Buckling of Bucket Foundations During Installation

Søren Madsen

Buckling of Bucket Foundations During Installation

Søren Madsen

Forskningsgruppen for Geoteknik og Offshore Fundamenter

Publikation: Ph.d.-afhandling

Abstract

Der er i disse tider et stort politisk ønske om at udvide den grønne energisektor. En velfungerende grøn energiform er vindmøller. Et stort antal vindmøller er installeret på land i løbet af de sidste årtier. Dog resulterer den nuværende udvikling i mølledesign i stadigt større vindmøller, for at reducere energiprisen. Dette begrænser anvendelsen af vindmøller på land både pga. begrænsninger ift. transport men også pga. uvilje fra potentielle naboer til kæmpevindmøller. Derfor er der indenfor de sidste par år for alvor kommet gang i udviklingen af havvindmøller. Selv om det løser de førnævnte problemer at installere vindmøllerne til havs, forøger det energiprisen, primært på grund af øgede installations- og vedligeholdelsesomkostninger. En meget stor andel af installationsomkostningerne, op mod 30 - 50 % ved brug af den nuværende teknologi, stammer fra udgifter forbundet med installation af fundamenter. Et nyt funderingskoncept, bøttefundamentet, tegner til at kunne reducere omkostingerne til fundamenter i en havvindmøllepark. Bøttefundamentet forventes at blive billigere, da det er både hurtigere at installere i forhold til den nuværende teknologi, og det kræver ikke tungt installationsudstyr. Da et bøttefundament dybest set er en relativt tynd skalkonstruktion udsat for både aksialt og latteralt tryk under installationen, bliver foldning et vitalt designkriterium. I 2005 skulle en stor vindmølle installeres i et kystnært område ved havnen i Wilhelmshaven i Tyskland. Vindmøllen blev aldrig opført da der, under nedsugningprocessen, opstod foldning i bøttefundamentets skørt. I denne afhandling er fænomenet foldning af bøttefundamenter under installation undersøgt ved hjælp af Finite Element-analyser. Indflydelsen af randbetinglser på bøttefundamentet er undersøgt, tillige med effekten af at inkludere den omgivende jord og jord-struktur interaktion. Undersøgelser er foretaget for foldningshændelsen i Wilhelmshaven, og resultaterne er sammenlignet med gældende DNV og Eurocode standarder. Endeligt præsenteres analyser af en ny udformning af et tværsnit, betegnetmulti-shell,med en højere foldningskapacitet end et tilsvarende cirkulært tværsnit. Hovedresultaterne er listet nedenfor:
• Låget på bøttefundamentet øger foldningskapaciteten i forhold til en simpelt understøttet rand,
• Antallet af lågafstivninger påvirker ikke den elastiske foldningskapacitet væsentligt,
• Den omgivende jord øger foldningskapaciteten væsentligt under installationen,
• Kun at betragte den første svigtform fra en lineær analyse som imperfekt geometri i en ikkelineær foldningsanalyse er ikke tilstrækkelig til at opfange foldningshændelsen i Wilhelmshaven. Men det er det at betragte de første 21 svigtformer og indføre den mest kritiske som en imperfekt geometri,
• Multi-shell tværsnittet er lige så følsom over for imperfektioner som en cirkulær cylinder,
• Det nye multi-shell design giver en væsentligt større (75 %) foldningskapacitet i forhold til det traditionelle design.

