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Replace or Repair? – Exploring Wind Turbine Blade Failures

Critical blade defects leading to failure of blades can be caused by excessive force or fatigue damage. This poses a significant risk and creates extreme economic impact for a wind turbine owner. Therefore, it is important to ensure wind turbine blades (WTBs) last their full designed lifetime.

 Blades can fail via different types of failure modes. This is highly dependent on the individual blade design as selected by the Original Equipment Manufacturer (OEM). Failures can be related to design, manufacturing defects, environmental factors, lightning strikes, external impacts, and fatigue. These damages will lead to complex blade repairs or full blade replacements.

 To understand if a blade should be repaired or replaced, first the blade must be investigated after a structural damage is detected. In a Root Cause Analysis (RCA) investigation, different initial hypotheses are enlisted if failure is related to design, manufacturing, fatigue, or control issues. Therefore, the blade design type, the location of the failure, defect characteristics and SCADA data should be investigated thoroughly to proximate the cause. Initially, the structural damage in the final failure should be categorized if it is a:

 ·        Potential initiator of the failure: Key evidence to point out that the potential initiator damages are the defined patterns of cracks, delamination surrounding the damage, and stress whitening of laminate.

·        Damages caused by the damage initiator: The damage initiator can cause effect on its neighboring structures. This has an undefined crack pattern and shows more of a laminate tear appearance which occurs during collapse.

 Once damages are categorized, more in-depth investigation will be applied to the potential initiator of the failure if it is associated to manufacturing, fatigue or irregular loading caused by faulty control systems. Therefore, it is of importance to build a failure scenario supported by facts, carry out a proper investigation which highlights the failure mechanism, and understand how it has progressed leading to the final failure.

 To restore the blade structural integrity, the repair must consider the following:

·        Damage severity level: specification of how much the damage has affected the blade in terms of material damage and extension of area affected, i.e., how big, and deep the damage is.

·        Damaged region: correlated to severity level but, it is important to understand that blades are composed of different structural components. If these structural regions are affected, then severity level becomes higher.

·        Aerofoil geometry requirements: since blades are designed with complex geometry, ensuring that aerodynamic efficiency is reached to cultivate maximum power from the wind resource.

 If the blade’s structural components are affected, and if the damage extension is too big and deep, then a repair is less likely to be successful. It is important to highlight that when a repair is performed, it must restore the blade’s structural integrity and should endure loads during the blade’s remaining lifetime.

 Taking these considerations into account, a repair scope can be built where other factors are accounted for, including material and tools requirement, labour, logistics and weather. These factors will be the base of the cost of the repair. That is why it is highly suggested that a repair cost breakdown must be requested, to mitigate the cost of repair in comparison to the cost of blade replacement scenario.

 When deciding if a repair of a blade is feasible, it is important to determine the affected areas of the blade. If the damage is confined to the blade shells, it is in most cases possible to repair the blade. The main driver of repair time and complexity is determined by the area the defect covers.

If the defect is located at the blade beam, root, and web structure, it affects load carrying parts of the blade. This increases the complexity and size of a repair significantly, as the overlap between laminate layers in the repair ensures that the repair can handle the loads in the structural parts of the blade.

To determine if a repair exceeds the value of the blade, it is necessary to establish the position of the blade damage, effect on the blade structure and size of the repair. Then, repair time and cost are estimated and matched against cost of a blade replacement.

If a replacement has been determined as the best course of action, then it is essential to find matching blade(s) that balance with the remaining blades on the turbine. Turbine blade manufacturing is still to a larger degree a handcraft, it is therefore common to encounter discrepancies between blades of the same type. This is due to the manufacturing process, where the glass fiber layers are impregnated with epoxy resin.

It is common to see difference in saturation levels of composite laminates, and resin pools in the blades are also an expected occurrence during the manufacturing process. This creates variation in blade weight and load distribution across the blade span. Therefore, it is a good indication that if two blades match in weight but, the blade center of gravity (CoG) and loads at the blade root, also known as root bending moment (RbM), need to match for the blade set to balance. If a selected blade does not fit to the other blades in the set, it will cause rotor imbalance that causes unwarranted loads to the blades, bearings, and the drivetrain. It is possible to adjust some discrepancy in the blade attributes using weight blocks or ballast boxes inside the blade. These weights are often placed at the midspan of the blade. There is a limitation to the amount of weight that is possible to add, as it increases loads locally in the weight position. This can cause early life fatigue if the loads exceed the allowed threshold. It is therefore true that not all blades are possible to matched if their weight, CoG or RbM is exceeding the adjustment range of the blade set.

Authors – Wind Power LAB

Morten E. Handberg, Chief Blade Specialist at Wind Power LAB

Aura Vanessa Guzmann, Senior Blade Specialist at Wind Power LAB

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