1- Introduction
A renewed interest for clean and renewable wind energy sources is witnessed in the past two decades. Increasing number of large size and high power wind turbines are used for harvesting energy. Due to their enormous size and due to the fact that wind turbines may be subjected to extreme wind conditions during their lifetime, it is imperative that proper assessment of wind load should be carried from structural safety point of view. It is observed that very little information is available in published literature. Generally there are three main sources of information to establish relevant wind loading schemes for wind turbine design. These include (i) experimental studies, (ii) theoretical/numerical studies and (iii) field and full-scale measurements. Experimental studies: An example that fits under this category is the Unsteady Aerodynamics Experiment (UAE) study carried out by the National Renewable Energy Laboratory (NREL), which comprised tests of an extensively instrumented onshore wind turbine blades in the large NASA-Ames 24.4-m (80 ft) by 36.6-m (120 ft) wind tunnel (Simms et al., 2001). The objective was to estimate the load paths and to provide comparisons with analytical techniques used to evaluate wind loads. It was reported that the blind comparison results were not favorable. In another study, wind load at the base of wind turbines was estimated using a 1:50 scale models representing several 5 MW floating offshore wind turbines (FOWT) in a wave tank under combined wind and wave loading at the Maritime Research Institute Netherlands (MARIN) (De Ridder et al., 2011). Skaare et al. (2007) also carried out model scale experiments at the Ocean Basin Laboratory in Trondheim in order to validate the motion characteristics of the HYWIND concept, a full-scale …show more content…
This information is used to characterize the force and moment exerted by the turbine at the top of the foundation column. Svensson (2010) used typical wind loads for onshore wind turbines situated on the west coast of Sweden, for the design aspects of different types of foundations. The actual loads acting on the foundation (provided by the manufacturer) were given in the form of two forces, one horizontal and one vertical, one bending moment and one twisting moment. Both manual calculations and numerical analyses were performed for the design procedure of a case study of an 80 m high wind turbine with actual loads. In a report published by the National Renewable Energy Laboratory (NREL), all load data were presented for a 5 MW NREL reference wind turbine (Jonkman et al., 2009). The report includes rotor thrust and torque for different wind speeds. These loads were obtained numerically by the FAST (Fatigue, Aerodynamics, Structures, and Turbulence) program (Buhl Jr and Manjock, 2006). Ragan and Manuel (2007) presented statistical extrapolation methods for estimating wind turbine extreme loads for a utility-scale 1.5 MW turbine sited in Colorado to compare the performance of several alternative techniques for statistical extrapolation of rotor and tower loads. These statistical methods were based on field data from the utility-scale turbine to address statistical load extrapolation issues. The study suggests that the method should be useful in simulation-based attempts at deriving wind turbine design loads. Xu and Ishihara (2014) used analytical formulae to estimate the maximum value for along-wind and cross-wind loads on the wind turbine towers by using the quasi-steady analysis. The formulae showed favorable agreements with the full dynamic simulation. Simplified formula to calculate the maximum base moment
A renewed interest for clean and renewable wind energy sources is witnessed in the past two decades. Increasing number of large size and high power wind turbines are used for harvesting energy. Due to their enormous size and due to the fact that wind turbines may be subjected to extreme wind conditions during their lifetime, it is imperative that proper assessment of wind load should be carried from structural safety point of view. It is observed that very little information is available in published literature. Generally there are three main sources of information to establish relevant wind loading schemes for wind turbine design. These include (i) experimental studies, (ii) theoretical/numerical studies and (iii) field and full-scale measurements. Experimental studies: An example that fits under this category is the Unsteady Aerodynamics Experiment (UAE) study carried out by the National Renewable Energy Laboratory (NREL), which comprised tests of an extensively instrumented onshore wind turbine blades in the large NASA-Ames 24.4-m (80 ft) by 36.6-m (120 ft) wind tunnel (Simms et al., 2001). The objective was to estimate the load paths and to provide comparisons with analytical techniques used to evaluate wind loads. It was reported that the blind comparison results were not favorable. In another study, wind load at the base of wind turbines was estimated using a 1:50 scale models representing several 5 MW floating offshore wind turbines (FOWT) in a wave tank under combined wind and wave loading at the Maritime Research Institute Netherlands (MARIN) (De Ridder et al., 2011). Skaare et al. (2007) also carried out model scale experiments at the Ocean Basin Laboratory in Trondheim in order to validate the motion characteristics of the HYWIND concept, a full-scale …show more content…
This information is used to characterize the force and moment exerted by the turbine at the top of the foundation column. Svensson (2010) used typical wind loads for onshore wind turbines situated on the west coast of Sweden, for the design aspects of different types of foundations. The actual loads acting on the foundation (provided by the manufacturer) were given in the form of two forces, one horizontal and one vertical, one bending moment and one twisting moment. Both manual calculations and numerical analyses were performed for the design procedure of a case study of an 80 m high wind turbine with actual loads. In a report published by the National Renewable Energy Laboratory (NREL), all load data were presented for a 5 MW NREL reference wind turbine (Jonkman et al., 2009). The report includes rotor thrust and torque for different wind speeds. These loads were obtained numerically by the FAST (Fatigue, Aerodynamics, Structures, and Turbulence) program (Buhl Jr and Manjock, 2006). Ragan and Manuel (2007) presented statistical extrapolation methods for estimating wind turbine extreme loads for a utility-scale 1.5 MW turbine sited in Colorado to compare the performance of several alternative techniques for statistical extrapolation of rotor and tower loads. These statistical methods were based on field data from the utility-scale turbine to address statistical load extrapolation issues. The study suggests that the method should be useful in simulation-based attempts at deriving wind turbine design loads. Xu and Ishihara (2014) used analytical formulae to estimate the maximum value for along-wind and cross-wind loads on the wind turbine towers by using the quasi-steady analysis. The formulae showed favorable agreements with the full dynamic simulation. Simplified formula to calculate the maximum base moment