Originalsprog	Engelsk
Vejledere	Andersen, Lars Vabbersgaard, Hovedvejleder Ibsen, Lars Bo, Bivejleder
Udgivelsessted	Aalborg
Udgiver	Department of Civil Engineering, Aalborg University
Status	Udgivet - 2014

Emneord

Wind turbine foundations
Bucket foundations
Buckling
Offshore wind turbines
Wind energy
Cost reductions
Finite element analysis

AUB Link

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Citationsformater

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title = "Buckling of Bucket Foundations During Installation",

abstract = "There is a great politically will to expand the green energy market in these times. A proven green technology is wind turbines. Wind turbines have been installed in great numbers on land over the last decades. However, the current development in wind turbine design leads to larger turbine sizes in order to reduce the cost of energy. This limits the on land application due to transportation limitations and unwillingness from prospect neighbours. Thus, offshore wind energy started developing over the last couple of years. Although installing the wind turbines offshore resolves the before men tioned issues, it brings up the cost of energy mainly due to increased installation and maintenance costs. A very large part—up to 30–50% using current technology—of the installation cost origins from the expenses related to the installation of foundations. A new foundation concept—the bucket foundation—has been proposed to reduce the foundation costs. The bucket foundation promises to be cheaper as it is both faster to install compared to current technology and it does not require heavy installation equipment. Since the bucket foundation is essentially a relatively thin shell structure subjected to both axial and latteral pressure during the installation process buckling becomes a crucial design consideration. In 2005, a large wind turbine was to be installed nearshore the harbour in Wilhelmshaven, Germany. The windturbine was never erected since the skirt of the bucket foundation buckled during the suction assisted installation process. In this thesis, the phenomenon of buckling of the bucket foundation during installation is investigated by means of Finite Element Analysis. The influence of boundary conditions on the bucket foundation is adressed as well as the effect of including the surrounding soil and soil–structure interaction. Investigations are made regarding the buckling incident in Wilhelmshaven and the results are compared to current DNV and Eurocode standards. Finally, analysis of a new design of the cross section with a higher buckling capacity than an equivalent circular cross seciton—denoted multi-shell—is presented. The main findings are listed below:• The bucket lid shows an increase in the buckling load compared to a pinned end,• Number of lid stiffeners does not substantially affect the elastic buckling load,• The soil restraint increases the buckling load significantly during penetration,• Considering the first mode shape from a linear buckling analysis as imperfect geometryin a nonlinear buckling analysis is not sufficient for capturing the buckling incident in Wilhelmshaven; however, considering the first 21 mode shapes and introducing themost critical one as an imperfect geometry is,• The multi-shell cross section is just as sensitive to imperfections as a circular cylinder,• The new multi-shell design provides a significantly larger (75%) buckling load compared to the traditional design.",

keywords = "Wind turbine foundations, Bucket foundations, Buckling, Offshore wind turbines, Wind energy, Cost reductions, Finite element analysis, Wind turbine foundations, Bucket foundations, Buckling, Offshore wind turbines, Wind energy, Cost reductions, Finite element analysis",

author = "S{\o}ren Madsen",

year = "2014",

language = "English",

series = "DCE Thesis",

publisher = "Department of Civil Engineering, Aalborg University",

number = "64",

address = "Denmark",

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T1 - Buckling of Bucket Foundations During Installation

AU - Madsen, Søren

PY - 2014

Y1 - 2014

N2 - There is a great politically will to expand the green energy market in these times. A proven green technology is wind turbines. Wind turbines have been installed in great numbers on land over the last decades. However, the current development in wind turbine design leads to larger turbine sizes in order to reduce the cost of energy. This limits the on land application due to transportation limitations and unwillingness from prospect neighbours. Thus, offshore wind energy started developing over the last couple of years. Although installing the wind turbines offshore resolves the before men tioned issues, it brings up the cost of energy mainly due to increased installation and maintenance costs. A very large part—up to 30–50% using current technology—of the installation cost origins from the expenses related to the installation of foundations. A new foundation concept—the bucket foundation—has been proposed to reduce the foundation costs. The bucket foundation promises to be cheaper as it is both faster to install compared to current technology and it does not require heavy installation equipment. Since the bucket foundation is essentially a relatively thin shell structure subjected to both axial and latteral pressure during the installation process buckling becomes a crucial design consideration. In 2005, a large wind turbine was to be installed nearshore the harbour in Wilhelmshaven, Germany. The windturbine was never erected since the skirt of the bucket foundation buckled during the suction assisted installation process. In this thesis, the phenomenon of buckling of the bucket foundation during installation is investigated by means of Finite Element Analysis. The influence of boundary conditions on the bucket foundation is adressed as well as the effect of including the surrounding soil and soil–structure interaction. Investigations are made regarding the buckling incident in Wilhelmshaven and the results are compared to current DNV and Eurocode standards. Finally, analysis of a new design of the cross section with a higher buckling capacity than an equivalent circular cross seciton—denoted multi-shell—is presented. The main findings are listed below:• The bucket lid shows an increase in the buckling load compared to a pinned end,• Number of lid stiffeners does not substantially affect the elastic buckling load,• The soil restraint increases the buckling load significantly during penetration,• Considering the first mode shape from a linear buckling analysis as imperfect geometryin a nonlinear buckling analysis is not sufficient for capturing the buckling incident in Wilhelmshaven; however, considering the first 21 mode shapes and introducing themost critical one as an imperfect geometry is,• The multi-shell cross section is just as sensitive to imperfections as a circular cylinder,• The new multi-shell design provides a significantly larger (75%) buckling load compared to the traditional design.

