Judging from comments at a recent conference, there is progress to report. Speakers at the Oildoc conference in Rosenheim, Germany, said industry has made substantial strides not only in pinpointing the reasons for turbine failures but also in determining how lubricants might prevent them. But some added that more work remains. Some speakers contended that lubricant standards should still be more robust. Others said that the process for approving turbine oils is so laborious and expensive that it significantly delays bringing better lubricants to market.

Tough Environment

Wind turbines may seem bucolic, but they create a tumultuous environment for the oils that lubricate them. In fact, operating conditions are becoming more severe. In their efforts to generate more power, manufacturers have built taller towers that accommodate longer rotor blades. Longer blades allow them to capture more wind, which after all represents the potential energy they are trying to convert. But more energy from the rotors also means greater forces brought to bear on turbine drivetrains. Torques and pressures on gearboxes were already substantial, and now they have increased.

Frank-Dieter Krull, of Eickhoff Antriebstechnik, told Januarys Oil-doc conference that the variability of wind is another factor making things difficult for wind turbine lubes. Eick-hoff is a gearbox manufacturer based in Bochum, Germany. Krull said equipment manufacturers in general try to design machines so that components will have lubricating films of the thickness needed to ensure proper protection from wear. And film thickness depends on several factors, he said, including velocity of moving parts, loads on components, temperature and lubricant properties.

In many machines, operating speeds are relatively consistent, or at least predictable, and designers can take this into account. But winds are very unpredictable, subject to changes in direction, wide ranges of speed, and gusts rather than steady currents. These further complicate the forces that turbines must handle, but they also affect oil thickness.

Dynamic loads and accelerating speeds have impacts on the stability of surface-protecting fluid films, K rull said. Those conditions lead to critical lubrication conditions.

Climate is also a challenge for wind turbine lubricants. Regions with very hot and very cold climates often have [windy] conditions, and this makes those sites attractive for windparks, Krull said. But this means that components of turbines located in such places are impacted by strong, gusty winds, extreme temperatures, ice, snow or hot climates. Many turbines are built in locations where they are also exposed to salt-water.

Foam, Wear and Pitting

Wind turbines contain a number of systems and parts that are lubri-cated, but Oildoc speakers agreed that breakdowns usually involve the gearboxes that transmit power from the rotor to the generator. There also seems to be consensus about the biggest causes of turbine breakdowns.

The most relevant failures and downtimes in wind turbine gearboxes are caused by foaming issues, gear wear, as well as bearing failures, said BPs Kirsten Tschauder. Most gearboxes have a combination of planetary and helical gears, and these can be subjected to sliding wear caused by high speeds under low loads, or extreme pressure caused by high loads at low speeds. The former causes scuffing, while the latter can lead to stress fractures.

The bearings for the main gear shaft can be subjected to heavy loads, and if lubricating films are inadequate these can cause micropitting. Micropitting begins as microscopic cracks on the surface of bearings or gears, but continued exposure to the same forces causes pitting inside the cracks. This weakens the surface and can eventually cause it to crumble.

Foaming occurs when fast-moving components roil the oil, causing entrainment of air that compromises the lubricants performance.

Testing to Avoid Problems

Industry has developed turbine gear oil standards in an attempt to identify products that prevent these and other potential problems. The most widely recognized industry specification is IEC 61400-4 FDIS [I-1], developed by the International Electrotechnical Commission as part of IEC 61400-4, the standard for wind turbine gear-boxes themselves. Some turbine and gearbox manufacturers also maintain their own specifications.

Tschauder said those manufacturers paid attention to the blame placed on foaming and bearing and gear wear. This resulted in the implementation of requirements to evaluate those behaviors of gear oils in some of the most recent field trial specifications of wind turbine or gearbox manufacturers, she said.

The product approval processes outlined in the specifications are exceedingly thorough. Krull cited IEC 61400-4 FDIS [I-1] as an example. It begins with lubricant or additive companies conducting simple screening tests to gauge physical and chemical characteristics – such as viscosity and corrosion inhibition – and submitting results to gearbox suppliers for review. That step alone can take a year, Krull said. Next comes a series of dynamic performance tests measuring things such ability to prevent micropitting or scuffing, performed by additive or lubricant companies or independent labs and submitted to gearbox or wind turbine suppliers. Another year for the second round, Krull said.

