2411
Cyclic loading of caisson supported offshore wind structures in sand
Chargement cyclique des éoliennes offshore soutenues par des caissons à succion en sable
Versteele H.
Cathie Associates SA/NV, Diegem, Belgium (formerly Université de Liège, Liège, Belgium)
Stuyts B., Cathie D.
Cathie Associates SA/NV, Diegem, Belgium
Charlier R.
Université de Liège, Liège, Belgium
ABSTRACT: With the number of offshore wind turbines in Europe growing rapidly, offshore wind farm developers are looking for
support structures which are relatively light, easy to produce and install and are suited for water depths in excess of 30m. Suction
caissons could offer a solution for these requirements. Since cyclic environmental loads form an important part of the loading
conditions, the cyclic degradation of the caisson capacity needs to be evaluated in detail. During storm events, pore pressure build-up
inside and around the caisson can lead to degradation of capacity and stiffness. To date, there are no generally accepted material
models which combine generation and dissipation of pore pressure with the mechanical response of the sand. Existing methods for
analyzing pore pressure build-up are reviewed. Subsequently, a numerical model is proposed which captures the phenomena of pore
pressure generation and dissipation around the caisson. Pore pressure increases under storm load cycles are calculated from cyclic
laboratory tests and are added to existing pore pressures in the numerical model. The influence of cyclic loading history and drainage
effects on the caisson performance is assessed using the 3D FE model. Implications for suction caisson design in sand are outlined.
RÉSUMÉ : Vu la croissance rapide du nombre d'éoliennes offshore en Europe, les développeurs des parcs éoliens offshore sont
intéressés par des structures combinant légèreté, facilité de fabrication et qui sont adaptées à des profondeurs d'eau supérieures à 30m.
Les caissons à succion répondent à ces critères. Comme les charges environnementales cycliques constituent une partie importante du
chargement total, la dégradation cyclique de la capacité portante du caisson doit être évaluée en détail. Lors de tempêtes,
l'accumulation de pressions d’eau interstitielle à l’intérieur et autour du caisson peut induire une dégradation de la capacité et de la
raideur. A ce jour, il n’existe pas de modèle de matériau unanimement accepté qui combine génération et dissipation de pression
interstitielle et comportement mécanique du sable. Les méthodes existantes d'analyse de génération de pression interstitielle sont
examinées dans un premier temps. Ensuite, un modèle numérique intégrant les principaux mécanismes de génération et dissipation de
ces surpressions autour du caisson est introduit. L'augmentation de pressions interstitielles résultants des charges cycliques dues aux
tempêtes est estimée de manière indirecte sur base des résultats d'essais cycliques en laboratoire; ces surpressions sont ensuite
ajoutées aux pressions interstitielles existantes dans le modèle numérique. L’influence de l’historique de chargement cyclique et des
conditions de drainage est évaluée à l’aide du modèle éléments finis 3D. Enfin, les implications de ces résultats pour la conception de
caissons à succion sont exposées.
KEYWORDS: suction caisson, cyclic loading, liquefaction analysis, offshore wind turbine, marine geotechnics
1 INTRODUCTION
1.1
Suction caisson as foundations for offshore wind turbines
The European Wind Energy Association expects that the
installed offshore wind capacity within the EU will increase
from 4GW to 40 GW by 2020 (EWEA 2011) requiring the
installation of approximately 6000 6MW turbines located ever
further offshore in consequently deeper waters. Due the
demanding working conditions at sea and the limited
availability of offshore installation vessels, the foundation
system typically accounts for up to 25-30 % of the total cost of
an offshore wind farm. This makes the choice and design of the
foundation an important factor in the overall cost effectiveness
of offshore wind farms.
Offshore wind farm developers are thus looking for support
structures which are relatively light, easy to produce and install
and are suited for water depths in excess of 30m. Suction
caissons could offer a solution for these requirements.
A suction caisson is a steel structure consisting of a circular
top plate with peripheral vertical skirts (Figure 1). In operation
it is similar to a skirted gravity foundation, but the skirt length is
significant compared to the diameter.
Installation of the caisson is achieved in two phases. After
initial penetration under the self-weight of the caisson, water is
pumped out. The induced pressure difference pushes the caisson
into the soil, while the induced seepage forces and reduced
effective stress near the skirt tips facilitate penetration.
Advantages
of the caisson include a potentially lower cost
than equivalent piled foundations (Senders 2008) and relatively
easy installation and removal, not restricted by water-depth.
