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Gearbox Failure Investigation  

Author:  | Rhode Island, Technology

EXECUTIVE SUMMARY

The town of Portsmouth, Rhode Island (Portsmouth) commissioned a new AAER 1500-77-65 1.5 Megawatt wind turbine on March 24, 2009. On May 18, 2012, significant amounts of metal were found in the gearbox oil filter housing and significant internal damage was observed with a borescope. The filter element was replaced on May 25, and the turbine was returned to service. The turbine was removed from service on June 18, 2012 after significant additional metal was discovered in the filter housing.

The Portsmouth wind turbine gearbox has suffered a significant, premature failure of the first and second planetary stages. The gearbox must be replaced in order to return the turbine to service.

The failure occurred after 3 years of operation [emphasis added] despite a design life of 20 years. Total operating time on the turbine is approximately 21,000 hours, and total production in accordance with the production meter is 9,898 MWh. Depending upon future gearbox reliability, the cost of one or more replacements may exceed the potential future profit for the life of the turbine. Therefore, a clear understanding of the root cause of failure and the reliability of the turbine moving forward is critical to Portsmouth.

Due to the extent of the damage, replacement is most likely preferable to repair, depending upon repair vs. replacement cost.

The root cause of failure was not determined during this investigation. Several potential causes were ruled out as follows:

  • Structural damage to critical support structure or looseness of components such as blades, hub, tower foundation, and generator
  • Generator misalignment
  • Blade exterior condition such as cracks, delamination, or missing vortex generators
  • High frequency of yaw error exceeding 10 degrees (nacelle direction vs. prevailing wind direction

Possible root causes of failure that cannot be confirmed or ruled out are categorized as follows:

  • High: Gearbox manufacturing error with respect to assembly quality and choice of materials and specifications—complete teardown inspection is required as well as review of design and build records
  • High: Gearbox design error with respect to structural stiffness of housing—analysis verification required
  • Low: Wind turbine support structural stiffness—analysis verification required of unique ring mount design
  • Low: Pitch asymmetry due to incorrect blade offsets
  • Low: Service quality, especially after May 2011. However, failure to identify failure in a timely manner resulted in unplanned failure and extensive gearbox collateral internal damage.
  • Low: Siting—possible wind shear from freeway cut. Nearby 47 foot-tall water tank is not a likely contributor.

The gearbox configuration is not conventional by US industry standards. The make and model of gearbox have a poor track record, with at least 3 of 5 installed in the US having gearbox failures within the first 3-4 years of operation. Supply of replacement gearboxes is likely to be limited, and the turbine is destined to become an orphan due to bankruptcy of the manufacturer (AAER) and sluggish support from the designer (AMSC/Windtec). AAER purchased a license to build the 1650 kW AMSC/Wintec design but built a 1500 kW machine instead.

Download original document: “Gearbox Failure Investigation

This article is the work of the author(s) indicated. Any opinions expressed in it are not necessarily those of National Wind Watch.

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