This thesis concerns the problematics of estimating
the soil-structure interaction for a monopile using
the current design guidelines in the offshore sector.
These recommendations are implemented in a simple
1D Finite Element model used to estimate the first
eigenfrequency of the OWT analysed.
The thesis is divided into four stages. The first stage
serves as a base of knowledge, where the design
tendencies within the field of OWT foundations are
investigated and the theory for basic dynamics of soil
is covered. Stage 2 considers the set up of a simple
Finite Element model, representing the behaviour of
an OWT by modelling a vertical stick with a mass
on top and to analyse the importance of the input
parameters. The purpose of Stage 3 is to incorporate
the effect from the soil surrounding the foundation of
the OWT, by modelling the soil-structure interaction
using the original API p-y curves recommended
in the current design guidelines. Investigations of
improvements to the current design guidelines and the
method proposed by the PISA project for estimating
the soil-structure interaction are explored. Also, the
Finite Element model is separated in order to model
the OWT and the foundation separately and still being
able to estimate the first eigenfrequency of the full
system. Lastly, Stage 4 considers the utilisation of
the commercial program Plaxis 3D to analyse the
arising subjects from the previous stages. The stage
will contain three analyses, concerning the estimation
of the first eigenfrequency for an OWT, the extraction
of p-y curves from a 3D model and the change in the
displacement field of the soil around the monopile as
the diameter of a monopile is increasing.