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Pile foundations for offshore wind turbines: Numerical and experimental investigations on the behaviour under short-term and long-term cyclic loading [Elektronische Ressource] / Pablo Cuéllar. Betreuer: Stavros Savidis

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Pile Foundations for Offshore Wind Turbines: Numerical and Experimental Investigations on the Behaviour under Short-Term and Long-Term Cyclic Loading von Pablo Cuéllar *)Ingeniero de Caminos, Canales y Puertos geboren am 16. Januar 1978 in Madrid von der Fakultät VI - Planen Bauen Umwelt der Technischen Universität Berlin zur Erlangung des Akademischen Grades Doktor der Ingenieurwissenschaften (Dr.-Ing.) genehmigte Dissertation Promotionsausschuß: Vorsitzender: Prof. Dr.-Ing. Karsten Geißler Gutachter: Prof. Dr.-Ing. Stavros Savidis Prof. Dr.-Ing. Werner Rücker Gutachter: Prof. Dr.-Ing. Manuel Pastor Tag der Einreichung: 15. Dezember 2010 Tag der Prüfung: 11. Februar 2011 Berlin 2011 D 83 *) Bauingenieur / Civil engineer ABSTRACT The behaviour of pile foundations for offshore wind turbines deviates from classical assumptions and accumulated experience mainly due to their large diameter, reduced slenderness and elevated ratio of lateral to vertical loads. The offshore environment poses the additional challenge of large numbers of load cycles from wind and waves and the possible influence of transient changes of pore water pressure around the pile, both of them issues that are still not well understood and also not being contemplated in current design guidelines.
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Pile Foundations for Offshore Wind Turbines: Numerical and
Experimental Investigations on the Behaviour under Short-
Term and Long-Term Cyclic Loading


von Pablo Cuéllar
*)Ingeniero de Caminos, Canales y Puertos
geboren am 16. Januar 1978 in Madrid


von der Fakultät VI - Planen Bauen Umwelt
der Technischen Universität Berlin
zur Erlangung des Akademischen Grades
Doktor der Ingenieurwissenschaften (Dr.-Ing.)
genehmigte Dissertation

Promotionsausschuß:
Vorsitzender: Prof. Dr.-Ing. Karsten Geißler
Gutachter: Prof. Dr.-Ing. Stavros Savidis Prof. Dr.-Ing. Werner Rücker
Gutachter: Prof. Dr.-Ing. Manuel Pastor

Tag der Einreichung: 15. Dezember 2010
Tag der Prüfung: 11. Februar 2011

Berlin 2011
D 83

*) Bauingenieur / Civil engineer
ABSTRACT

The behaviour of pile foundations for offshore wind turbines deviates from classical assumptions
and accumulated experience mainly due to their large diameter, reduced slenderness and elevated
ratio of lateral to vertical loads. The offshore environment poses the additional challenge of large
numbers of load cycles from wind and waves and the possible influence of transient changes of
pore water pressure around the pile, both of them issues that are still not well understood and
also not being contemplated in current design guidelines.
In saturated soils, short-term cyclic loading within the extreme regime often involves a pore-
pressure build up that eventually can lead to liquefaction phenomena and foundation failure. On
the other hand, the effects of the long-term cyclic loading on the foundation's stability and espe-
cially on its serviceability must also be studied.
The purpose of this work was to gain an insight into both aspects, while developing a practicable
numerical tool for a short-term prognosis and deriving useful criteria for design. To achieve such
goals, the investigations have been broadly divided into a first part with a theoretical orientation
for the analysis of short-term transient effects in the frame of the Finite Element Method, and a
second empirical block for the study of long-term phenomena by means of model tests in a re-
duced scale.
The general picture that arises from these investigations is that of a foundation subject to harden-
ing and soil densification in the long term, but also affected by transient episodes of significant
softening during the storms as a consequence of pore pressure accumulation.
Hence the importance of considering the coupling effects between soil stress and pore water
pressure in design, even for those cases where a soil liquefaction could in principle be disregarded
on the grounds of a high relative density of the soil.
For practical design purposes, it seems sensible to distinguish between piles in fully drained con-
ditions, in partially drained conditions and in undrained conditions, and analyse them correspon-
dingly employing different strategies. Some considerations in this respect have been given in the
final sections of this thesis.
i
KURZFASSUNG

