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Comparing Sources of Damping of cross-wind Motion. Niels Jacob Tarp-Johansen, DONG Energy Christian Mørch, DONG Energy Lars Andersen, AUU Erik D. Christensen, DHI Sten Frandsen, Risø/DTU Bjarne Kallesøe, Risø/DTU Danish R&D project: HAVDIM Coordinator Risø/DTU. Outline.
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Comparing Sources of Damping of cross-wind Motion Niels Jacob Tarp-Johansen, DONG Energy Christian Mørch, DONG Energy Lars Andersen, AUU Erik D. Christensen, DHI Sten Frandsen, Risø/DTU Bjarne Kallesøe, Risø/DTU Danish R&D project: HAVDIM Coordinator Risø/DTU
Outline • Background: the wind-wave misalignment problem • Measurements of over-all damping • Cautious theoretical reconstruction of damping contributions • Conclusions • (Approach to load calculations)
The Wind-wave misalignment Problem • Random input • Resonant response is governed by damping • Current approach assumes e.g. log. decr. modal = 6% ( 2, = damping ratio) • Assuming aligned loading is not necessarily conservative
Measurements • Emergency stops, i.e. no aerodynamic damping • Horns Rev 1 • Burbo • modal > 10%
Aero-dynamic Damping cross-wind • Inherent in BEM-simulations • Works through coupling • The aerodynamic mode is slightly misaligned width the mean wind direction, i.e. cross-wind motion is damped via the along wind aerodynamics too. • With coupling • 0.50 % • With-out coupling • 0.25 % • NREL-turbine down-scaled to 3.5 MW
Tower dampers • Various configurations exist, e.g. • Pendulum submerged in liquid • Liquid dampers • Key issues are • Requires tuning to 1st mode frequency • Efficiency dependents on damper mass vs. main (modal) mass • Consequently turbine design dependent. • Typically one sees modal > 2%
Structural • modal 1.2% • For a monopile this does NOT regard the grout-connection • I.e. it is a conservative value
Hydrodynamic • Wave radiation • Dissipation due to drag • Dissipation • Proportional to deflections squared • Introducing relative velocities in Morrison's eq. is NOT conservative • Wave radiation • Frequency domain (.e. linear) approach applying WAMIT • modal 1.5% 80 m 0.3 Hz 20 m D=4.7 m
Soil damping • Geometrical (wave radiation) • Vanishing for frequencies < about 1 Hz • Material • Non-linear hysteresis • Linear-visco-elastic • Approach • Soil volume rather than soil-springs • Frequency independent loss factor of 5% • modal 3% 80 m 0.3 Hz D=4.7 m • Conservative because elasto-plastic behaviour at sea-bed and toe. • Investigations accounting for this indicates modal 5% 20 m
Conclusions • Measurements show more damping than assumed in present design calculations • A cautious theoretical reconstruction has been made • Monopile in a sediment 80 m 0.3 Hz 20 m D=4.7 m 20 m