Article by Arie-Jan van Renswoude - geotechnical engineer
Recent years have been characterised by a strong demand for and commitment to renewable energy. In the Netherlands, this can be seen mainly in the large numbers of solar panels placed on roofs of companies and houses, and in the increase in wind turbines, which some-times seem to shoot up like mushrooms.
These new wind turbines are accompanied by many great developments. For example, they are becoming increasingly efficient, make less noise and sometimes even take into considera-tion the local fauna. Another development is the constant increase in the height of the tur-bines. This particular development presents us with a technical challenge: to build these tur-bines, larger and heavier cranes are required. The Dutch soil, however, remains as weak as ever.
Importance of crane hardstands
Because 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 hap-pens that hardstands are not stable enough or that they deform too much. As a result, pro-jects 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 hard-stands 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 reduce 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 defor-mations 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 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 1). Hereafter, the first model is called ‘model A’ and the second model is called ‘model B’.