Senvion Blade Bearing Failures and what you can do about them

Why do blade bearings fail – Senvion MM82/92 Wind Turbines

Most, if not all, of wind turbine OEMs have issues with blade bearings. It remains a weakness in wind turbine design. They see many dynamic forces: bending from the wind, torsional from pitch systems as well as rotation of the rotor and gravity. They also see repetitive movement and partial rotation, with the same areas of the bearing handling concentrated stress. Stiffness issues can occur and unfortunately there isn’t a blue pill that you can take for this. Incorrect hardening of the raceways during bearing production is also common but difficult to detect. In addition to all this, particular to Senvion WTGs, it is believed that corrosion protection was not applied to the bearings before 2012, and then poorly applied after discovery.

Challenged maintenance

By their nature as well as its location there is very little maintenance that can be done with pitch bearings. Oil leaking from pitch systems can sometimes flush the grease from the blade bearing, compromising the lubricant health. Without an auto-lubrication or biannual re-greasing the bearing could be running dry. Grease sampling can be problematic as well as inconclusive. Blade bearings can be operated into the tens of thousands of Fe ppm without changeout and, due to partial and slow rotation, they do not lend itself to vibration condition monitoring.

Flying blind

Periodic inspections from the outside of the bearing are worthwhile but require confined space training and rescue considerations. Visually inspecting the outside of the bearing at top dead centre is advisable as this is most likely where the cracks will occur. You can also visually inspect blade bearing bolt holes for signs of grease ingress, and this is a sure sign there is a crack along the bolt hole. However, because of the design of the bearings it is impossible to inspect with an endoscope. Cage failures, blade bolts or even rolling elements can be discovered externally and will make themselves known, like a brick in a washing machine. But these are late-stage failures and by this time your counter measures are limited.

With electric pitch wind turbines such as the GE, Enercon and Senvion, you can sometimes see pitch errors arising in the SCADA data and this can be used as a medium-term warning for blade bearing failure.

Senvion pioneering monitoring in wind

Sadly, Senvion are no longer producing wind turbines in Germany and the rest of the major components are not without their issues. These include main bearing failures, gearbox failures on the high-speed stage and with the sun pinion circlip, and generator failures due to slip rings and poor earthing. Other concerns include the blades having issues with the stiffener or “spoiler” on the LM40 blades. Apart from that, I still think they are good wind turbines, but I am a classic car enthusiast so probably have an unhealthy tolerance for poor reliability. One thing Senvion did well is they acknowledged the potential of CMS to better manage their failures. Their gearboxes came with Gastops particle counters as standard which allowed for early warning of gearbox failure in an accessible way that technicians and site managers could understand. They also fitted the blade bearings with the CMBB (condition monitoring blade bearings) system for crack detection. These utilised nut sensors and continuity to provide advanced warning of failure but this resulted in a lot of false alarms and downtime. Most operators have removed them or, if not, have replaced them with the upgraded version.

Upgrade options

Some bearing manufacturer specialists have come up with some retrofit and upgrade options for these troubled components. Stiffeners or bearing braces can be applied to provide additional strength to the loaded zones of the bearing. Retrofitted in a half moon shape, this is possible to do up-tower and can increase the life of the bearing. For those wind farms where changeout is an inevitability some bearing manufacturers offer upgrades such as labyrinth seals, coated rollers or an upgrade from 2-row-4 contact ball bearings to a 90 degree angular contact ball bearing. This reduces ring cracking and contact ellipse truncation. No cage issues, lower hoop stresses and fatigue issues. It is a blade-off job but may be an option if you still have a long period of time to operate.

Semi-portable CMS for managing your next failure

There are also better quality blade bearing condition monitoring systems (CMS) that can detect crack initiation and crack propagation continuously, so you are combining remote monitoring data with bearing specialists that can analyse inspection images and advise operators on replacement strategies. The CMS utilises fibre optics for continuity to monitor for crack formation on 360 degrees of the outer ring. The system is independent of the wind turbine control system, meaning that downtime is optimised for bearing investigation as opposed to CMS troubleshooting. For blade bearings with existing cracks, there is a version of the system which utilises proximity to determine when the crack is propagating; and this is highly accurate to the order of 1/100mm. Stiffening plates can be applied to increase the remaining service life of the defective blade bearing, to enable the turbine to continue operating despite the crack.

Summary of advantages of blade bearing CMS on Senvion WTGs

• prevention of catastrophic failure such as rotor blade loss and accidents
• prevention of downtime and loss of generation
• peace of mind from 24/7 monitoring
• system resets possible remotely
• no system downtime due t0o false positives
• system is DNV-GL approved