Did you know that ~80% of bearing failures are as a result of lubrication issues? And that monitoring the lubricant, especially monitoring for wear metals will give a significantly earlier indication of bearing distress than vibration monitoring? (In this failure mode the damege must have already occurred before vibration starts). With stats like that it's no wonder oil analysis is fundamental to wind turbine monitoring and maintenance.

The problem is that there is an overreliance on traditional methodologies aka. accelerometer based vibration monitoring. Monitoring will not prevent failure, it is your best defence against a very expensive one.

Our view is different.

For the very lowest cost but comprehensive monitoring, start with Acoustic Emissions sensors (aka Holroyd), they are inexpensive, do not need complex data analysis or a baseline signature or careful orientation. They WILL easily detect scuffing, cracking, fatigue and spalling on bearing surfaces BEFORE vibration becomes obvious.
For largest installations a more comprehensive solution would also include either a moisture sensor in the lubricant and/or a sensitive metallic particle sensor.
Only after this is it appropriate to consider an Off-Line analysis and only then for the more problematic installations.

Above: Scottish Power Trial Graphs (click to enlarge)

The data above is taken from a large wind turbine fitted with a Kittiwake Magnetic Particle detector. The graph shows progression in a failure in the main gearbox. Firstly we see the generation of relatively small particles than a gradual increase in occurrence of larger particles as damage becomes more severe. The failure was identified as initially frictional wear at the gear teeth that had initialed in frequent high winds. The gearbox brake is operated leading to shock loads in the drive train between turbine shaft and the brake and under conditions of little of no lubrication.

As wind turbines become larger and more complex, the power produced from them increases, but so can rectification cost of failures. One such event was the failure of all 30x2MW turbines installed at Scroby Sands in the UK. All failures occurred within one year of installation. Data provided by a wind turbine gearbox manufacturer shows the total cost of removing, overhauling and reinstalling a gear box can be up to €450,000. This can be due to the wind turbine's inaccessibility, crane rental, man hours used and loss of earnings on the wind turbine. If wind turbine monitoring using sensors detects a potentially serious problem before it becomes too advanced, the repair bill could be a fraction of the final cost.

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