October 2012

During the early years of the North American Wind Industry, the general misconception regarding blade maintenance was that once the blades were in the air, routine maintenance was not required. As the industry matured, some tough lessons were learned. Maintenance, especially on the leading edge of the blade, is not only recommended but essential if blades are to reach their design life expectancy.

In this article, CWC offers answers to the most frequently asked questions by owner/operators using real-life experience data. Those questions are:  

• What measures can be taken to reduce the costs associated with rotor blade maintenance?

• At what point in the life of a rotor blade does the leading edge require attention?

• What detrimental effects will result if the blade leading edge isn’t maintained?

As the name suggests, the leading edge of a blade is the foremost edge of the airfoil or the part of the blade that first contacts air. Any turbine located in terrain that has an abundance of dust, bugs, rain, sleet, snow or any other abrasive air particulate is more susceptible to erosion than a turbine located in an area where these factors are not present. Since wind farms are typically constructed in rural areas where farmers turn the soil multiple times every year, and weather pattern changes are a common occurrence, the majority of all blades manufactured are subjected to abrasives daily. Image 1

To mitigate leading edge erosion, many manufacturers offer leading edge protection in the form of a tape or paintable coating; however, not all do. Companies purchasing blades should be sure to make it clear to the original equipment manufacturer that leading edge protection should not be an optional feature. The small increase in cost to apply leading edge protection during manufacturing will be worthwhile throughout the life of the wind farm. The application of leading edge protection will aid in the prevention of erosion, provided the protection isn’t damaged during shipping or construction. The smallest tear or scratch in leading edge protection will result in a weak link and a point where the erosion will initiate. There are also significant differences in each form of protection, so it’s best to do your research before choosing the protection that best suits your needs. It’s important to understand that even the best leading edge protection system isn’t infallible; therefore blades should be inspected periodically to make sure they are still in good shape.

Unfortunately, leading edge erosion begins early in the life of a blade. If leading edge protection has not been applied during the manufacturing process, blades as young as three years old will begin to show signs of wear. The tip of the blade is more susceptible to wear due to the operating speed of the blade at the tip; however, erosion does present itself inboard of the tip at this stage as well. Even if leading edge protection has been applied, quality issues, perceived as minor in the factory, also play a role in the rate at which a leading edge degenerates. Small air pockets along the leading edge are routinely overlooked and covered up with coating. As the blade bends and flexes, these air pockets expand and contract causing the leading edge to erode from the interior of the coating. Image 2 Simultaneously, external debris causes deterioration on the leading edge from the exterior. When erosion is identified in the early stage, repair is relatively inexpensive when compared to waiting until more severe coating damage appears. If repairs are not done early, damage to the underlying laminate will be present as early as year five.  Regrettably, there remains a large gap in the knowledge and experience of those performing rotor blade repairs, and often the wrong choice of materials is used to repair laminate damage.  Generally, cost is the driving factor when improper materials and an insufficient repair program are chosen. Although the cost of repair is much lower, a repair using incorrect materials will only mask the damage for a short period of time before a compounded problem returns.

When leading edge erosion is not repaired or is repaired incorrectly, the health of the blade and the turbine is jeopardized. Vibrations create undesirable loads that neither the turbine nor the blades were designed to carry. Chordwise and spanwise cracks in the blade coating occur.  A blade shell is made of a “sandwich structure”, either balsa or foam core with laminate layers on either side. Over time, the laminate under the coating cracks and water enters the core material of the blade. As freeze-thaw cycles occur, the blade core expands and contracts separating the laminate from the core. A waterlogged core is also cause for an unbalanced rotor which leads to more unwanted vibrations throughout the drivetrain. When a blade sustains this level of damage, repair options are limited. Either the blade must be brought to the ground to be repaired, which will cost a minimum of $75,000 per set, excluding crane costs; or new blades can be purchased, which is an even more expensive option than that of repair.

The information provided, is based on 16 years of experience in rotor blade inspection and repair and portrays an accurate picture of what can be expected if blades are not maintained properly. Even though there are factors other than leading edge erosion that can cause a blade to fail, the chances that a blade will reach its desired life expectancy are greatly increased if the leading edges are maintained. 

