IMPROVING REALIBILITY OF WIND TURBINE GEARBOXES THROUGHT OIL ANALISIS

Transcription

1 PAPER REF: 3142 IMPROVING REALIBILITY OF WIND TURBINE GEARBOXES THROUGHT OIL ANALISIS Beatriz Graça 1(*), Jorge Seabra 2 1 Institute of Mechanical Engineering and Industrial Management (INEGI), Unit of Tribology, Vibrations and Industrial Maintenance (CETRIB), Porto, Portugal 2 Department of Mechanical Engineering and Industrial Management (DEMEGI), University of Porto, Portugal (*) bmg@fe.up.pt ABSTRACT Wind turbine gearbox failures represent dramatic consequences in terms of turbine availability to generate energy as well on maintenance and operating costs. In order to improve reliability, several approaches are being applied and continually developed. One of those, rely on the lubrication field: the lubricating oil in the gearbox must guarantee a good lubrication of their surfaces independently of the operating conditions. For that, it is essential to keep it free of contamination and with capabilities to protect surfaces from abnormal wear, extending the life of the wind turbine gearbox. This paper will focus the important contributions that an effective oil analysis program can provide to increase reliability and availability of wind turbines, minimizing maintenance costs associated with oil change outs, labour, repairs and downtime. Information to assess lubricant condition, contamination and mechanical wear can be successfully achieved if there is a strategic combination of techniques and, more important, an accurately understood of the results obtained. 1. INTRODUCTION The fast-growing wind industry is developing larger, increasingly efficient and reliable wind turbines that require equally capable and durable lubricants (Magats, 2007). The most expensive components of a wind turbine, besides tower and blades, are gearbox and bearings, requiring about 13% of the total costs. In Western Europe, the average lifetime of these important components is between 5 to 8 years and the costs to replace the main gear multiplier of a 1.5MW wind turbine after an unexpected failure could cost around 250,000 euros. According to Wind Operation and Management Report (The Wind Energy Update s, 2011), as much as 66% of the operation and maintenance (O&M) costs for some offshore wind turbines may be related to unplanned maintenance actions. Given that gearbox is one of the most costly component in the turbine drivetrain to maintain through the expected 20-year design life of a wind turbine, the implementation of an effective oil analysis program could be a necessary step to provide a high performance of the turbine lubricant which will maximize the equipment productivity while maintenance costs will be minimized. An effective monitoring program will highlight trends within lubricants, identify potential problems and help to optimize equipment maintenance plans (Barrett, 2009). Wear debris analysis enables a precise diagnosis of the health of equipment, including recognising when abnormal wear has started, the type of wear and identification of the wearing component (Tschauder, PES: Europe). Combined with infrared spectroscopy analysis (FTIR), which in turn gives information about the lubricant degradation process, a reliable ICEM15 1