Das Verhalten von Pfahlgründungen für Offshore-Windenergieanlagen weicht von den klassi-
schen Annahmen und gesammelten Erfahrungen vor allem wegen der großen Pfahldurchmesser,
geringeren Schlankheit und hohen Verhältnisse von lateralen zu vertikalen Lasten ab. Zudem
entsteht im Offshore-Bereich die zusätzliche Herausforderung einer großen Anzahl von Last-
wechseln aus Wind und Wellen und des möglichen Einflusses von vorübergehenden Verände-
rungen des Porenwasserdrucks um den Pfahl. Da in diesem Hinsicht noch viel Unkenntniss
herrscht, wurden beide Aspekte in den Bemessungsrichtlinien bisher nicht ausreichend in Be-
tracht gezogen.
Es ist aber bekannt, dass wassergesättigte Böden unter zyklischer Belastung oft einen Porenwas-
serdruckaufbau zeigen, der schließlich bis zur Bodenverflüssigung und einem Gründungsversa-
gen führen kann. Andererseits können grosse Lastspielzahlen erhebliche bleibende Verformun-
gen zur Folge haben, die die Gebrauchstauglichkeit der Gründung progressiv gefährden.
Wesentliche Ziele dieser Arbeit waren, einen Einblick in beide Aspekte zu gewinnen, ihre Folgen
aufzuzeigen und sinnvolle Kriterien für die Pfahlbemessung herzuleiten. Dabei wurde aber auch
ein praktikables numerisches Werkzeug entwickelt, um den Einfluss von Kurzzeit-
Sturmereignissen auf die Offshore-Pfähle prognostizieren zu können. Die Untersuchungen wur-
den deswegen in zwei Hauptrichtungen gegliedert: ein erster Teil, mit einer theoretischen Orien-
tierung, der der Analyse von kurzzeitigen Ereignisse anhand der Finite-Elemente-Methode ge-
widmet wurde, und ein zweiter empirischer Teil für die Betrachtung von Langzeitphänomene
anhand Modellversuchen in kleinem Maßstab.
In diesen Untersuchungen konnte gezeigt werden, wie sich der Boden an der Pfahlgründung
langfristig progressiv verfestigt (long-time hardening). Vorübergehend kann es aber auch zu einer
Entfestigung während der Sturmereignisse kommen (short-time transient softening). Sowohl die
abgeminderte Bettungssteifigkeit während der Entfestigung, als auch ihre kummulativen Effekte
im Sinne von bleibenden Verformungen, zeigen die Relevanz und Notwendigkeit der Berücksich-
tigung der Wechselwirkung zwischen Porenwasserdruck und Bodenverformung in der Pfahlbe-
messung. In der Summe werden die Phänomene und praktischen Folgen für die Bemessung der
Pfähle in dieser Dissertation hergeleitet.
ii
PROJECT FUNDING

BAM's doctoral program and the German Federal Ministry for the Environment, Nature Con-
servation and Nuclear Safety (BMU) have kindly provided the funding for these investigations,
which were carried out in the frame of the RAVE (research at Alpha Ventus) research program.


PERSONAL ACKNOWLEDGEMENTS

For the completion of this work I have enjoyed the assistance and support of many people, to all
of whom I am greatly indebted. First of all, my deepest gratitude is due to my academic advisor,
Prof. Manuel Pastor, for his enthusiasm, help and constant encouragement and for always mak-
ing everything so easy. With him there was never a problem that could not be solved.
For the past years I have also had the great privilege of working at BAM's department for Build-
ings and Structures under the direction of Prof. Werner Rücker who watched over my work and
granted me permanent support, assistance and understanding, especially during the final stages of
the doctoral endeavour. I am also very grateful to Prof. Stavros Savidis from the Technical Uni-
versity of Berlin for his interest on my work, his advice on academic matters and for supervising
this thesis. Finally, I am grateful to Prof. Karsten Geißler for assuming the chairmanship of the
doctoral committee and to Prof. Yuri Petryna for his disposition and gentle substitution at the
scientific defense.
Among my direct colleagues, I am particularly indebted to Pablo Mira and José Antonio
Fernández Merodo for their selfless support, advice and help. Together with Manolo Pastor, they
provided the sources of the GeHoMadrid Finite Element code and were a decisive help for deal-
ing with the intricacies of computational geomechanics. Fernando Pardo and the research group
within the Geotechnical Laboratory of CEDEX in Madrid have kindly hosted my "research-and-
training" stays at CEDEX.
For the experimental investigations I was able to rely on the experience and wisdom of Fred
Ziegler, Wilfried Wuttke and Hans-Joachim Peschke, who, along with many other individuals at
BAM (a.o. Margrit Kayser, Steven Georgi, Krassimire Karabeliov, Christian Knaust and all the
staff at BAM's division VII.2) made my work in Berlin much easier and particularly rewarding.
iii
Finally, my special appreciation is due to Matthias Baeßler, the leader of my work-group, who has
coordinated all of my German projects and has gradually guided me towards becoming an inde-
pendent researcher. His inquisitive style, sharp insight and critical judgement have been a perma-
nent stimulus and never precluded his unconditional assistance and friendship.
Last but not least, I want to acknowledge the marvellous support provided by my family. I cannot
thank them enough for their love and care. This dissertation, which is the product of their con-
stant encouragement and support, is dedicated to them.

