Close up - LM's 73.5-metre blade

Eize de Vries meets LM Wind Power's chief technology officer Frank Nielsen to examine the 73.5-metre blades developed for Alstom's 6MW direct drive Haliade 150-6MW offshore turbine.

The LM 73.5-metre blade mould

The LM 73.5 blade is a result of a strategic partnership between the two companies and has been designed to specifically fit the Alsom Haliade turbine. The rollout comes seven years after the launch of LM previous longest 61.5-metre LM 61.5 P blade, which was the world’s longest series manufactured rotor blade and is fitted on REpower 5M/6M turbines.

What is immediately striking about the blades is the enormous length compared to the small root diameter. Prominently visible at the round blade section measured from the root is a surprisingly simple looking spoiler attachment. Nielsen explained that the spoiler geometry fits well with the inner blade section, enhancing aerodynamic-lift performance without substantially increasing resistance or drag.

Another interesting surface feature is the polyurethane tape. This has the feel of rubber and is used as an erosion protection measure around the rounded leading edge. Nielsen said: "This specific tape was first applied on our blades fitted at turbines in the Middelgrunden offshore wind farm near Copenhagen and the solution has proven itself already for over ten years. The Haliade by comparison operates at a substantially higher 90m/s rated tip speed but this value represents a solid experience offshore."

The blade technology itself builds upon LM’s slender blade GloBlade programme, which first resulted in a 42.1-metre blade followed by larger 3MW sister products designed for rotor diameters up to 125-metres.

GloBlade combines advanced aerodynamics with in-house developed proprietary airfoils and proven materials technology. Nielsen said: "Materials technology is a decisive parameter to design and manufacture this relatively lightweight fibreglass blade for its length. We had to push glass/polyester blade technology to the next level for creating a 73.5-metre blade for offshore application without using carbon. We did that because of this composite technology’s strong track record, an ability to reduce overall wind turbine cost and because it would reduce risks in terms of manufacturability."

LM’s new airfoils are developed and tested in their own low-speed wind tunnel, with a capability to test blade sections facing real environmental conditions. "This offers superior and validated aerodynamics in every blade design", Nielsen concluded on the topic.

The combination of the blade monitoring system and turbine individual pitch control enabled a 10% reduction in loads and blade deflection. "Thanks to the work done on the design together with Alstom experts," Nielsen says, "we were also able to retain a 3.2-metre root diameter similar to the LM 61.5 P, while limiting blade mass to only 26 tons."

Nielsen further called a compact relative lightweight rotor hub design, the use of smaller diameter pitch bearings, and easier road transportation logistics as distinct added benefits of the modest blade root diameter concept.

A specialised team manufactures the giant upper and lower shells, and separate shear-web reinforcements, inside a hall dedicated to prototypes. After the different glass fabric layers and balsa wood fillings have been put precisely into a mould, it is sealed by plastic foil. As a next step polyester resin is pulled into the mould by negative pressure and impregnates the fibres already laid out in the mould followed by a curing process. Nielsen said: "A key advantage of applying polyester against epoxy resin is that it infuses very well and cures at room temperature, which makes it a safe and secure process."

Each new developed rotor blade is finally first exposed to a static extreme loads bending test followed by a dynamic two-directional fatigue full 20 – 25 year life test. The programme is completed by a post fatigue static test lasting in total about one year. LM said the duration of this testing programme compares well with the actual turbine field test validation program.