national Centre for Tribology at Southampton (nCATS), School of Engineering Sciences, University of Southampton,Southampton, UK.www.southampton.ac.uk/ncats

Wind Energy - Challenges for Materials, Mechanics and Surface Science, 28th Oct 2010, IoP, London

Robert J.K. Wood

Tribology and corrosion aspects of wind turbines

Tribology

• Science of lubrication, friction and wear

• Sand and liquid particle erosion of blades

• Gearbox performance

• Bearings ( blade pitch and yaw, shaft etc)

• Oil/grease lubrication systems

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Estimated life of wind turbines is about 20 years, compared to conventional steam turbine generator units that have averaged 40 years

The failure rate of wind turbines is about 3 times higher than that of conventional generators (due to transient loading).

Therefore, reliability is essential to the success of wind energy systems and this demands good tribological designs and practice coupled with appropriate condition monitoring. This would also reduce maintenance costs. 3

Introduction

Introduction

Sustainability of turbines needs to be increased

Offshore wind turbines require a suitable support structure

Tribology of offshore energy conversion technologies should consider the whole system not just the tribologicalcomponents in isolation

Corrosion now a growing focus.

Talk outline

• Overview of wind turbine industry:

• Tribological challenges

- Drive train and gearbox

- Rolling element bearings

- Premature subsurface fatigue

- Butterfly and White Etching Cracking (WEC)

• Corrosion control (a growing concern)

• Research inputs for alleviation

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Tribology of wind turbines

• Includes solid particle (hail, sand, dust/salt particles etc) and foreign object damage of blades,

• tribocorrosion of blades and bearings,

• biofouling of tower and ice on blades causing increased drag,

• unknown and unusual loadings on bearings and seals (40°misalignment),

• extremely high torques through the drive train as well as lubricant degradation particularly from salt water contamination and atmospheric corrosion.

• All of these add uncertainty to the lifetime of tidal and wind turbines

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Main tribological components

Generator

Brake

GearboxMain shaft and bearings

Coupling

Blade pitch bearings

Yaw bearing

Lubrication system

Size effect on Power and Torque

• The shaft power is proportional to R2 but probably more important to the tribological components is the torque which is proportional to R3.

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Wind turbine size development

470 MNm

70 MNm

Economics of large turbines

unproven

After RS Heemskerk, TC Wind, SKF

Challenges

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Gearbox: Step up from 10 to ~1500+rpm. Low-speed bearing is subjected to cyclic loads due to the transient loading. High-speed bearings are highly susceptible to failure being subjected to braking loads and grid coupling loads.

Yaw Drive: Adjusts blades in the direction of the wind at increments of a few degrees. A rolling slewing bearing and their control system suffer from premature failure by wear.

Pitch Drive: The blade pitch control bearings (typically Ø2m+) and the attendant control system also experience premature failure by wear.

Filtration: The gearbox oil used to lubricate bearings & gears is part of acirculatory system contained within the nacelle, which regulates oil temperature and filtering of contaminates. Pressure loss across the filters can reduce contaminate removal efficiency, which can be exasperated by cold starts.

Size: Yaw and Pitch bearings

• Large slew rings bearings are designed to be flexible and modelled to match the blade and hub stiffness to understand the real internal load distribution.

• Premature wear is mainly due to poor grease properties that do not prevent false brinelling (damage caused by fretting, with or without corrosion, that causes imprints that look similar to brinelling) or poor sealing. Solvable

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Wind turbine gearboxes are designed to last 20years!

Average life is only 5 years.

Bearing fail = GBX replaced.Costs £0.1-0.3M.

Gearbox replacement cost

Gearbox

25-70% of downtime due to major component failure. Gearboxes failures

are below these figures while 60-85% of gearbox failures are due to bearing

failures.

Planetary gear system

~2m

Non-classical tribology

• Problem is not poor application of tribology but

• Variable and sometimes high wind loads and many stop-starts (transient loading).

