Influence of initial soil state and installation parameters on the lateral behaviour of vibratory-driven monopiles in sand

Abstract

To support offshore wind turbines
(OWT), monopiles are currently the most frequently used foundation method.
These monopiles are open-ended steel tubes with a diameter of 5 - 12 m, which
are most often installed in the seabed by means of hydraulic impact hammering
usually mounted on a specialised ship. This method is produces high noise
levels and releases shockwaves into the water, which can damage the sea life’s
hearing or even outright kill them. Additional measures can be taken to reduce noise
emissions and adhere to strict environmental regulations written for marine
biology protection, but they are costly and slow down projects significantly. There
are other installation methods which have the potential to be more silent but
are not well understood yet from a noise generation, driveability, and lateral
behaviour point of view. These are aspects that will be investigated within the
SIMOX Joint Industry Project (JIP). One of these methods is vibratory
installation.

A monopile used as a foundation for
an OWT will experience a multitude of loads during its service life, the most
important of which are the lateral loads. While the behaviour of impact
hammered piles under lateral loading has been researched extensively, the
behaviour of vibratory driven piles is still relatively unknown. The influence
of different parameters used during installation (frequency, penetration speed)
and other conditions (wall thickness and soil conditions) on that behaviour
must be understood to be able to accurately predict how a vibrated monopile
will react to both cyclic and monotonic loading.

The present research explores the
behaviour of monopiles under lateral loading and the impact different
installation parameters have on it. To achieve this, a laboratory testing
campaign was carried out with model piles. The purpose of these tests is to
produce qualitative results to be used in following field-testing campaigns
within SIMOX. Piles were installed with differing installation parameters in
sand beds with different density. These piles were then subjected to initial
monotonic loading, followed by cyclic loading, and then monotonic loading
again. The data obtained during the experiments was then interpreted and
analysed. By comparing the results as well as measurements taken during and
after installation, conclusions are made concerning installation parameters and
other factors that may play a role on the lateral behaviour of monopiles.

The interpretation focused mainly on
pile head displacement during initial lateral loading depending on penetration
speed and frequency. The loading tests have shown that impact hammered piles underwent
lower displacements than vibrated piles after being loaded laterally. When
solely considering vibrated piles, piles with a larger wall thickness showed
lower displacements in general than thin-walled piles.

Frequency and penetration speed were
found to play a role in the lateral behaviour of monopiles. In the experiments
considered for this thesis, it seemed in dense sand crane-controlled piles showed
lower displacements than free-hanging piles. In medium dense sand, lower
penetration speed led to lower displacement during loading, but more research
is needed on this topic to be able to formulate clear conclusions on the exact
role of each installation parameters. The experiments also show an interesting
phenomenon regarding measured soil elevation that might link compaction around
the monopile to lower lateral displacements. The difference in elevation before
and after installation seemed to correlate with lateral displacements. In
general, piles with larger compaction around the pile displaced less during
initial loading.

In conclusion, this paper provides a
range of observations regarding the impact of installation parameters and other
conditions such as wall thickness and sand density on the behaviour of
monopiles under lateral loading, as well as offering a comparison with impact
hammered piles. Recommendations and suggestions are given for further research
and for the field testing experiments, so that the analysis made here may be
used to predict the lateral behaviour of vibrated monopiles more accurately.