2391
Experimental Testing of Monopiles in Sand Subjected to One-Way Long-Term
Cyclic Lateral Loading
Étude expérimentale de monopiles dans le sable soumis à un chargement cyclique transversal
non alterné
Roesen H.R., Ibsen L.B., Andersen L.V.
Aalborg University, Aalborg, Denmark
ABSTRACT: In the offshore wind turbine industry the most widely used foundation type is the monopile. Due to the wave and wind
forces the monopile is subjected to a strong cyclic loading with varying amplitude, maximum loading level, and varying loading
period. In this paper the soil–pile interaction of a monopile in sand subjected to a long-term cyclic lateral loading is investigated by
means of small scale tests. The tests are conducted with a mechanical loading rig capable of applying the cyclic loading as a sine
signal with varying amplitude, mean loading level, and loading period for more than 60 000 cycles. The tests are conducted in dense
saturated sand. The maximum moment applied in the cyclic tests is varied from 18% to 36% of the ultimate lateral resistance found in
a static loading test. The tests reveal that the accumulated rotation can be expressed by use of a power function. Further, static tests
conducted post cyclic loading indicate that the static ultimate capacity increases with the magnitude of cyclic loading.
RÉSUMÉ: Dans l'industrie éolienne maritime, les fondations de type monopile sont les plus largement utilisées. En raison de la force
des vagues et du vent, ces fondations sont soumises à des charges cycliques élevées dont l’amplitude, le niveau maximal et la
fréquence varient. Dans cet article, l'interaction sol-pieu d'une fondation de type monopile implantée dans du sable et soumise à un
chargement transversal cyclique est étudiée au moyen d'essais à échelle réduite. Les tests sont effectués avec une grue de chargement
mécanique capable d'appliquer un chargement cyclique sinusoïdal avec amplitude, niveau moyen et période de chargement variable
pendant plus de 60 000 cycles. Les tests sont effectués dans un sable dense saturé. Le moment maximal appliqué durant les essais
cycliques varie de 18% à 36% de la résistance transversale ultime obtenue lors d’essais de chargement statique. Les essais montrent
que la rotation accumulée peut être exprimée par une fonction puissance. En outre, des essais statiques menés après le chargement
cyclique indiquent que la capacité statique ultime augmente avec le niveau du chargement cyclique.
KEYWORDS: Experimental, wind turbine foundation, monopile, long-term cyclic loading, dense sand.
1 INTRODUCTION
In the offshore wind turbine industry, the most widely used
foundation type is the monopile, i.e. a large diameter stiff pile.
During the lifetime of a wind turbine, the monopile foundation
is subjected to few load cycles with large amplitudes, caused by
the strong storms, and also to millions of lateral load cycles
with low or intermediate amplitudes due to the wave loading.
This loading may cause failure in the fatigue or serviceability
limit states, FLS and SLS respectively (Wichtmann et al. 2008).
The cyclic loading might induce a change in the soil stiffness
and a permanent accumulated rocking rotation (tilt) of the
turbine. Due to the efficiency of the wind turbine, strict
demands for the rotation and the stiffness of the entire structure
are normally made and thus, the change in stiffness and rotation
becomes key issues in the design. However, the current design
guidance, DNV (2011), on this long-term loading is limited and
a procedure for designing large diameter piles is yet to be fully
expressed and confirmed. The development of a reliable design
method requires verification and for that in-situ and large-scale
testing is by far the best tool. However, this is also the most
expensive and time-consuming tool. Therefore, the recent
choice for evaluating the cyclic behaviour has been numerical
modelling and small scale testing. Several authors has
investigated this e.g. Niemunis et al. (2005), Achmus et al.
(2005), Peng et al. (2006), LeBlanc et al. (2010) and Achmus et
al. (2011). However, the research has mainly been based on
cyclic triaxial tests, FEM-calculations and 1g experimental
setups in dry sand.
In this paper, a 1g testing rig for modelling the
environmental loading on a stiff monopile foundation in dense
saturated sand is described and the results from four one-way
cyclic loading tests are presented. The purpose of the cyclic
tests is to evaluate the influence of the number of load cycles,
N
,
on the accumulated rocking rotation of the pile at seabed, under
long-term cyclic loading with constant frequency but different
loading amplitude and mean loading level.
The characteristic of the cyclic loading can be described by
the ratios
and as defined by LeBlanc et al (2010).
expresses the magnitude of the loading as the ratio between the
maximum load in a load cycle and the maximum static lateral
capacity,
.
will take a value between 0 and
1. The cyclic load ratio
defines the direction of the loading
on the basis of the minimum and maximum load in a load
cycle,
.
will take the value 1 for a static
test, 0 for one-way loading, and -1 for two-way loading.
2 EXPERIMENTAL MODEL TESTS
The 1g small scale tests are carried out at the geotechnical
laboratory at Aalborg University, Denmark. In the tests an open
ended aluminium pipe pile is used. The pile is scaled
approximately 1:50 in relation to a typical offshore monopile. In
Table 1 the dimensions of the model pile are presented.
Table 1. Dimensions of the open ended aluminium pipe pile.
Diameter Embedded length
Wall thickness
Load eccentricity
(mm)
(mm)
(mm)
(mm)
100
500
5
600
The bending stiffness of the model pile is similar to a scaled
prototype steel pile, however, the behaviour of the pile during
loading also depends on the stiffness of the surrounding soil.
