Root cause analysis for structural problems in the rear frames of 2.0 MW wind turbines
Robust design of wind turbine structural components is essential to achieve a life expectancy beyond 20 years. Otherwise, major inspection, maintenance and repair campaigns will be required to meet such target.
There are two fundamental aspects that will influence the design of the component: in the first place, the correct prediction of the loads that affect the wind turbine and, second, an accurate obtaining of the structural response of the system.
Today, we encounter a number of components that, due to inadequate design, suffer structural problems before the 20 year period. An example of such is the rear frame of some 2.0 MW platforms, which often have severe fatigue failures in the rear welds.
The main function of the rear frame is to support the weight of different main components, such as the electrical generator, the transformer or the converter. In addition to this, and due to the accelerations of the nacelle during operation, the rear frame suffers from the inertial forces of the elements that are suspended on it. Furthermore, in some cases, there is the possibility of an eccentricity in the rotor of the electric generator, which can cause a centrifugal force, resulting in a negative effect on the structural integrity of the wind turbine.
For a better understanding of this complex reality, we recommend performing a strength analysis, which first step would consist in obtaining the loads through aeroelastic simulations, considering the specific conditions of the wind farm and the wind turbine operation data.
In the second step of the analysis, a dynamic analysis of the structure is performed. This analysis begins with the development of the Finite Element Model (FEM) of the structure, which requires a reverse engineering process to capture the geometries of the component and assign its physical properties.
Once the model is completed, the modal analysis continues, where the natural frequencies and modes of vibration of the component are identified. These frequencies are compared with the main harmonics of the excitation sources, and it is thanks to this comparison that possible resonance phenomena caused by the accelerations, or the centrifugal force of the generator rotor are identified.
After the dynamic analysis, fatigue analysis is carried out, where a basic analysis based on Von Mises equivalent stress and a more advanced analysis using critical plane-based methodologies are simulated. In our specific case study and after the stress analysis, it is verified that the wind turbine welds are not adequately designed to withstand 20 years of operation.
Thanks to this methodology, the root cause of the component's fatigue problems is identified, and a possible solution is studied. In this particular case, NWH recommends the installation of certain cross beams and brackets near the welds, stiffening the structure, thus solving the resonance problem, and lowering the stresses in the welds, extending the life of the component up to 20 years or more.
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