Analysing the lifetime of a wind turbine – operation past design life

Introduction

The lifetime of a wind turbine is minimum 20 years according to IEC 61400. However, differences between the design loads and the actual loads on-site can lead to the possibility to operate the Wind Energy Converter (WEC) longer than the design life. Using an aero elastic simulation the individual overall lifetime can be calculated per main component.

Approach

Each WEC has an individual lifetime which is affected by the on-site wind conditions. Using an analytical approach, the lifetime of each WEC main component can be calculated. Consequently, the weak points of a WEC can be determined. Knowing the overall lifetime serves as a basis for organisational and financial decisions.

Main body of abstract

The individual WEC type is modelled using aero elastic software. Afterwards, the model is exposed to the site-specific loads to calculate fatigue load spectra. The site-specific loads result from the wind conditions and include the average wind speed and the turbulences.
The fatigue load spectra of the design process are calculated. Next, the fatigue load spectra of the site, taking into account ambient turbulences and surrounding wind turbines, are calculated. For both configurations the fatigue load spectra are determined in cross sections for the blade hub, centre of the hub, tower head and foundation respectively and compared with each other.
From the fatigue load spectra the damage equivalent loads are derived and the resulting overall lifetime of the cross sections are calculated. The values for the overall lifetime are given per main component.
The components of wind turbines are made of various materials. Therefore, the analysis is carried out irrespective of the material. The specific resistance against fatigue of each material is characterized by the slope of the S-N curve.
Different settings for the calculations are imaginable. The simulations can be carried out during the planning process of a wind farm if an advanced planning reliability is needed. Moreover, the analysis helps to estimate financial and structural risks when the conditions on-site change e.g. in the case of new WECs built in the direct vicinity of existing WECs. The new wind turbines influence the turbulence on-site and cause a wake effect. This might lead to lower energy yields of the existing wind turbines and even influence the structural safety in a negative way. The analysis offers a direct comparison between the two settings. In addition, the results of the simulation can be used to give advice on how to operate a WEC more safely and adjust it to the individual on-site conditions.
Also, the individual maintenance plan can be adjusted according to the lifetime of the main components and repairs of critical components can be planned in a long-term schedule. In the regular inspections it is possible to check the weak points more closely and notice anomalies at an early stage. Knowing the weak points of a WEC the operation mode can be adjusted, thus reducing the loads and resulting in a longer lifetime of the wind turbine.


Conclusion

In summary, each WEC got its individual lifetime which can be analysed based on the on-site wind conditions. Thus, the analysis enables operators, project developers and investors to plan individually and with a high reliability.


Learning objectives
So far, more than 30 projects show that the simulated WECs can be operated past their design lives. The results lie between 23 and 39 years of overall lifetime for a design life of 20 years each. In most cases a retrofit or renewal of the weakest component can easily be carried out after a few years of operation. That way it even prolongs the overall lifetime for several years. Various scenarios for the analysis are imaginable and lead to a base for financial and organisational decisions.