Power plant operation is a bottom line driven business. Once an asset is built and providing power, every dollar spent on operation and maintenance affects that bottom line. Improvements to the plant reliability and performance can be made, but only with sufficent cost justitifcation to demonstrate return on investment. For condition monitoring (CMS), one factor is the known reliability of the particular wind farm, for example, take three onshore wind farms with MW class turbines with three point mounted drivetrain turbines with double spherical roller main bearings (DSRB). This type of turbine arrangement is known to have its issues, with failure of the downwind raceways and rollers. In this article we use a case study using site data to examine the bottom line. First from Weibull curves (a well known reliability analysis method) from several United States wind farms we have the data in Table 1. Note that main bearing failures are dependent on site conditions, bearing type, maintenance practices, etc. – this data is for problematic sites.

From these three farms we can construct a typical US wind farm with this issue for studying costs. The farm has 80 turbines, the size of turbine is approximately. 2MW and over ten years the average number of main bearing failures per year is 2.91 (based on the average data from Table 1), PPA is $55/MWhr, capacity factor is 0.35 and availability sits at 97% (excluding downtime from any main bearing failure). Each wind turbine brings $900 per day in revenue.

A question for the asset manager is: will condition monitoring improve the bottom line for main bearing issues on this wind farm? We paint a few scenarios in Table 2. First compare Scenario A and B, both with no condition monitoring, in each case the bearing fails and little warning was received. Only when the bearing availability is scarce (Scenario B), with three months lead time for repair, is the downtime a significant part of the cost. The cost is largely driven by crane mobilization and repair or replacement costs (costs quoted are typical but vary from site to site and are based on a refurbished bearing). Compare to Scenario C, the CMS system gives early warning of the failure, yet there are still separate crane mobilizations. Savings thanks to CMS are in the order of $40K where bearings are readily available and up to $300K when a lot of downtime is prevented (Scenario C versus A and B). More savings made when maintenance is scheduled more effectively due to the CMS (Scenario D). Given the early warning on the bearings (and it takes months for the DSRB failure mode to progress) repairs for the two failures are now scheduled for the same week, the crane is only mobilized once and savings are now in the order of $150-400K, depending on downtime saved (Scenario D versus A and B). Here the CMS system has added value with payback for retrofit installation and monitoring over three to four years, without considering potential benefits to the gearbox and generator. For different turbines and sites costs vary and where a known reliability issue exists, ROI calculations for CMS systems can be completed to evaluate benefits.

With the slow rotation it is often said that a damaged main bearing is difficult to detect with vibration; this is true where a standard envelope spectra or vibration RMS may not discern the damage from other vibration sources. However with the right techniques the condition monitoring is very effective for the main bearing and the progression of damage can be monitored over three to six months while the bearing deteriorates, finally when the temperature rises the bearing condition is very poor and replacement within a few weeks is recommended. Figure 1 provides an example where a low frequency accelerometer recorded time series data which was then processed in the Romax InSight iDS software for early detection of main bearing failure. This typical example illustrates how no special hardware is needed for main bearings, just good signal processing. By point D in the diagram the condition of the bearings is very poor, typical to a set (Figure 2) inspected in a factory teardown.

SCADA Monitoring

Aside from condition monitoring (via accelerometers, oil particle counters and so on) the turbine SCADA system is a valuable source of data for monitoring the turbine. From the 100’s of channels of data coming in (typically at a 10s sample rate) there are sets of parameters useful for checking the health of machine and also the performance. In many cases they are related, e.g. a high yaw misalignment will reduce performance, but also effect the health of the turbine longer term, as the rotor bending moments are increased and accelerate structural fatigue for all major components in the load path: pitch bearing, rotor hub, main bearing, main shaft, main frame, yaw bearing and depending on the drivetrain architecture also the gearbox. Analyzing SCADA data together with the maintenance records is also an effective way for the wind farm operator to monitor and control the maintenance quality.

Figure 3 shows a bearing temperature histogram from a European offshore wind farm analyzed in the Romax InSight SCADA software. After replacement of the high speed shaft bearing, the temperature histogram for the following months shifted to a higher temperature range with maximums up to 85°C. Unfortunately, this temperature shift was not identified at that time. As a result, the bearing had been running under this unhealthy condition and failed the gearbox within 12 months. Due to access and availability issues offshore, the gearbox wasn’t replaced until seven months later and the cost of downtime and repair was in excess of $1M.

