Development of tower and foundation assessment tools for utility-scale wind turbines
One key component for assessing ongoing wind turbine health and lifespan is monitoring the turbine foundation. Operators assess turbine foundation structural health to make informed decisions about turbine operation after extreme events, such as tornadoes, hurricanes, or damaged gearboxes. Wind turbines also experience wear and tear through normal use; as a wind turbine turns to meet incoming wind, the weight of the nacelle and blades is unevenly distributed over the wind turbine tower and foundation. Thus, it is also important to assess the turbine foundation before ‘repower’ projects, where the top unit of the turbine (including the rotor and blades) is replaced and the foundation and tower are reused beyond their initial design life.
The purpose of this project was to assemble a noninvasive, inexpensive, and portable system for taking wind turbine foundation and tower measurements, develop a method of assessing foundation health based on those measurements, and test the device on several wind turbines.
From fall 2016 to fall 2018, researchers at the St. Anthony Falls Laboratory developed and tested a portable sensor and data acquisition system with the following components:
- One tiltmeter, to measure foundation tilt
- Three strain gauges, to measure tower strain
- One cup anemometer at an elevation of 10ft used to estimate the wind speed at turbine hub height
- One datalogger
The tower strain and foundation tilt values are used to determine the foundation rotational stiffness, foundation fatigue and damage equivalent load (DEL). Overall these values are used to determine the remaining life span of the turbine foundation. The anemometer measures the ambient conditions to ensure that strain and tilt data are collected for a wide enough range of weather conditions. The datalogger records and saves the sensor values over time.
Each component of the structural health monitoring (SHM) system was tested in a controlled laboratory environment to validate the sensitivity and performance of the sensors. The research team then used the permanently installed foundation sensor system at the University of Minnesota Eolos wind turbine to optimize the new SHM system. The Eolos sensor system includes 20 strain gauges measuring at 20 Hz, and testing different subsets of SHM sensors against the robust Eolos system helped determine the minimum number of sensors and the sampling rate necessary to collect adequate data.
Finally, the system was used to estimate current structural health and the remaining useful life of three turbines at Xcel Energy’s Grand Meadow Wind Farm.
The structural health monitoring system is portable, so it can be used on multiple turbines. The system can be installed by a trained technician in under 4 hours and left unattended on the wind turbine while data collection occurs for approximately 5-7 days. The SHM system is also noninvasive, meaning the foundation is not unearthed during the testing process. The SHM system is enclosed in a weatherproof case for protection, durability, and portability. The prototype components cost approximately $7,500 for the portable SHM system. In comparison, the component cost for a permanent system, like one found at the Eolos wind turbine, is approximately $16,000. When used on ten wind turbines, the portable system would save operators over $150,000, which could also result in an overall decrease in the price of wind energy.
The project team is still producing structural health monitoring systems for various clients. New systems are modified to fit specific client needs and to streamline the installation and data collection process. New systems are also being integrated into existing site data collection systems.