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Gearbox problems have been a significant challenge to wind turbine operators since the birth of the industry. Over the years, each stage of turbine development has been accompanied by an increase in rotor diameter, resulting in the input loads — which increase with the cube of wind speed and the square of rotor diameter - often being underestimated by those choosing the technical specifications for the gearbox.

Sometime a new turbine type and accompanying gearbox design work well across a whole fleet of turbines. But this is not guaranteed. Problems can still crop up unexpectedly even where, in general, performance of the turbine/gearbox combination has otherwise been good. A prime example is the so-called white etching crack (see box, overleaf), where the problem itself needs to be fully understood before solutions can be found.

"Although wind turbine gearboxes are designed for an operational life of 20 years, in practice an onshore turbine gearbox typically used to last five to eight years before needing to be exchanged or repaired in-situ," says Martin Elliott, senior associate at renewable-energy consultancy BVG Associates (BVGA) with responsibilities that include the Lowca and Askam wind farms in Cumbria, north-west England. Lowca and Askam, each with seven Vestas V47 660kW turbines, were commissioned in 1999 and 2000.

"But now, the general expectation is eight to ten years. In the light of experience, if you only have to change a gearbox twice in 20 years then you are doing well," says Elliott.

Staying up-tower

Cutting the costs of gearbox failure is vital to wind operators. They are pushing hard to reduce operations and maintenance (O&M) costs to minimise the levelised cost of energy, and there is a constant demand for improved performance, points out Mikko Sillanpaa, global head of field service at gearbox manufacturer and service provider Moventas.

"The number of types of different gearbox repairs that are possible in the tower has increased tremendously over the past seven years or so," he says. Aside from the cost of the gearbox repair itself, if the work can be done up-tower, or at the bottom of the tower, substantial cost savings are made possible by avoiding the use of a large crane and minimising the transport logistical requirements.

Looking back some 30 years, up-tower repairs were limited to areas that were accessible without having to open the whole gearbox, such as a simple bearing or shaft exchange. But about 10 to 15 years ago, the industry began considering how larger and more complex components that were not easily accessible could be repaired in-situ.

Although up-tower repairs were often possible mechanically, there was a reluctance to open the whole gearbox housing, especially if the nacelle roofing had to be opened to allow access. This was because of fears that impurities would get into the gearbox oil, with negative consequences. Precision of assembly could also be compromised by the narrow space in the nacelle, explains Sillanpaa.

These risks were weighed up against the high cost of shipping the gearbox to a workshop, which would be clean, used high-precision tooling, and allow for good test runs for all necessary parameters as well as condition monitoring.

Crane costs

The situation is rather different today. Up-tower repairs are facilitated with optimised tooling, systemised repairs, improving cleanliness up-tower and using oil-flushing units to remove impurities. Portable online condition monitoring systems allow tests to be run and the gearbox parameters to be measured before full production restarts.

And, as turbines and gearboxes get bigger, up-tower repair is increasingly important "as it adds more to the bottom line, and can save a lot of money", says Sillanpaa.

In Europe, a crane for lifting a 2MW gearbox can cost up to €50,000 ($54,500) a day. In the US, because of the long distances that need to be travelled, it can be as high as $150,000 (€137,000) a day, depending on the urgency and location. Pressure to avoid the use of big cranes is therefore higher in the US than in Europe.

The potential savings are substantial. Repairing the gearbox in the nacelle eliminates the need for a 50-tonne crane to carry out an 80-metre lift, requiring instead only a 10-tonne crane for the 80-metre lift, which can cut crane costs by half, says Sillanpaa.

And repairing the gearbox on site rather than replacing it can save thousands of euros in logistics costs, he adds. If an exchange is required, it is typical for the operator to arrange exchange pooling with a gearbox supplier, such as Moventas, so that the replacement unit is ready on-site, says Sillanpaa. This avoids unplanned costs in the logistics of the replacement or damaged unit.

Full service

The gears usually consist of one or two planetary stages, and one or two helical stages. It has been possible since the early days of wind-turbine deployment to do around half the repairs for the helical stage in the nacelle. Now that the potential risks of opening the gearbox housing in the nacelle have been minimised, it is possible to go much further with helical stage repairs, says Sillanpaa.

Moventas performed its first full helical service in 2011 and has been developing up-tower processes, tooling and safety procedures ever since. In 2012 it started performing on-site repairs in the US using its mobile service units. These are self-contained, climate-controlled mobile workshops fitted with a retractable roof, a power generator, plumbed air line, a parts wash basin and an induction heater.

