Environmental Engineering Reference
In-Depth Information
• On the other hand, the compliance of the soil attributes a certain additional
damping properties to the structural SSI system compared with the fixed base
model, leading to energy dissipation.
• Moreover, large motions of the foundation may disturb the control processes of
the
machine
leading
to
an
inefficient
production
or
even
an
emergency
shutdown.
It is obvious from the above discussed points that the SSI effects can act in
opposite directions and it is not possible to establish deductively whether the
displacements of the tower will decrease or not. In general, for each specific
configuration of wind unit and underlying soil, an exhaustive investigation is
necessary. Norms and guidelines [
6
,
13
,
14
] provide only few suggestions in
regard to the SSI analysis. Indeed, the awareness of these sophisticated aspects
encounters
hardly
a
straightforward
application
in
practical
regulations
and
therefore it is only roughly mentioned.
An all-encompassing dynamic analysis of a soil-wind turbine system appears
certainly intricate. Anyway, it is possible to isolate the decisive aspects of the
problem and reduce the number of variables at play.
In dynamic SSI, the modeling procedures can be classified into rigorous and
approximate. The former are usually used in the substructure method, while the
latter lend themselves to the direct method of analysis. In the substructure method,
the soil and the structure are analyzed separately with different approaches and
then coupled at the interface enforcing compatibility and equilibrium conditions,
while in the direct methods the whole system is solved through a unique one-step
technique.
Rigorous modeling satisfies the radiation condition for unbounded media,
which enforce that the wave propagation energy is radiated toward infinity. This
condition can be used to find the fundamental solution (also called Green's
functions) of the wave propagation problem in each specific medium configuration
(i.e., half space and full space). Finally, boundary integral equations can be built
upon the fundamental solutions. The problem can be solved also numerically
discretizing the interface in boundary elements. This procedure is called boundary
element method (BEM). The main advantage of rigorous modeling is that the
radiation condition is automatically satisfied and the problem size is reduced by
one, as only the interface between soil and structure must be taken into account.
Further explanations can be found in [
24
]. However, the fundamental solution is
only available for the homogenous isotropic half space. For complicated soil
configurations, the fundamental solutions can be found only numerically and no
closed analytical form is available. For example, for the horizontally layered
unbounded medium, one may resort to the thin layer method (TLM) [
9
] or to the
precise integration method (PIM) [
20
] for computing the Green's functions.
Among approximate modeling methods, there are several combinations of finite
element method (FEM) and artificial boundaries (transmitting, viscous, paraxial,
etc.), which have highly energy absorption capabilities. Also, infinite elements can
be used as an extension of the finite element method. The drawbacks of these