Results for all tests are then reviewed by gearbox and turbine suppliers, as well as standard certification bodies, like IEC. IEC 61400-4 FDIS [I-1] states that all three must agree that requirements have been met for the candidate oil to receive preliminary approval. But that is not the end. The lubricant still must undergo field testing that includes continuous monitoring for wear and other factors plus more thorough inspections every three to 12 months. Field tests can take one to three years, Krull said.

Obviously there are good reasons to have a thorough approval process, but there are drawbacks, too.

The process takes a very long time, Krull said. Qualified test facilities and laboratories for mechanical oil tests are booked frequently, and waiting times for test capacities are long. The availability of prototype wind turbines for field tests is rare. After the initial success, field tests for one and two years with a higher number of turbines – 19 to 20 – are required.

This leads to long product launches. Often the time to market lasts five years or more.

Room for Improvement

As thorough as they are, some Oildoc speakers contended that the existing specifications still have shortcomings in terms of identifying oils that will protect gearboxes.

According to Krull, it is difficult to devise a test that informs about a lubricants ability to prevent micro-pitting during actual use. This is because of the number and complexity of factors that affect this form of wear – gear toothing technologies, surface finish and operating conditions, not to mention characteristics of the lubricant itself.

Compared to calculations for pitting or scuffing, [tests for micropit-ting] are not well defined, he said. He also argued that industry should consider conducting some tests at 60 degrees C. in addition to the higher temperatures for which they are currently defined. He offered two reasons. First, cooling systems in some turbines are designed to prevent operating temperatures above 65 C. Second, he said, some additives function best within a certain temperature regime.

[T]his means that important performance criteria might be met at higher test temperatures, but at lower temperatures the lubricant fails.

Tschauder suggested that it would be a good idea to conduct key mechanical tests on used oils as well as fresh fluids. She cited research in which BP conducted standard tests for gear wear, bearing micropitting and foaming on used oil samples of three commercially available wind turbine oils. As products carrying industry or OEM approvals, all three would have passed fresh-oil tests along with field trial requirements. When the same mechanical tests were conducted on used samples, however, some passed while others did not. Tschauder said this implies that operators using approved oils may not receive the performance they expect in the latter stages of drain intervals.

The development and strict application of performance tests… during the development of new lubricants, as well as for the benchmarking and suitability of market-available lubricants, helps to increase reliable operation and for units to achieve higher power outputs, Tschauder said. Combining both will help to set up drivetrain systems which are able to cope with the latest and future requirements on wind turbine operation, reliability and availability.

Next Generation Oil

Steffen Homberg, of lubricant manufacturer Addinol Lube Oil GmbH, said it is becoming apparent what type of oil it will take to protect wind turbine gearboxes.

In order to comply with the new requirements of more efficient gearboxes and wind turbines of the future, both novel base oils and a groundbreaking combination of additives are necessary, he told the Oildoc conference.

Homberg said an effective turbine oil should have a viscosity index above 180 to help the lubricant maintain its characteristics in the face of wide temperature swings. He said such a high viscosity index will also ensure a consistently sufficient oil film for the annulus gear, which is subject to some of the widest swings in operating speed.

In this [environment], the viscosity-temperature behavior of the gear oil in the relevant rotation range is highly important.

Homberg said this model gear oil should be made with a base stock that has a very high viscosity index, in order to achieve the finished lubricant target without needing too much V.I. improver. The base stock should also be very shear stable, have excellent low-temperature performance and very good air release. Finally, it should not interact adversely with other component materials. Addinol, which is based in Leuna, Germany, suggested polyalphaolefin as one possible choice.

Because of the heavy loads on gearboxes, Homberg said the additive package will need friction modifiers that focus on protecting components in the mixed lubrication regime. It is very likely that these will require new technology, he added.

Industry seems to be developing a better idea of the lubricants needed to help reduce breakdowns of wind turbines. Operators are certainly eager to have them.