Full-scale fatigue testing is part of the certification process for large wind turbine blades. That testing is usually performed about the flapwise and edgewise axes independently but a new method for resonant fatigue testing has been developed in which the flapwise and edgewise directions are tested simultaneously, thus also allowing the interactions between the two mutually perpendicular loads to be investigated. The method has been evaluated by comparing the Palmgren-Miner damage sum around the cross-section at selected points along the blade length that results from a simulated service life, as specified in the design standards, and testing. Bending moments at each point were generated using wind turbine simulation software and the test loads were designed to cause the same amount of damage as the true service life. The mode shape of the blade was tuned by optimising the position of the excitation equipment, so that the bending moment distribution was as close as possible to the target loads. The loads were converted to strain-time histories using strength of materials approach, and fatigue analysis was performed. The results show that if the bending moment distribution is correct along the length of the blade, then dual-axis resonant testing tests the blade much more thoroughly than sequential tests in the flapwise and edgewise directions. This approach is shown to be more representative of the loading seen in service and can thus contribute to a potential reduction in the weight of wind turbine blades and the duration of fatigue tests leading to reduced cost.

In this paper, the sensitivity of the structural integrity of wind turbine blades to debonding of the shear web from the spar cap was investigated. In this regard, modal analysis, static and fatigue testing were performed on a 45.7 m blade for three states of the blade: (i) as received blade (ii) when a crack of 200 mm was introduced between the web and the spar cap and (iii) when the crack was extended to 1000 mm. Calibration pull-tests for all three states of the blade were performed to obtain the strain-bending moment relationship of the blade according to the estimated target bending moment (BM) which the blade is expected to experience in its service life. The resultant data was used to apply appropriate load in the fatigue tests. The blade natural frequencies in flapwise and edgewise directions over a range of frequency domain were found by modal testing for all three states of the blade. The blade first natural frequency for each state was used for the flapwise fatigue tests. These were performed in accordance with technical specification IEC TS 61400-23. The fatigue results showed that, for a 200 mm crack between the web and spar cap at 9 m from the blade root, the crack did not propagate at 50% of the target BM up to 62,110 cycles. However, when the load was increased to 70% of target BM, some damages were detected on the pressure side of the blade. When the 200 mm crack was extended to 1000 mm, the crack began to propagate when the applied load exceeded 100% of target BM and the blade experienced delaminations, adhesive joint failure, compression failure and sandwich core failure.

Upscaling wind turbines has resulted in levelised cost of energy (LCoE) reductions. However, larger turbine diameters pose significant design challenges, often with conflicting requirements. For example, non-linear dynamics of aeroelastic tailored blades must be accurately predicted whilst, for the sake of efficient gradient-based design, it is also desirable to simplify the numerical definition of such blades—keeping design variables (DVs) to a minimum. This work presents and validates two features of the ATOM code (Aeroelastic Turbine Optimisation Methods), developed at the University of Bristol, that enable accurate and efficient modelling of large-scale wind turbine blades. Both an efficient parameterisation method and high-order beam elements illustrate the capacity for increasing the speed of gradient evaluations whilst accurately predicting blade dynamics—either by reducing DVs or simulation time. As a preliminary validation, aero-servo-elastic simulations from ATOM and an industry-standard software—DNV GL Bladed—are compared against field measurements gathered from an existing 7 MW turbine.

This paper presents, through the structural design of a 20 MW wind turbine blade, a selection of novel analysis and optimisation methods for wind turbines. These methods are integrated in the software—Aeroelastic Turbine Optimisation Methods (ATOM). A key feature is the novel, computationally-efficient piecewise linear model for running rapid design load case simulations (up to 16 times speed-up over conventional methods). Further, a comprehensive set of realistic design constraints is also proposed to ensure structural feasibility and aeroelastic stability. To demonstrate these methods, a sequential gradient-based optimisation process is employed, relying on the globally convergent method of moving asymptotes (GCMMA). The process begins with an aerodynamic optimisation to generate twist distributions, followed by an iterative loop during which load envelope updates and ‘frozen-load’ blade structural optimisations are performed independently. Aeroelastic loads are therefore considered, but are not directly optimised for. The present study investigates the structural design of a 20 MW wind turbine blade with hybrid carbon-glass spar caps. The optimised 122 m blade is found to have a mass of 83,622 kg, which decreases to 81,396 kg (-2.66%) with the addition of sweep. The GCMMA is found to converge successfully at each structural optimisation step. By contrast, the iterative loop is observed to oscillate, albeit within small bounds. Finally, results suggest that convergence of multi-step optimisation methods for aeroelastic blade design may not be guaranteed if design variables inducing aeroelastic couplings are considered.

No abstract