2 Porto/Portugal, July 2012 diagnostic of the turbine condition can be attained. This gives the service/repair department the option of performing a closer inspection of the equipment to quickly detect and amend the source of the problem. In this case study, the lubricant of the main gearbox of two wind turbines (VESTAS V80 2MW) have been analysed through different analysis techniques, including Ferrography and Fourier Transform Infrared Spectroscopy (FTIR). Results had shown that lubricant analysis can be used as a tool to provide important information about the operating condition of critical components (bearings, gears, etc.) and lubricant degradation. Improved reliability and availability of wind turbines can be definitely achieved with founded decisions and proactive strategies. 2. IMPROVING WIND TURBINE RELIABILITY Considering the extreme environmental and mechanical pressures at which wind turbines operate, their reliability is quite high. This doesn t mean that there is a high availability achieved by mostly wind turbines. However, through their expected-life time, the O&M costs have to be under control, and some efforts are applied to achieve a decreasing path. One strategy to accomplish a reduction of wind turbines O&M costs can be based on the following strategies: Identify Critical Components turbine critical components are those showing failure high-risk, either because they demonstrate to be failure prone, the failure diagnostic and repair are expensive and time-consuming or they are absolutely essential to turbine operation. Gearboxes are certainly one of these critical components and so, a close monitoring will provide strategic information for the O&M staff; Failure Modes Characterization the understanding of the failure mode allows the O&M staff to focus in the monitoring assets to identify the problem and potentially delay or prevent catastrophic failure. Rolling bearings and gear wear can be detected early with scrupulous oil analysis techniques and actions can be taken to avoid progression of the damage (p.e. more frequent oil changing or better filtering); Root Cause Identification any failure analysis represents always an opportunity for improvement. Evaluating the root cause of a major component failure is essential to determining if the failure is due to poor manufacturing quality, inappropriate design or product misapplication (Christopher, 2006). Bearing and gear failures are frequently lubrication related. Consequently, root cause analysis should be focused on the information revealed through the wear debris generated and carried by the lubricant. 3. IMPORTANCE OF THE OIL ANALYSIS Wind turbine`s reliability greatly depends on the proper operation of the gearboxes. The gearboxes are constituted by planetary gears and bearings which are submitted recurrently to extreme operating conditions in terms of high variation of loads, contamination and large difference in external temperatures. Consequently, the oil used to lubricate gearboxes reached, in the last years, a primordial importance to provide effective gear and bearing lubrication and an increase in the gearbox lifetime. The lubricating oil of any mechanical application carries always a great deal of information about the operating condition of the equipment as well as provides a leading indicator of what could be the condition if no corrective action is taken. Lubricant analyses answer to several 2

3 questions, such as: Is there severe wear particles? Is there contamination? Has the chemistry of the oil been compromised? Such answers combined with other data permits the identification of premature failures in advance allowing a well-documented approach to maintenance decisions. Proactive actions can securely be implemented and/or the corrective work can be well planned and scheduled. This approach will conduct to minimize O&M costs and simultaneously extend the wind turbine gearbox lifetime. Some of the more common failure modes that Oil Analysis can detect include: Accelerated oil degradation; External contamination (water, dust particles, etc.); Component fatigue and wear (sliding, abrasive, corrosive, etc.); Over extended oil drains; Poor oil handling/sampling practices; Incorrect lubricant use/selection; Inadequate contamination control measures (filtration, leakage, etc.). To understand the contents enclosed in the combined information obtained through several oil analysis techniques, a certain expertise is required in either, tribological fundamentals and in maintenance engineering. Lubricant failure causes equipment failure and vice-versa. The oil analysis program should be designed to recognize both modes of failure. Fig.1 Combination of oil analysis techniques to improve wind turbine gearbox reliability. ICEM15 3

