To establish a general accepted procedure how to obtain and combine all six internal blade loads in order to determine fatigue stress including design safety factors for life time predictions.
In order to calculate fatigue stresses for wind turbines, a set of preliminary calculations were first performed. These included calculations of:
aerodynamic power coefficient;
rotor shaft torque.
Some discrepancies were found between the resuls calculated by different partners. This is probably due to the fact that different models are being used.
A series of load calculations, specifically of the cumulative flat and egdewise bending moment and the rotor shaft torque spectra for the design loading conditions, were performed. Again there were discrepancies in the results possibly due to differences in wind turbulence modelling.
The fatigue stress factor (FSF) was then determined for 3 chosen crosssections:
rotor blade inner cross section (material steel);
rotor blade outer cross section (material glass fibre reinforced plastic);
rotor shaft at first bearing.
The load cases normal (composed of operation, start, stop and idling) combined with 2 fault conditions (yaw and pitch) over the design pipe of the wind turbine were considered.
As a preliminary conclusion it may be said that the spread in the FSF for the inner cross section is too large and reasonable for the other locations. Further assessment is necessary to draw any firm conclusions.
The calculation of rotor blade loads by means of computer programmes have reached a certain degree of maturity. This is also the case with regard to design requirements imposed by licensing or equivalent authorities. Most of these requirements are stated as general conditions, such as: "that normal, extreme and faulted conditions shall be taken into account".
As large size wind turbines will be designed on basis of analyses, a balanced implementation of all steps to establish fatigue stresses shall be aimed at, in order to estimate reliable life time predictions.
The work procedure is directly at gaining insight and conclusive proposals with regard to the following topics in-view of its impact on the estimated design life of horizontal axis wind turbines.
1) inventory of load conditions;
2) wind input;
3) wind turbine (wind farm aspects are not considered);
4) load calculations, viz. calculation of internal load spectrum in time or frequency domain;
5) stress calculations;
6) fatigue damage model;
7) design life.
Note that item 6) is not to be elaborated as a research subject belonging to this contract, but as a mean for performing sensitivity assessments.
The work procedure will be based on mutual comparison of the implementation of procedures and calculation methods.
Funding SchemeCSC - Cost-sharing contracts
BS18 9JB Bristol
1000 AJ Amsterdam