THE IMPORTANCE OF USING VERTICAL OSCILLATION TESTING TO DETECT WIND TURBINE BASE MOVEMENT
Fatigue loads are important for the overall yield from a wind turbine. Loading on the tower is one of the more fundamental loads, as the tower is the most essential component of any wind turbine. Consequently, it is important to detect tower loads, which are larger than necessary in order to prevent excessive tower movement that may eventually lead to displacement.
Currently, the majority of wind turbines are supported by conical tubular steel towers. These towers represent 65% of the turbine weight, highlighting the importance of having adequate fixing and stability between the wind turbine tower and its foundation.
Due to the increasing age of wind farms, asset owners and operators are finding that excessive movement between the wind turbine tower and the turbine base is becoming a common issue, with the majority these issues only being identified when visual damage such as concrete cracking or displacement of large chunks of concrete becomes apparent. Unfortunately, by the time this type of visual damage is identified, excessive movement will have progressed to a much higher level leading to a degree of displacement that leaves your wind asset structurally unsafe and unable to operate safely.
Vertical oscillation testing is essential in order to ensure continues and safe operation of your wind assets, consisting of a typical 3 sensor setup placed on the wind turbine tower, detecting the movement between your wind turbine tower and the wind turbine foundation under pre-determined conditions. The data is then processed, and the moving average is recorded. The follow up actions are determined by the amount of movement that is present between the wind turbine tower and the wind turbine base, the following chart is the typically recommended by most wind turbine manufacturers.
Measurements below 1mm are typically seen as not requiring intervention but should be monitored with annual measurements conducted in order to identify if the level of displacement has increased.
Measurements of between 1mm and 5mm recommend intervention within a 6–12-month period, with semi-annual oscillation testing repeated in order to ensure that the wind turbine is not structurally unsafe, and that operation can continue.
Measurements between 5mm and 10mm are seen as being within the high-risk category, with repair recommended within a 3-month period and monthly oscillation testing conducted in order to ensure full displacement and structural failure does not occur.
Measurements over 10mm recommend that the turbine is stopped until a thorough structural assessment and repair is conducted, it is unsafe for a wind turbine to operate with this amount of displacement, as this poses a substantial risk of structural failure.
Annual vertical oscillation testing is pivotal for safe and economic operation of your wind turbines. Vertical tower oscillation measurement, allows changes within the structure to be identified at the earliest stage of degradation, mitigating the risk of high-level structural failure.
Vertical oscillation measurement can support damage detection on turbine towers and document progression of fatigue, allowing asset owners to react before high-level failure can occur, reducing the inflated cost of repairs and the associated downtime resulting in lost revenue.