4 Porto/Portugal, July OIL CONDITION The oil quality does not directly reveal any damage but gives the chance to avoid damage from developing at all. Lubricating oils undergo complex chemical changes when in-service as a result of thermal, physical, and oxidative stresses imposed on them as they lubricate moving surfaces. Keeping the oil in a proper condition is one of the main premises to attain the optimum lifetime of a component. There are several modes of oil degradation, depending mainly on the exposure to contaminants (dirt, moisture, wear particles, etc.), the oil formulation (base oil and additive package) and the severity of the application operating conditions (load, speed and temperature). Two common modes of failure in wind turbine gear oils are additive depletion and the formation of sludge and deposits (Livingstone, 2012). Both modes directly impact the performance of the lubricant and its ability to protect the gears. The recommended tests to evaluate windmill gearboxes should include, at least, Fourier Transform Infrared Spectroscopy (FTIR) and Viscosity measurements. Inductively Coupled Plasma Spectroscopy (ICP) could also help to assess the oil condition determining the source of the problem by elemental analysis. Fourier Transform Infrared Spectroscopy (FTIR) is an instrumental technique that has the ability to track a wide range of functional groups associated with chemical changes in lubricants allowing an overall quality assessment.various in-service oil condition monitoring parameters, such as oxidation, nitration, soot, water, ethylene glycol, fuel dilution, gasoline dilution, sulfate by-products and phosphate antiwear additives, can be measured by FTIR spectroscopy. Changes in the values of these parameters over operating time can then be used to help diagnose the operational condition of the windmill gearboxes and to indicate when an oil change should take place. Recently, ASTM International introduced a series of standard practices and test methods for Condition Monitoring of Used Lubricants (ASTM E2412, D7412, D7414, D7415 and D7418). Viscosity is the most important property of a lubricant since it determines the film thickness that prevents contact between moving surfaces. Consequently, the viscosity measurement and trending analysis is important and may indicate degradation of the base oil, additive depletion, contamination and the eventual addition of an incorrect lubricant. The kinematic viscosity is routinely measured at 40 C (ASTM D445) and the results are compared with specifications and trended. In applications where the operating temperature varies considerably, such as in wind turbines, could be important to obtain the Viscosity Index (VI), which measures the variation of viscosity with temperature. For that, another kinematic viscosity measurement at 100 C will be required. The VI change could be related with lubricant degradation and/or contamination. ICP Spectroscopy is a technique that measures the concentrations of various chemical elements in the oil. Wind turbine gear oils are formulated with extreme pressure (EP), anti-wear (AW) and anti-oxidants (R&O) additives containing several metallic elements (phosphorus, zinc, etc.). Some of these compounds react and form inactive byproducts with similar elemental compositions but in a different molecular form. This should be considered when ICP spectroscopy is used to monitor additive depletion. The concentration of additive elements does not necessarily represent the presence of active chemical compounds. Thus, judgement and experience is required when trending these values. 4

5 3.2. OIL CONTAMINATION Contamination is the Nº 1 cause of lubricant-related machine failure. It is also the Nº 1 cause of lubricant degradation (Fitch, 2012). Over the last decades, wind turbine manufacturers have increasingly focused on oil quality and cleanliness, which has an enormous impact on the bearings lifetime and gearbox performance. This is because higher output means more strain on gears, increased mechanical wear and a greater chance of oil contamination. The three main sources of oil contamination are solid particles, moisture and oil oxidation by products. Contamination can enter gearboxes during manufacturing, be internally generated, ingested through breathers and seals, and accidentally added during maintenance. New oil contamination also occurs as result of wrong manner in which the oil is stored. All of these sources must be addressed in order to successfully contain the potential negative impact of contamination on equipment components (Muller, 2002). Contaminants can promote the severe wear rate, accelerate fluid degradation and cause reliability problems. Therefore, the oil cleanliness monitoring and the identification of external contamination in the lubricant are important issues in a proactive maintenance program. The following tests are fundamental to monitor contamination problems in wind turbine gear oils: Fourier Transform Infrared Spectroscopy (FTIR) can detect the presence of water, identify specific chemistries of foreign fluids and the occurrence of varnish and sludge. Water ingression depends upon climate, operating and maintenance conditions where gearboxes are put into service. Small percentages of water can significantly degrade the gearbox oil by causing the lubricant to foam or lose its ability to generate a film between contacting surfaces, contributing to gear micropitting and additionally short the bearing life. Particle Counters are used to measures insoluble dirt and hard particles in fresh or inservice oil, to evaluate the effectiveness of lubricant filters and to determine the oil cleanliness level under the ISO 4406:99 Standard (International Organization for Standardization, 1999). The ISO code obtained represents the number of particles in three different size categories, >4 µm, >6 µm and >14 µm determined in one milliliter of oil sample. It is important ensure that oil used for factory tests and service oil meets cleanliness requirements. If not, it is imperative to prefilter it. Use a 3-µm filter during factory testing to remove any contamination remaining after assembly or added during testing. After the factory test, drain the oil, flush the gearbox and install a new filter element. If the oil does not meet cleanliness standards after the factory test, then gearbox assembly cleanliness should be improved (Muller, 2002). As part of preventive maintenance, oil is cleaned to obtain longer lifetimes of mechanical parts and oil. Table 1 Required Oil Cleanliness for Wind Turbine Gearboxes Source of Oil Sample Required Cleanliness ISO 4406:99 New Oil 16/14/11 Gearbox after factory testing 17/15/12 Gearbox during service 18/16/13 ICP Spectroscopy can detect the ingression of metallic components in an in-service lubricant. From a contamination perspective, this is often silicon from dirt. However, silicon is also used as anti-foaming additive in wind turbine gear oils. Other metallic ICEM15 5