Pablo Cuéllar. February 2011, Berlin.
iv
STUDENT: MEPHISTO: The threads flow on, unseen and subtle,
I have arrived quite recently Before we get into my views – Each blow effects a thousands ties.
And come, full of humility, What Department do you choose? The philosopher comes with analysis
To meet the giant intelect STUDENT: And proves it had to be like this:
Whom all refer to with respect I should like to be erudite, The first was so, the second so,
MEPHISTO: And from the earth to heaven’s height And hence the third and fourth was so,
This is a charming pleasantry. Know every law and every action: And were not the first and second here,
A man as others are, you see. – Nature and science is what I need. Then the third and fourth could never ap-
Have you already called elsewhere? MEPHISTO: pear.
STUDENT: That is the way; you just proceed That is what all the students believe,
I pray you, take me in your care. And scrupulously shun distraction. But they have never learned to weave.
I am, believe me, quite sincere, STUDENT: Who would study and describe the living,
Have some odd cash and lots of cheer; Body and soul, I am a devotee; starts
My mother scarcely let me go, Though, naturally, everybody prays By driving the spirit out of the parts:
But there is much I hope to know. For some free time and liberty In the palm of his hand he holds all the
MEPHISTO: On pleasant summer holidays. sections,
This is just the place for you to stay. MEPHISTO: Lacks nothing, except the spirit’s connec-
STUDENT: Use well your time, so swiftly it runs on! tions.
To be frank, I should like to run away. Be orderly, and time is won! Encheirisis naturae the chemists baptize it,
I cannot say I like these walls, My friend, I shall be pedagogic, Mock themselves and don’t realize it.
These gloomy rooms and somber halls. And say you ought to start with Logic STUDENT:
It seems so narrow, and I see For thus your mind is trained and braced, I did not quite get everything.
No patch of green, no single tree; In Spanish boots it will be laced, MEPHISTO:
And in the auditorium That on the road of thought maybe That will improve with studying:
My hearing, sight, and thought grow numb. It henceforth creep more thoughtfully, You will reduce things by and by
MEPHISTO: And does not crisscross here and there, And also learn to classify.
That is a question of mere habit. Will-o’-the-wisping through the air. STUDENT:
The child, offered the mother’s breast, Days will be spent to let you know I feel so dazed by all you said
Will not in the beginning grab it; That what you once did at one blow, As if a mill went around my head.
But soon it clings to it with zest. Like eating and drinking so easy and free,
And thus at wisdom’s copious breasts Can only be done with One, Two, Three.
You’ll drink each day with greater zest. Yet the web of thought has no such creases
STUDENT: And is more like a weaver’s masterpieces:
I’ll hang around her neck, enraptured; One step, a thousand threads arise,
But tell me first: how is she captured? Hither and thither shoots each shuttle,

