To study wind characteristics in complex terrain, we installed three dimensional sonic
anemometers at 67 m and 45 m above ground level in Iwaya wind park of Aomori prefecture in Japan.
High resolution analysis of wind direction shows that the flow distortion arises along the undulating
topography. Absolute values of vertical wind speeds observed at lower observation point are more or less
larger than those at higher point, which is dependent on wind directions. The difference of vertical flow
between different heights would relate to increase of the wind shear and the turbulent intensity. Due to
such flow characteristics and large roughness on the ground, the turbulent intensity seems to be larger
than IEC category A, in the wide range of wind direction keeping steep slope and complex topography.
In this study, the relationship between tightening torque and axial force of bolt at the tower top flange
was investigated through a laboratory test with flange section models and a 750 kW wind turbine which had
experienced the damages of bolt at the tower top flange. Tightening torques and axial forces of bolt in these
tests were measured directly by using torque wrenches and strain gauges inserted in the bolts. Existence of
lubricant had a great influence on the axial force of bolt, while types of torque wrench had a small impact on
those. The expected axial force of bolt was obtained by torque wrenches and the variation was around 10%.
A method for quantitatively evaluating decrease in axial force of bolt was proposed by measuring rotation of
bolt. The damaged bolts in the wind turbine were also investigated and the axial forces at the damaged bolts
were completely lost, while those at the loosed bolts were more than 30% of the initial value. It was found
that the axial force of bolt decreased by about 20% after 500 hours from the exchange of bolts. This
indicates that retightening is necessary for wind turbine tower bolts.
We conducted feasibility study of detection system for unexpected lightning damage for wind
turbine blade. In this study, puncture of blade skin and disconnection of down conductor are considered as
the lightning damage. Aimed at detect these damages, three systems are examined. The first system
consists of two down conductors and current measuring devices that installed at root part of the
conductors. The first system is appropriate to detection of puncture of blade skin caused by lightning
strike of wintertime thunderbolt. The second system consists of two down conductors and optical fiber
current sensors which are installed at tip and root of each conductors. The second system has the merits of
enable to simple judgement system for lightning damage and periodic inspection (to confirm the
soundness of the down conductors). The third system is based on acceleration measurement by sensors
that installed at the blade skin. The acceleration sensors measure the elastic wave caused by lightning
strike (puncture of the blade skin). Feasibility of the third system was confirmed by model experiment.
The third system can be retrofit to an existing wind turbine blade.
L-type flanges used for wind turbine towers need high bearing resistance when they install in storm
and earthquake active areas. Petersen, Seidel, and RISO/DNV introduced methods for evaluation of
L-type flange ultimate strength based on elastoplastic theory, however they didn’t consider the elastic
deformation of flange and caused underestimation of the load acting on bolts. VDI2330, Petersen, and
Seidel also showed the location of equivalent flange reaction force by using flange thickness direction
bending moment equilibrium, but these methods are complicated. An action point correction factor of
L-type flange is proposed to consider the effect of flange deformation and to expand the applicable range
of flange dimension. The results show good agreement with those by the formula of Seidel at the ultimate
condition by using load transmitting factor for L-type flange proposed in this paper.