Figure 4 provides the SCADA alarm history for another offshore wind farm. Here Turbine 78 has a large number of generator bearing temperature alarms and shutdowns. The total downtime for this machine from this one fault accumulated to 780 hours (an 8.9% availability reduction) over 12 months. This many production hours is around $75k at the site but as the alarms were more regular during high wind speeds, the real production loss was nearly $150k. In order to reduce the downtime, a reactive generator bearing replacement was carried out in September. However, the high temperature alarms continued. Another generator bearing replacement program was carried out two months later. The alarm problem still wasn’t solved after the second bearing exchange program, and with the issue prevalent on other machines at the site, an investigation was carried out by Romax to solve the problem. The replaced bearings were examined and a root cause analysis was conducted. This RCA determined the alarm wasn’t triggered by faulty bearings, but due to other factors. The misdiagnosis not only caused production loss, but also incurred unnecessary maintenance cost, with totals nearing $500k. Romax InSight SCADA brings together maintenance records, inspection reports and analysis reports with SCADA data to provide a powerful tool for assessing the site. This allows for a clear visualization (Insight) so the site manager or control center operators can easily see such correlations and make more informed decisions in operations and maintenance. Figure 5

It seemed that only a few short years ago much of the wind industry didn’t have a plan for when or how they would change the factory fill oil that was in their newly commissioned wind turbines.

“I can remember pulling into an O&M barn at a wind site back in 2007 and asking, ‘Is there oil up there?’” said Gary Sage, CEO of Sage Oil Vac.

Before that, Sage had been building mobile oil change equipment for a number of industries for more than 10 years prior to this impromptu wind farm stop and he was curious if there was a market for such a system for the wind industry. The site manager responded that there were 80 gallons of synthetic gear oil in each turbine and his crew didn’t know when they would be required to change the oil. Sage was more astonished when he realized that the site didn’t have tools and processes for replacing the oil in the turbine.

 
“I guess we will have to do the bucket brigade when it comes time,” the site manager said. 

Sage and his team immediately started working with different wind companies to develop an effective tool to automate this important task and developed the Sage Oil Vac Gear Oil Exchange System.

Gearbox oil change decisions are sometimes based on service times such as every two or every three years, while some rely on oil analysis samples taken from the gearbox. The industry has evaluated and debated oil change procedures and frequencies over the last few years, ranging from getting as many years out of their factory fill synthetic as possible, to some determining that they will change the oil at the one or two year commissioning anniversary regardless of oil samples. Over the last four to five years, much has changed in the processes and technology related to oil changes in gearboxes.

“Evaluation of lubricating oils in wind towers, is at an all-time high,” says Randy Andersen of Dakota Fluid Power in Sioux Falls, SD. “The need for understanding oil types, oil properties, proper sampling and maintaining target cleanliness levels are all factors for determining the life of the oil. Oil is key to any gearbox or hydraulic system and if applied, the 3C’s (Clean, Cool, and Contain) the results can be very positive.”

Justifying Equipment for the Oil Change

The investment in equipment to perform gearbox oil changes and flushes, like any business decision, must be weighed against economic benefit. The investment in a trailer or truck mounted gear oil exchange system has an easy, purely numbers analysis side to it, and four or five years ago, it was a simple formula for the analysis. The evaluation was using a gear oil exchange system versus manual bucket oil changes. An 80 gallon gear oil replace and fill can be done in under two hours with a gear oil exchange system, versus up to two men working 12 hours to do it by hand.

“Our first system was a simple two-tank, two-step system — vacuum the used oil down the tower, and pump new oil back up,” says Phil Seidenberger, vice president of product development for Sage Oil Vac. “Customers wanted to know how many towers they could get done in a day. The more they could get done, the more easily they could justify the purchase.”

Another big change in the industry came when gearbox life cycle started to become an issue for then industry and it became evident that gear oil cleanliness and oil change frequencies could be the difference in a gearbox failing or lasting. Image 1

Flushing

The industry started to realize that a basic two-step oil change process didn’t necessarily guarantee clean oil. Also, using buckets up and down tower left a lot of room for contamination of the fresh gear oil. As it became clear that more than a basic oil exchange was needed and the gearbox needed to be flushed, then using a gear oil exchange system is necessary. “It’s not economically feasible to change oil and perform an effective flush on a gearbox using buckets manually,” says Gary Sage.

