Fatigue Loads (Forces)
Because a wind turbine will be subjected to varying wind speeds, and therefore fluctuating forces it needs to be able to withstand the varying load. This is especially the case where a turbine is situated in an area with a turbulent wind climate.
The components of a turbine will be subjected to repeated bending, particularly the rotor blades. This flexing and relaxing will eventually cause cracks to appear in the component which can ultimately lead to the component breaking. As an example of this is, the huge Growian machine (a German turbine with a 100 m rotor diameter) had to be taken out of service due to stress fatigue of the rotor blades after being in operation for less than three weeks. Turbine manufacturers are well aware of metal fatigue, as it is a well known problem which occurs in many different industries. Because of this, metal is not generally used for rotor blades, as there are other, more suitable materials which withstand the repeated flexing.
During the design of a new wind turbine, care is taken to accurately predict the stresses on all of the different components. How much they will vibrate, bend or stretch, both individually, and as a whole, is calculated in advance. This is structural dynamics, a specialist field which calculates the forces which act on a structure such as a wind turbine. Engineers have developed mathematical models to determine the behaviour of a structure under dynamic forces and use computers that can analyse the behaviour of all of the components in wind turbine before it is even built.
Using these models means that wind turbine manufacturers can design their machines to operate safely in even extreme wind conditions, and determine the expected life-span of a turbine by accurately calculating the amount of stress it can withstand before it needs to be replaced.
Fatigue conditions at a particular site are normally characterised by a turbulence value calculated from the standard deviation of the wind speed. Cup anemometers are well-proven in their ability to characterise turbulence at a site but LIDAR measurement of turbulence is still in its infancy.
Structural Dynamics: An Example
If a wind turbine tower is 50 metres tall it will, for example, have an innate tendency to bend backwards and forwards every three seconds. This frequency of oscillation is known as the eigenfrequency (eigen is from the German for innate) of the tower. The eigenfrequency is dictated by the height and diameter of the tower, the thickness to which its walls are built, the specific type and arrangement of steel and the weight placed upon it by the nacelle and rotor blade assembly.
Every time one of the rotor blades passes in front of the tower it enters the 'wind shade' of the tower which means that the winds force on the rotor is slightly reduced for a fraction of a second. During this moment the rotor will exert slightly less force against the tower. If this coincides with the eigenfrequency of the tower, in other words the rotor turns with a speed that means that a rotor blade passes in front of the tower each time the tower is flexed to an extreme position, then the reduced force from the rotor blade can either reduce or increase the swaying of the tower. This can have a dramatic effect on the life expectancy of the tower.
Because the rotor blades themselves are also made of a slightly flexible material, they will have a tendency to vibrate. If this is once per second, for example, then this vibration, coupled with the frequency of oscillation of the tower and the reduced load as the rotor passes the tower, can all coincide and cause significant damage which quickly puts the tower outside of safe working conditions. This is why it is very important to calculate the eigenfreqencies of each and every component so as to design a turbine which is safe and does not oscillate out of control.