Mobil supplies, amongst other lubricants and greases, the synthetic gearbox oil Mobilgear SHC XMP 320, which is used in more than 30,000 wind turbines across the world.

Kevin J. Harrington, Mobil SHC Technical Manager, says: “If you look at the top 12 wind turbine manufacturers – the majority of them use our oil in their gearboxes at the factory.

“And what we find as an increasing trend is that as more and more turbines come off warranty – even if they’re not filled with our oil at the factory – the wind farm operators recognise these benefits and are choosing to switch to our oil in order to improve their productivity by reducing their lubricant-related downtime.”

So what properties are needed? For a gearbox to work efficiently, a lubricant has to strike a balance between: micro-pitting protection, gear and bearing protection, oxidative stability, viscometrics and low temperature performance, filterability, water tolerance, foam and air release, and rust and corrosion performance.

“There are other lubricants that provide micro-pitting protection, but they don’t necessarily have low temperature performance. Because a gearbox that’s operating on a factory floor in a temperature controlled environment doesn’t need to have excellent low temperature viscometrics,” Harrington explains.

“The biggest driver, I think, for wind turbine operators to move to a synthetic lubricant like Mobil Lubricant SHC, is oil life. Because when you talk about a wind turbine that’s 100 m up in the air, there’s no such thing as routine maintenance. Every maintenance activity is a major project.”

Harrington says average service life is 18 months for a mineral oil, whereas a synthetic oil and do three years or more if monitored.

Another issue for wind turbine gearbox oil is moisture.

“The gearboxes go through a thermal cycle almost every day. It’s very typical that they don’t run 24 hours a day and that there’s a lull in the wind speed; so the turbines shuts down, the gearboxes coold off, and they pull air from the atmosphere into the gearboxes – which brings moisture into the gearboxes.

“The next day, the gearboxes warm up, and you repeat the cycle over and over again. So the oil needs to be able to handle water contamination. It needs to maintain its filterability,” Harrington explains.

Mobil therefore formulated its gearbox lubricant so it separates from water.

“When we look at a typical case of oil analysis for our product, we find that the water levels are below 100 parts per million. When we look at some competitive oils, we see 50% of the samples maybe above 900 parts per million,” he says.

“One of the most crucial components of a wind turbine is the bearings in the gearbox. And most gearbox failures start as a bearing problem, and then the bearing problem becomes a gearbox problem.

“High levels of water in oil contribute to reduce bearing fatigue life. These bearings are designed to last at least 20 years, but with higher levels of moisture in the oil, the fatigue life calculations would show that they would not last 20 years.”

Offshore vs. onshore requirements

Asked whether there are different requirements for offshore and onshore wind turbines, Harrington says that in general, they are quite similar.

“A key difference is when we talk about corrosion performance. The salt that is present in the mist offshore – that makes the challenge around rust protection much more significant,” Harrington says.

“Protecting a wind turbine onshore from rust is, from our perspective, an easy technological challenge, provided that we maintain the balance of the rest of the formulation.

“When we go offshore, we have the salts that are in seawater – that makes it that more difficult.”

Mobil Lubricants has experience from several offshore units, including 6 REpower 5M offshore wind turbines at Thornton Bank offshore wind farm in Belgium.

“I have never seen any issues of rust with any of those turbines,” Harrington adds.

Staying ahead

In contaminant tests, “our oil today is still the reference oil,” says Harrington. And although “we don’t see anybody who delivers the same comprehensive balance of performance as we do,” he sees no reason to rest on their laurels.

“We call on the engineering divisions of the major builders we do business with, and we also talk to their gearbox suppliers and the key components suppliers to the gearbox suppliers. We want to make sure they understand lubricants – how lubricants can help them as they design their equipment.

“We also want to understand where they are going – what they see as the trends and their new lubrication requirements for their equipment.”

The GE specifications for gearbox oil to be used in its wind turbines, for example, is 50 pages long. Mobil Lubricants has nonetheless been approved for use in GE’s wind turbines.

“The challenge is not to improve one dimension or another,” Harrington continues. “It’s to maintain excellent performance on all parameters without sacrificing something somewhere else.”

China next?

Asked what the main markets are for Mobil Industrial Lubricants, Global Brand and Marketing Manager Michael J. Hawkins, says currently, the USA is the main market, followed by Germany and China.

China is a fast growing market, and will be attractive in the future. But Hawkins cannot say when. “We follow the heart of the market,” he adds.

Either way, Mobil is already planning its next products. “We already have the next-generation of oil ready to go,” Hawkins reveals without disclosing further information.

“You can always improve. Gearbox technology is improving very rapidly – we want to be at the forefront of that curve.”