How to plan proactive blade maintenance

With 194GW of global wind power capacity installed at the end of 2010, well over 400,000 turbine blades are rotating every day around the world.

High maintenance...repairs make financial sense
High maintenance...repairs make financial sense

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While there is no question that engineering developments associated with blade construction have advanced significantly over the past ten years, it remains true that every blade in operation is vulnerable to wear and fatigue.

If this is overlooked or downplayed, it can cost a company dear.

With improvements in inspection and repair methods — including better access, whether via ropes, platform or high-lift cranes — the industry now knows that blade erosion is a reality.

There is still some debate over the direct performance implications of eroded blades but if left unchecked, they will absorb water and oils.

This will adversely affect rotor balance and cause fatigue, so it is critical for owners and service providers to have a proactive blade management strategy.

A new set of blades can cost the equivalent of 20–25% of the original price of the wind turbine.

A minor blade repair, on the other hand, will cost just 10% of a replacement blade.

So the relatively small expense of a well thought-out blade management programme is money well spent.

This is the essence of lifecycle cost analysis and management.

Most manufacturers’ turbine warranties will cover blade repairs and/or replacements under normal operating conditions.

Typical planned maintenance relies mostly on visual inspections, but because blades should be considered as wear components (based on what is actually being seen in operation), it is clear that a more proactive approach is necessary. The industry is now recognising this and we see more and more customers working to implement proactive blade monitoring, inspection and repair.
There is no one-size-fits-all answer when it comes to a blade management strategy.

Money is limited so owners and operators will be looking for a cost-effective solution that meets their specific needs and provides the right level of monitoring, timely inspection and repair methods with reduced blade risk.

Even so, there are certain components of any blade management programme that are accepted as vital across the industry.

These are: visual inspections; proactive up-tower inspections; proactive repairs; rotor balancing; and reporting tools and a data repository.

Most operations and maintenance manuals call for annual visual inspections. These include ground-based inspections for blades, up-close visual inspections of bolted connections in the hub and a look at pitch bearings and motors for visible damage, wear, corrosion and adequate lubrication.

A quick scan of the blades for any visible damage or unusual sounds by technicians every time they visit the turbine is also recommended.

At some point, a technician will have to get close up to the blade and assess its health.

This is where proactive up-tower inspections come in, although the frequency of internal and external inspections will be highly dependent on the environment.

As well as the obvious — age, moisture, ice, dust and prevalence of lightning — high turbulence intensity is also a factor that affects blade health.

An understanding of the influence of these variables for a particular project will determine the most appropriate intervals at which up-tower blade inspections should be conducted.

In fact, owners and operators should place greater consideration on their site environmental conditions when selecting and specifying new unit tubines and blades in the first place.

Make full use of up-tower time

Up-tower inspections should not be limited to external checks; internal inspections should be part of the inspection strategy as they provide significant insight into overall blade health.

For obvious safety reasons a turbine has to be shut down during an up-tower inspection.

As this incurs costs associated with lost production, mobilisation and equipment, it is logical to make efficient use of that downtime and so prepare for more than just an inspection.

Technicians should come equipped to complete an array of repairs, and carry out proactive maintenance, notably to combat erosion.

It is recognised that minimising the effects of erosion is critical to extending the life of blades. Erosion creates pits and crevices for moisture (and oils) to seep into the blade, which is the start of more significant problems and more costly repairs.

Leading-edge protection, whether using tape or other means, is considered an effective means for minimising erosion.
Similarly, imbalance in blades, caused by water intrusion, blocked drain holes or lightning strikes, can lead to excessive wear in drivetrain components if not rectified.

Monitoring the rotor for these imbalances will reduce overall lifetime costs.

There are several real-time monitoring and sensing devices that can detect imbalances, as well as field balancing methods to restore proper rotor balance.

Companies should also invest in a quality reporting system that records and transmits images of damaged sections of the blades along with post-repair images.

Over time, the ability to retrieve images and a full description of the repairs will provide valuable insight into overall blade health.

Whole-system planning

All these elements should also be considered and accommodated when it comes to planning a performance upgrade of units.

Wind farm owners are under continuing pressure to maximise or increase annual energy production from existing assets.

The key to any performance upgrade is coordination between hardware and software.

Any material modification, be it serrations, vortex generators or leading-edge protection, should be evaluated at a system level perspective and combined with controls logic adjustments.

Taking action to increase energy production without this systems-level analysis may create unforeseen and potentially expensive problems in other components within the drivetrain.

Therefore, owners and operators should take care to ensure that the integrity of the overall system is considered and the resultant configuration stays within the design practice of the manufacturer.

So the message is clear: proactive measures are vital to prolonging the life and efficiency of wind turbine blades.

And the entire value chain — blade manufacturer, wind turbine supplier, service provider and owners/operators — has a role to play.

Turbine suppliers and blade manufacturers are specifying and designing blades that provide better lifecycle cost with erosion-resistant materials or applications.

Service providers and owner/operators should plan proactive maintenance to reduce the risk of reactive and costly blade repairs.

Finally, turbine suppliers are best positioned to provide system-level upgrades that deliver increases in reliability and production.   

John McGuinness is wind service platform leader for GE Renewable Energy

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