• Flexible structures with often oscillatory behaviour

• Low maintenance regimes

• Premature failure modes have not been seen before

• Some incorrect choice of bearings and variable quality of supply

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Gearbox bearings

~2m

Cylindrical roller bearing

~0.25m

Rolling elements & race

Stresses induced by rolling contact

Cylindrical roller bearing Gearbox bearingsRolling element/race

~5cm~5mm

Cross section

~2m~20cm

Small area

y (mm)z

(mm

)

Example contour plot of the maximum

orthogonal shear stress (τo) in outer

ring subsurface with a central Hertzianpressure of 1.7GPa.

Zone most likely for subsurface yield and rolling contact fatigue

Cylindrical roller bearing Gearbox bearingsRolling element/race

~5cm~5mm

Cross section

~2m~20cm

Small area

1mm

ForcePressure created in

oil film

Approx shear stressdistribution with depth

max

Zone likely to initiate subsurface

fatigue

Premature subsurface fatigue:

Butterflies and White Etching Cracks (WEC)

Butterflies – (10-250µm) WEC – (100µm – 5mm+)

Literature findings

Cylindrical roller bearing Gearbox bearingsRolling element/race

~5cm~5mm

Cross section

~2m~20cm

Small area

Cylindrical roller bearing Gearbox bearingsRolling element/race

~5cm~5mm

Cross section

~2m~20cm

Small area

Factors driving premature subsurface fatigue

- Slip between contact

- Surface shear stresses

- Metal-to-metal contact

- High water content in oil

- Certain oil additive types

- Static electric potential

- Standstill corrosion

50% of literature quote hydrogen as an

accelerator or root cause

Cylindrical roller bearing Gearbox bearingsRolling element/race

~5cm

Cross section

~2m~20cm

Small area

1mm

~1cm

~5mm

1mm

Cylindrical roller bearing Gearbox bearingsRolling element/race

~5cm

Cross section

~2m~20cm

Small area

~10cm

~5mm

Corrosion

• Seawater is a saline solution and a ionic conducting electrolyte.

• Invokes electrochemical reactions that drive metallic corrosion processes.

• Salt spray/droplets

• Flow corrosion

• Type and concentration of salt, dissolved oxygen concentration, temperature, orientation of surface etc

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Interior corrosion protection

• improved painting systems

• maintaining a dry environment inside the machine via sealing.

• The gear and generator are cooled by heat exchangers recycling the air used in the air-cooling system.

• De-humidifying devices can be placed in the tower and nacelle room.

• The de-humidifying system maintains the interior relative humidity below the limit of any steel corrosion risk limit (60%).

• For additional protection, the main electric components (generator, control systems, etc.) have standby heating systems, preventing condensation, even during sudden variations in temperature.

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External Corrosion Zones

Atmospheric corrosion zone

splash zone

Submerged zone

Rotor

Nacelle

Tower

sub-structuremoorings

Sea level

Structural

• Atmospheric Area -multilayered paint system Zinc primer (Thermal sprayed), silicon epoxy sealant, 2 part liquid epoxy, UV resistant top coat of polyurethane. 370 microns thick.

• The Splash Zone – 1 mm thick polyester sprayed coating –this zone needs specialist inspections/monitoring.

• The submerged area – coating plus CP (sacrificial anodes or impressed current)

• 10 to 20 year coating life

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Power generation • Use of VCI (Vapour Corrosion Inhibitor)

Corrosion accelerated damage

• Stand-still corrosion can affect bearing life

• Small amounts of water ingress into lubricant coupled with modest temperature cycles between running (frictional/flash temperature heating) and stand-still.

• Axial cracks on bearing surfaces and spalls correlate to extent of surface corrosion.

• Extent of visual corrosion correlates with subsurface damage (Cracking).

• Possible link between corrosion and hydrogen generation and associated acceleration of WEA/WEC and reduced fatigue life.

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Condition monitoring

Condition-based maintenance for wind turbines is relatively new but is a fast developing research area.

