Wind farms are part of our surroundings and therefore are in general fairly accessible power generation facilities. Safety is key to both continuation of operations and a corporate responsibility towards workers and third parties. It must be safeguarded through a comprehensive process including analytical RUL calculation, inspections and certification to confirm that there is a limited risk exposure while the installation continues operating. Once the real status of the wind turbines is characterised, smart operational strategies can be deployed to maximise the return on the investment.
Jose Javier Ripa / Business Development Manager / UL DEWI, Spain
RUL is calculated by comparing the number of cycles, performed at critical locations (load stations) on the aeroelastic model, under two scenarios of power production and external conditions. The scenarios are:
- Those corresponding to certification (design basis of the turbine), and
- Those estimated for the site-specific conditions.
The lifetime extension (LTE) factors can be determined with a certain accuracy level, which is heavily influenced by the quantity and quality of available data (from the design/certification phase and gathered from on-site conditions), wind turbine models and simulations. A refined treatment of all available data and simulations can reduce the uncertainty and increase the accuracy of the LTE estimation within the possibilities.
According to ISO 2394: General principles on reliability for structures, reliability is ‘the ability of a structure or structural element to fulfil the specified requirements, including the working life for which it has been designed’. The probability of failure can be calculated as the inverse of the reliability.
Wind turbine components are usually designed considering extreme and fatigue loads. Extreme loads are defined based on the materials’ ultimate strength, which from the design point of view equates to sizing the section’s area, or using materials set to the proper strength.
Fatigue is more complex to manage. Loads are defined by Wöhler curves (S–N curve = fatigue curve). Damage (how the fatigue margin is consumed) is calculated by the rainflow cycles counting method, adding the damage of each cycle in line with the Palmgren–Miner theory, exampled by the Endurance curve above.
In practical terms, instead of using a discrete calculation for each load level, the fatigue history is normally summarised in a damage equivalent load or DEL for a specific number of cycles (usually 10 million cycles as the reference).
As part of uncertainty reduction, correct wind modelling is a critical aspect. The use of a proper turbulence model such as the Kaimal model, and a proper number of turbulence seeds in order to model the wind power spectrum and spatial coherence, is a must in analytical LTE modelling. IEC 61400-1 ed. 3 standardises the number and requirements using several turbulence seeds per wind speed. The IEC proposed method is key to reducing LTE estimation uncertainty.
Figure 2 shows an example of the synthetic wind seeds with spatial coherence generated for three different turbulence intensity levels (0.5%, 5% and 18%).
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