Kevin Dinwiddie of Amsoil was instrumental in developing a two-phase flush procedure, drain and fill and the four-phase flush procedures (Drain, Flush, Rinse and Fill).

“In the past, wind turbine companies depended on normal oil analysis iron levels as a determination of a good oil change. It seemed that just draining and filling the gearbox was enough. We know that normal ICP oil analysis can only see particles about five microns in size and smaller, whereas Analytical Ferrography can detect all ferrous particles of all sizes and is a much better evaluation in this case.

A recent field test comparison study was performed on eight turbines and the result was that just draining and filling will leave 50 percent of the particles in the gearbox. The study compared normal ICP iron analysis and Analytical Ferrography in two types of oil change procedures; drain and fill and the four-phase flush procedure. The four-phase procedure left just over .1 percent of wear particles compared to the 50 percent in the two-step drain and fill.

When the benefits of flushing the gearbox were realized, then essentially a single oil change became two or three oil changes at one tower. With the four-phase process, you are vacuuming and dispensing used oil, flush oil, rinse oil, and the final fill oil a number of times.

“Over the course of the year starting in 2007, customers went from the basic two tank system to asking about three and four tank systems with flushing capabilities. They weren’t as concerned about how many they could get done; they wanted to know how to replace the oil as cleanly as possible in a multi-step process. Customers didn’t care if they only got one turbine done a day, as long as they ended up with clean oil in the turbine. The industry shifted from quantity to quality, so to speak,” Seidenberger said. “The payback analysis went from ‘how many can we get done in one day?’ to, ‘what is it worth to see how clean can I keep my gearbox and oil during uptime and during oil changes?’” Image 2

Hire, Rent or Buy

The industry has options to obtain proper gearbox flush and oil changes. Equipment can be rented or bought outright or they can choose to contract with a third party who has equipment.

“Our customers include integrated wind farm owner-operators who have their own personnel running and maintaining the wind farm, third-party service providers, startup businesses that specifically offer oil change and flushing capability, manufacturers with personnel on-site during warranty periods, and O&M companies,” Seidenberger said.

The decision to rent or buy equipment versus contracting the service has pros and cons on each side of the equation. If a wind farm with 60 towers wants to have their own unit, it might be difficult to justify the $60,000 to $70,000 investment just to be used on their own farm. But they may be able to rent a unit only during the time period that it is needed, then use it for a month or two, and they are good until the next oil change is due. Some wind farms want to have ultimate control over the gearbox oil change and flush, from selecting the oil all the way down to using their own trained technicians to perform the work. 

On the other hand, some wind farms want the expertise of a third party crew who are experts on the operation of the system and who have practice leaving the gearbox free of contaminates with very clean oil. Image 3 

Cleanliness of Oil

Sage’s Gear Oil Exchange System has an onboard CC Jensen 54/27 Fine filter system that will filter fresh gear oil down to ISO 16/14/11 levels. It is necessary in some cases to filter the new oil that is put in the gearbox as final fill. New gear oil out of the tote or drum can be less clean than the wind farm wishes to put in as the final fill.

“New oil is not always clean oil and needs to be considered when changing oil. If possible, it is recommended to run the oil through a filter every time you transfer and provide good housekeeping with storage and containment,” says Randy Anderson, sales manager of Dakota Fluid Power of Sioux Falls, South Dakota. “The ease and handling of oil is key to promoting cleanliness and eliminating accidents. Sage Oil Vac is a leader that has provided the equipment to handle your change out and filtration requirements of oil with ease. Sage has provided an air system for simple filling with the storage of new oil and ease of removing the waste oil, while also including a flush and a rinse for increasing the end result of a complete oil change in the field.”

