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Comparing Sources of Damping of cross-wind Motion

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

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  1. 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

  2. Outline • Background: the wind-wave misalignment problem • Measurements of over-all damping • Cautious theoretical reconstruction of damping contributions • Conclusions • (Approach to load calculations)

  3. 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

  4. Measurements • Emergency stops, i.e. no aerodynamic damping • Horns Rev 1 • Burbo •  modal > 10%

  5. 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

  6. 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%

  7. Structural •  modal  1.2% • For a monopile this does NOT regard the grout-connection • I.e. it is a conservative value

  8. 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

  9. 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

  10. 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

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