While scheduled wind turbine maintenance costs are relatively low, unscheduled maintenance can be another story. Under the maintenance and warranty agreements, original equipment manufacturers (OEMs) have shouldered these costs. But now, for the first time in the US wind industry, the trend will start to shift in 2010 as more turbines come off warranty than are currently covered, causing maintenance and repair to become bigger issues for project owners. Unscheduled maintenance costs will become the responsibility of the owners, negatively affecting a wind company's profitability.
As an indication of what is coming next, project owners should seek to learn the real value of the warranty service they have been receiving, such as the cost of parts, labour and repair time. And, with high repair costs and potential downtime on the line, an end-of-warranty inspection can pay back with just a single finding while turbines are still under warranty terms.
With a potential conflict of interests, the turbine OEM may not be the most suitable candidate to perform such an inspection - although their representation during the inspection process may avoid disagreements on future warranty claims. Qualified internal resources are, instead, a common choice for current operators, while financial entities and owners less familiar with wind O&M often turn to an independent service provider (ISP). Choose one with experience of your specific turbine platform and define your expectations prior to the inspection.
What to include
Turbine manufacturers provide manuals with checklists for mechanical completion, commissioning and routine service quality assurance. These can be used to help find issues that are common to a project or a turbine platform. Qualitative checks on items such as bolting torque settings, component oil leaks, pitch linkage wear, cable routing, electrical terminations and installed safety equipment are all commonly performed during end-of-warranty inspections. While these checks will expose obvious deficiencies, there are other areas worth including, too.
Gearbox and generator rebuilds include components that are expensive to replace and often involve the additional cost of mobilising a crane, plus a long lead time to get to the site and set up, resulting in longer downtime and higher revenue loss. Failing to notice a $1,000 bearing problem can lead to a $100,000 gearbox replacement, a $50,000 generator rewind and a $75,000 crane hire. Add lost production revenue, and one post-warranty gearbox failure can cost 10-15% of the price of the turbine. Avoid this by conducting a thorough gearbox inspection during the end-of-warranty walk down with oil analysis, vibration monitoring and a borescope camera inspection, which looks inside hard-to-reach areas of the drivetrain.
A wind turbine gearbox, typically with six or more gears and rolling element bearings, commonly fails in high-speed shaft bearings, planet bearings and intermediate shaft-locating bearings. Many gearbox failures are thought to be related to poor lubrication and lack of maintenance. Contamination can enter gearboxes in a number of ways: during manufacture; wear of gear or bearing; airborne through breathers and seals; and introduced during maintenance. This contamination can damage the gearbox bearings during the warranty period, so an oil and grease analysis should be conducted to determine the health of gearboxes, bearings and generators. It is also a good idea to test regularly for water, contaminants and metal wear.
Gear mesh vibration frequencies are easy to recognise, but while it may be clear that a turbine gearbox is noisy, because it is not normally easy to place a transducer close to the problem gears and there is an abundance of vibration sources, it can be difficult to tell which bearing is responsible, or if something else is causing the problem. So, including qualified gearbox vibration monitoring in an end-of-warranty inspection can pay huge rewards if anomalies are discovered. Furthermore, the information gathered can support additional borescope inspections that can search in problem areas and digitally capture the condition of the internal gears and bearings.
Though balance of plant equipment may not be included in a turbine service agreement, it is a convenient time during end-of-warranty inspections to conduct a site-wide infrared analysis of electrical components. This quick in-service inspection can identify transformer hot spots, overheated electrical connections and any pending failures on elbows, arrestors and circuit breakers. This may be the first time that thermography is even conducted on the site, and can serve as a baseline for future condition studies.
A good end-of-warranty inspection should also include a review of all service reports and parts usage during the warranty period. This will bring to light recurring issues, especially repeated parts replacement or service. It will also determine if the warranty provider has performed all service work that was required, and in accordance with the scheduling stipulations of the manufacturer.
The final report of inspection results should be provided in both a summary format and a detailed description of the findings, including digital photographs for all discrepancies noted. Specific reports for oil analysis, vibration monitoring and thermography services should also be provided if these services were part of the inspection.
Once a final report is provided, an owner will have sufficient information to fully evaluate the condition of their project. The information can also be referred to during future, post-warranty maintenance for issues that may have existed from the warranty period.
Knowing the material history of the turbines helps to gain insight on issues that will carry beyond the end of warranty. This knowledge of possible serial defects, predictive failure rates and inventory discrepancies that must be addressed will greatly affect the final terms at end of warranty. Remember, once the OEM hands over the turbine maintenance to you, the owner, you inherit any unresolved issues, if there are any. As the saying goes, "you broke it, you bought it".