6 Porto/Portugal, July 2012 contaminants that may be found are potassium, sodium, calcium, zinc, boron and barium. It is important to analyse a new oil sample to be used as reference, since some of these metallic elements are from additive compounds and/or steel metal alloys. Analytical Ferrography is a qualitative technique that provides valuable information about particulate debris present in an oil sample, extracted and deposited on a microscopic slide called a Ferrogram. This technique requires some skill and knowledge on the microscopic examination, to recognize metal from non-metal particles (organic/inorganic) and ferrous from non-ferrous particles. A recent standard method was developed to characterize the particles using polarized dichromatic microscope (ASTM D ). The microscope uses both reflected (top) and transmitted (bottom) light to distinguish the size, shape, composition and surface condition of ferrous and nonferrous particles. Caution must be exercised when drawing conclusion from the particles found, especially if the sample being examined is the first from that type of machine ABNORMAL WEAR Some parts of wind power equipment often produce abnormal wear because of assembly, severe operating conditions and poor oil quality. Thus, analyzing the causes of abnormal wear is very important point for the operation and management staff. In terms of a wind turbine gearbox, the vast majority of the particulates suspended are wear particles which have become detached from different gearbox components. So, wear failure condition diagnosis can be effectively made by the quantitative and qualitative analysis of wear metal particles in the oil, which can guide the maintenance personal to take timely measures to carry out condition maintenance, and to avoid further deterioration of the accident to ensure the safe operation of the wind turbine. Ferrography is one of the most reliable techniques in providing valuable information about the wear evolution in gearbox components. Direct Reading Ferrography is the instrument that quantitatively measures the concentration of wear particles in lubricating oil using a magnetic method. The index readings are indicated as density small (DS) and density large (DL). DS represents all particles measuring up to 5µm in size whereas DL indicates all particles greater than 5µm in size. When there is a sharp increase of the DL index, it is indicative that abnormal wear is in progress and an analytical ferrography is needed. ICP spectroscopy could also be used to quantify wear elements (Fe, Cr, Cu, Al, Ti, etc.). However, its detection limits due to particle size (>3 micron particles are not efficiently detected) turns this technique blinded or unable to detect large wear debris particles form abnormal wear. Analytical Ferrography is often referred to as the oil analysis equivalent of criminal forensic science (Huysman, 2011). Analytical Ferrography utilizes microscopic analysis to identify the composition of the material present. This technology will differentiate the type of material contained within the sample and determine the wearing component from which it was generated. Wear particles can indicate the degree and type of damage that is being experienced by different gearbox components. The size and shape of particles are dependent on the type of wear they have experienced whilst the number can signify the degree of damage that has occurred 6

