Zur Modellierung der Verflüssigung von Sandböden

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

70 m

0 m

-23 m

Basic Monitoring System

MONOPILE FOUNDATION

x y70 m

0 m

-23 m

MONOPILE FOUNDATION

e.g. 3D-Accelerometer

@ 3 levels


Structural response (time and frequency domain)

MONOPILE FOUNDATION

e.g. 3D-Accelerometer

@ 3 levels

Signals during operation

(here computed)


Structure

identification

Mode 1: f = 0,352 Hz

ζ = 0,7 %

x y


Structural response (time and frequency domain)

Measurement

(simulation) of

structural response

Analysis of

Eigenfrequencies,

Eigenmodes, Damping

Analysis of vibration

energy distribution

Statistical analysis and

definition of reference

values and tolerances

Learning Phase

Basic Monitoring System - Analysis

Measurement

(simulation) of

structural response

Analysis of

Eigenfrequencies,

Eigenmodes, Damping


energy distribution




Learning Phase


Comparison with

reference values

Structural assessment

Monitoring Phase

Measurement

(simulation) of

structural response

Analysis of

Eigenfrequencies,

Eigenmodes, Damping


energy distribution

Measurement

(simulation) of

structural response

Analysis of

Eigenfrequencies,

Eigenmodes, Damping


energy distribution




Learning Phase


Comparison with

reference values

Structural assessment

Monitoring Phase

Measurement

(simulation) of

structural response

Analysis of

Eigenfrequencies,

Eigenmodes, Damping


energy distribution

?

Introduction



Multiaxial load

Geometry effects


Summary

Folie 11

Folie 12

Cyclic response of granular material

undraineddrained

CompactionPore presure

accumulation

Folie 13


undraineddrained

Hardening

Liquefaction

Folie 14


undraineddrained

HardeningSoftening

Introduction



Multiaxial load

Geometry effects


Summary

Folie 15

Folie 17

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

Introduction



Multiaxial load

Geometry effects


Summary

Folie 18

Folie 19

wave wind

S

N

W O W

S

N

O

Example from Sedlacek et al., 2012

Load direction

Folie 20

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

Introduction



Multiaxial load

Geometry effects


Summary

Folie 21

Folie 22

Simplified model

Folie 23

Homogener Boden

D =

2m-10m

L/D=

3 … 15

Vollpfahl mit modifizierter

Steifigkeit für Stahlrohr t = 7cm

Interface-Elemente

„weiche“ Balkenelemente

(soft beam)

30m

40m

50m40m

100m

Lasteinleitung

FE Model – (HS small)

Folie 24

FE Model – initial stiffness profile

1. Standard API p-y-curves

2. mod. API p-y-curves with small strain stiffeness 1)

Folie 25

Analytical approach (p-y-curves)

ult

ultpA

yzkpAp tanh

sd sE E

zkE reds

1

/11mod

s

sdultred

E

E

A

ppkk

1

1) nach Kirsch, Richter, Coronel (2014)

1. Standard API p-y-curves

2. mod. API p-y-curves with small strain stiffeness 1)

3. mod. API p-y-curves with small strain stiffness and geometry

softening (large diameter) 1)

Folie 27

Analytical approach (p-y-curves)

ult

ultpA

yzkpAp tanh

' ' ( 2)red F D

sd sE E

zkE reds

1

/11mod

s

sdultred

E

E

A

ppkk

1

1) nach Kirsch, Richter, Coronel (2014)

Folie 28

Standard API andmodified p-y-curves 1)

D=8m

1) nach Schädlich, Kirsch, Richter (2015)

Folie 29

D=8m

(initial portion of response)

1) nach Schädlich, Kirsch, Richter (2015)

Pile head load displacement curves(mudline) 1)

Folie 30

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

Introduction



Multiaxial load

Geometry effects


Summary

Folie 31

Validation of WTG Eigenfrequencies

Offshore Wind Farm BelWind:

Turbines: Vestas V90 90m rotor diameter 72m hub height

Location: Bligh Bank (Belgium) 46km from shore line water depth: 20m to 37m

Monopile: 5m diameter 70mm wall thickness 20.6m pile penetration length

(published at http://www.owi-lab.be)

Monitoring system at BelWind running for

more than 3 years

Measured overall

eigenfrequencies

(tower and monopile)

found significantly

higher than design

eigenfrequencies!

Soil properties at reference location:

Source:

Predominant: Typical North Sea sand as presentfor many offshore wind farms.


Comparison of Soil Approximation:

Displacement [m]

Depth

belo

w m

udlin

e [

m]

ModifiedStandard API

Lateral Displacement [m]

ModifiedStandard API

Depth: 7.5m below mudline

Pile Deflection at SLS Load Level(10% of ultimate strength)

Dis

crete

spring f

orc

e [

N]

Nonlinear equivalent spring andlinearization by secant module


Resulting first overall eigenfrequencies for location BBCO1:

Soil discretization

(in BLADED)Standard API

Modified

p-y Curves

20 discrete

equivalent springs0.350 Hz 0.354 Hz

Equivalent pile head

stiffness matrix0.350 Hz 0.355 Hz

Information

Initial BelWind Design: 0.350 HzMeasured: 0.361 Hz

Conclusion: Better Approximation of first overalleigenfrequency by modified p-y curves!


Introduction



Multiaxial load

Geometry effects


Summary

Folie 37

Folie 38

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?

Zur Modellierung der Verflüssigung von Sandböden

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