As the offshore wind industry grows, the demand for larger wind turbines and foundations increases. The most common foundation type for a wind turbine is a monopile which is currently installed by large hydraulic hammers. This installation method generates a lot of underwater noise which may harm marine life. To solve this problem and its unwanted consequences, GBM aims to implement a new silent installation technique. By applying fluidization, jetting and by inducing vibrations by harmonically exciting the bottom of the pile, both the dynamic tip resistance and the shaft resistance are reduced so that the monopile can penetrate the soil. These techniques are aimed to produce less harmful underwater noise than conventional hammering. Little is known about the penetration performance of a pile which is harmonically excited at the bottom. This thesis aims to provide more insight on the penetration performance when exciting a pile at the tip. A literature study is performed on existing pile penetration models. From this study it is concluded that, currently, there are no available penetration models capable of describing the penetration performance of the pile when exciting the system at the bottom with varying harmonic excitations. Therefore, the aim of this thesis is to develop a new penetration model. The purpose of this model is to describe the penetration performance at different harmonic force parameters. The developed model is based on finite elements by using the FEMAP software. The pile is represented by shell elements. The interaction between the pile and the soil is modeled using multiple spring-damper-slider elements which are spread along the pile surface. The slider elements allow the relative motion between the pile and the surrounding soil. The different soil-structure interaction elements are uncoupled and the sliding resistance is assumed linearly elastic, perfectly plastic. Energy radiation due to elastic waves is captured by simple dashpot elements.
The developed model is used to analyze the effect of certain parameters on the penetration of the pile. This is done by changing the amplitude, the frequency or the direction of the harmonic force for a specific set of soil parameters. For each variation, the model calculates the pile displacement at a certain depth from which the penetration speed is determined. This penetration speed is then compared to the other results to determine the effect of each chosen parameter. Also, the location of excitation is analyzed. The model is used to analyze a pile which is excited from the top or at the bottom. From the results it is concluded that an increase in the amplitude and the frequency of the excitation has a positive effect on the penetration speed. The dependence of the direction of the vibration on the pile penetration is complex. Therefore, a clear correlation between the two could not be obtained within the time framework of this research. This research provides is a first step towards understanding of the Vibro-drill system performance. As this is an investigation which is still in progress there are some recommendations for further research on this topic. One important recommendation is to improve the soil reaction in the model to a coupled system where it is now uncoupled. Furthermore field tests can be performed providing more knowledge on the effects of the soil and to validate the model. ... Read more

The energy transition results in large offshore windfarms being installed. The most common support structure for a wind turbine at sea is a monopile. These are currently installed by large hydraulic hammers. However, hammering produces a lot of underwater noise which may harm marine life (Kastelein, 2014). Therefore, the industry is looking for new installation methods that are more environment friendly. GBM Works aims to implement such a new installation technique called the Vibro-drill. They introduce a revolutionary new design which is to replace the hammering technique. Vibrations, jetting and liquefaction are used to reduce the soil resistance. This reduction is so powerful that the monopile sinks into the ground under its own weight without producing the harmful underwater noise. Before each monopile is installed an analysis is made of the factors determining the chance of a successful installation. This process is called a penetration prediction. For the industry to accept the Vibro-drill installation method, penetration predictions are required to prove the applicability of the new technique. The radical changes in the installation process used by this technique make it impossible to use current penetration prediction methods. This means that a new prediction method needs to be developed that takes into account the specific features of the Vibro-drill. This thesis aims to address this problem by way of providing building blocks for the development of a fully-fledged penetration prediction model that is applicable to the Vibro-drill method. To this end, this thesis will focus on delivering a two-dimensional soil model in penetrated by a pile. The two-dimensional approach, which is the basis of this thesis, has of course its limitations as only a part of the pile and soil are included in the model. The assumed point symmetry over the centreline of the monopile leads to only a slice of the pile being modelled. The soil is captured using a lattice model. A well-known practice for modelling soil in a two-dimensional way. The properties of the lattice are determined using Cone Penetration Test data. As a result the model represents different layers of soil along the depth. The model uses a rigid pile to penetrate the soil. The penetration of the pile causes a reaction force in the soil. This force is used to calculate the friction forces. The reaction force and the friction force are the basis for calculating the penetration speed of the pile. From this it follows that the penetration speed and final installation depth depend on the different soil layers within the soil model. A first step towards validation is made. The pile/lattice interaction is verified and the results are compared to test data. A sensitivity analysis is performed to verify the independence of the lattice size and resolution on the modelled penetration speed of the pile. Subsequently the penetration predictions of the model in its current form are analysed. The results are compared to the real-life outcome of tests with the Vibro-drill. From this first attempt to validation the conclusion can be drawn that the two-dimensional model, as developed in this thesis, could be a suitable basis for further work on a viable penetration prediction method for the Vibro-drill technique. Finally, recommendations are made for further enhancing the model with a view to its eventual implementation as a fully functional penetration prediction model for the Vibro-drill. ... Read more