7 (Maslach, 1996). In a windmill gearbox several wear mechanism may occur simultaneously (see Table 2). A trained ferrographic analyst is able to use the size, shape, concentration and composition to identify the wear mode and status of the equipment. This allows a skilled diagnostician to determine the root cause of a specific tribological problem. Not only the overall equipment condition can be determined, but the lubricant condition can also be analysed by looking for particles such as those listed in Table 3 (Maslach, 1996). Table 2 Wear types and particles which identify abnormal wear Wear type Particle characteristics Size (µm) Possible cause Rubbing Wear Platelets < 15 Sliding of components Cutting Wear Severe Sliding Wear Bearing Wear Gear Wear Fine chunks/spirals like machining swarf Deep grooves within irregular-shaped particles, chunks Fatigue spalls Laminar particles Fatigue chips from gear teeth Scuffing/Scoring particles > 5 Misaligned components, abrasive contamination, cracks in the wear surfaces > 15 Poor lubrication, severe operating conditions (speed/load) > 15 Abnormal rolling wear in rolling bearings or between gear teeth > 15 Scuffing and scoring of the gear teeth around the pichline Spherical Wear Smooth and rough spheres < 5 Early indication of an abnormal rolling contact Table 3 Particles which identify lubricant condition. Particle type Characteristics Possible causes Black Oxides (Fe 3 O 4 ) Black and irregular in shape High operating temperatures due to inadequate lubrication Red Oxides (Fe 2 O 3 ) Red and irregular in shape Rust (water presence) or dirt contamination Corrosive Wear (FeO) < 1 micron in size Acidic lubricant due to additive depletion Lubricant Degradation Irregular shape amorphous matrix containing ferrous particles Overstress causing breakdown of the lubricant structure Dirt Sand, dirt, fibers, etc. Filter or breather leak ICEM15 7

8 Porto/Portugal, July WIND TURBINE OIL ANALYSIS Case Studies The lubricants of the main gearbox from two identical wind turbines (VESTAS V80 2MW) located in different wind farms, have been evaluated through different analysis techniques, including Viscometry (IP 212/92), Total Acid Number (TAN) (ASTM 974:04), Water (ASTM D95), Ferrometry (DRIII), Analytical Ferrography (FMIII), Particle Counting (ISO 4406:99) and Fourier Transform Infrared Spectroscopy (FTIR). These analysis have been performed to evaluate the oil condition and the gearbox wear. Since no historical data has been supplied and no previous oil analyses have been made before (in our), new oil samples have been analyzed and the results have been used as reference. The following table presents some information and results obtained for each sample. Table 4 Oil analysis results obtained from each sample Wind Turbine 1 Wind Turbine 2 NEW OIL1 USED OIL1 NEW OIL2 USED OIL2 Hours/Oil 0? Base Oil Saturated Ester Mineral VISCOSITY (cst@40 C) 320,0 317,6 322,7 341,6 TAN (mgkoh/g) 1,75 13,2 0,56 1,02 WATER (%) - < 0,05 - < 0,05 FERROMETRY DL - 27,5-80,2 DS - 18,1-31,6 PARTICLE COUNTING 17/15/13 23/22/ The Used Oil from Wind Turbine 1 reached a high value of TAN (13,2 mgkoh/g) and a high ISO Cleanliness Code (23/22/19). However, the viscosity didn t change and the Ferrometry results show relatively high wear indexes, but these results are not considered critical values for gear applications. In Wind Turbine 2 there was a significant viscosity increase (6%) but still inside the recommended limits defined by DIN /ISO 3448 for the class ISO VG320 (288cSt < ν < 352cSt). The wear indexes obtained are very high showing the presence of an abnormal wear condition. These results can be seen that the oil from the Wind Turbine 1 presents an advanced degradation problem while in the Wind Turbine 2 the problem is wear related. The additional analyses that have been made will provide some insight into these two problems and will help to identify their root cause WIND TURBINE 1 In order to determine the cause of such high oil TAN without any change of the viscosity and no water contamination, Fourier Transform Infrared Spectroscopy (FTIR) analyses have been performed. Figures 2 and 3 show the infrared (IR) spectrums obtained. 8

