With increasing diameters of offshore monopile foundations, impact hammers require more energy per blow in order to drive them into the seabed. This does not only increase the fatigue loading on the monopile, but it also increases underwater noise that affects marine life. The increase in installation costs due to additional material on the monopile, noise mitigation and longer installation times drives contractors to look for alternative installation methods. GBM’s Vibro-drill strives to install monopile foundations through a combination of jetting, liquefaction and fluidization to reduce the soil resistance to such low levels that the monopile will penetrate the soil under its own weight and thereby significantly reducing underwater noise. Although the working principles are still being tested, the Vibro-drill design needs to be improved for commercial application. The design must include the three operational functionalities stated above, but to make the Vibro-drill re-usable for consequential monopile installations, additionally retrieval and structural rigidity are required. For this thesis, the Vibro-drill is re-designed to include these functionalities. Starting from a process description, all load cases and additional design requirements were found. Based on a first global load case, a first selection was made for the connection mechanism. Subsequently, a Multi-Criteria Analysis was performed yielding two suitable concepts: the Vibro-Boxer and the Vibro-Polyp. The Vibro-Boxer relies on three extending arms that connect with a specially designed pile shoe using twelve hydraulic grippers. To feed the system with hydraulic fluid, water and air, an umbilical connects the installation vessel with the central hub of the Vibro-drill. At the end of each arm, eccentric weights are used to vibrate the pile tip. The Vibro-Polyp has a similar lay-out except for the arms and connection mechanism. Nine hinged arms form three sets of folding parallelograms to expand or contract the Vibro-drill. The connection is made through a set of pins on each arm that slide into holes in the pile shoe. Due to unknown soil properties and effectiveness of jetting, liquefaction and fluidization, the loads on the structure cannot be determined deterministically. Therefore, a Monte Carlo simulation is used to perform the structural analysis based on a best guesstimate for the distribution of these unknown parameters. Based on this approach, a tool is created that aims at performing the structural analysis while iteratively improving the design. As test results will become available after finalizing this thesis, input for the unknown parameters can be updated to improve the results of the structural analysis. Subsequently, based on this progressive insight, the design of the Vibro-drill can be optimized further. Note here that for the structural analysis, the system is assumed to be quasi-static which is a gross simplification of reality. Although this imposes limitations to the reliability of the results, this assumption is allowable for the preliminary re-design phase considered in this thesis. The structural analysis described here has been applied to both suitable concepts. For each concept, the external loads were identified and the internal loads were calculated. A series of structural checks in the Monte Carlo simulation provided a probability of failure. For both concepts the probability of failure was initially too high and therefore improvements to the structural components were made. Based on the design criteria, the two optimized design concepts were compared. Due to the smaller frontal area of the Vibro-Polyp its soil resistance on the tip of the monopile and the Vibro-drill is the smallest and therefore requires the least eccentric force. Nevertheless, the structural analysis shows that its hinged arms are fatigue sensitive yielding a higher probability of failure. Therefore, despite its larger frontal area, the Vibro-Boxer has a higher reliability and is the most suitable concept for the Vibro-drill. Finally, a more detailed design of the Vibro-Boxer concept is delivered that complies with all design requirements. For this design, recommendations are given on how to update the design based on the dynamic analysis and future test results.

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.

Offshore wind turbines are commonly supported by monopile foundations. Impact hammering is the conventional method for installing offshore monopiles but has several disadvantages. Time-consuming installation procedures, high noise emissions and steel fatigue due to the high impact are problems resulting from this installation procedure. GBM Works is a start-up that is currently developing the Vibro-drill: a new method for installing monopiles, aiming to solve the stated problems associated with the hammering method. The Vibro-drill is mounted at the bottom of monopile and uses vibrating elements that simultaneously jet water, aiming to reduce the soil resistance. As a result, the monopile can penetrate the soil to target depth under its own weight. The monopile foundation is designed to support an offshore wind turbine throughout its lifetime of about 25 years. As the wind turbine is primarily subjected to lateral loads, one of the major design requirements is the stability of the pile in terms of lateral bearing capacity. Hammered piles have shown to provide sufficient lateral bearing capacity and this method has been used world-wide to install monopile foundations. Because the Vibro-drill machine is a unique and new concept, there is insufficient knowledge about the installation effects. Therefore, the lateral bearing capacity of a Vibro-drill installed pile is yet unknown. Before this new method can enter the offshore wind market, it needs to be researched and demonstrated that this new method provides sufficient bearing capacity for an offshore wind turbine. To that end, this thesis contains a study regarding the possible soil effects and resulting bearing capacity of Vibro-drill installed monopiles. Predictions of the monopile response were done with a finite element model in Plaxis 3D. The numerical model was verified based on data from monopile tests in Cuxhaven from 2014. During these full-scale field tests, three pile pairs were installed in dense sand by vibro-driving and hammering. The effect of the methods was quantified by lateral load tests on both piles. The input parameters for the Plaxis model were derived from CPT measurements using empirical relations based on relative density recommended for monopile design. The FEM predictions show a fit with the field-measurements during initial loading where after the curves start to deviate. The differences between the FEM prediction and the field measurements are discussed, expected is that they are caused by creep effects that occur in the field. As no soil effects as result from Vibro-drill installation have yet been measured, a worst-case approach was taken to identify the risks concerning the lateral bearing capacity. A hypothetic soil disturbance as result of Vibro-drill installation was generated based on studies regarding soil effect due to existing vibrating and jetting methods. Different worst-case scenarios of possible soil effects were sketched to study the resulting pile reaction due to a horizontal applied load. This was done by comparing the monopile response of a hammered and vibro-drill installed pile and increasing the pile size of the Vibro-drilled pile iteratively until a similar reaction to the hammered pile was obtained. In this way, the effect of a Vibro-drill installation was quantified in terms of additional pile material. For the studied scenarios, an additional proportion ranging from 17% to 29% of the embedded pile material was required to compensate for the softer pile reaction. Also, a study on the effect of pile size has shown that for larger piles, a lower proportion of additional pile material is required to compensate for a possible softer pile response resulting from Vibro-drill installation.

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.