Figure 1: Typical three-stage planetary-helical gearbox used for 3-point suspension wind turbines
Most geared wind turbine drivetrains employ a planetary gearset in the gearbox. Planetary gearsets provide higher power density than a parallel gearset, while producing a step in the shaft speed (RPM) in a multitude of gearing options. However, planetary gearsets are more challenging for inspection with only partial access by borescope (above oil line, those bearings that can be reached).
The planetary bearings need to withstand high loading as well as transient loads and speeds, which has proven to be problematic in wind turbine applications. These bearings can suffer from rolling contact fatigue manifested as spalling of the raceways, and typically wear out later in life. It’s generally the life limiting component in the gearbox design life calculations.
This failure mode, often referred to as bearing spalling, has been consistently observed in 3-point and 4-point machines, on both onshore and offshore wind farms. Fortunately, with the latest analytics and technology, these bearing faults can be reliably detected from vibration signals. More importantly, vibration provides a diagnostic, that tells you which bearing to inspect. In comparison, an oil sample or an oil sensor high reading just indicates that there is “something in the gearbox” that needs inspection.
Planet bearing spalling – readily detectible by vibration
Figure 2 shows one example where the failure was detected on a multi-megawatt offshore wind turbine with a double planetary and parallel staged gearbox. Specifically, indentations and abrasive wear were found on the outer raceway of the 2nd stage planet bearings. This fault signature (Figure 3) was visible in the vibration data more than 4 months before the turbine was shutdown for proactive gearbox replacement. Most interesting is just how early the vibration was in detecting the progression of the failure. The inspection turned up debris indentations from a spall that was “somewhere” in the bearing, but the damage was not progressed far enough to see widespread spalling.
This predictive maintenance is useful in many ways. First, with the diagnostic, time and money is not wasted searching for a needle in a haystack. The borescope technician is pointed to the correct part of the gearbox to inspect. One mobilisation offshore is sufficient. Secondly, the repair can be planned months in advance and in fair weather.
Figure 2: Borescope photos showing abrasive wear and debris indentations. The analytics performed on the vibration signal identified the fault in very early stages. The planet bearings are difficult to inspect but fortunately vibration diagnostics points you to the correct part of the gearbox to search for the damage.
Figure 3: Early detection of 2nd stage planet bearing fault in fleetMONITOR. The trend shows over 8 years of operation with the rise in the trend over 4 months before a “fair-weather” repair
In the above detection example, the turbine continued to run at full power for more than 4 months until the turbine was shutdown for maintenance and the gearbox was replaced.
In most cases, an asset manager is going to wonder:
Could more remaining useful life (RUL) and annual energy production (AEP) be captured through operational strategies such as derating?
Can I delay this repair until another budget period? Or wait for better weather? Or combine with other works?
This is exactly what can be achieved. Vibration data, and any other available data, combined with engineering expertise allows O&M decision support that fulfills the predictive maintenance paradigm.
How about de-rating for life extension?
De-rating is a common approach for sites with major component reliability issues such as failing pitch bearings, hubs cracking, and serial defects in the drivetrain components.
Once a serial failure mode is properly understood, one can consider whether the de-rate of those aging assets may offer an improved RUL. For example, in the main bearings, derating isn’t that effective. Thrust and radial loads can remain high with power (torque) de-rated. For planet bearings however, it is certainly effective, as bearing load is proportional to the drivetrain torque.
Recently we examined operational data from twenty megawatt-class gearboxes. All gearboxes were the same make/model and experienced the same failure mode (planet bearing spalling). In this real-life example, 10 gearboxes were operated normally while 10 gearboxes were de-rated in order to extend remaining life.
Note: all 20 gearboxes had the fault detected by vibration analysis with rising vibration trends and various inspections to confirm damage and plan repairs.
This excellent set of field data allows for a valuable study and Figure 4 shows two box plots comparing the actual life. The mean gearbox remaining life increased by 75% and the median remaining life increased by 130%.
Yes, de-rating already failing planet bearings has a massive impact on life.
This is just one of several ways in which ONYX’s database of failures and life modelling tools can be used to provide O&M decision support that maximises the profits of a wind farm.
Figure 4: Box plot results of 20 failed gearboxes due to planet bearing spalling, during vibration monitoring. The 10 de-rated gearboxes (blue) had a 130% increase in the median remaining life when compared to the 10 normal, uncurtailed gearboxes (red).
If you’re interested in seeing the details of this study and you’d like to learn more, then please join us for our 4th Annual North American Wind Turbine Technical Symposium where our Head of Global Monitoring, Noah Myrent, will be presenting on managing aging assets and similar case studies.