9 New Oil Used Oil Fig.2 IR Spectra in transmittance/wavenumber (cm -1 ) of New and Used Wind Turbine Oil. Upon analysis of the spectrum overlays obtained through FTIR, shown in Fig.2, there are clearly visible relative molecular changes in the oxidation/nitration peaks ( cm -1 ). Water contamination can be identified through typical spectrum peaks ( cm -1 ) as well as the formation of deposits due to the presence of insoluble products resulting from the degradation process in the lubricating oil (2000cm -1 ). The differential IR spectra, obtained by subtracting the IR spectrum and presented in Figure 3, show the main chemical changes occurred and clearly confirm the advanced degradation of the oil. Water Insoluble/Soot Oxidation Nitration Fig.3 Differential IR Spectra in transmittance/wavenumber (cm -1 ) identifying the changes in the used oil in relation to new oil. The IR analysis brought to evidence several degradation mechanisms: Water contamination since the base oil of this windmill gearbox being a saturated ester, the presence of water could be untraceable. Saturated ester oils have the ability to absorb relatively high amounts of water. This could prove to be beneficial when there is some water emulsified, which is dangerous to any lubricated systems. However, the reactions occurred through water absorption by the oil results in molecular changes in it, promoting its oxidation; ICEM15 9

10 Porto/Portugal, July 2012 Oxidation results from the reaction of the oil molecules with oxygen. High temperatures, presence of water and metal particles accelerate the oxidation mechanism. This will promote the generation of carbonyl acids (degradation byproducts) which increase the acid number (TAN), thus converting the oil in a corrosive agent. Nitration - similar to oxidation, but at low operating temperatures, nitration results from the reaction of oil hydrocarbons, with nitrogen oxides that are produced as a result of the oxidation of atmospheric nitrogen. Nitration products (nitrate esters), are the major cause of varnish and lacquer build-up. As the insoluble/soot content of the used oil increases, so does the nitration level. Insoluble/soot is the result of the previous reactions, which generates insoluble particles causing deposits. The increased acid number combined with a change in the FTIR oxidation/nitration/sulfation value is a strong indicator of fluid degradation (Johnson, 2009). The high cleanliness code obtained is explained also by the large amounts of contamination resulting from the presence of insoluble. Analytical Ferrography was conducted to verify if the oxidised condition of the lubricant had promoted any abnormal mechanism of wear in the windmill gearbox components. These results are presented in Figure 4 through the following microphotographs. Photo 1 Photo 2 Photo 3 Photo 4 Photo 5 Photo 6 Fig.4 Analytical Ferrography analysis from the Used Oil of Wind Turbine 1. 10

11 The ferrous wear particles deposited on the ferrogram and observed under low magnification (Photo 1 at 200x) were not abundant but some revealed having been generated by severe wear. Under higher magnification (1000x) more detailed wear particle information can be obtained: Photo 2: large ferrous wear particle typical from fatigue wear; Photo 3: large ferrous particle from fatigue wear and some spherical particles (a); Photo 4: black iron oxides (b); Photo 5 and 6: medium size ferrous fatigue wear particles thermally oxidized. Although the size and quantity of the wear particles doesn t shown strong evidences of severe wear, the presence of significant amounts of medium size ferrous particles severely oxidized reveals the presence of the following kind of oxidation mechanisms: thermal oxidation, due to high operating temperatures and/or poor lubrication or chemical oxidation, produced by the advanced degradation of the lubricant. This lubricant has to be immediately changed to prevent a major damage in the windmill gearbox WIND TURBINE 2 Analytical Ferrography was performed to identify the cause of high wear indexes obtained through Ferrometry. The wear particles observed under microscopic analysis are presented in Figure 5. Photo 1 (a) Photo 2 (b) (c) Photo 3 Photo 4 Photo 5 Photo 6 Fig.5 Analytical Ferrography analysis from the Used Oil of Wind Turbine 2. ICEM15 11

