A geotechnical centrifuge is used to conduct tests on small-scale models to obtain data on the response of real-life structures. In the field of earthquake engineering, centrifuges can be used to model soil-structure interaction and to analyse structural response to earthquakes through scaled models. By rotating the centrifuge at high angular velocities, the model inside the centrifuge can experience the stresses corresponding to a full-scale prototype. Through simplified scaling laws, the dimensions of the small-scale model are estimated, and the results from these tests can be extrapolated to study the behaviour of the prototype.

When the small-scale model of a dynamically sensitive structure, such as an offshore wind turbine, is subjected to high angular velocities inside a centrifuge, it is essential that the model remains stable to obtain meaningful results from the test. Instabilities, such as divergence or flutter, may arise in the small-scale model depending on the system’s parameters due to the action of pseudo-forces and coupling between the displacement fields, which may lead to the failure of the model. Moreover, it is unclear whether the simplified scaling laws can still be applied for such slender small-scale models due to the action of these pseudo-forces and the coupling terms. Therefore, understanding the behaviour of these dynamically sensitive small-scale models under centrifuge conditions is important to ensure their stability and to obtain conclusive results about the prototype.

This research represents the small-scale model of a monopile-founded offshore wind turbine inside a centrifuge as a homogeneous Rayleigh cantilever beam rotating about a vertical axis, with each point along the beam consisting of three translational displacements. A mathematical model is formulated using Lagrangian formalism that incorporates all relevant pseudo-forces and coupling of displacement fields that may affect the stability of the small-scale model. Euler-Lagrange equations are then applied to formulate the governing equations of motion, and a dimensionless form of the equations is presented to draw general conclusions. Moreover, the effect of soil-structure interaction is considered using a lumped spring model.

Subsequently, an eigenvalue analysis is performed using analytical and numerical approaches. The numerical approach is more robust and is applied to calculate the eigenproperties of the system, although it requires a good initial guess. The technique of representing the response as a summation of assumed modes is explored in the analytical approach, and a convergence study is conducted. Afterwards, a parametric study is performed to identify the factors influencing stability. The angular velocity at which the onset of instability happens is also determined. Finally, the findings are applied to a case study of a small-scale model of an offshore wind turbine tested inside the centrifuge facility at ETH Zürich as part of the research project, DONISIS, led by TU Delft.

The results of this study demonstrate that, for the small-scale beam model of a monopile-founded offshore wind turbine tested inside a centrifuge, the dominant instability mode is the divergence of chordwise lateral bending. This instability arises from the axial compression induced by the centrifugal forces acting along the beam. The coupling of displacement fields through Coriolis forces does not affect the onset of instability, although its influence on the eigenproperties becomes apparent only at angular velocities far exceeding the practical operating range of a centrifuge. Since the Coriolis terms have no significant impact on system behaviour, their effect on scaling laws can be regarded as negligible. Consequently, the existing scaling laws remain applicable to small-scale models of rotating beams within the practically operating range of angular velocities of the centrifuge. ... Read more

