Gründungen von Offshore Windenergieanlagen: Von der Planung bis zur Lebensdauerüberwachung

Gründungen von Offshore Windenergieanlagen: Von der Planung bis zur Lebensdauerüberwachung Dr.-Ing. Fabian Kirsch GuD Geotechnik und Dynamik Consult GmbH, Berlin Offshoretage, 18. März 2016, TU Berlin

Übersicht Introduction Soil behaviour under regular cyclic loading Irregular load history Multiaxial load Geometry effects Validation example and dynamic behaviour Summary Folie 2

e.g. Monopile Foundation Structural assessment for: Prognosis of remaining service life Planning of inspections Planning of maintenance or retrofit

Basic Monitoring System 70 m MONOPILE FOUNDATION 0 m -23 m

Basic Monitoring System x y 70 m MONOPILE FOUNDATION e.g. 3D-Accelerometer @ 3 levels 0 m -23 m

Basic Monitoring System Structural response (time and frequency domain) MONOPILE FOUNDATION e.g. 3D-Accelerometer @ 3 levels Signals during operation (here computed)

Basic Monitoring System Structural response (time and frequency domain) Structure identification Mode 1: f = 0,352 Hz ζ = 0,7 % x y

Basic Monitoring System - Analysis Learning Phase Measurement (simulation) of structural response Analysis of Eigenfrequencies, Eigenmodes, Damping Analysis of vibration energy distribution Statistical analysis and definition of reference values and tolerances Monitoring Phase Measurement (simulation) of structural response Analysis of Eigenfrequencies, Eigenmodes, Damping Analysis of vibration energy distribution Comparison with reference values Structural assessment

Introduction Soil behaviour under regular cyclic loading Irregular load history Multiaxial load Geometry effects Validation example and dynamic behaviour Summary Folie 11

Cyclic response of granular material drained undrained Compaction Pore presure accumulation Folie 12

Cyclic response of granular material drained undrained Hardening Liquefaction Folie 13

Cyclic response of granular material drained undrained Hardening Softening Folie 14

Introduction Soil behaviour under regular cyclic loading Irregular load history Multiaxial load Geometry effects Validation example and dynamic behaviour Summary Folie 15

Influence of package order Short intermediate summary: - at constant mean stress level the influence of the package order is negligible Palmgren-Miner applies - after large cyclic load situations smaller loading leads to almost no additional deformation - with increasing load packages the effect of preceeding cyclic loading has a large effect Folie 17

Introduction Soil behaviour under regular cyclic loading Irregular load history Multiaxial load Geometry effects Validation example and dynamic behaviour Summary Folie 18

Load direction N N W O W O S wave S wind Example from Sedlacek et al., 2012 Folie 19

Influence of multidirectional loading Short intermediate summary : - changes in load direction lead to sudden increase of deformation - the increase is dependent on the angle of the change - in terms of total accumulated deformation the time of directional change is not relevant Folie 20

Introduction Soil behaviour under regular cyclic loading Irregular load history Multiaxial load Geometry effects Validation example and dynamic behaviour Summary Folie 21

Simplified model Folie 22

FE Model (HS small) Lasteinleitung weiche Balkenelemente (soft beam) 40m Interface-Elemente D = 50m 30m 40m 2m-10m L/D= 100m Homogener Boden 3 15 Vollpfahl mit modifizierter Steifigkeit für Stahlrohr t = 7cm Folie 23

FE Model initial stiffness profile Folie 24

Analytical approach (p-y-curves) 1. Standard API p-y-curves p A p ult tanh k z y A pult 2. mod. API p-y-curves with small strain stiffeness 1) E E sd s k red E s z 1 k mod p / p ult Esd k 1 red 1 1 A Es 1) nach Kirsch, Richter, Coronel (2014) Folie 25

Analytical approach (p-y-curves) 1. Standard API p-y-curves p A p ult tanh k z y A pult 2. mod. API p-y-curves with small strain stiffeness 1) E E sd s k red E 3. mod. API p-y-curves with small strain stiffness and geometry softening (large diameter) 1) ' red ' F ( D 2) s z 1 k mod p / p ult Esd k 1 red 1 1 A Es 1) nach Kirsch, Richter, Coronel (2014) Folie 27

Standard API and modified p-y-curves 1) D=8m 1) nach Schädlich, Kirsch, Richter (2015) Folie 28

Pile head load displacement curves (mudline) 1) D=8m (initial portion of response) 1) nach Schädlich, Kirsch, Richter (2015) Folie 29

Geometry effect Short intermediate summary : - Current monopile geometries leave basis of embedded pile solution for slender piles (p-y approaches) - especially at smaller strains (important for load simulation and operational modes) soil behaves stiffer than predicted by standard API p-y - Modifications of standard API p-y for small strains and geometry effects were developed Folie 30

Introduction Soil behaviour under regular cyclic loading Irregular load history Multiaxial load Geometry effects Validation example and dynamic behaviour Summary Folie 31

Validation of WTG Eigenfrequencies Offshore Wind Farm BelWind: Turbines: Location: Vestas V90 90m rotor diameter 72m hub height Bligh Bank (Belgium) 46km from shore line water depth: 20m to 37m Monopile: 5m diameter 70mm wall thickness 20.6m pile penetration length Monitoring system at BelWind running for more than 3 years Measured overall eigenfrequencies (tower and monopile) found significantly higher than design eigenfrequencies! (published at http://www.owi-lab.be)

Validation of WTG Eigenfrequencies Soil properties at reference location: Source: Predominant: Typical North Sea sand as present for many offshore wind farms.

Discrete spring force [N] Depth below mudline [m] Validation of WTG Eigenfrequencies Displacement [m] Comparison of Soil Approximation: Nonlinear equivalent spring and linearization by secant module Modified Standard API Modified Standard API Pile Deflection at SLS Load Level (10% of ultimate strength) Depth: 7.5m below mudline Lateral Displacement [m]

Validation of WTG Eigenfrequencies Resulting first overall eigenfrequencies for location BBCO1: Soil discretization (in BLADED) 20 discrete equivalent springs Equivalent pile head stiffness matrix Standard API Modified p-y Curves 0.350 Hz 0.354 Hz 0.350 Hz 0.355 Hz Information Initial BelWind Design: Measured: 0.350 Hz 0.361 Hz Conclusion: Better Approximation of first overall eigenfrequency by modified p-y curves!

Introduction Soil behaviour under regular cyclic loading Irregular load history Multiaxial load Geometry effects Validation example and dynamic behaviour Summary Folie 37

Summary 1. Interpretation of monitoring (SHM) requires knowledge of system development 2. Soils behave differently under different loading stages, cyclic load history influences stiffness development 3. Soil structure interaction is not constant over the lifetime 4. Modifications of simple p-y-approaches are necessary 5. Back calculation of measured and published values are promising, but 6. Better models might do better? Folie 38