As wind turbine technology has evolved and more experience has been gained in the field, both wind turbine manufacturers and O&M professionals have become acutely aware of the importance of high-quality lubrication. Jackies Jones interviews Mike Hawkins from ExxonMobil Lubricants & Petroleum Specialities.
“They do now appreciate the significant role that lubrication can play in maximising turbine lifetimes and productivity,” says Mike Hawkins, of ExxonMobil Lubricants & Petroleum Specialties. This is true for the ‘first fill’ with lubricant that is done by the turbine manufacturer, and then for the subsequent ‘service fills’.
Wind turbines use several forms of lubrication: gear oils, hydraulic oils and greases. “But it’s the gearbox, that is the heart of wind turbine, and is generally the most challenging item when it comes to lubrication,” says Hawkins, who is Global Brand Manager, for ExxonMobil’s flagship Mobil SHC brand of high-performance synthetic lubricants.
Gearbox warranties generally last anywhere from two to three years, after which it is normally O&M professionals — often contract companies — which become responsible the operation of the turbine for remainder of its service life. “We’re talking about an asset that’s going to operate for over 20 years,” says Hawkins. And it’s an asset which, should the gearbox fail, is very expensive to repair.
Hawkins talks about some of the particular characteristics of wind turbines that make optimised lubrication important. Again, it’s the gearbox that is most challenging, and the drive amongst turbine manufacturers to minimise nacelle weight has resulted in more compact gearbox designs that now need to cope with increased stresses.
As well as coping with significant temperature variations they have to handle a wide range of differences in wind velocity, resulting in heavy loads and extreme load changes. Maintaining the optimum thickness of the protective film of lubricant throughout this is very important to minimising wear. He adds that the same is true for the bearings, which are also put under a lot of stress due to fluctuating and extreme ambient temperature conditions, big loads and vibrations. They, too, have to be protected against any sort of metal-to-metal contact, which would result in excess wear.
The challenges faced by turbines increase as they are used in a wider range of environments — often harsher environments, with bigger temperature fluctuations, and offshore. And in those environments, especially offshore or in remote locations, the challenge of carrying out repairs is greater too. Add to that the size and heights of these machines, plus access issues, and it becomes clear that minimising the need for repairs is absolutely essential: “You have to think of what’s involved in replacing this kind of part — and the cost — to know how important that is,” says Hawkins.
“When you consider the demanding conditions that wind turbines are subjected to, it’s no wonder that wind turbine operators are increasingly turning to high-performance lubricants that are expertly formulated to deliver a wide range of benefits, from enhanced energy efficiency and excellent protection against conventional wear modes, such as micropitting,” adds Hawkins.
Micropitting is a surface fatigue issue that creates numerous surface cracks that propagate at a shallow incline angle to the gear or bearing surface and form extremely small micropits. Over time, these micropits aggregate to produce a continuous fractured surface, reducing tooth accuracy and leading to breakage.
As the early wind turbines developed two decades ago often used gear oils formulated with conventional, mineral-oil basestocks, micropitting was a common issue. So, to help wind turbine operators better protect their equipment from micropitting and maximise turbines’ productive lifetimes, ExxonMobil entered into a research and development collaboration with a major gearbox manufacturer to explore a synthetic solution to the problem, leading to the introduction, in 1998, of the company’s first synthetic gear oil for wind turbines.
That product has been further improved over time, and the current product, Mobilgear SHC XMP 320, is now being used in more than 30,000 wind turbines around the globe, says Hawkins.
While micropitting remains one of the problems that can occur in gearboxes and bearings, “lubrication technology has progressed so far over the past two decades that micropitting can be significantly minimised through the selection of the right lubricant solutions,” explains Hawkins.
“And, synthetic oils are more effective for this than mineral-based oils. They really do outperform the mineral-oil based products of the past.”
In addition to minimising micropitting, the product claims enhanced oxidation resistance, and additives that minimise wear, or scuffing. “It also has a high viscosity index —which means it maintains optimum viscosity, and optimum film thickness, at a wide range of temperatures,” says Hawkins. That means that at very high temperatures the lubricant remains viscous enough to provide the necessary protective thickness of film, but at low temperatures it does not become too thick to offer effective protection. Nor does it clog filters, says Hawkins. He adds that another advantage of synthetic lubricants is that they can actually reduce the operating temperature within the gearbox, which is advantageous.
Yet Hawkins is keen to emphasise that effective lubrication is a process that involves more than just the lubricant itself. Running a proactive oil analysis programme — a regular ‘health check’ — by sampling and analysing the lubricant every few months, is an essential part of the process. These days it is carried out widely throughout the industry. So what can oil analysis indicate tell us about what has been going on inside the turbine? Broadly speaking, it will measure levels of oxidation, evidence of high operating temperatures, any kind of condensation or water infiltration, and signs of wear metals.
ExxonMobil offers customers its own proprietary oil analysis programme. This involves a scientific breakdown analysis: first in the laboratory, and then comparing the results with a large database of results from similar equipment, allowing the experts working on this to identify and analyse patterns that might be early indicators of trouble. This allows appropriate action to be taken. By acting on any early warning signs that are detected, turbine lifetime can be extended. The oil analysis specialists will also advise the ideal interval between sampling. Depending on the equipment, application and location, this is usually at intervals of a few months.
This also means that the interval between oil changes can also be extended with confidence, with consequent savings in maintenance. “There’s no such thing as a simple oil change in a wind turbine,” says Hawkins, referring to their location, height and access issues. He explains that the Mobilgear product typically extends the gap between oil changes to three years or more — this compares with 18 months for conventional, mineral-based products.
Mobil Industrial Lubricants’ products are selected by the majority of the world’s leading turbine manufacturers for the ‘first fill’, and the company continues to work closely with them in further refining and designing lubrication products. Hawkins explains that the company’s Equipment Builder Group has a long history of working with equipment builders across many engineering sectors — to help them take their products to the next level, even contributing to new designs. Close co-operation of this kind helps leverage expertise, says Hawkins. It doesn’t mean that they produce tailor-made products for different manufacturers or equipment — rather, collaborating to create better solutions results in an ongoing improvement in the lubrication products. “We’re always challenging the status quo,” says Hawkins.
Mike Hawkins is clearly excited by the contribution his company is making to the wind industry by helping turbine manufacturers and plant operators improve performance and reliability: “Advances in these lubricants are not only helping individual customers become more productive, but are helping wind power become more competitive compared with other forms of power generation,” he concludes.
Jackie Jones is consulting editor to Renewable Energy World
Mike Hawkins is global brand manager for Exxon Mobil Mobil Lubricants & Petroleum Specialities Company