Offshore wind turbines are designed and analyzed using comprehensive simulation tools (or codes) that account for the coupled dynamics of the wind inflow, aerodynamics, elasticity, and controls of the turbine, along with the incident waves, sea current, hydrodynamics, mooring dynamics, and foundation dynamics of the support structure. This paper describes the latest findings of the code-to-code verification activities of the Offshore Code Comparison Collaboration Continuation project, which operates under the International Energy Agency Wind Task 30. In the latest phase of the project, participants used an assortment of simulation codes to model the coupled dynamic response of a 5-MW wind turbine installed on a floating semisubmersible in 200 m of water. Code predictions were compared from load case simulations selected to test different model features. The comparisons have resulted in a greater understanding of offshore floating wind turbine dynamics and modeling techniques, and better knowledge of the validity of various approximations. The lessons learned from this exercise have improved the participants’ codes, thus improving the standard of offshore wind turbine modeling.

Today, structural integrity inspections of wind turbine blades are typically carried out by the use of rope or platform access. Since these inspection approaches are both tedious and extremely costly, a need for a method facilitating reliable, remote monitoring of the blades has been identified. In this article, it is examined whether a vibration-based damage localization approach proposed by the authors can provide such reliable monitoring of the location of a structural damage in a wind turbine blade. The blade, which is analyzed in idle condition, is subjected to unmeasured hits from a mounted actuator, yielding vibrations that are measured with a total of 12 accelerometers; of which 11 are used for damage localization. The employed damage localization method is an extended version of the stochastic dynamic damage location vector method, which, in its origin, is a model-based method that interrogates damage-induced changes in a surrogate of the transfer matrix. The surrogate’s quasi-null vector associated with the lowest singular value is converted into a pseudo-load vector and applied to a numerical model of the healthy structure in question, hereby, theoretically, yielding characteristic stress resultants approaching zero in the damaged elements. The proposed extension is based on outlier analysis of the characteristic stress resultants to discriminate between damaged elements and healthy ones; a procedure that previously, in the context of experiments with a small-scale blade, has proved to mitigate noise-induced anomalies and systematic, non-damage-associated adverse effects.

Aeroelastic design and fatigue analysis of large utility-scale wind turbine blades have been performed to investigate the applicability of different types of materials in a fatigue environment. The blade designs used in the study are developed according to an iterative numerical design process for realistic wind turbine blades, and the software tool FAST is used for advanced aero-servo-elastic simulations. Elementary beam theory is used to calculate strain time series from these simulations, and the material fatigue is evaluated using established methods. Following wind turbine design standards, the fatigue evaluation is based on a turbulent wind load case. Fatigue damage is estimated based on 100% availability and a site-specific annual wind distribution. Rainflow cycle counting and Miner's sum for cumulative damage prediction is used together with constant life diagrams tailored to actual material S-N data. Material properties are based on 95% survival probability, 95% confidence level, and additional material safety factors to maintain conservative results. Fatigue performance is first evaluated for a baseline blade design of the 10 MW NOWITECH reference wind turbine. Results show that blade damage is dominated by tensile stresses due to poorer tensile fatigue characteristics of the shell glass fiber material. The interaction between turbulent wind and gravitational fluctuations is demonstrated to greatly influence the damage. The need for relevant S-N data to reliably predict fatigue damage accumulation and to avoid nonconservative conclusions is demonstrated. State-of-art wind turbine blade trends are discussed and different design varieties of the baseline blade are analyzed in a parametric study focusing on fatigue performance and material costs. It is observed that higher performance material is more favorable in the spar-cap construction of large blades which are designed for lower wind speeds.

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This article presents a simplified, integrated method for design studies of blades for offshore wind turbines. The method applies to variable speed horizontal axis wind turbines with pitch control, and allows designing the rotor blades based on a very limited set of input parameters. The purpose of the method is to allow parametric studies of different design configurations of the rotor at a reasonable effort. The resulting wind turbine models are at a level of detail suitable for preliminary design considerations using e.g. aero-elastic simulations in the time domain. The aerodynamic design is based on blade element momentum (BEM) considerations using a distribution of 2D airfoil characteristics. The structural design of the blades is based on aerodynamic forces calculated from a small number of load cases. The design procedure is facilitated by using simplified cross-section definitions and iterative approaches. The resulting blade designs are shown to compare well with data from available turbine models.