12 Porto/Portugal, July 2012 As can be seen, the wear particles deposited on this ferrogram were generated by a very severe wear. Under low magnification (Photo 1 at 200x), it can be observed that the size, the shape and the concentration of particles (ferrous and non-ferrous) are typical from: (a) severe fatigue wear; (b) gear scuffing; (c) severe sliding wear (chunk). These particles were submitted to heat treatment (325 C during 90sec.) to identify their metallurgy. As can be observed in Photo 2, the fatigue and scuffing wear particles are low alloy steel material (gear teeth) and the large sliding particle is copper alloy, from a severe bearing damage. Additional information about the wear mechanisms present can be reached using higher magnification and observing other areas of the ferrogram: Photo 3: high magnification (1000x) of the ferrogram entry shows the oxidized surface of a severe fatigue particle (a), evidencing high temperatures in the contact; Photo 4: high magnification (1000x) of the mid-section of the ferrogram, shows a large sliding chunk from copper alloy and small ferrous wear particles; Photo 5: low magnification (200x) of the ferrogram end shows the presence of very small wear particles, typical from corrosive wear; Photo 6: high magnification of Photo 5 (1000x) shows a copper alloy particle over the corrosive wear particles. This case study identifies a possible abnormal wear condition in the rolling elements. As already evidenced by the National Renewable Energy Laboratory (NREL) in USA, the majority of the wind turbine gearbox failures appear to initiate in the bearings. However, if this problem is not been detected and located early on, further damage can occur. As shown previously, the ferrous wear particles analyzed already shown an advanced fatigue wear process in the gear teeth. A suspected cause (misalignment, oil contamination, etc.) need to be confirmed to enable a strategic and corrective action to avoid a potential gear failure. 5. CONCLUSION Considering the limited accessibility of wind turbines and the long lead times for supplying gearbox components, oil lubricant analysis offers an attractive, proactive way of maintaining and servicing wind turbine units, leading to improve their reliability. If an oil analysis program is correctly implemented and performed, important information can be obtained, which will in turn, provide the maintenance personnel with the ability to improve equipment availability rates, lower downtime and save significant amounts of money that could be used on other maintenance requirements. The combination of wear particles analysis with molecular evaluation through infrared spectroscopy is a powerful tool to identify prematurely problems in wind turbine gearboxes and to understand the alterations occurred in the lubricant chemistry that could related with the problem origin. 12

13 REFERENCES Magats, Shalini, Wind Turbine Oil Trends and Best Practices, Monograph, Eric Bevenino, Industrial Lubricants & Solutions, North American Lubricants. Chevron Texaco, USA, The Wind Energy Update, The Wind Energy Operations & Maintenance Report, London, UK, Barrett, M., Stover, J., Understanding Oil Analysis and How it Can Improve the Reliability of Wind Turbine Gearboxes, Insight Services, TESTOIL, USA, Tschauder, K., Deutsche BP AG, Industrial Lubricants & Services, PES: Europe. Christopher A. Walford, Wind Turbine realiability: understanding and Minimizing Wind Turbine Operation and Maintenance Costs, SANDIA Report, SAND , USA, Livingstone, G., Pradhan, A. and M. Mary, Wind Turbine Gearbox Oil Analysis Strategy, FLUITEC, Technology + Expertise, USA, April Fitch, Jim, Linking Enhanced Reliability to the State of Lubrication, Machinery Lubrication magazine, April International Organization for Standardization, ISO 4406:1999(E), Hydraulic fluid power Fluids Method for coding the level of contamination by solid particles. Muller, J. and Errichello, R., Oil Cleanliness in Wind Turbine Gearboxers, Machinery Lubrication magazine, July Huysman, W., Utilizing Analytical Ferrography for Root Cause Analysis and Failure Prevention, ReliabilityWeb.com, Maslach, J., Ferrographic Analysis of Grease-Lubricated Systems: An Analysis of Greases in Roller Bearings, Lubrication Engineering, September Johnson, M. and Spurlock, M., Best Practices, Strategic oil analysis: Developing the test slates, Tribology & Lubrication Technology, July ICEM15 13