Segmented wind turbine blades

As the wind industry strives for higher wind turbine efficiency and output, rotor diameters grow larger and larger. This has been evident over the last decade as the standard rotor diameter has grown from 89m to 113m and now to 133m onshore in the United States and over 200m onshore in China. This has created logistical challenges getting 100m blades delivered to onshore sites. An increasingly common solution to this challenge has been designing segmented blades. Segmented blade designs seek to solve these logistical hurdles, reducing the cost of road transport including permits, escorts, daylight restrictions, and rail limitations, among others.

Because of bends, twists, and turns in railroad lines, the upper limit for transporting single-piece land-based blades by rail is currently 75 meters. Manufacturing blades that can bend with “controlled flexing” will allow railroads to ship longer blades around the United States. Other potential benefits of segmented designs include blade extensions and tip replacements after lightning damage.[1]

The history of segmented blades, their designs, their applications in the industry, and challenges to wide-spread adoption are discussed below.

THE HISTORY OF SEGMENTED BLADES

Early research and development efforts into segmented concentrated on two distinct joining methodologies: bonded and bolted.[2] In bonded joints the two segments are chemically adhered together with a one-time, non-reversible connection. Bolted joints use mechanical fasteners, typically bolts or studs, to secure the two segments together.

In March of 2011, WindEurope-UpWind, which at the time was the EU’s largest research and development project, published a study called “Design Limits and Solutions for Very Large Wind Turbines: A 20 MW Turbine is Feasible”, where it determined that a bolted joint using channel fittings was the optimal design when considering weight, cost, assembly, and reliability.[3] It applied this methodology to a 42.5m Gamesa blade. A channel fitting is two metallic fittings bolted together, effectively transferring the load between modules axially, where fittings are bonded to the spar by embedding the fittings into the laminate.

Since that study, many turbine OEMs and blade suppliers have designed segmented wind turbine blades. These efforts are in various stages of development, only one of which has made it to market in the United States. Vestas has patented a segmented blade design with a bonded finger joint but has not yet installed them on any projects; SGRE offers a segmented blade design that uses fiber-reinforced epoxy and carbon pultruded profiles; Enercon has developed a blade with a bolted connection for installation on its E-126 turbine; and Nebrawind, a European technology company, offers a NebraJoint design to turbine manufacturers that has a bolted connection.[4]

Since 2021, GE has deployed a so-called “pin joint” technology in its Cypress onshore platform: 158m and 164m rotors with segmented blades. The design of the pin joint, which is based on segmented glider wing designs and was patented by GE in 2010, entails a rigid spar component with metal radial and chord pins which connect a relatively long root section with a smaller tip section. The same design is currently available in the US market as part of GE’s Sierra onshore platform (GE3.x-140 and GE3.x-154 turbine models). Although initial production runs of the Cypress blades were made by GE subsidiary LM, subsequent production of the Sierra blades were made by third party blade supplier TPI Composites.

CHALLENGES

While bonded joints are typically less expensive, lighter, and have higher fatigue strength once cured when compared to bolted joints, the process of joining the segments happens in uncontrolled field settings where environmental factors can have a significant impact on bond quality. The temperature, humidity, and any contamination at the time of assembly can affect the bond strength. Bolted joints, while heavier and more costly, are easier to assemble, can be disassembled and reassembled as needed, and are less affected by environmental factors. One inherent risk in bolted joints however is that the strength of the joint itself is solely dependent upon the fatigue strength of the steel bolts causing additional inspection and torquing activities for operators. Pin joint technology is also dependent upon fatigue strength of its metal components. Although these do not require preloading to achieve sufficient joint strength, the metal components are not visible for inspection which poses some inherent risk. All of these risks are compounded by the increasing size of turbine blades, manufacturing complexities, the introduction of materials with different risk profiles such as carbon fiber or metallic components in more lightning-prone areas of a blade, and overall market pressures to reduce cost.

HOW DO I KNOW IF THIS TECHNOLOGY IS IN MY WIND TURBINES?

It is not always evident from promotional or publicly available material what technology is present in any particular wind turbine model. However, regarding novel features like segmented blades, wind turbine suppliers are generally willing to share such information with relevant parties upon request. As of Q4 2023, GE is the only turbine manufacturer offering segmented blades on turbines for the North American market. As with other novel technology introductions, project developers should consider undertaking additional technical diligence when selecting turbines. Third-party turbine technology reviews from industry consultants such as Natural Power can provide useful insights into important details such as what technology is included in a particular turbine model, what risk profiles are associated with different design features, and an independent view of a turbine’s operational track record.

WHAT WAYS OF MITIGATING THESE RISKS ARE WITHIN MY CONTROL?

In addition to turbine selection, there are opportunities throughout the project development cycle for project developers and owners to mitigate novel turbine technology risks such as segmented blades. These include negotiating appropriate turbine supply and O&M contracts, third-party design review, manufacturing oversight, pre-installation inspections, and technology-specific, risk-based operating strategies. In contracting, attention should be paid to the details of blade health monitoring and should include specific coverage and responsibility for blade inspection, repairs, defect or failure investigations, and replacements. A design review and design for manufacturing review conducted by a qualified third party prior to project construction can enhance an owner’s understanding of the specific design or manufacturing aspects of a segmented blade that contribute to potential operating risks, allowing that owner more options in the construction and operation of the project.

Inspections can be an effective mitigation tool, especially when deployed early such as during blade manufacturing, storage prior to construction, or even if appropriately planned for, during turbine delivery or construction. It is relatively common for inspections to be part of a blade health monitoring program during operations. Pre-installation inspections are less common and tend to be cursory external inspections intended to identify severe transportation damage prior to turbine erection. As the size and complexity of blades increase, there is also increasing value in more detailed pre-installation internal and external inspections by skilled inspectors. This can often be conducted in the form of manufacturing oversight, performed at the relevant factories during original equipment manufacturing. Engagement of this sort by an owner’s representative allows owners the deepest visibility into the quality of manufacturing and can motivate more diligent adherence to manufacturing quality processes in factories but is not always logistically or commercially feasible. Alternatively, turbine owners can conduct detailed visual internal and external inspections of blades after manufacturing but prior to construction either in a storage or laydown yard. Such inspections can also identify manufacturing quality anomalies, provide baseline images and documents for longer-term monitoring, and generally provide insight into the overall quality and condition of blades before operations begin.

Regardless of what pre-operations blade diligence is performed, a long-term operating strategy that includes comprehensive blade health monitoring is the final element in minimizing risk associated with novel blade technology. The current state of blade health monitoring generally consists of annual external drone or zoom photography of the blade surface, combined with learning algorithm technology to assess this high volume of data for blade damage or defects. While this has been a cost-effective and valuable method of mitigating blade risk, there is a growing consensus for improved best practices that reduce the risk of blade separation and potential turbine collapse associated with blade failures. Some of these practices include:

• Establishing a baseline blade condition through pre-operation internal and external inspections;
• Employing a flexible, risk-based approach to blade inspection intervals and populations;
• Equipping site technicians with skills to be active players in blade health management;
• Expanding the criteria that determine when and why blade damage or wear warrants repairs.

References

[1: NREL 2021]

[2: Design and Analysis of a Segmented Blade for a 50 MW Wind Turbine Rotor: Alejandra S Escalera Mendoza, Shulong Yao, Mayank Chetan, Daniel Todd Griffith: January 16, 2022]

[3: Design Limits and Solutions for Very Large Wind Turbines: A 20 MW Turbine is Feasible: UpWind]

[4: Siemens Gamesa Onshore Technological Solutions, February 2023]