Feature

O&M for wind turbine blades


Jack Wallace Jr., Mark Dawson

Although many different processes are used to build wind turbine blades, they all share the same maintenance challenges. Understanding how and why a blade is made a certain way will improve the chances of making good decisions on blade maintenance, say Jack Wallace Jr. and Mark Dawson of Frontier Pro Services.

Blades are getting bigger. Over the last 20 years, we have seen the standard blade size grow from 7.5m to well over 60m. In the future these tools that gather energy from the wind will only be limited in size and performance by materials and our innovation. Meanwhile, let's see what we can do to maintain those in operation now.

We work with three different aspects of wind turbine blades. For the past two decades our experience with blade work has included:

  • Time in the field performing blade maintenance on wind farms;
  • High-tech telescopic blade design for manufacturers;
  • Blade repairs for operators and manufacturers in the wind turbine industry.

Before discussing effective operation and maintenance techniques for maximum power generation, it is useful to present a brief exploration of blade technology.

Introduction to blade technology

Blades are the main components that differentiate wind turbines from other machinery, acting as the “respiratory centre” of a wind turbine. The length of the blade determines the amount of power that can be extracted from the wind, because it affects the swept area of the rotor. The airfoil determines the blade's subtle characteristics.

Blades are interesting in part because they can be made several ways: hand-carved from wood, or made from injection-moulded plastic or pultruded fiberglass. Pultrusion is a process for producing continuous fibres for advanced composites. Blades can also be large composite structures over 60m (200 ft) long. This article will focus on hand-laid fiberglass blades, as they are typically used on most large turbines today.

Blades are not only strong, but they are built to be strong in specific directions. This is usually done with fibres laid in specific orientations. Each and every fibre of material placed in a blade is engineered for specific strength characteristics. The type and weight and the direction of orientation of each layer of fiberglass (or other fibre) are carefully selected to meet engineering blade design characteristics in both strength and modulus (stiffness/flexibility).

Most of the strength of the blade is in the spar caps and support spars. This is essentially a long composite “I beam” which handles most of the load. Problems with the spar and spar caps usually signal very serious problems ahead.

If most of the strength of the blade is in the spar caps and support spars, then you may ask what the rest of the blade is doing. The rest of the blade is designed to extract power from the wind. From the shape of the leading edge to the width of the blade itself, it all works to pull power from the wind. This is called the airfoil.

The airfoil has different shapes and sizes depending on which part of the blade you are examining. The airfoil at the tip of the blade is not the same as the airfoil 10m from the tip or 10m from the root of the blade. At each location along the blade, the airfoil is designed to be the most efficient for the speed of the blade at that point, and the resultant angle of attack with the incoming wind.

Blade O&M concerns and recommendations for longer blade life and performance
 

Lightning

As lightning strikes can cause various amounts of damage to wind turbines, this is a focal point for engineers working to improve blade survivability. Typical methods of controlling lightning consist of bare metal pucks near the tips of the blades.

These pucks are attached to heavy wires that carry current from a strike to the root of the blade. From there, current passes into the hub and main shaft where brushes carry it to the nacelle bedplate and down to the ground. However, even with a good conductive path to ground, blades may delaminate when hit by lightning because any moisture present in the blade is turned to steam.

Oil leakage

Since oil leakage can penetrate into the blade laminate layers and cause the blade to come apart over time, leaks inside blades need to be cleaned up and controlled. Oil leaks on the outside of blades can attract dirt and bug build up causing reduced performance.

Cracks

Visible blade cracks are the easiest way to see that a blade has problems. All cracks should be reported to ensure that the crack can be repaired before it becomes a bigger problem. As cracks tend to propagate, the repairs only get more expensive with time. Cracks can allow water to enter the blade, which can cause damage in freeze-thaw climates.

Dirt

Airfoils do not perform as well when they are dirty. Just like a car windshield, blades can collect bugs and dirt rapidly. Stall regulated turbine airfoils are designed to stall in high winds to protect the turbine. These airfoils are sensitive to dirt or bugs on the leading edge because they cause the blade to stall prematurely.

A dirty stall regulated airfoil may lose 20% efficiency, so it is important to keep it clean. In some locations, it is economical to clean blades every couple of weeks. As pitch regulated turbine airfoils are designed to avoid stalling, they are not as affected by dirt as stall regulated blades.

Leading edge erosion

In some parts of the world, leading edge erosion is a serious problem and at other sites it is not a problem at all. If you find that leading edge erosion is a problem at your site, we recommend using leading edge tapes that can be applied to the blade's leading edge. Such tapes are very durable and will definitely prevent erosion.

Metal fatigue

Although most of the blade is fibre, some blade components are made of metal. As the areas of concern are the component mounts for pitch and blade tip mechanisms, look for cracks in the metal supports on which these items are mounted. Be careful when tightening blade fastening hardware as both over and under tightening of blade bolts can have serious consequences.

Ice

As ice build up on blades can be very dangerous, it is best practice to stay clear of the machine until all the ice is gone. Ice reduces the efficiency of the airfoil, and can unbalance the rotor. Turbines are often shut down during extreme icing conditions.

Deployable tips

These are movable tips that are used as braking devices to prevent runaway situations and are typically found on fixed pitch turbines. Blades with deployable tips require tip maintenance.