The current Hardening Soil small-strain (HSsmall) model, developed by Benz (2007), is capable of capturing the behaviour of a wide range of soil types, including their small-strain response. However, under specific conditions, the model exhibits overshooting, which is the overestimation of the material’s elastic stiffness upon the closure of a small unloading-reloading (UL-RL) cycle interrupting monotonic loading, rather than recovering the stiffness corresponding to the onset of the cycle, as observed in real soil behaviour. Overshooting leads to underestimated deformations and consequently non-conservative results.
To address this issue, PLAXIS (part of Seequent, the Bentley Subsurface company) proposed two formulations: the Continuous Brick (CB) formulation, which replaces the small-strain component of the HSsmall model, and a Memory-Surface-Based (MSB) formulation, which extends it. Both have been implemented in the source code of the existing HSsmall constitutive model. However, their implementations had not yet been verified in the literature, nor had an assessment of overshooting, the motivation behind their development, been conducted.
This research aims to close that gap and to determine which of the two formulations provides the most suitable approach. Implementing the best-performing formulation into the HSsmall model results in a new more robust state-of-the-art model.
Accordingly, the following research question is posed:
‘To what extent do the Continuous Brick formulation, as a new formulation for small-strain stiffness, and the Memory-Surface-based formulation, as an extension to the existing small-strain stiffness formulation within the Hardening Soil small-strain model, reduce the overshooting observed in the current formulation?’
This question is addressed through a structured test plan consisting of two main components: (i) single stress point simulations to verify whether the formulations behave as expected at the most fundamental level, and (ii) a boundary value problem to evaluate their performance under more numerically demanding conditions representative of practical applications in the pre-failure range, where small strain stiffness strongly influences the magnitude of deformations.
It was found that both formulations reduce overshooting to a negligible level. However, their effectiveness decreases in simulations involving nested cycles, as both formulations exhibit the limitation of retaining the memory of only a single UL-RL cycle. Although both formulations perform well, the MSB formulation proves to be the most suitable approach: it is easier to interpret, appears more robust, retains the small-strain component of the original HSsmall model, and yields a response consistent with the HSsmall model under monotonic loading.
Extending the HSsmall model with the MSB formulation leads to more accurate deformation estimations in geotechnical problems especially within the pre-failure range, as deformations will no longer be underestimated, without introducing additional model parameters. Moreover, adopting this formulation will not have major consequences for the end user, since its behaviour is consistent with that of the HSsmall model, except that overshooting no longer occurs.
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This report evaluates the influence of dynamic motions, more specifically earthquake motions, on the stiffness of offshore soil. As this stiffness reduces over the course of an earthquake, the structural response of offshore technology is affected. The focus of the investigation presented is on the influence of this seismically triggered stiffness degradation on the simplified design procedure of jack-up vessels, a vessel used to install and maintain offshore structures such as wind turbines, in the offshore area of Japan and the East and South Chinese Sea.
A literature study was conducted to outline the shortcomings in the current screening process of a jack-up vessel, as well as to get acquainted with some geotechnical concepts. These concepts were used to develop a soil model with a simplified structure on top. Earthquake motions of different intensity were applied to the base of this soil deposit and the response of the structure was compared to the response of a three-dimensional jack-up model, in which the soil was represented by linear springs and dashpots.
The results show that for lower intensity levels, defined as the earthquake’s level of peak ground acceleration, there is barely any degradation in the soil. For earthquakes with a peak ground acceleration of around 0.1g, there sometimes is degradation in the soil, however, almost never significant enough to alter the structural response. For more intense earthquakes, there is a clear degradation in the soil deposit, which affects the response of the jack-up vessel. For the case presented in this research project, an equivalent stiffness degradation factor is found for these more intense earthquakes, so that soil stiffness degradation can be implemented in the simplified design procedure of a jack-up vessel. Furthermore, the results imply that linking the earthquake’s signal intensity to both the peak ground acceleration and the Arias intensity level will lead to a more accurate subdivision of the equivalent stiffness degradation factors ... Read more

The early 20th century marked a period of significant infrastructural development in Europe, resulting in the construction of numerous bridges and structures that have lasted to the present day. Many of these foundations, often designed as piled rafts but analysed as pile groups, were constructed using simplistic methodologies. These methods neglected the interactions between the raft, soil, and piles. Despite advancements in geotechnical engineering, current standards like Eurocode 7 still rely on this outdated approach, potentially leading to unnecessarily conservative and expensive retrofitting solutions.

Recent research highlights the significant contribution of rafts to the overall bearing capacity of foundations, suggesting that current design standards may underestimate the capacity provided by rafts. This thesis investigates the behaviour of piled raft foundations compared to pile groups and rafts, utilizing both 3D finite element analysis with the SANISAND-MS constitutive model and centrifuge testing. Furthermore, it explores the feasibility of using a viscoelastic model to capture the cyclic behaviour of piled raft foundations. The study focuses on dry sand conditions and quasi-static cyclic loading to address questions regarding load-displacement behaviour, load and moment distribution, internal forces on piles, and the modelling of cyclic loading conditions.

