Design and development of megawatt wind turbines - Wind Power Generation and Wind Turbine Design - page 235

greater than the upper sections, but the average value yields an alternative view for
the lowest possible tower mass design goal; i.e. target the average tower mass per
meter for the solid line or lower.
The turbine designer must keep a large number of additional tower design goals
in perspective when searching for the overall cost-effective solution. Some of these
considerations include:

1. Dynamics, structural, machinery vibration damping and seismic

2. Internals, electrical cables, climb systems, platforms, packaged power modules

(PPMs) or down-tower assemblies (DTAs)
Installation, erection methods, joints, fasteners and crane requirements 3.

Future large turbine tower technology will likely incorporate lightweight space-
frame construction incorporating multiple support legs that are spread apart. These
confi gurations must remain simple; yet meet all of the design goals while comply-
ing with personnel health and safety requirements. Additionally, one should not
underestimate the internals as these can signifi cantly affect overall tower design;
e.g. potential impact of a welded studs, Kt (stress concentration factor) on the
tower plate thickness specifi cation.
The use of hybrid materials and structural design confi gurations should become
more prevalent for larger turbine sizes. Better tower and foundation integration
should increase overall functional capabilities for lower total cost.

4.4.16 Structural bolted connections
Construction of today's WTs uses a large number of structural bolted assemblies
including:

1. Blade attachment

2. Hub attachment

3. Drivetrain components

4. Bedplates

5. MH and yaw bearing attachment

6. Tower sections

7. Tower to foundation attachment

Flanged joints and machine elements are bolted together with pre-loads such
that the fl ange does not separate. The fl ange friction takes all shear force and
the bolts are only in tension. In practice, small amounts of bending moment
due to fl ange machining, separation and misalignment can exist which needs
to be accounted.
To fi nish a bolted connection, the torque process is the most commonly applied
process. However, the torque process works against the frictional resistance on the
bolt or nut face and in the thread resulting in inaccuracies of up to 100%. Another
popular method is called “turn of the nut” or turn-angle. Bolts that are tensioned
using the turn-angle method are one step better than the torque process. When the
turn-angle process is used, the initial tension is provided by torque. The torque part
of the process involves friction and provides about 20 − 30% of the maximum value.