Diameter
Skirt length
Figure 1: Cross-section sketch of a suction caisson and installation
principle
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Cyclic loading of caisson supported offshore wind structures in sand
Chargement cyclique des éoliennes offshore soutenues par des caissons à succion en sable
H. Versteele
Cathie Associates SA/NV, Diegem, Belgium (formerly Université de Liège, Liège, Belgium)
B. Stuyts & D. Cathie
athi Associates SA/NV, Diegem, Belgium
R. Charlier
Université de Liège, Liège, Belgium
ABSTRACT: With the number of offshore wind turbines in Europe growing rapidly, offshore wind farm developers are looking for
support structures which are relatively light, easy to produce and install and are suited for water depths in excess of 30m. Suction
caissons could offer a solution for these requirements. Since cyclic environmental loads form an important part of the loading
conditions, the cyclic degradation of the caisson capacity needs to be evaluated in detail. During storm events, pore pressure build-up
inside and around the caisson can lead to degradation of capacity and stiffness. To date, there are no generally accepted material
models which combine generation and dissipation of pore pressure with the mechanical response of the sand. Existing methods for
analyzing pore pressure build-up are reviewed. Subsequently, a numerical model is proposed which captures the phenomena of pore
pressure generation and dissipation around the caisson. Pore pressure increases under storm load cycles are calculated from cyclic
laboratory tests and are added to existing pore pressures in the numerical model. The influence of cyclic loading history and drainage
effects on the caisson performance is assessed using the 3D FE model. Implications for suction caisson design in sand are outlined.
RÉSUMÉ : Vu la croissance rapide du nombre d'éoliennes offshore en Europe, les développeurs des parcs éoliens offshore sont
intéressés par des structures combinant légèreté, facilité de fabrication et qui sont adaptées à des profondeurs d'eau supérieures à 30m.
Les caissons à succion répondent à ces critères. Comme les charges environnementales cycliques constituent une partie importante du
chargement total, la dégradation cyclique de la capacité portante du caisson doit être évaluée en détail. Lors de tempêtes,
l'accumulation de pressions d’eau interstitielle à l’intérieur et autour du caisson peut induire une dégradation de la capacité et de la
raideur. A ce jour, il n’existe pas de modèle de matériau unanimement accepté qui combine génération et dissipation de pression
interstitielle et comportement mécanique du sable. Les méthodes existantes d'analyse de génération de pression interstitielle sont
examinées dans un premier temps. Ensuite, un modèle numérique intégrant les principaux mécanismes de génération et dissipation de
ces surpressions autour du caisson est introduit. L'augmentation de pressions interstitielles résultants des charges cycliques dues aux
tempêtes est estimée de manière indirecte sur base des résultats d'essais cycliques en laboratoire; ces surpressions sont ensuite
ajoutées aux pressions interstitielles exista tes dans le modèle numérique. L’influence d l’historique de chargement cyclique et des
conditions de drainage est évaluée à l’aide du modèle éléments finis 3D. Enfin, les implications de ces résultats pour la conception de
caissons à succion sont exposées.
KEYWORDS: suction caisson, cyclic loading, liquefaction analysis, offshore wind turbine, marine geotechnics
1 INTRODUCTION
1.1
Suction caisson as foundations for offshore wind turbines
The European Wind Energy Association expects that the
installed offshore wind capacity within the EU will increase
from 4GW to 40 GW by 2020 (EWEA 2011) requiring the
installation of approximately 6000 6MW turbines located ever
further offshore in consequently deeper waters. Due the
demanding working conditions at sea and the limited
availability of offshore installation vessels, the foundation
system typically accounts for up to 25-30 % of the total cost of
an offshore wind farm. This makes the choice and design of the
foundation an important factor in the overall cost effectiveness
of offshore wind farms.
Offshore wind farm developers are thus looking for support
structures which are relatively light, easy to produce and install
and are suited for water depths in excess of 30m. Suction
caissons could offer a solution for these requirements.
A suction caisson is a steel structure consisting of a circular
top plate with peripheral vertical skirts (Figure 1). In operation
it is similar to a skirted gravity foundation, but the skirt length is
significant compared to the diameter.
Installation of the caisson is achieved in two phases. After
initial penetration under the self-weight of the caisson, water is
pumped out. The induced pressure difference pushes the caisson
into the soil, while the induced seepage forces and reduced
effective stress near the skirt tips facilitate penetration.
Advantages of the caisson include a potentially lower cost
than equivalent piled foundations (Senders 2008) and relatively
easy installation and removal, not restricted by water-depth.
Figure 1: Cross-section sketch of a suction caisson and installation
principle
Diameter
Skirt length
