The advantages of absolute analysis for wind farm life extension
In wind farm life extension processes, the most common approach is the application of relative load analysis. These analyses perform a comparison between design loads and wind turbine site loads, obtained from the wind and operating conditions, using validated aeroelastic models that clone the behavior of the wind turbines not only replicating the design but also considering specific site conditions (as is or under curtailments).
The relative analysis assumes that the component has a useful life of 20-25 years (depending on the case), the average number of years at which most wind turbines are certified. Based on this assumption and an accurate calculation of the loads on the wind turbine, the operational lifetime of the same, at the specific site, can be obtained.
But what happens when the 20-year premise or the target that the wind farm owner looks for in a specific component is not met? Is there room for more? That is, when there is a design error and the life of a given structural component is below 20 years, what should be done? Or what if the real-life expectancy is well above those 20 years?
In the following article we will explain how to deal with these common scenarios by detailing one of the latest absolute analyses we have performed on a 1.5 MW wind turbine hub.
The case presented was as follows: due to the site loads and operating and wind conditions of one of our client’s wind farms, the lifetime of the hub according to a relative analysis carried out by an external company was less than 20 years. An unusual situation, and one that may come as a surprise to any wind farm operator.
After almost 10 years of experience in wind farm analysis, very rare are the cases where we see structural failures in the hub. Given the criticality of such kind of forecasted failure, we recommended to perform an absolute analysis to see and analyze in detail where the problem came from. In this analysis, using finite element techniques, we directly validate the geometry and material’ properties against the loads suffered by the component.
This analysis began with a Reverse Engineering process where, thanks to Nabla Wind Hub's know-how based on laser scanning manual measurements and ultrasound measurements, the entire hub geometry was obtained.
The Hub component
While our CAD team worked on the geometry, our aeroelastic loads department got to work, obtaining, according to the design conditions of the wind farm, the load cases that the wind turbine would suffer throughout its life cycle.
Wind modeling in aeroelastic simulations
Once the geometry was finalized, it was then used to develop the finite element model, where key aspects such as elastic material mechanical properties, preload, contacts, and boundary conditions were included.
The Hub’s Finite Element Model
The first step of the absolute analysis is to obtain the vibration modes of the component. For that we performed a modal analysis alongside a Fourier analysis of the input forces. The Fourier analysis allows us to obtain the main harmonics of the excitation signals, ensuring they are not close to the natural frequencies of the component to be analyzed.
Hub first mode of vibration
After having defined the dynamic characteristics of the hub, we began with the ultimate state analysis. This analysis allowed us to identify the critical points of the structure, which will later be analyzed using fatigue criteria. The critical points of the hub were predictable, as those are areas where stress concentrations or load application points exist, such as blind threads or the transition zone between the hub and the main shaft.
The Hub's critical areas
Finally, we reached the last step of the process, the fatigue analysis, in which we post-process the stresses at the most critical points of the component using mechanical fatigue criteria. This analysis has two approaches: (I) The basic approach, where we analyze the equivalent Von Mises stress at the selected points; (II) The advanced analysis, carried out by means of critical plane methods, where the interactions between the different components of the stress tensor are captured.
Equivalent Von Mises Stress Analysis
Thanks to this methodology, we analyze the critical points, already identified in a previous relative analysis, extrapolating the specific component's real life, and obtaining the real hub life expectancy which in the end exceeded the 40 years.
The application of absolute analysis in Ageing Management Plans for specific wind components, such as the hub of our case study, becomes key due to its low percentage of uncertainty. These analyses provide highly accurate life expectancy estimates for each wind turbine component, which in return unlocks the full potential of wind turbines and reveals the true value of wind farm assets.
The absolute analysis is complementary to the relative analysis, where a general overview of the state of the turbine is obtained and the critical points are identified as mentioned above.
If you would like to receive more information about Nabla Wind Hub's Structural Integrity processes and analysis, please contact our Structural Integrity Leader Adrián López at adrian.lopez@nablawindhub.com.