“Building reliability starts with effective contamination control,” said Justin Stover, sales manger for C.C. Jensen Filtration. “The importance of clean oil in gearboxes has been well documented over the last four decades. Contamination in a wind turbine gearbox must be controlled throughout its life cycle. Contaminants are defined as anything foreign to the lubricant that alter its performance. Over 90 percent of the damage done to a gearbox is a direct result of particle and moisture contamination. Dirt and water are inevitably introduced during service and maintenance. These “wrecking balls” create wear particles that act as catalysts — in effect microscopic spark plugs — that cause more wear, tear and lubricant failure. While it is important that particles large enough to be seen or felt are not exposed to lubricants, more frequently it is the invisible particles that cause the most damage. That is why it is essential that a filter with a fine micron rating and water removal capabilities be selected and installed. Lubricants are what we buy – lubrication and contamination control are what we do to ensure optimum lifetime and reliability. We cannot wish greater reliability into existence. It must be the right lubricant, in the right place, in the right amount, at the right time, keeping it clean, and keeping it dry. Their design allows for ISO 320 Gear Oil to be vacuumed from a drum or a tote at a rate of 15 to 20 gallons per minute. This is done without a fluid pump or venting of tanks. The operator builds vacuum on the fresh oil tank and connects the barrel tote or straw and quickly vacuums fluid.  This is a time saving step that most people don’t factor in. The customer does not have to have a pump to transfer from the tote or drum to the Gear Oil Exchange unit.”

Once oil has been brought onto the Sage Oil Vac system, the customer can filter the new oil using a kidney loop fine filtration system, while simultaneously passing oil through a shell and tube heat exchanger adding heat to the oil for easier pumping capability.

The use of a 1000 PSI modified diaphragm pump with the Sage Gear Oil Exchange System leaves no gear pump wear metals in the filtered clean fresh oil. The unit also has a flushing wand that allows the uptower technician to insert the wand inside the inspection plate and wash down individual gears and ledges inside the box.

Certainly, much has changed in just a short period of time when it comes to gearbox maintenance, from the “bucket brigade” mentality to the gear oil exchange trailer two-step process of flushing and filtration. Whatever comes next, there is no doubt that much more thought and care has gone into extending the life of the gearbox.

Houston, Texas-based EDP Renewables North America (EDPR NA), is the third largest renewable energy company in the United States. We design, develop, construct and operate 28 wind farms throughout North America, and have developed more than 3,400 megawatts (MW) of wind energy. Our parent company, which operates facilities in Europe and South America, is the third largest wind energy company in the world.

At EDPR NA we are committed to enabling a clean energy future for the U.S. Current market trends, including increasing economic efficiency and acceleration of technological developments in renewable energy, support that vision. With a sound development pipeline, first class assets and market-leading operating capacity, we have undergone exceptional development in recent years. In addition, we are committed to employing the most innovative strategies and technologies in the industry to maximize our company’s performance and the value we deliver to our customers.

Turbine Performance – The Key to Wind Energy Competitiveness

According to the American Wind Energy Association’s First Quarter 2012 Market Report, the United States wind industry has added more than 35 percent of all new generating capacity over the past five years, second only to natural gas, and more than nuclear and coal combined. Today, U.S. wind power capacity represents more than 20 percent of the world’s installed wind power. The report also states that the U.S. wind industry represents not only a large market for wind power capacity installations, but also a growing market for American manufacturing. More than 470 manufacturing facilities across the U.S. make components for wind turbines, and dedicated wind facilities that manufacture major components such as towers, blades and assembled nacelles can be found in every region.

So wind energy has become a serious contributor to our national energy market, and the economy itself. This growth has been fueled in part by rapid improvements in wind turbine technology. However, despite these technological advances, competitive market pressures continue to demand that wind energy suppliers pursue innovation throughout their organizations to operate as efficiently and reliably as possible.

Like most wind energy companies, EDPR NA’s assets are in very remote locations dispersed across the United States. Modern technology enables companies to accurately forecast wind speeds and generation potential, but the industry as a whole must still grapple with prioritizing performance issues based on severity and wind patterns, communicating knowledge across a geographically dispersed organization, and the challenge of working around the availability of resources to schedule maintenance – a more dynamic process than what traditional energy generation sources face.

At EDPR NA, this relentless focus on performance improvement has led us to think of turbine issue management in terms of a Dynamic Case Management approach that links governance to operations to ensure that farms perform optimally while the wind is blowing.