Last summer the company began tackling planetary stage repairs on site, starting in the US with its high crane costs. The work may take place entirely up-tower, depending on the accessibility of the problem part. Or the gearbox can be disassembled into two modules to allow either or both the planetary and helical stages to be lifted down separately to the bottom of the tower to be exchanged.

In September last year Moventas announced that all failed "low-speed" planetary stage gears and bearings in most GE 1.5MW turbines could be repaired up-tower at a far lower cost than replacing the gearbox. In January, the company offered a similar service for 2.3MW Siemens turbines in the US, confirming it will "offer the same capability in Europe this year for this popular fleet". Moventas also plans to launch the mobile service units used in the US for the European market.

For each repair, customers can opt for longer-life components by upgrading the gearbox up-tower - replacing through-hardened ring wheels and old bearing designs with state-of-the-art case-carburised ring wheels and integrated planet wheel bearings, for example, says Sillanpaa.

Today, gearboxes are still being taken down and exchanged as technicians battle with daily pressures to keep turbine downtimes as short as possible. But the costs can be huge, especially in urgent situations.

Balance of risk

Ideally, a preventive approach using gearbox condition monitoring systems (CMS), allows issues to be detected early for better planning of repairs up-tower or on-site.

"How bad is it? How long can we run the turbine before we have to stop? Can it be fixed up-tower or do we have to take the gear unit down?" says Sillanpaa. "These questions can be answered swiftly enough to avoid catastrophic failure and prevent the worst-case scenario - scrapping the gearbox."

The use of CMS to flag up potential gearbox problems is becoming increasingly cost-effective, agrees BVGA's Elliott. But retro-fitting CMS on to turbines with a rating of under 1MW is still rare, he points out, simply because of the cost. In the UK, it costs £4,000-6,000 (€5,000-8,000) per turbine for the kit and installation, and a further £750-£1,250 a year for the monitoring.

"It's a balance of risk against cost," says Elliott. "For a 15 to 20-year-old wind farm, the CMS may reduce a £50,000 gearbox exchange to a £10,000 up-tower fix. Or it might not ..."

If a gearbox replacement becomes necessary, the costs break down into the price of the replacement component, the technician and crane costs, including labour and materials and, of course, the lost revenue. Weather conditions can make a big difference to the final bill. A loss of three weeks revenue is not uncommon if high winds prevent the work proceeding, says Elliott.

For a sub-megawatt turbine, a gearbox exchange takes around three days: one to prepare the turbine for the work, another for the exchange to take place, and the third to tidy up and recommission. In principle, says Elliott, the schedule is roughly the same for a 3MW turbine.

Citing experience with the Vestas V47 660kW machine, Elliott says a refurbished gearbox costs about £40,000-£45,000. But if taken in exchange for the failed but repairable gearbox, which could be valued at around £15,000, depending on the extent of the damage, then the replacement component from a supplier such as ZF Wind Service or DB Wind (part of Moventas), may in effect cost the operator as little as £30,000.

Aside from the component, the three-to-four technician crew will cost £12,000-£15,000 for the whole job. Adding in a 60-100-tonne crane able to lift up to 50 metres will cost another £1,500-£2,000 a day. This raises the average outlay for a gearbox exchange to around £55,000 to £65,000, not including the income lost due to the interruption of electricity generation. The full cost impact on the wind-turbine owner will depend on what warranty and insurance cover applies.

UNSOLVED PROBLEMS — WHITE ETCHING

Although gearbox quality and repair options have improved over the years, some problems still leave engineers scratching their heads. One that is still challenging gearbox experts is the formation of so-called white etching cracks in the 1-millimetre zone below the steel-bearing contact surface, which leads to a premature wear failure known as white structure flaking. This can occur within as little as six to 24 months of the gearbox operation.

It could be due to hydrogen diffusion into bearing steel, sourced from the hydrocarbon lubricant or water contamination, according to research carried out at the National Centre for Advanced Tribology (nCATS) at the UK's Southampton University. Butterfly cracks can also form during rolling contact fatigue of the bearings, but whether there is a link with white structure flaking is not clear.

NCATS is currently investigating the root causes of white etching cracks in gearbox bearings on several fronts. "In particular, the role of hydrogen and oil additives, the role of carbides in steels, the influence of heat treatment on steels and bearings, and the influence of severe plastic deformation are being researched," says Ling Wang, associate professor in tribology condition monitoring at nCATS.