AB - There is a great politically will to expand the green energy market in these times. A proven green technology is wind turbines. Wind turbines have been installed in great numbers on land over the last decades. However, the current development in wind turbine design leads to larger turbine sizes in order to reduce the cost of energy. This limits the on land application due to transportation limitations and unwillingness from prospect neighbours. Thus, offshore wind energy started developing over the last couple of years. Although installing the wind turbines offshore resolves the before men tioned issues, it brings up the cost of energy mainly due to increased installation and maintenance costs. A very large part—up to 30–50% using current technology—of the installation cost origins from the expenses related to the installation of foundations. A new foundation concept—the bucket foundation—has been proposed to reduce the foundation costs. The bucket foundation promises to be cheaper as it is both faster to install compared to current technology and it does not require heavy installation equipment. Since the bucket foundation is essentially a relatively thin shell structure subjected to both axial and latteral pressure during the installation process buckling becomes a crucial design consideration. In 2005, a large wind turbine was to be installed nearshore the harbour in Wilhelmshaven, Germany. The windturbine was never erected since the skirt of the bucket foundation buckled during the suction assisted installation process. In this thesis, the phenomenon of buckling of the bucket foundation during installation is investigated by means of Finite Element Analysis. The influence of boundary conditions on the bucket foundation is adressed as well as the effect of including the surrounding soil and soil–structure interaction. Investigations are made regarding the buckling incident in Wilhelmshaven and the results are compared to current DNV and Eurocode standards. Finally, analysis of a new design of the cross section with a higher buckling capacity than an equivalent circular cross seciton—denoted multi-shell—is presented. The main findings are listed below:• The bucket lid shows an increase in the buckling load compared to a pinned end,• Number of lid stiffeners does not substantially affect the elastic buckling load,• The soil restraint increases the buckling load significantly during penetration,• Considering the first mode shape from a linear buckling analysis as imperfect geometryin a nonlinear buckling analysis is not sufficient for capturing the buckling incident in Wilhelmshaven; however, considering the first 21 mode shapes and introducing themost critical one as an imperfect geometry is,• The multi-shell cross section is just as sensitive to imperfections as a circular cylinder,• The new multi-shell design provides a significantly larger (75%) buckling load compared to the traditional design.

KW - Wind turbine foundations

KW - Bucket foundations

KW - Buckling

KW - Offshore wind turbines

KW - Wind energy

KW - Cost reductions

KW - Finite element analysis

KW - Wind turbine foundations

KW - Bucket foundations

KW - Buckling

KW - Offshore wind turbines

KW - Wind energy

KW - Cost reductions

KW - Finite element analysis

M3 - PhD thesis

T3 - DCE Thesis

PB - Department of Civil Engineering, Aalborg University

CY - Aalborg

ER -

Buckling of Bucket Foundations During Installation

Abstract

Emneord

AUB Link

Fingeraftryk

Citationsformater