刚度和阻尼单桩基础在砂PPT课件

1、Wind-wave misalignment cause the resonant response to be governed by soil damping Damping (excl. aerodynamic damping) become a design driver Consequence of underestimation soil damping: increased use of steel higher costs1Soil dampingIncreasing water depths and larger turbines reduces the 1st natura

2、l frequency Soil stiffness become a major design driverConsequence of underestimating soil stiffness :increased use of steelhigher costsapplicable range of monopile foundations limited to water depths less than 30 m.Soil stiffnessImpact on design第1页/共20页 Damping presently used for design calculation

3、s is based on a theoretical reconstruction of damping contributions2Soil damping - theoretical20 m20 m80 mD=4.7 mSource1st mode (%) RemarkSoil3-5Visco-elasticHydrodynamic0.75Radiation onlySteel tower + pile1.2Disregarding groutTower damperTypically 2Turbine dependentAerodynamic1Inherent in BEM0.3 Hz

4、Table adapted from: Niels Jacob Tarp-Johansen et al. Comparing Sources of Damping of cross-wind Motion, European Offshore Wind 2009, Stockholm( modal = Modal log. decr., 2, = damping ratio)第2页/共20页3Soil damping - measurements Emergency stops, i.e. no aerodynamic damping Horns Rev 1 Offshore Wind Far

5、m Burbo Offshore Wind FarmMeasurements show more damping (excl. aerodynamic damping) than assumed in present design calculationsCurrent design = Theoretical approach: modal = 8 %Measurements: modal 10 %第3页/共20页Soil stiffness - theoreticalThe py curves for piles in sand described by Reese et al. (197

6、4) and ONeill & Murchison (1983) led to recommendations in the standards (DNV, 1977; API, 1993)Scale effects?Is a monopile a pile?第4页/共20页Soil stiffness - measurementsFigure: Scour hole depth from xyz-point cloud Gunfleet Sands Offshore Wind Farm Soil: sand and clay layers 1st Natural Frequency:

7、 Calculated value: 0.302-0.308 Hz Measured value: 0.314 Hz 60-150% higher than predicted Closest prediction in wind farm Back-calculation on soil stiffness:第5页/共20页Introduction, stiffness and damping Impact on design ExperienceSoil damping Overview of soil damping Theoretical derivation of damping d

8、ue to pore pressure dissipation ConclusionsOn-going and future work Full-scale measurements6Outline第6页/共20页7Soil dampingTypeParticle size mmClay0-0.002Silt0.002-0.06Sand0.06-2Gravel2-60Geometrical damping (wave radiation)Vanishing for frequencies 1 HzMaterial dampingNon-linear hysteresis. Investigat

9、ions indicates modal 3-5%Rodenhausen, Moritz (2010), Soil Response of Offshore Wind Turbines - Stiffness and damping of monopile foundations, Master Thesis, University of StuttgartDamping contribution from pore pressure dissipation? 第7页/共20页8Grid of two-dimensional soil model in Flac3DSoil damping f

10、rom pore pressure dissipationDfluxy p FLAC3D, Itasca 3D disc model Partially drained simulation Linear elastic soil / Darcy flowDecreasing permeabilityFully drained responseUndrained response第8页/共20页9 kGshear-stiffnesskKbulk-stiffnessDamping due to Darcys flow dySoil damping from pore pressure dissi

11、pation Comparison results (marker) with simple spring-dashpot model (solid lines) Replication FLAC3D results using a simple spring-dashpot modeSpring-dashpot constants calibrated to FLAC3D results.第9页/共20页10Soil damping from pore pressure dissipation Pile diameter: 5 m Natural frequency: 0.3 Hz Soil

12、 stiffness representative of a typical sandConclusions: Transition range over two orders of magnitude of permeability Undrained (stiffer) response in typical sand and silts Up to modal 1%, however Significant damping only in gravels and highly permeable sandsSchematic illustration of soil response a

13、f function of soil permeability第10页/共20页Introduction, stiffness and damping Impact on design ExperienceSoil damping Overview of soil damping Theoretical derivation of damping due to pore pressure dissipation ConclusionsOn-going and future work Full-scale measurements11Outline第11页/共20页Full-scale meas

14、urements 1 Monopile Commisioning in spring 2011 Soil profile: Sand L/D = 412On-going and future work - Walney Offshore Wind FarmMono pile36 strain gauges (9 levels)Transition Piece8 strain gauges (2 levels)6 accelerometers (2 levels)12 displacement transducers (6H 6V)2 manual inclinometers (NS &

15、 EW)第12页/共20页Sensor installation at Walney Offshore Wind Farm13Monopile sensorsRostock, GermanyEEW (steel work) / HBM (sensors)第13页/共20页Sensor installation at Walney Offshore Wind Farm14Monopile sensorsRostock, GermanyEEW (steel work) / HBM (sensors)第14页/共20页Sensor installation at Walney Offshore Wind Farm15Transition piece sensorsBarrow, UKHBM (Sensors) 第15页/共20页Sensor installation at Walney Offshore Wind Farm16第16页/共20页Sensor installation at Walney Offshore Wind Farm17第17页/共20页Full-scale measurements 1 or 2 Monopile(s) Commisioning in 2012 Soil profil