Year on year, offshore wind turbines are getting more powerful. In this article, SKF Philipp Schmid describes how the industry is rising to the challenges that this poses for the service life and reliability of wind turbine drive trains and their bearings.

Since 2012, offshore turbines with total power outputs in excess of one GigaWatt (GW) have been coming on stream in European waters every year. According to WindEUROPE, Offshore wind in Europe saw a net 1,558 MW of additional installed grid-connected capacity in 2016. 



Despite the challenges of constructing offshore turbines, the installed capacity is expected to grow as suitable land based sites become scarcer and operators take advantage of the greater consistency of wind at sea. The output power of offshore turbines also tends to be greater than their land based counterparts. According to WindEUROPE the average power output of offshore turbines installed in 2016 was 4.8 MW. Turbines of 9MW or more capacity are now in the launching stage – Vestas’ V164-9.5 MW development being a prime example. 



Offshore wind turbines tend to have longer blades, which impose greater forces on drive trains; at the same time, drive trains and their bearings are at greater risk of corrosion due to the saltwater environment. Conducting maintenance offshore is difficult, potentially dangerous and very costly, so operators are keen to reduce the frequency of maintenance visits, which places considerable demands on the rotor bearings and their ability to continue to function reliably in these conditions for long periods. 



There are four common bearing concepts for turbine rotor shafts. The first is a two-point suspension concept with a toroidal roller bearing on the rotor side and a spherical roller bearing on the generator side. This is used for turbines in the 6MW category for example. 



For higher performance classes, the trend is to use ‘rigid’ bearing arrangement that feature a non-locating and locating bearing combining a cylindrical roller bearing and a double-row tapered roller bearing. Alternatively, a purpose designed bearing that combines the two into one bearing, such as SKF’s Nautilus can be used, or arrangements comprising two adjusted tapered roller bearings. In all cases, the design, materials of construction and mechanical geometries of these bearings will have a considerable impact on their ability to function reliably between maintenance intervals. 



Bearing cages, for example, are generally made of machined brass or sheet metal, the latter being more often found in larger bearings. Where possible, cages are always installed in one piece, but for larger bearings, they may comprise rows of segments which are manufactured individually and positioned one behind the other. All types of cage can be inner-ring centred, which produces less wear and thus prolongs the service life of the bearing - clearly important where offshore wind turbines are concerned. 



Technological advances have also been made in recent years in the field of gearbox bearings and these developments are now being exploited in next generation offshore wind turbine gearbox developments. One of the most important advances is black oxidation of the raceway, a surface chemical treatment process that produces a black oxidised layer on the surface of the bearing steel. 



Compared with untreated bearings, black oxidised bearings offer a range of benefits for wind turbine applications, including reduced risk of premature bearing failure caused by white etching cracks, greater resistance to chemical attack by the more aggressive components of some lubricants, lower hydrogen permeation and improved resistance to corrosion. Moreover, black oxidised bearing surfaces offer reduced friction, decreased risk if slip damage and greater tolerance of poor bearing lubrication conditions. 



In addition to all the mechanical and environmental stresses that bearings must overcome in the offshore environment, they must also be able to cope with the potentially damaging effects of high electrical currents. For example, wind turbine generators are equipped with frequency converters, which pose a whole new set of problems for bearings. The three-phase AC voltage outputs of the converter take the form of a series of rectangular pulses, rather than true sine waves, with the result that the total of these voltages is not zero and that there is a common mode voltage. This common mode voltage can result in leakage currents to the generator rotor via its bearings, damaging raceways and compromising lubricant properties. 



In order to prevent the passage of these leakage currents, the rolling elements of generator bearings are constructed from ceramic materials, which also allow significantly higher speed operation than is possible with equivalent steel ball bearings. In addition to non-conductive rolling elements, bearings are also available with ceramic-coated rings, which provide additional insulation to prolong bearing service life. 



So far, we have looked at bearings serving the turbine main rotor and generator; of equal importance is the ability of the turbine nacelle to align itself according to the wind direction and the blades to ‘feather’ in response to wind speed. These functions are supported by slewing bearings, which have also become larger with the rise in turbine power capacity. Double-row four-point contact bearings are normally deployed in blade feathering mechanisms, while the tower bearings are usually single-row, four-point contact bearings. These bearings are spray galvanised to prevent corrosion, and because of the extreme weather conditions of the offshore environment, they are also fitted with special seals. 



A test bench for the future



With the output of offshore multi-megawatt wind turbines expected to top the current 10MW limit in the not-too-distant future, the bearings described above will be subject to even greater forces. In preparation for this, SKF has built a test facility in Schweinfurt that can subject bearings with an outer diameter up to 6m to the forces that they are likely to encounter in the offshore environment. This test bench can accommodate both individual main bearings and complete bearing assemblies, enabling future generations of large bearings to be designed more quickly and subsequently tested under more realistic conditions than has been possible hitherto. 



Source: WindEURope: he European offshore wind industry – key trends and statistics 2016, https://windeurope.org/about-wind/statistics/offshore/european-offshore-wind-industry-key-trends-and-statistics-2016/, retrieved on 23.11.2017 



MHI VESTAS OFFSHORE WIND: The world’s most powerful available wind turbine gets major power boost. http://www.mhivestasoffshore.com/worlds-most-powerful-available-wind-turbine-gets-major-power-boost/, 06.06.2017, Retrieved 23-11-2017 



AB SKF, Testing to the limit: Evolution online http://evolution.skf.com/worlds-most-powerful-large-size-bearing-test-centre-in-operation/ 22.09.2017 Retrieved 23-11-2017