The findings demonstrate the superior performance of piled rafts in terms of stiffness and resistance compared to pile groups and rafts. Specifically, the vertical resistance ratio (β_PR,V ) and combined resistance ratio (ξ_PR,V ) indicate that accounting for the raft nearly doubles the vertical resistance of the piled raft foundation and aligns closely with the combined resistance of the pile group and raft as separate foundations. In lateral loading, the horizontal resistance ratio (β_PR,H) and moment resistance ratio (β_PR,M ) suggest that the piled raft requires more horizontal load and moment to achieve the same displacement and rotation compared to the pile group alone. The raft contributes approximately 45% of the vertical load and 50% of the moment, reducing the axial load on individual piles by around 40%.

Analysis of internal forces reveals that shaft resistance is relatively small. Larger moments are observed in edge piles under axial loading, indicating the deformability of the raft and suggesting a likelihood of failure in the edge piles due to moments. Under lateral loading, the tension piles quickly reach maximum tensile resistance, with moments continuing to increase, highlighting a high probability of failure in the edge piles due to moments.

The study also demonstrates that modelling the elastoplastic cyclic behaviour of piled rafts using a viscoelastic model with frequency-independent viscosity is feasible. While not fully capturing all aspects of foundation behaviour, this approach adequately replicates stiffness and damping characteristics, making it useful for seismic design using a pseudostatic approach.

These findings offer potential for more efficient foundation designs, improved retrofitting strategies for existing structures, and increased understanding of the foundation behaviour of piled raft systems. ... Read more

The engineering response of fibrous soft soils, such as peats, is significantly influenced by the presence of fibres. Interpretation of laboratory tests performed on such soils is complicated due to the reinforcement provided by the fibres, which results in the material behaving as a composite, with properties different from traditional inorganic soils. The fibre matrix interaction complicates the estimation of geotechnical parameters such as shear strength and frictional properties from standard laboratory tests. Some empirical criteria to account for the contribution of fibres have been proposed, but there is no general consensus in practice. In this work, a numerical approach using finite element method and accounting for the kinematics of fibres is used to gain physical insights into the role of fibres in the mechanical behaviour of soft soils. Numerical replicas of common geotechnical laboratory tests were simulated as boundary value problems by superimposition of a soil matrix with a fibrous network. The contribution of fibres to the mechanical response of the soil sample is quantified for various stress and strain paths relevant to the engineering practice. The influence of two constitutive stress-strain relationships for the fibres upon stress reversal are compared and results suggest that fibres engage before they return to their original configuration, thus implying an influence of their compression history that could be linked to local tensioning of the fibres after compression. Further, laboratory tests on peat are used to validate the numerical approach, modelling the big fibres explicitly in the model and using an advanced model for the matrix, capable of reproducing behaviours such as distrotional hardening expected from the microscopic fibres in the matrix. The comparison highlights the effectiveness of the proposed numerical approach in accurately capturing the mechanical behaviour of natural fibrous peat as observed in laboratory tests. This approach provides valuable physical insights into the composite behaviour of peat. Furthermore, it offers a novel tool for interpreting extensive experimental data on peat samples, enabling the derivation of critical geotechnical parameters essential for the design and assessment of infrastructure founded on this unique soil. ... Read more

The Netherlands, known for its extensive coastal regions and significant portions of land below sea level, faces escalating flood risks due to climate change and sea level rise. To address these threats, the country relies on the construction of dykes founded on soft soils like peat. Traditionally, peat has been regarded as the critical soil layer for infrastructure stability; however, recent studies suggest that the interface between peat and clay is crucial for assessing failure mechanisms. This research focuses on the mechanical response of peat-clay interface samples to better understand the interaction between these two materials.

