2016. And when they're installed, TIV cranes and jackup
barges won't be necessary. According to Dagher, VolturnUS-style
hulls, composite towers and turbines can be assembled dockside,
then be towed to the deepwater farm site by conventional tugboat.
"Tis is a big deal," Hettick concludes. "It's a win-win situation
for composites." |
CT |
DeepCWind partners have already tested an Ershigs-built 1:2-scale
bolted composite tower (one tower section is shown during testing).
The VolturnUS' patent-pending concrete semisubmersible hull design
can be seen here, prior to the foating turbine system's launch.
Technical Editor
Sara Black is CT's technical editor and has
served on the CT staf for 15 years.
sara@compositesworld.com
Source
|
UMaine
Source
|
UMaine
4 8
4 8
Read this article online | short.compositesworld.com/FloaterEI.
See a video of the VolturnUS launch at www.cianbro.com/NewsMedia/
TabId/244/VideoId/178/VolturnUS-Offshore-Wind-Launch-Video.aspx.
Read previous
CT editorial on offshore wind in "Wind over deep water" | CT
October 2011 (p. 20) or visit http://short.compositesworld.com/o7Cmv6P4.
For more information about NREL's open-source FAST coupled modeling
software, visit http://www.nrel.gov/wind/offshore_tools_methods.html.
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