Inspect all the mechanisms for cracks and worn items. Most large blades with tips use cables to attach the blade tip to an actuation mechanism located at the root of the blade. Typical problems with these involve breaking off of the tip locks, break off (and wear and breaking) of the cables. While blades can and will function without the tip locks, they will not function with a broken cable, as the blade tip will deploy.

In our view, it is worth asking if all blades should have deployable tips as a final safety precaution to prevent runaways as there have been runaways on turbines with independently pitched blades.

Handling damage

It is important to properly protect the fragile portions of the airfoil when handling blades. We regularly see trailing edge damage as a result of carelessness while moving blades around. We protect trailing edges with a “taco shell” protector before strapping it up to move.

Blade balance

Blades must be balanced so they do not cause excessive loads on the rest of the turbine or tower. Just like the wheels on a car, rotating blades cause repetitive swinging loads if they are not balanced.

Dead weight moment

This is the weight of the blade hanging on its root. Every time the hub rotates 180 degrees, this weight reverses. Reversing loads cause a lot of damage, and if the blade is incorrectly designed or constructed, it can fail near the root where these loads are greatest. As blades get longer and longer, this becomes a key design load.

Blade vibration faults

As blades get larger and wind turbines get more expensive, more safety devices are used. Blade vibrations can be detected with accelerometers, and the controller can change blade pitch, turbine speed, or other parameters to lessen unwanted vibrations. If your turbine is faulting on this fault, you need to inform your engineer. This can be a serious event if you fail to investigate why it is occurring. As blade vibration faults usually require special detection tools which most field technicians do not have, we recommend using a turbine engineer to collect and analyse data in order to properly discover its cause.

Resonant frequencies

These occur when the natural frequency of vibration of an object coincides with the speed of rotation of the turbine. When blades are designed, their natural frequencies must differ from those of the rotor rpm and tower swaying frequencies. Otherwise, normal blade bounces are magnified as they resonate with other parts of the turbine, causing extreme loads in the structure of the blade. Because blades are shaped like wings, they vibrate at different frequencies edge to edge as against a flap wise direction.

Both of these frequencies need to be understood, and the problem of resonance gets trickier when the blades are mounted on variable speed turbines. Large repairs or deviations from design during blade construction can change the weight of a blade, and this can in turn change its resonant frequency. This is why turbines may have a blade vibration sensor which can fault the turbine if the blades run near any natural frequency.

Runaways

Runaways are turbines that fail to stop. A runaway may be caused by faulty brakes or pitch systems. They can also be caused by controller or operator error. These are dangerous situations because the power produced by the blades continues to rise as the rpm increases. If the generator is no longer on line, there is no load to prevent the rpm from rising.

As the rpm increases, several things can occur. Blades may flex back and strike the tower or simply fly apart because of increased centrifugal force. If this happens, the rotor is instantly thrown out of balance, and the turbine may shake itself off of the tower. Since none of the systems are designed to handle extreme over-speeds, the tower or foundation may also fail, toppling the entire turbine. Do not approach a runaway turbine, as parts may be thrown hundreds of feet during a failure. A modern turbine can actually fall further than a football field due to its height!

Blade to tower clearance

This is an important consideration in modern wind turbine design, since most turbines face upwind. As blades fly, they tend to bend back towards the tower.

The clearance between the tip of the blade and the tower is affected by the following: blade stiffness, rotor rpm, wind speed, blade coming away from the tower, the tilt of the nacelle, slop in the yaw bearing, and the shape of the tower. Over time, yaw bearing wear and aging of the blade may reduce the available clearance between the blade and the tower. If a blade strikes the tower, it is a catastrophic event for the blade that can destroy the entire turbine and tower.

Locating blade defects

Look, listen and feel. Good technicians have learned to listen to the turbine and blade sounds.

Any change in the sound of a rotating blade means that something has changed. If your turbine has a blade that suddenly begins to whistle, then it is likely that something on that blade has changed.

Again, if you notice your turbine swaying side to side in a manner that it never did before, then maybe something in the blade has changed. We recommend using a good pair of binoculars to scan your blade skins regularly. You can use a video recorder to record scans of blade skins and review and zoom on suspicious areas at later times. But we recommend that the same person scans the same skins or recording so that minor changes are noticed. Such changes may not be readily evident to the unskilled eye.

Blade finishes

A good blade finish costs money. Since blades cost you so much anyway, we recommend a good finish after any repair or blade purchase. There is no reason to settle for a finish with roller or brush marks on your blades.

Some finish work can cause you to lose energy production, such as brush marks in the gel coat on the leading edges of blades. Unless the aerodynamic engineers built these brush marks into the airfoil they shouldn't be there. You may want to take time to sand them out of your new blades. It is the same as having clean blades versus dirty blades.

Blade pitch marks

These are usually located on the outside of the blades. This is fine for smaller blades but for larger blades where you work on the inside of the hubs, the pitch marks should be placed on both the outside and the inside. If a blade is pitched incorrectly by as little as half a degree, its performance is noticeably changed.

Blade identification

We recommend marking blades with large identifiers so inspections from the ground or by camera can easily identify each blade. This makes tracking blades much less of a problem.
 

About the authors

Jack Wallace Jr. is a wind turbine technical advisor with Frontier Pro Services.
+1 951-849-3194
Mark Dawson is a wind turbine engineer with Frontier Pro Services.
+1 951-849-3194

 

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