Johann Wolfgang von Goethe, 1808, Faust. Eine Tragödie


Write to be understood, speak to be heard, read to grow
Lawrence Clark Powell

v

TABLE OF CONTENTS


Page
1  INTRODUCTION 1  
1.1  Project Framework. Pile Foundations for Offshore Wind Turbines 3 
1.2  Scope of this Work. Offshore Piles under Cyclic Lateral Loading 5 
1.3  Research Structure and Methodology 6  
1.4  Terminology and Conventions employed here 8  
2  CYCLIC LATERALLY LOADED PILES. AN OVERVIEW 11 
2.1  Static Behaviour and Design Approaches 12 
2.1.1  Ultimate lateral resistance 12 
2.1.2  Deformational behaviour 15 
2.2  Effects of Cyclic Loading. Considerations in Design 22 
2.2.1  General cyclic behaviour. Lower bound design methods 23 
2.2.2  Particular aspects of cycling 25 
2.2.3  Explicit considerations in design 33 
2.3  Open Issues for the Large-Diameter Offshore Piles 37 
2.3.1  Short-term. Pore pressure accumulation during a storm 38 
2.3.2  Long-term. Incremental collapse and embedment stiffening 42 
3  SHORT TERM BEHAVIOUR: NUMERICAL INVESTIGATIONS 45 
3.1  Model Description 46 
3.1.1  Mathematical model: The u-p formulation 46 W
3.1.2  Numerical model: The Finite Element approach 54 
3.1.2.1  Discretization in space 55 
3.1.2.2  Discretization in time 58 
3.1.2.3  Solution of the non-linear system 60 
3.1.3  Constitutive model 62 
3.1.3.1  Sand behaviour and the Generalized Plasticity Theory 62 
3.1.3.2  The Pastor-Zienkiewicz model for sands 65 
3.1.3.3  Parameter calibration 72 
3.2  Element Technology 77 
3.2.1  Special issues for the transient simulation of an offshore pile 78 
3.2.2  The stabilized enhanced strain Simo-Rifai formulation 80 
3.2.3  Pile soil interface and contact problem 83 
3.3  Optimization of the Computational Cost 89 
3.3.1  Solution strategies 89 
3.3.2  Handling of the sparse matrices 91 
3.3.3  Parallelism and super-computing 93 
v Table of Contents

3.3.4  Functional software: external solver libraries 95 
3.4  Validation 96 
3.4.1  Stabilized coupled element 97 
3.4.2  No-tension joint element 99 
3.5  A Case Study 100 
3.5.1  Evolution of Pore Water Pressure 102 
3.5.2  Soil stresses and strains. Cyclic mobility 105 
3.5.3  Pile deflection and soil settlement 106 
3.6  Parameter Relevance for the Accumulation of PWP 108 
3.6.1  Influence of load level 110 
3.6.2  Influence of soil permeability 114 
3.6.3  Influence of loading frequency 118 
3.6.4  Influence of pile diameter 120 
3.6.5  Comprehensive PWP accumulation trend 124 
3.6.6  A note on irregular loading and order effects 126 
3.6.7  A note on the computational cost 131 
3.6.8  A note on the choice of constitutive model 132 
3.7  Computation of a real storm 133 
3.8  Recapitulation 136 
4  LONG TERM BEHAVIOUR: EXPERIMENTAL INVESTIGATIONS 139 
4.1  Motivation and Scope 140 
4.1.1  Long-term stability: cyclic Axial vs. cyclic Lateral loading 141 
4.1.2  Soil subsidence and grain migration 142 
4.1.3  Study object: Large diameter offshore monopile 144 
4.2  Model Tests 146 
4.2.1  Model testing and 1-g scaling laws 146 
4.2.2  Experimental setup and testing program 150 
4.2.3  Results of test series. Attenuation of displacement rate 155 
4.2.4  Displacement accumulation pattern 158 
4.2.4.1  Accumulation laws from the literature 162 
4.2.4.2  Generalisation of the Log-Linear accumulation law 163 
4.3  Soil Densification 166 
4.3.1  Photogrammetry: Structured-light 3D scanning 167 
4.3.2  Topographic evolution of soil surface 168 
4.3.3  Densification and embedment stiffening 170 
4.3.3.1  Volume loss and subsidence depth 173 
4.3.3.2  Final soil density 174 
4.3.3.3  Concluding remarks 174 
4.4  Sand Ratcheting Convection 176 
4.4.1  Granular convective cells 178 
4.4.2  Quasi-static convection and granular ratcheting 183 
4.4.3  Incremental collapse vs. localised failure 185 
4.5  Practical Implications. Bi-Phasic Model of Long Term Behaviour 186 
4.5.1  Densification-dominated phase 186 
vi

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