Progressive structural scaling of a 20 MW two-bladed offshore wind turbine rotor blade examined by finite element analyses

Abstract

Two-bladed turbines offer a promising opportunity for rotor cost savings, especially considering the ongoing growth trend in rotor size. An increased chord and airfoil thickness of a two-bladed turbine’s blade results in potential structural improvements caused by a rapidly growing second moment of area. Compared to a three-bladed turbine’s blade, the blade structure would theoretically require less material, while withstanding 50% higher flapwise loads. An analytical method of progressive structural scaling for three-dimensional rotor blade structures, based on equal material stresses, is introduced to calculate the modified structural thickness properties of the two-bladed turbine’s blade. It simplifies the airfoil-shaped structure to a thin-walled rectangle, utilizes a fixed initial flapwise load factor, and scales the edgewise loads proportionally to the required blade mass. To evaluate the validity of this analytical approach, a progressively scaled and an iterated 20 MW two-bladed turbine’s blade are examined with finite element analyses for static loads. The outcomes are then compared to corresponding analyses of a three-bladed turbine’s reference blade. Overall, the static stress comparisons at different blade positions show good agreement with the analytical results. Nevertheless, the buckling analyses performed reveal stability issues, which subsequently will lead to a readjustment of the blade mass.