Real Life Examples of Recent Wind Turbine Failures and their Root Cause

In many aspects, wind turbines are more reliable than ever today as common issues have been investigated, understood and then solved or at least mitigated in a practical manner over the years. Yet, with a growing installation base and developments in technological innovation, we see new challenges emerging, as well as repeats of past problems, within the rotating machinery. Some failures are due to aging turbines reaching fatigue life limits, some are quality related, such as material defects or lapses in manufacturing process control, and some are new failure modes related to increased rotor size or the introduction of new technology. A culture in the industry of root cause investigation and problem solving helps manage the cost of machinery failures long term.

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Why conduct a root cause analysis (RCA)?


It depends on the stakeholder, but some typical reasons include:

  • Turbine or Component OEM - Improve product or process; reduce LCOE (levilized cost of energy); put in place a solution to reduce future liabilities
  • Owner - Obtain independent third party assessment; address risk position; find solutions to reduce the cost of failure
  • Operator - Find mitigation strategies to reduce the ongoing cost of failures their effect on asset availability
  • Designer - Close the loop between product performance and design; improve design
  • Insurer - Determine if the failure is covered under the policy
  • Sub-component supplier - Understand if the subcomponent is the problem (e.g. bearing) or the component or environment; improve application engineering; improve product or process

The RCA methodology is physics based and data driven, typically utilizing analytical tools and executed by a multi-discipline team that thoroughly documents each step. The process begins with defining the problem, then reviewing existing documentation and machine data. More information is gathered via up-tower inspections and measurements, lubrication analysis and factory teardown. With the causes narrowed down, simulations and metallurgical testing are applied to narrow the root cause. Corrective actions are validated and implemented, while risks are mitigated in the susceptible installed assets. This article provides highlights of two recent wind turbine drivetrain failures to illustrate the RCA process.

Example 1: Intermediate Pinion Failure

THE PROBLEM

Multiple wind turbine gearboxes had intermediate pinion failures during a similar timeframe. The gearboxes were refurbished by an aftermarket supplier other than the original equipment manufacturer (OEM) and these replacement gearboxes had been in service less than 3 years. The intermediate pinions had failed catastrophically with multiple tooth fractures resulting in liberated teeth and consequential damage to other components.


Intermediate pinion tooth fracture cause by a material defect

THE ROOT CAUSE

Metallurgical analysis was performed in the laboratory on cross-sections of the shaft taken from locations within the fractured teeth. Fractographic examinations found that the cracks originated at subsurface material defects and these were identified as oxide inclusions containing aluminium and silicon. During normal cyclic loading the inclusions act as localized stress concentrations which cause cracks to form prematurely in the gear’s life. While inclusions exist in all steels, the ISO 6336-5 standard specifies the steel cleanliness grade according to the size and quantity of allowed inclusions. For the wind turbine gearbox application the standard recommends a minimum steel cleanliness grade of MQ, which is comparable to AGMA Grade 2. The failed pinion was evaluated in the lab per ASTM E45 (for characterizing non-metallic inclusions) and found to not meet this MQ cleanliness requirement. The gear supplier had a poor material batch issue that caused these intermediate pinion failures.

     
Scanning electron microscopy of inclusion sizes and counts adjacent to a fracture

THE SOLUTION

The site owner identified the turbines operating with pinions manufactured from the same batch of steel and up-tower inspections determined the turbines that needed in-situ shaft replacements, mitigating the risk of significant and costly future failures. As an outcome of the RCA, to improve manufacturing, a gearbox repair specification was established for the suppliers of the refurbished gearboxes. The specification defines that material cleanliness must meet MQ grade and gear manufacturing quality control processes must be added using non-destructive testing. An additional corrective action would require a design change to the type of steel used and the composition of its alloying elements. A material with improved core toughness and hardenability characteristics can reduce the likelihood of cracks propagating in the presence of inclusions. This option remains if problems persist after addressing process quality.

THE BENEFITS OF CONDUCTING RCA

Gearboxes in the same batch identified and proactively repaired, saving them from significant consequential damage. Supplier processes were improved by tighter repair specification; an excellent outcome for both the owner and the supplier.

Example 2: Blade Bearing Failure

THE PROBLEM

The operators of a mega-watt class wind turbine removed a blade bearing from service due to pitch angle asymmetry. Upon disassembly, the primary failure was identified as spalling at the edge of the bearing race way, in the loaded zone. Spalling, also referred to as macropitting, is a progressive failure mode driven by fatigue of the steel material.