Dynamic Case Management (DCM) is a term coined by global IT analysis firm Forrester Research. Forrester describes DCM as “a new way of thinking about how complex work gets done. For enterprises, DCM provides a transformational opportunity to take the drudgery out of work and enable high-value, ad hoc, knowledge work.” [1]

Forrester also said, “Functional and industry-specific packaged applications have worked well for well-defined, highly structured processes where volume, scale, and straight-through processing dominate system design. But these apps are difficult to change, appear increasingly less relevant, and form a barrier to innovation for companies in fast-moving industries like energy, healthcare, and financial services…These [industries] will leverage advanced dynamic case management (DCM) and business process management (BPM) suites. [2]

Our Approach to Dynamic Case Management

EDPR NA decided that embracing Dynamic Case Management would give us a competitive advantage in overall process management against our energy industry competitors. To prove the concept, the company initially focused specifically on the area of operations issue management. In using technology as an engagement vehicle to boost asset performance and maximize potential earnings, EDPR NA targeted three specific areas:

• Prioritization – Which problems are the most critical to address and solve first?

• Geographic Dispersion & Department Specialization – EDPR NA’s distributed assets require that our technical service department assist multiple wind farms from a centralized location.

• Knowledge Retention and Learning – given the pace of technological advancement, EDPR NA needed to document maintenance techniques, technical solutions, and best practices. Figure 1

Business Process Management Platform

To transform the industry-standard approach to case and process management, we designed a solution called COBRA (short for “COlaBoRAtion”). COBRA is a system for Dynamic Case Management built of a Business Process Management (BPM) software platform from Appian. The platform provides the typical core BPM capabilities, such as a comprehensive process engine, integrated business rules, event management and real-time analytics. Importantly, Appian also combines these with native mobility and social business collaboration. Taken in sum, these capabilities allow us to conduct fast and intelligent resolution of dynamic case scenarios. 

Prioritization

In COBRA, issues encountered in remote sites are logged and assigned a priority for solution creation. COBRA uses real-time data about a problem’s size, combined with location-specific pricing information in order to determine the issue’s ability to affect the company’s bottom line. Through a complex algorithm, issues are assigned a score from 1-1000, which provides an absolute basis for objective comparison. We use this data to make transparent decisions about where to assign resources and focus improvement efforts.

Geographic Dispersion & Department Specialization 

Wind turbines are highly complex machines, and problems that affect the fleet can have wide-ranging consequences. COBRA gives us a platform for structured collaboration around each issue, allowing departments to weigh in on the ramifications (technical, engineering, wind resource assessment, environmental, safety, procurement, legal, regulatory, and others) of problems, as well as their potential solutions. COBRA also assigns internal resources to each case that serve as a dedicated collaboration contact to assist on-site managers in reaching solutions. COBRA makes collaboration between disparate locations easy by also serving as a platform to discuss updates and new developments. Figure 2

Knowledge Retention and Learning

Not only does COBRA assign dedicated resources to serve as a point of contact for each case, but the system also centralizes the information generated, creating a constantly expanding knowledge base of solutions for future problems. This ensures that EDPR NA staff can rapidly respond to similar problems encountered in the future, and prevents the company from having to constantly reinvent the wheel for solution creation.

Results
With COBRA we were able to capture over one hundred million dollars worth of issue solutions in the first nine months of use. It gives us a modern mobile and social platform for Dynamic Case Management that drives business performance by turning tacit knowledge into explicit knowledge.

The system has created a searchable knowledge base of detailed solutions for potential future issues, including the original case’s context, to aid decision-making. COBRA also allows us to track the frequency of issues related to specific equipment suppliers. COBRA is also rapidly eliminating our reliance on ad-hoc email-based issue tracking, collaboration and document sharing. The files in the COBRA system were accessed more than 3,000 times in those first nine months. Each of those points of access is part of a structured and auditable record of a particular solution instance.

Based on this success, we are now looking at other core areas of our operations that would benefit from this combination of Dynamic Case Management and Business Process Management software. There are many candidates, including our processes for Energy Assessment (site design/suitability analysis for meteorological tower installations), Procurement (RFP issuance and supplier qualification) and Origination (Utility Bid Response including Investment Analysis, Estimation, and Energy Assessment). 

References:

1) Forrester Research, “Dynamic Case Management: Definitely Not Your Dad’s Old-School Workflow/Imaging System,” Sept. 2011

2) Forrester Research, “Stuck In Cement: When Packaged Apps Create Barriers To Innovation,” Jan. 2012