INTEGRATED FATIGUE LOADING FOR WIND TURBINES IN WIND FARMS BY COMBINING AMBIENT TURBULENCE AND WAKES.pdf

INTEGRATED FATIGUE LOADING FOR WIND TURBINES IN WIND FARMS BY COMBINING AMBIENT TURBULENCE AND WAKES Sten Frandsen ? and Morten L. Th?gersen Ris? National Laboratory DK-4000 Roskilde Denmark e-mail: sten.frandsen@risoe.dk and morten.thoegersen@risoe.dk ABSTRACT A study is presented of an offshore application of a fatigue model for wind turbines operating partly in ambient turbulence and partly wakes. In order limit the number of aeroelelastic calculations in design verification, it is necessary to reduce the measured distribution of turbulence intensities into specific design turbulence. In this study, the observation that the fatigue loading is proportional to the turbulence is used. Thus, turbulence is used as primary design parameter, substantially describing the fatigue effects. The design turbulence is defined as the turbulence that gives an equal amount of fatigue damage as the combination of true distribution of ambient turbulence and turbulence generated from wake effects. The single most important parameter responsible for the increase in fatigue loading is very short distances between turbines. KEYWORDS Turbulence, fatigue analysis, wakes, offshore, added turbulence, ambient turbulence. NOMENCLATURE a Slope parameter for the turbulence standard deviation model a Constant in model for added turbulence “over” wind farm b Constant in model for added turbulence “over” wind farm b Constant in expression for “effective” width of wake CT Thrust coefficient of wind turbine rotor ct Drag coefficient for wind farm, corresponding to drag-per-m 2 e Equivalent stress range [N/m 2 ] f’ Coriolis parameter multiplied by e 4 f(*) Normal distribution f0 Probability as function of wind direction h Hub-height [m] iaddwf(y) Added turbulence intensity as function of downstream distance to wind turbine row I Turbulence intensity I’ Spatially averaged turbulence in wind farm I15 Characteristic value of hub-height turbulence intensity at a wind speed of 15 m/s Im Fatigue-weigh