Spalling damage originating from the raceway edge of a blade bearing
Spalling damage originating from the raceway edge of a blade bearing

THE ROOT CAUSE

The damage was caused by excessive contact stress at the edge of the raceway. The contact area of the rolling elements (the balls) had shifted beyond the raceway edge, which is referred to by bearing designers as “ellipse truncation”. Ball bearings have an elliptical-shaped zone of contact between the rolling elements and raceways. This region has a very contact high pressure as it is where the load is transferred from the inner raceway, across the balls and into the outer raceway.
Under the large loading from the blades, the bearing can deflect such that the contact runs over the lip of the raceway and the contact area is reduced, or “truncated”. Stress is the relationship of applied force over contact area, thus the contact ellipse truncation (reduced area) increases the stress on the steel material at the raceway edge. The higher stress reduces fatigue life leading to early failure.


FE analysis shows the high contact stresses at the edge of the raceway

THE SOLUTION

Contact ellipse truncation is likely to occur in blade bearings during certain extreme load conditions, else the bearing may be too heavy and costly. However it should be prevented from occurring during general loading. Design improvements can be made to achieve this. The “blob” in each of the below plots is made up of the vertical lines of contact for each ball against the raceway: many dozens of balls, thus appearing as a “blob”. For the original design (top) the red lines show where the contacts have gone over the raceway edge; truncated. The improved design (bottom) has balls in contact and the contact is no longer truncated. This was achieved by stiffening the bearing raceways, adjusting ball pre-load and optimizing the micro-geometry of the raceway profile.

RomaxWIND contact area plots of the improvement from design changes, thus avoiding ellipse truncation in blade pitch bearings. Figure shows the 360 degrees of the bearing raceway “unwrapped”
RomaxWIND contact area plots of the improvement from design changes, thus avoiding ellipse truncation in blade pitch bearings. Figure shows the 360 degrees of the bearing raceway “unwrapped”

This type of failure to blade pitch bearings will become more common as turbines age. If failures become frequent there will be demand for up-tower in-situ servicing. Where possible the debris-contaminated grease needs to be flushed clean and re-packed. The bearing could be rotated and remounted to relocate the load zone in a raceway region that has accumulated lower cyclic stresses. An aftermarket upgraded bearing may address the problem by applying the design modifications.

THE BENEFITS OF CONDUCTING RCA

With a full understanding of the failure mode, the owner knows their options. Costs can be alleviated by putting in place various early detection approaches to reduce both downtime and crane costs. Informed decisions can be made in regard to changing one or three blade bearings, based on remaining life expectations. Changing maintenance practices can be evaluated for their effectiveness, as well as seeking aftermarket design improvements to minimise the threat of re-occurrence.


Takeaways


The root causes of the two failures was determined by systematically executing the RCA process. Much of the detail is outside the bounds of this article, including the often challenging RCA steps of correctly eliminating the non-contributors to the failure. There are many things in the RCA tool bag (see table below) for the reliability engineer to use to carefully work through the investigation. Often the root cause is found quickly, with the obvious “smoking gun”, sometimes the failure is so spectacular the evidence is unclear, and occasionally multiple root causes are to blame, making an investigation complex or, depending on the stakeholder, only worth conducting up to certain point, so as to resolve a commercial outcome or decision.                                                                                                        

RCA Tool

 

Application

Teardown

 

Locate the failed component and establish the failure sequence.

Operational data review

 

Comb through SCADA data and controller fault logs to determine the onset of failure or extreme events.

Lubrication Analysis

 

A deficiency in the oil or grease can cause a failure even when all other factors are correct.

Inspection & Non-Destructive Testing

 

Identify and document the failure mode on other turbines.

Metallurgical analysis

 

Determine if the material was within specification or if defects caused the failure.

Instrumentation

 

Collect quantitative data for evaluating possible contributors to the failure.

Terrain and Obstacle Assessment       

 

Determine if micro-siting of a specific wind turbine results in loading or environmental contributors to the failure.

Desk Review

 

Thorough review of existing maintenance, manufacturing and inspection documents can reveal a simple cause of the failure.

Quality Audit

 

Visit manufacturers to determine if quality issues or manufacturing errors have contributed. Implement corrective actions.

Statistical Analysis

 

Use empirical data to determine the timeframe and quantity of future failures.

Design Simulation

 

Simulation model is created to recreate the failure and to validate the corrective action.

Main Shaft or Blade Root Loads Measurement   

 

When the more simple causes have been ruled out, advanced instrumentation can determine if loads are within design limits.

Contact Romax InSight at insight@romaxtech.com for solutions to your wind turbine failures.