The study integrates comprehensive laboratory experiments with advanced numerical modelling to investigate the peat-clay interface. The findings underscore the critical role of the clay layer in the mobilised shear strength, which is essential in dyke stability assessment. The study highlights that accurate modelling of the interface requires considering the volumetric fractions of peat and clay and the reinforcing effects of fibres. This research enhances the understanding of peat-clay interfaces in dyke systems and contributes to improving geo-infrastructure assessment. ... Read more

The Global Wind Energy Council (GWEC) forecasts an accelerated increase in wind installations as a result of the aggressive climate change and green energy objectives set by the majority of countries. As a consequence, new regions worldwide that were previously thought to be unsuitable for the construction of offshore wind farms due to the soil characteristics are planned to be explored. This is the case of glauconite sands, which include glauconite pellets that are very susceptible to crushing. The crushing transforms the coarse-grained sand into a fine-grained soil, changing the geotechnical characteristics of the soil. This poses particular geotechnical challenges, since the soil-structure interaction between the pile being driven and the sand changes, the side friction increases with depth and the prediction of blow counts in drivability analyses is underpredicted. In addition to this, the glauconitic sand influences the response in cone penetration tests (CPTs), characterised by high cone tip resistance (qc) and sleeve frictions (fs), leading to high friction ratios (Rf) and resulting in misleading soil classifications. IQIP, as a company specialised in installation and foundation projects internationally that provides innovative and high-quality equipment and solutions for construction projects related to Offshore Wind industry, is interested in better understanding the mechanical behaviour of glauconite sand during pile installation due to the risks posed by this sand, e.g. premature pile installation refusal. In order to understand this soil that poses geotechnical challenges, laboratory test programs should be designed in order to capture the soil behaviour and physical modelling is a cost effective and valuable tool. This research aims to obtain detailed information on the soil response of glauconitic sand in saturated conditions during pile installation with a new sample preparation method for glauconitic sand containing fines. It will investigate the feasibility of cone penetration testing, cyclic loading, pile driving, as well as the analysis of the shear band and grain crushing around the instrumentation penetrated in the sample in a calibration chamber (CC). Based on the results from the three performed tests it could be concluded that the preparation method led to a good procedure to prepare repeatable and homogeneous samples under fully saturated conditions, as well to a representative response to pile penetration. With this, it is possible to perform further research on the sand fraction of glauconitic sands using the same sample preparation method and testing equipment. ... Read more

Offshore wind energy is expanding rapidly as governments aim for net-zero emissions, with monopiles and jackets being the primary foundation methods for offshore wind turbines (OWTs). Supply chains, heavier turbines and deeper waters influence the efficiency of jacket foundations. This research considers a jacket-founded OWT. The development of OWTs has sparked its expansion to building sites with a proclivity to earthquakes. Such loading involves soil-structure interaction of which more needs to be known to improve accurate computational modelling.

Current practice of Siemens Gamesa Renewable Energy (SGRE) is to perform aeroelastic simulations in their computational tool BHawC. The Foundation Designer delivers a foundation Superelement to maintain secrecy. The equation of motion for the jacket foundation can then be solved linearly with a reduced amount of unknowns. However, prevalence of nonlinearity in soil raises the question to what degree a linear model adequately captures the response.

This study investigates the impact of soil nonlinearity on OWT dynamic behavior under seismic loads by comparing linear and nonlinear soil models. The analysis involves performing seismic simulations using multiple earthquakes. Another distinction is made through soil models with different characteristics. Nonlinearity is introduced to the soil stiffness and energy dissipation mechanism under cyclic loading. The investigated variations are a linear (elastic), geometrically nonlinear (nonlinear elastic) or both geometrically and physically nonlinear soil model (nonlinear plastic).

The numerical model consists of a Rotor Nacelle Assembly (RNA), tower, transition piece, jacket, piles and soil springs. Beam elements are used for the tower, jacket and piles. The transition piece is simulated by stiffening the top jacket braces. The RNA is modelled using a lumped mass with rotational inertia. It is vertically eccentric to the tower top and connected with a rigid link. The earthquake is applied uniform over depth and only horizontal movements are considered.

