WPNL 202101

Transport & Installation
Arie-Jan van Renswoude, Geotechnical engineer, Windbase
Supporting increasingly larger and heavier cranes
Advanced simulations for
crane hardstands
One of the developments in wind turbines is the constant increase in height. This
particular development presents us with a technical challenge: to build these
turbines, larger and heavier cranes are required. The Dutch soil, however,
remains as weak as ever.
B
ecause of the weak soil, a solid
design of the crane hardstands is
of utmost importance. The
crane must stand stable and
should not settle or rotate too much. After
all, a small rotation will result in a relatively
large displacement at the top.
Unfortunately, it still frequently happens
that hardstands are not stable enough or
that they deform too much. As a result,
projects are delayed, resulting in financial
setbacks that can quickly mount up to tons.
This again emphasises the importance of a
sound hardstand.
Challenges in the design of
crane hardstands
Windbase has long been involved in the
design, consultancy and realisation of crane
hardstands and turbine foundations, and
has also been involved in the development
of the STOWA guideline for the design of
crane hardstands. This guideline brings
together the different worlds within the
wind energy sector; from crane suppliers
and geotechnicians to wind turbine
manufacturers and project developers. The
aforementioned trend has therefore not
gone unnoticed within Windbase.
Windbase’s expertise is also regularly called
in when a hardstand turns out to be
insufficiently stable or shows too much
deformations. A quick and robust solution
is then required to limit the damage
incurred, often in the form of applying a
larger mat area under the crane, which
increases the load capacity and reduces the
deformations. Such a mat area is made of
long steel mats which are believed to
distribute the crane’s load over a larger
surface area. But to what extent is the load
from the crane distributed by the mats?
And will the full length of the mats or the
edges of a larger area of mats transfer the
load to the ground? Furthermore, in the
design of crane stands, the question has
always been whether determining the
deformations with calculation sheets gives
a realistic outcome, given the character and
duration of the loads. Isn’t there a method
to better deal with this?
Advanced simulations for
hardstands
A real-life situation where the
aforementioned problems arose was
analysed by Windbase by making a
PLAXIS 3D model. In this situation, one
layer of steel mats was placed under a
crawler crane. In this model, the steel mats
- with a length of 18 m - were modelled as
separate volume elements, with stiffnesses
in accordance with the supplier’s data.
Subsequently, the load from the crawlers
was modelled on the mats. The mats in the
model could move separately from each
other and were mobilised based on the load
and the ratio of their own (bending)
stiffness to that of the hardstand and
further subsoil. After that, a second model
was made where the 18 m mats were
applied in two layers in order to obtain a
better distribution of the loads (see Figure
3). Hereafter, the first model is called
‘model A’ and the second model is called
‘model B’. As far as the governing situation
is concerned, the raising of the crane’s
boom was taken. Usually, the hardstand is
tested by keeping the boom raised for
fifteen minutes, while measuring the
deformation of the crane. When the crane
remains stable, the construction of the
turbine can begin. The phases prior to
raising the boom are omitted here.
The model showed that the mats are
sufficiently stiff to spread the load over the
entire length of a mat; of course, the centre
of the mat area will transfer most of the
load. Under the governing situation it
appears that the load in model A is
concentrated on a small number of mats
(see Figure 1), after which large
deformations and failure occur. In model B
- because of the 2nd layer of mats - a
considerably larger number of mats is
mobilised, so that the deformations remain
smaller and a higher factor of safety is
reached (see Figure 2). In principle, these
results do not differ from the expectations.
In fact, they correspond well with
experiences from outside with regard to,
among other things, the total and
differential settlements between loaded and
unloaded mats.
Thus the analyses performed, fairly
accurately reflect what happens during the
operational phase of the hardstands. This
makes it possible to calculate the loadbearing
capacity and deformations of crane
hardstands to a high degree of accuracy,
also with regard to the duration of the
loads. It remains to be seen if this is
possible with the common, traditional
calculation tools.
22 | 01-2021