Catastrophic wind turbine blade failures are typically seen as uncommon events. Watch any local newscast of a reporter standing uncomfortably in a cornfield, a turbine with half its blade missing standing ominously in the background, you will often hear the question “How often does this type of thing happen?”
From a recently published study , blade failures were 40% of the value for insurance claims for the US onshore market. From an anecdotal perspective, across ONYX’s customer base, many are experiencing serial issues, in some cases having multiple failures at individual wind farms within the last year.
As the installed base has grown and whether failures are serial issues, early life, or aging-related, understanding the risks and future costs of major repairs is ever more crucial for turbine owners. The downtime associated with a catastrophic blade failure can often be months, especially for out-of-production/hard-to-source blades, plus hundreds of thousands of dollars, to purchase the blade and for erection.
Getting to know your blades
Blade architectures have different design philosophies and understanding them can help owners assign risk levels to each technology type as well as informing their blade maintenance strategy.
Figure 2 A photo from inside of a blade – what am I looking at?
Stepping into the blade, for an internal inspection, with the task of prevention of a potential catastrophic failure can be a confusing assignment. The photo illustrates, “what am I looking at?”, yet understanding and interpreting thousands of photos from an expensive internal inspection campaign can be crucial.
In the diagrams below, a few simple blade architectures are described and compared to help bring context. Blades are constructed from a shell, with the spar caps, trailing edge, and a shear web forming the load supporting structure. Shells are typically semi-structural and provide torsional rigidity.
The nomenclature for a blade root internal inspection has the shells designated as the suction side (SS) and pressure side (PS) and trailing edge (TE) spar or TE UD.
“UD” stands for unidirectional, in describing the type of fiberglass used in this region, with its fibres orientated along the axis of the blade. Whereas biaxial and triaxial are fibreglass fabrics found in the shear web and shell. LE denotes the leading edge of the blade.
Figure 3 Blade A Figure 4 Blade B
So what are the differences between Blade A & Blade B shown in Figure 3-4?
The single shear web architecture, Blade A, has only one web bonded to both the SS and PS spar caps and is the simpler architecture. For inspection there is only one main web bondline (where the spar cap abuts the shear web), however, there is no redundancy. If there are defects or damages in or near this bondline then the shear web can potentially detach. The resulting loss of structural integrity will lead to buckling of the shells and catastrophic failure.
The architecture with two main webs, Blade B, provides two bondlines to split the shear load that is transferred between the shear webs and the spar cap. Therefore, damages or defects found in a bondline with two main webs may be less likely to cause catastrophic failure. The other shear web provides another load path for that local region. This type of design is also more stable, providing a smaller trailing edge panel (the section between the shear web and trailing edge of the blade), which minimizes the potential of buckling of the panel under high loading or a defect in that region. The design is often heavier but brings benefits to reliability. The inspection needs to consider both bondlines.
There are many other architectures for blades and they differ by manufacturer, age, load requirements, and the market they were designed for. The landscape is continually changing, as new technology is designed, so OEM’s can sell turbines with competitive CAPEX to and reduce the levelized cost of energy.
To help understand your fleet, we provide a brief description of architectures and pros and cons, as well as inspection tips below:
|Single Shear Web Bonded||Simple, light weight||Highly stressed bondlines, no redundancy||Main shear web bondline, trailing edge, trailing edge panel near max chord|
|1.5 Shear Webs Bonded||Less core material, likely lower cost||Half web/Panel web – web foot (start of web) complex geometry/stresses||Shear web foot, all bondlines|
|Double Shear Webs Bonded||Stable redundant design||Complex manufacturing – quality of bondline risk||Shear web bondlines|
|2.5 Shear Webs Bonded||Stable redundant design||Complex manufacturing – quality of bondline risk||Shear web bondlines|
|Box Beam Bonded||Stable design, large bond area||Complex, specialized manufacturing||Spar bondline, trailing edge, trailing edge panel near max chord|
|Double Shear Webs Double Spar Caps Bonded||Reduces buckling risk in carbon spar caps||Less efficient, core in high stressed region||Core between spar caps, shear web bondlines, trailing edge, trailing edge panel|
|Single Shear Web Infused||No adhesive/bondlines||Complex, specialized manufacturing, infusion defects||All panels, internal inspection for infusion defects root to max chord/mid-span|
Need more information on your blades?
ONYX InSight is pioneering wind O&M combining practical engineering with technological solutions, working to solve your challenges now and in the future. If you would like to gain insight into your specific blade technology type, the risks associated with that design philosophy, and determine a blade maintenance schedule (such as when should I start doing internal inspections?) – reach out to ONYX Insight.
 Annual blade failures estimated at around 3800. Campbell, Shaun. WindPower Monthly. https://www.windpowermonthly.com/article/1347145/annual-blade-failures-estimated-around-3800