The findings underscore a difference in results between the linear and nonlinear models. The evaluated results from simulations consist of forces, displacements and dynamic characteristics of the structure. Also noted should be that the computational time of the linear model is significantly lower. The results found in models can differ greatly due to the loading spectrum with highly varying frequency peaks. Another factor is softening of the stiffness. The frequency domain of the elastic model results consists of narrow peaks at the system's natural frequencies. The peaks for the nonlinear elastic model are wider due to softening of the stiffness. For the structure used in this research, softening introduces coupled modes with greater displacements along the height of the structure. This makes it possible for the evaluated results to have higher values, even with less energy put into the system. The plastic models' peaks are of a width in between the elastic and nonlinear elastic model due to the combined use of isotropic hardening and nonlinear stiffness. When the model falls back on its initial stiffness upon unloading, the eigenfrequencies related to that stiffness become more pronounced. To match the occupancy of wider frequency peaks, loading and unloading should both happen nonlinearly. This can be achieved by using kinematic hardening instead of isotropic hardening. Plasticity generally reduces peak displacement and sectional moment values and nonlinear stiffness broadens the response frequency spectrum. Careful consideration of cyclic material behaviour, eigenfrequencies and loading characteristics are essential for a realistic model. ... Read more

The offshore wind market has become a substantially growing industry to meet the worldwide increasing energy demands. This results in heavier crane operations on offshore jack-up vessels due to the increasing size of wind turbine components. To ensure the seabed can support the loads on the jack-up legs, a temporary foundation for the leg's footings is established before crane operations through a process known as preloading. Where the two diagonal opposing leg pairs are loaded alternatively by the weight of the vessel until a stable condition is reached. Traditionally, the capacity of those foundations is determined based on storm loads adopted from offshore oil and gas jack-up guidelines.

The main objective of this thesis is to develop a robust method for analysing the applied preload in past jack-up crane operations conducted by Jan de Nul's Vole au vent. This analysis aims to provide a better understanding of the effectiveness of traditional offshore jack-up guidelines within the rapidly growing offshore wind industry, where heavy crane operations are now performed on a daily basis. Such understanding is crucial, as these guidelines were not originally intended for the advanced state of the current offshore wind sector and are not specifically calibrated for its unique demands.

To accomplish this objective, a method is designed to assess the preload safety factors of past jacking operations and determine their optimal value for heavy lifting operations. This safety factor provides the ratio between operational leg reactions and applied preload. The developed models are then applied to a case study using measuring data from jacking operations of the Vole au vent to validate their effectiveness.

This method is based on reliability analyses which evaluate the probability of failure by assessing if a certain limit state is exceeded. Failure for jacking operations can occur when the leg reactions during crane operations become larger than the applied preload. From the acquired measuring data, certain probability distributions of the leg reactions can be obtained, which are used by a Monte Carlo Simulation to assess the probability of preload exceedance through the defined limit states. This probability of preload exceedance quantifies the reliability of the applied preload in different soil types at three offshore wind farm sites.

This research then defines optimal targets for the annual probability of preload exceedance based on the consequences of failure of operations in both low-risk and high-risk soil profiles. These targets provide a balance between operational efficiency and safety. From those targets, an optimal preload safety factor is obtained and compared to what was originally applied during the jack-up operations.

The findings of this thesis indicate the need to evaluate and improve the standards to better align with the industry's evolving requirements.

It is shown that the currently used preload safety factor from traditional offshore jack-up guidelines is not yet correctly calibrated for heavy crane operations on jack-up vessels. To achieve an optimal balance between operational efficiency and safety, the applied preload, with respect to the experienced loads from heavy crane operations, should be slightly lowered compared to what is currently applied.

In addition, this research observed that the measured conditions during jack-up operations are not correctly estimated, leading to operational and preload uncertainties. ... Read more

Large-diameter monopiles serve as foundations for offshore wind turbines (OWT), and the diameters are now up to 10 meters. These monopiles exhibit lower embedded length-to-diameter (L/D) ratios compared to the conventional monopile that is widely employed in offshore oil and gas platforms. They undergo rotation when subjected to lateral loading like rigid or semi-rigid bodies. This distinct geometry necessitates a different approach to describe the soil reaction that is induced when a lateral load is applied. Previous research introduced a 1D model incorporating four soil spring components to represent four aspects of soil reaction namely lateral soil reaction, distributed moment, base shear force, and base moment. However, questions have arisen regarding the contributions of base components, specifically the base shear force and base moment, to maintaining monopile stability. In response, a series of monotonic loading tests on monopiles in dry sand is conducted to assess the contributions of base shear force and base moment to monopile stability. Following this assessment of base components, parametric analyses are carried out to investigate the effect of pile diameter (D), the L/D ratio, load eccentricity (e), and sand relative density on monopile responses under cyclic and monotonic lateral loading. In this study, the SANISAND-MS material model is employed within 3D Finite Element (FE) software to model ratcheting during cyclic lateral loading. Finally, an investigation is conducted with the aim of constructing a 0D model to represent base shear force and base displacement responses under both monotonic and cyclic lateral loading. ... Read more

