reinforcement from the cable stays allows
the use of much larger turbines with
twice the generating capacity of Sway’s
competitors.
In normal operation, with the wind
putting a 50 tonne load onto the turbine,
the tower leans at an angle of about 8° to
the vertical. In this position, as Forland
says, the turbine blades are vertical,
which allows them to capture more of the
wind’s power and operate efficiently.
‘Big gusts and waves push the mast
away from its optimal position but we’re
only talking about 3° maximum,’ he said.
The tower is designed to withstand waves
more than 30m high.
Because the cable stays must always
be directly opposite the turbine blades
there can be no twist in the mast-turbine
assembly, unlike with conventional wind
turbines where the turbine rotates on top
of the mast to face into the wind. Instead,
the Sway turbine has a clutch swivel
rotor at the junction of mast and
tension leg, and two universal joints at
either end of the tension leg.
‘We can also force the blades to
feather, and with these two
mechanisms we can turn the rotor into
the wind with virtually no torsion at all
in the construction,’ said Forland. ‘This
is pretty standard technology from the
North Sea offshore oil industry.’
One disadvantage of downwind
turbines is that the blades rotate in the
wind shadow cast by the mast. Every
‘Onshore, you have a noise problem with
downwind turbines but that’s not an issue
20 miles out to sea’ Michal Forland, Sway
time a blade passes the mast, the wind
force on it drops. This sets up a
vibration in the blade that can lead to
fatigue and eventual cracking. To avoid
this, Sway has designed an aerody-
namic housing for the mast to reduce
pressure drop and prevent vibration.
‘Onshore, you also have a noise
problem with downwind turbines —
you hear a fluttering sound every time
a blade passes a mast,’ said Forland.
‘But that’s not an issue 20 miles out
to sea.’
And the downwind design should
not cause problems with the turbine
itself, he added. ‘Our likely turbine
the EnGIneeR 28 JULY–17 AUGUST 2008 23
supplier has done a feasibility study
and found no problems. The blades
have to face the opposite way and some
small changes are needed to the major
gear hub but the supplier classes it as a
standard turbine. It’s a big relief to us
that only small alterations to the
turbine are needed.’
Sway’s £14.8m research and
development effort made extensive
use of computer simulation, said
Forland. ‘We have an integrated
simulation system which couples
aerodynamics and hydrodynamics and
this helped us take a lot of weight out of
the construction.’
Sway is carrying out detail
engineering to enable it to build a
full-scale prototype that will be
deployed off the Norwegian coast in
2010, in full operational depth of water.
This puts it about a year behind
the schedule of its competitors
(one of which, StatoilHydro, is one of
the major investors in Sway) but
Forland is hopeful the greater
production capacity of the Sway design
will make it an attractive option.
See also Careers News, page 44
INSIGHT
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