Page 486 - Wind Energy Handbook

460 COMPONENT DESIGN

rotor thrust fluctuations above rated and the reduced extreme loading at standstill.
Fatigue is also more likely to be critical at low wind-speed sites, because the
percentage reduction in extreme loads is less than the percentage reduction in
fatigue equivalent load.

Tuning of tower natural frequency

Considerable scope exists, at least in theory, for adjusting the tower natural
frequency to a suitable value by varying the base diameter, while maintaining the
necessary strength against extreme and fatigue loading. The effect on natural
frequency of varying tower base diameter by a factor of two, for a case where
extreme loading governs, is illustrated for a 60 m diameter stall-regulated machine
at 50 m hub-height in Figure 7.40. The frequency increases from 0.517 Hz for a
2.5 m base diameter to 0.765 Hz for a 5.0 m diameter. Now the rotational speed of a
60 m diameter turbine to yield a 60 m/s tip speed is about 19 r.p.m. If we assume
that the machine is two speed, with a lower rotational speed of 19 3 2=3 ¼
12:67 r:p:m:, then the lower blade passing frequency will be 0.633 Hz, right in the
middle of the available tower natural frequency range. Adopting a þ15%/ 15%
frequency exclusion zone, the tower natural frequency is required to be less than
0.538 Hz or more than 0.728 Hz. However, a frequency of 0.728 Hz would require a
diameter of about 4.7 m (without making the tower wall thicker than necessary for
the strength requirement), which is likely to be ruled out by transport considera-
tions. Thus the only strength limited design option is one with a base diameter of
2.75 m, with a weight penalty of about 10 T compared with the 60 T optimum
design, giving a natural frequency of about 0.535 Hz. Alternatively a 4 m base
diameter could be chosen and the wall thickness increased by 37% to 27.5 mm to
give a frequency of about 0.728 Hz. However, the weight penalty in this case is over
15 T.
The above case study illustrates the fact that it is not always economic to satisfy
the natural frequency requirements for a particular combination of turbine and hub
height. In these circumstances it may well be preferable to change the hub height.
For example, a hub height of 55 m would work much better for the case described,
with a tower base diameter of 3.5 m yielding a natural frequency of 0.535 Hz and a
tower weight of 74 T.

Joints between tower sections

Towers are normally fabricated in several sections for transport reasons, so joints
are required. Welding on site is an expensive operation, so bolted joints are almost
always used, although sleeved joints, in which each tapered tower section is
threaded over the one beneath and forced into place by jacking, have been used
successfully.
The structurally most effective joint is made with friction grip bolted splice plates
oriented vertically and sandwiching the walls of the abutting tower sections be-
tween them. Provided the grip force is adequate, the joint will not slip even under