According to Poulus and Hull (1989) a pile behaves flexible if
and rigidly if
, where
is a critical length
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
1
Experimental Testing of Monopiles in Sand Subjected to One-Way Long-Term
Cyclic Later Loading
Test expérimental de monopiles implantées dans du sable et soumises à un chargement latéral
cyclique à long terme
H. R. Roesen, L. B. Ibsen and L. V. Andersen
Aalborg University, Aalborg Denmark
ABSTRACT: In the offshore wind turbine industry the most widely used foundation type is the monopile. Due to the wave and wind
forces the monopil is ubj cted to a strong cyclic loading th varying ampli ude, maximum l ading level, and varying load g
pe iod. In this pa er the soil–pile interacti of a mon pile in sand subjected to a long-term cyclic lateral loading is investigate by
m ans of small scale test . The tests are c ducted with a mechanical loading rig capable of applying the cyclic loading as a sine
signal with v rying amplitude, mean loading level, and loading period for more than 60 000 cycles. The tests are onducted in dens
atur ted sand. The maxim m moment pplied in the cyclic tests is varied from 18% to 36% of th ultima lateral resistance found in
a static loading test. The tests reveal th t the accumulated rota ion c n be expressed by use a power function. Further, static tests
condu ted post cyclic loading indic te that t static timate c pacity i cr ases with the magnitude f cyclic loading.
RÉSUMÉ: Dans l'industrie éolienne offshore, le type de fondation le plus largement utilisé est la monopile. En raison de la force des
vagues et du vent, la monopile est soumise à une charge cyclique élevée dont l’amplitude, le niveau de charge maximale et la
périodicité varient. Dans cet article, l'interaction sol-pieu d'une monopile implantées dans du sable et soumises à un chargement latéral
cyclique est étudiée au moyen d'essais à échelle réduite. Les tests sont effectués avec une grue de chargement mécanique capable
d'appliquer un chargement cyclique sinusoïdal avec amplitude, niveau moyen et période de chargement variable pendant plus de
60 000 cycles. Les tests sont effectués dans du sable dense saturé. Le moment maximum appliqué durant les essais cycliques varie de
18% à 36% de la résistance latérale ultime obtenue lors d’essais de chargement statique. Les essais montrent que la rotation
accumulée peut être exprimée par l'utilisation d'une fonction de puissance. En outre, des essais statiques menés après le chargement
cyclique indiquent que la capacité statique ultime augmente avec le niveau du chargement cyclique.
KEYWORDS: Experimental, wind turbine foundation, monopile, long-term cyclic loading, dense sand.
1 INTRODUCTION
In the offshor wind turbine industry, the most widely used
fo ndation type is the monopile, i.e. a large diamet r stiff p le.
During th lifetime f a wind turbine, the onopil foundation
is subjected t few loa cycles with arge amplitudes, caused by
the str ng storms, and also to millions of lat ral load cycles
with low or intermediate amplitudes due to the wave loading.
This lo ding may cause failur in the fat gue or serviceability
limit states, FLS and SLS respe tively (Wichtmann et al. 2008).
The cyclic loadi g might induce a change in the so stif ness
and a permanent accumulated rocking rotation (tilt) of the
turbine. Due to the efficiency o the wind turbine, strict
demands for the rot tion and t stiffn ss of the ntire structure
ar normally made and us, the change in stiffn ss and rotatio
becomes key issues in the design. However, the current design
guidance, DNV (2011), on this long-term loading is limited and
a procedure for des gning large diameter piles is y t to be fully
expressed and confi med. The development of a reliable design
me hod requires verification and f r that in-situ and larg -scale
t sting is by far the best tool. However, this is also the most
expensive and time-consuming tool. Therefor , the r cent
ch ice for evaluating the yclic behaviour has been numeric l
modelling and small scale tes ing Several authors h s
investigat d this e.g. Niemunis et al. (20 5), Achmus et al.
(2005), Peng et al. (2006), L Blanc et al. (2010) and Achmus et
al. (2011). How ver, the research has mainly be n based on
cyclic triaxial tests, FEM-calculations and 1g experimental
setups in dry sand.
In this paper, a 1g testing rig for modell g th
envi onmental loa ing on a stiff monopi e foundation in dense
saturated sa d is desc ibed and the results fr m f ur one-way
cyclic loading tests are pr sented. The purpose of the cyclic
tests is to evaluate the influence of the number of load cycles,
, on the accumulated rocking rotation of the pile at seabed,
under long-term cyclic loading with constant frequency but
different loading amplitude and mean loading level.
The characteristic of the cyclic loading can be described by
the ratios
and
as defined by LeBlanc et al (2010).
expresses the magnitude of the loading as the ratio between the
maximum load in a load cycle and the maximum static lateral
capacity,
=
/
.
will take a value between 0
and 1. The cyclic load ratio
defines the direction of the
loading on the basis of the minimum and maximum load in a
load cycle,
=
/
.
will take the value 1 for a
static test, 0 for one-way loading, and -1 for two-way loading.
2 EXPERIMENTAL MODEL TESTS
The 1g small scale tests ar carried out at the geotechnical
l bo atory at Aalborg University, Denm rk. In the tests an ope
ended aluminium pipe pile is used. The pile is scaled
approximately 1:50 in relation to a typical offshore monopile. In
l the dime sions of the model pile are present d.
Table 1. Dimensions of the open ended aluminium pipe pile.
Dia eter Embedded length Wall thickness Load eccentricity
(mm)
(mm)
(mm)
(mm)
100
500
5
600
