Demonstration of Requirements for Life Extension of Wind Turbines Beyond Their Design Life

The LifeWind project analyzed the inputs of several stakeholders and formulated procedures for extending the operational life of wind turbines. The following definitions for life extension were formulated:
• Design lifetime: The time period used in the strength verification of the turbine during its design process as per IEC 61400-1 (IEC 61400-1, 2019).
• Lifetime extension: Additional period beyond the original design lifetime that the turbine is operational.
• Remaining Useful Life: Additional period from the present for which the turbine may be operated within an acceptable reliability.
• Operating life: Lifetime from commissioning to decommissioning of the wind turbine or wind farm.
• Safety: Prevention of failure which can result in risk of human injury or social or economic consequences or is in violation of local regulations.
Inspections on several operating wind turbines were made both offshore and onshore and included Vestas V80, V52, V67, Bonus 1 MW and Nordtank turbines. The main inspections points were focused on bolts, blade erosion and effective repair of faults found in past inspection reports. Based on the findings made from the inspected 8 wind turbines, it was concluded that the design-lifetime of 20 years can be extended. Specific tools for the determination of tension in tower bolts were tested and found to be effective in measuring remaining tension of bolts as conducive for life extension.
Operational measurements as obtained from SCADA for several wind farms were analyzed along with the aeroelastic design basis of the turbines to predict life consumption within a wind farm. The prediction of damage consumption is based on training neural networks with input SCADA based measurements.
The neural networks reproduce time series of loads wind turbine structures within a wind farm. The predicted loads using the measured mean SCADA signals is validated both with measured loads on a single turbine and with measured power standard deviation as a proxy for loads within large wind farms in complex terrain. The ability to use generic aeroelastic design basis to scale existing turbine design data to different turbine capacities and thereby simulate the damage consumption on those turbines is also shown.

The existing standards (ISO, Eurocode etc.) relevant for the extension of life of wind turbines were examined and a sufficient list of applicable standards and key procedures therein were identified. For decisions on life extension for wind turbines, it is proposed that they be based on a cost-benefit approach, as this will result in economically responsible decisions for the interest of both the owners of the wind turbines and for the society. This might lead to lower target reliability levels than was used in the original design.

Based on the above a detailed list of recommendations was formulated as input to the IEC 61400-28 standard that is presently under development for life extension of wind turbines.