Various on-line and off-line monitoring techniques, including vibration, temperature, debris and electrical monitoring, are being developed for wind turbine condition monitoring. Potential additional techniques include Acoustic Emission, thermal imaging, ultrasonic, fibre optics etc

However, adaptation of these techniques has been challenging and the effectiveness is affected due to rapidly varying load, slow-speed rotation and non-rigid foundations.

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Currently PhD programmes:

– Intelligent health monitoring of oil lubricated hybrid bearings using multiple sensors and GMM approaches developed within the group.

– Premature subsurface failures in wind turbine gearbox bearings (sponsored by Vestas Wind Systems)

– oil age detection using integrated smart sensors (sponsored by Shell Global Solutions, UK)

Research strategy:

– Develop robust sensing techniques and advanced signal processing methods for tribological applications, e.g. subsurface crack and lubrication regime detection using acoustic emissions and electrostatic sensing.

– Develop low friction and wear resistant surfaces such as textured surfaces and self assembled monolayer.

– Modelling of interfaces.

Current Research

Green lubricants

Nano additives development (EP additives)

Solid lubricants and low friction coatings (for extreme environments)

• Use wet and dry depositions to produce v. low friction coatings • nanostructured and hydrophobic • with embedded Nano Particles

IF-WS2 Red - Synthetic oilBlue -Synthetic oil with 0.1% IF-WS2 NPs

Current Research

Corrosion monitoring systems - (BAE Systems, Rolls-Royce, Dstl, GE are all seeking critical information on the most damaging localised corrosion processes).•Provide early warning of costly corrosion damage and critical information on location, and importantly the rate of deterioration. Vital for the economic viability of offshore renewable power generation (wind, wave and tidal).•Effective condition monitoring is essential to make these system economical viable in this very challenging and unforgiving environment.

Antifouling / biocorrosion / biofilm sensing – exploring the bio-electrochemical interface (biologically catalyzed process – surface degradation to the autocatalytic generation of biocides).

Ship resistance effects of marine biofilms and slime fouling (enhance hydrodynamic performance and the science of biofilm attachment).

Natural Product AF Films and Coatings

Current Research Focus:

Incorporation of Natural Products (NP) in marine paints as environmentally acceptable antifoulants

Synthesis of polymeric self-polishing marine paint binders

Laboratory and marine exposure testing of novel antifouling (AF) coatings

Strategic Vision:

Future AF coatings will rely less on toxic compounds and more on surface effects to deter fouling on ships’ hulls

Formation of “slime” (a resistant film of bacteria and microalgae, e.g. diatoms) on the latest generation foul-release AF coatings leads to increased drag and fuel consumption. A solution to this problem is the next challenge for AF technology

Chondrus crispus – a source of NP

Polymer synthesis

Coating formulation and testing

The bearing issues due increasing bearing sizes and extremely high and variable loads. Quality of the steel used is an issue.

The additional cost of providing unexpected maintenance and/or repair significantly affects the cost of wind energy as a whole and undermines the reputation of this green energy source to producereliable energy.

Current bearing and gearbox standards are not good enough to ensure the turbine works well.

Corrosion a growing area of concern particularly corrosion-induced but dynamically driven failures. Drastic reduction in bearing life associated with stand-still corrosion.

Condition monitoring has potential to predict premature failures.

Conclusions

Further reading

R.J.K. Wood, A.S. Bahaj, S.R. Turnock, L. Wang, M. Evans, Tribological design constraints of marine renewable energy systems, Phil. Trans. R. Soc. A 28 October 2010 vol. 368 no. 1929 4807-4827.

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Thank You!

Prof Robert J K WoodnCATS Director

email: R.Wood@soton.ac.ukphone: +44 (0) 23 8059 4881web: www.soton.ac.uk/ncats

Advance invitation to: One day workshop on Marine energy systems: Tribology and corrosion, March 2011, nCATS, University of Southampton