Monopiles in SandStiffness and Damping Christian LeBlanc Thilsted, DONG Energy RenewablesNiels Jacob Tarp-Johansen, DONG Energy Renewables

EWEA 2011, 14-17 March, Brussels

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Introduction, soil stiffness and dampingImpact on designExperience

Soil dampingOverview of soil dampingTheoretical derivation of damping due to pore pressure dissipationConclusions

On-going and future workFull-scale measurements

*Outline

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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 costs

*Soil dampingIncreasing water depths and larger turbines reduces the 1st natural frequency Soil stiffness become a major design driver

Consequence of underestimating soil stiffness :increased use of steelhigher costsapplicable range of monopile foundations limited to water depths less than ~30 m.

Soil stiffness

Impact on design

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Damping presently used for design calculations is based on a theoretical reconstruction of damping contributions

*Soil damping - theoretical0.3 HzTable 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)

Source1st mode (%)RemarkSoil~3-5Visco-elasticHydrodynamic~0.75Radiation onlySteel tower + pile~1.2Disregarding groutTower damperTypically > 2Turbine dependentAerodynamic~1Inherent in BEM

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*Soil damping - measurementsEmergency stops, i.e. no aerodynamic damping

Horns Rev 1 Offshore Wind FarmBurbo Offshore Wind Farm

Measurements show more damping (excl. aerodynamic damping) than assumed in present design calculationsCurrent design = Theoretical approach: modal = 8 %Measurements: modal > 10 %

Soil stiffness - theoreticalThe py curves for piles in sand described by Reese et al. (1974) and ONeill & Murchison (1983) led to recommendations in the standards (DNV, 1977; API, 1993)

Scale effects?Is a monopile a pile?

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Soil stiffness - measurements

Figure: Scour hole depth from xyz-point cloud Gunfleet Sands Offshore Wind Farm Soil: sand and clay layers

1st Natural Frequency:

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:

Introduction, stiffness and dampingImpact on designExperience

Soil dampingOverview of soil dampingTheoretical derivation of damping due to pore pressure dissipationConclusions

On-going and future workFull-scale measurements

*Outline

*Soil damping

Geometrical damping (wave radiation)Vanishing for frequencies < ~1 Hz

Material dampingNon-linear hysteresis. Investigations indicates modal 3-5%

Rodenhausen, Moritz (2010), "Soil Response of Offshore Wind Turbines - Stiffness and damping of monopile foundations", Master Thesis, University of Stuttgart

Damping contribution from pore pressure dissipation?

TypeParticle size [mm]Clay0-0.002Silt0.002-0.06Sand0.06-2Gravel2-60

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Grid of two-dimensional soil model in Flac3DSoil damping from pore pressure dissipation

p FLAC3D, Itasca 3D disc model Partially drained simulation Linear elastic soil / Darcy flow

Decreasing permeabilityFully drained responseUndrained response

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Soil damping from pore pressure dissipation Comparison results (marker) with simple spring-dashpot model (solid lines)

Replication FLAC3D results using a simple spring-dashpot mode

Spring-dashpot constants calibrated to FLAC3D results.

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*Soil damping from pore pressure dissipation Pile diameter: 5 m Natural frequency: 0.3 Hz Soil stiffness representative of a typical sand

Conclusions: 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 sands

Schematic illustration of soil response af function of soil permeability

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Introduction, stiffness and dampingImpact on designExperience

Soil dampingOverview of soil dampingTheoretical derivation of damping due to pore pressure dissipationConclusions

On-going and future workFull-scale measurements

*Outline

Full-scale measurements1 MonopileCommisioning in spring 2011Soil profile: SandL/D = 4

*On-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 & EW)

Sensor installation at Walney Offshore Wind Farm

*Monopile sensorsRostock, GermanyEEW (steel work) / HBM (sensors)

Sensor installation at Walney Offshore Wind Farm

*Monopile sensorsRostock, GermanyEEW (steel work) / HBM (sensors)

Sensor installation at Walney Offshore Wind Farm*Transition piece sensorsBarrow, UKHBM (Sensors)

Sensor installation at Walney Offshore Wind Farm

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Sensor installation at Walney Offshore Wind Farm

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Full-scale measurements1 or 2 Monopile(s)Commisioning in 2012Soil profile: Clay

On-going and future work London Array

Monopiles in SandStiffness and Damping Christian LeBlanc Thilsted, DONG Energy RenewablesNiels Jacob Tarp-Johansen, DONG Energy Renewables

EWEA 2011, 14-17 March, Brussels

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