Speaking at a joint meeting of the Institute of Physics and the Institution of Engineering Technology in London, Arnoud Reininga, the engineering manager of energy industries for Swedish bearing manufacturer SKF, pointed out that in Europe lost production costs on average €500 per megawatt per day. Crane hire costs are €80,000 to €200,000 per week on shore or €500,000 per week offshore. Unscheduled maintenance is particularly painful for operators.

With such a demanding working environment and high maintenance costs, every part of the wind turbine must perform perfectly. Original equipment manufacturers and their suppliers are looking at the lubricant, alongside every mechanical component, and are paying close attention during the design phase to how they all work together. 

Rather than formulating a lubricant to meet the requirements of parts, parts are optimized for the lubricant.

“For the first time in my career, I’m hearing OEMs saying that for the next generation of synthetic industrial wind turbine gearbox oils, they will preselect components to be compatible with the new lubricants,” Helen Ryan, Afton Chemical’s distinguished advisor, told Lubes’n’Greases. But this was not always the case.

Carving a Niche

In 2011, the National Renewable Energy Laboratory, headquartered in Colorado, United States, made public a wind turbine gearbox failure database that gathered together data on the failure modes for wind turbines in the field at that time. The most prevalent mode of failure for bearings was listed as “cracking, roller and ring cracks, hardening cracks” with 14 instances in 36 failures. This was the largest group of failures fitting a single description, but not as large a group as the total failures due to wear (17 in three subcategories). 

There were also reports of cracking with “decoration,” which is usually where flakes of metal peel off the rolling element or ring. Such reports are common today. The decoration often appeared white when etched with nitric acid in alcohol solution. White etch cracking soon became the favored term to describe this form of failure. As this referred to a previously defined failure mode, there were some who doubted that all failures were genuinely white etch cracking, but the label stuck.

White etch cracking in wind turbines usually occurs in the bearing that supports the high-speed shaft (labelled as the high-speed coupling in the illustration on page 18) between the gearbox and the generator. The role of the gearbox is to convert the rotation of the turbine blades at about 10 revolutions per minute into around 1,000 rpm for electricity generation. 

Cost, Cost, Cost

“White etch cracks are old news,” said Amir Kadiric of Imperial College, London, at during a presentation at the IoP-IET meeting. Although deliberately provocative, it reflected the mood of many in the audience that tribologists and mechanical engineers had now worked out how to prevent it happening in wind turbine bearings by a mixture of surface finishes and bearing design. Additionally, there were many who doubted whether all instances of wind turbine bearing failures reported as white etch cracking were due to the failure mechanism. But ultimately that detail may no longer be an issue. Whether the observed features in failed bearings are white etch cracks or not, Kadiric referenced a 2019 white paper by SKF that states white etch cracks are a consequence, or symptom, of bearing failure and not a root cause.

Several studies have stated that hydrogen embrittlement is a possible failure mode of bearings. Lubricants are quoted in these studies as the hydrogen source. 

The historical issues around white etch cracking, hydrogen embrittlement and the move to even larger wind turbines has led the push for new materials in bearings and surface treatments of existing elements. Among these are “black oxide” coatings, which are said to prevent hydrogen ingress into the metal parts. Heat treatments that harden surfaces, such as case carburizing (adding carbon to harden the surface) or carbonitriding (hardening the surface with carbon and nitrogen), also appear to offer benefits, but as Jakub Rydel of Afton Chemical pointed out at the IoP-IET meeting, these surfaces have different chemistry, so the additives used in current formulations act differently at the surface.

According to Ryan, “some manufacturers have moved in the last few years from roller to journal bearings on the high-speed shaft, in order to address white etch cracking.” 

Rydel pointed out that journal bearings contain iron-free alloys, which changes the chemistry of the surface that the lubricant is supposed to protect. 

“There is a Chinese GB specification that fluids must pass if they are to be approved in most Chinese wind turbines. This has a pass limit of 250 kilograms for a four-ball weld load. To pass that, you need to have sulfur in your formulations, and some sulfur chemistry can attack the new alloys used in journal bearings. It’s key to select the correct extreme pressure agents,” said Ryan.

What’s Next?

“In common with many other lubricant applications, OEMs are looking for longer drain intervals, with many asking for ‘fill for life’ fluids that would spend 20 years in the next generation of wind turbines,” said Ryan. Reininga went further by claiming that OEMs are looking for 30 year lifetimes in their products. Such extended lifetimes could result in failure modes that have not been common so far, leading to further iterations of identifying the root causes of common failures and developing solutions to them. It is highly unlikely that the lubricant will provide the magic bullet for future failures, so the formulator will be working to optimize a mechanical engineering solution.

However, extending the lifetime of current assets will become a major issue. Grants and subsidies for the erection of wind farms are ending or are being significantly reduced in both the U.S. and China over the next two years. 

“This will bring greater focus on life extension for existing assets,” said Ryan.

As the U.S. and China are two of the top three markets for installed wind capacity, there is currently a forecast that predicts a rapid drop off in new installations globally in 2021 and 2022. This could make India, which currently has the fourth largest installed wind turbine capacity in the world, a more important player for new installations.

Reininga and others pointed out that Chinese OEMs are now producing their own wind turbine designs, rather than licensing them from the leading Western manufacturers, such as Siemens Gamesa or Vestas. These OEMs may specify different solutions inside the turbine’s nacelle for which new fluids will be required. But is white etch cracking a thing of the past? Not according to Ryan. 

“White etch cracking may be something that can be engineered out of new wind turbines, but cracking failures – whether they are genuinely [white etch cracking] or some other form of micro-pitting – will continue to be an issue with existing technology that is out in the field for another 10 to 20 years. Additionally, OEMs will want proof that new and existing fluid technologies don’t cause [white etch cracking] in new equipment. It will be an issue for many years that fluid suppliers have to address in gaining approvals,” she said.

Trevor Gauntlett has more than 25 years’ experience in blue chip chemicals and oil companies, including 18 years as the technical expert on Shell’s Lubricants Additives procurement team. He can be contacted at trevor@gauntlettconsulting.co.uk