The rapid growth of Floating Offshore Wind (FOW) has spurred intensive research across various aspects of the floating system. A standard practice in mooring system design within the industry is that anchors remain fixed on the seabed. In contrast, plate-type anchors exhibit mobility under loading, a crucial factor considering the thousands of loading cycles experienced by FOW turbines during operation. Understanding the strain accumulation mechanism during cyclic loading has substantial implications for design. This thesis delves into the behavior of Drag Embedded Anchors (DEAs) subjected to static monotonic and cyclic loading, utilizing 3-dimensional Finite Element simulations. The installation trajectory of DEAs is initially defined through established analytical methodologies. Subsequently, the movement of the anchor and soil response under monotonic and cyclic loads is elucidated. Analytical expressions for monotonic force-displacement curves are examined, identifying optimal models and offering relevant parameter values for different anchor trajectory points. Under cyclic loading, the influence of average load and cyclic load amplitude is studied, along with exploring the feasibility of applying an existing 1-dimensional model for predicting anchor response.
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Offshore wind energy's rapid expansion underscores the need for accurate and efficient methods to analyze the behavior of monopile foundations supporting wind turbines. While three-dimensional (3D) analyses provide comprehensive insights, their computational demands are significant. As an alternative, one-dimensional (1D) models with spring elements to simulate the interaction between the structure and the surrounding soil, offer efficiency and simplicity. Realistic soil behavior, characterized by elastoplasticity, necessitates proper calibration of the spring models in 1D analysis.
This thesis addresses the challenge of soil-monopile interaction analysis, specifically focusing on the monopile response under lateral static monotonic loading. The research commences by highlighting the development imperatives in monopile-founded offshore wind turbines. The first phase involves calibrating elastic springs through a comprehensive review of existing literature. This calibration accounts for variations in spring stiffness along the monopile's length. Subsequently, the study progresses to elastoplastic soil modelling, adopting a linear elastic perfectly plastic approach and employing only lateral shaft springs. Acknowledging the limitations of linear elastic perfectly plastic p-y response, new material models, namely a bilinear and an exponential model, are examined. A parametric analysis encompassing various monopile geometries and lateral load eccentricities is conducted. An optimization routine refines the bilinear and exponential model parameters to closely match 3D responses. The results demonstrate satisfactory agreement for the analyzed high L/D monopiles, yielding valuable insights and conclusions. However, the low L/D monopiles exhibit a less successful match, primarily attributed to the absence of rotational shaft springs in the analysis.
Furthermore, empirical design processes for applying the bilinear and exponential models are outlined. These processes are founded on the relationships between the model parameters and the length-to-diameter (L/D) ratio as well as the eccentricity-to-diameter (e/D) ratio. The study highlights the applicability of the bilinear model across various soil conditions, monopile geometries and lateral load eccentricities. In contrast, the exponential model's efficacy is constrained by the examined L/D ratios, warranting further analyses for expanded application.
In conclusion, this thesis presents a systematic transition from elastic to elastoplastic modelling for soil-monopile interaction analysis under static monotonic loading. The proposed bilinear and exponential models enhance the accuracy of 1D simulations, facilitating efficient design and analysis of monopile-founded offshore wind turbines. These methodologies contribute to the advancement of sustainable offshore wind energy, catering to diverse soil conditions and design scenarios. ... Read more

Reducing noise pollution during offshore monopile installation is crucial for marine life. Current installation methods create too much noise or do not have the certainty of reaching the required depths. By using a Vibrohammer combined with a Jet-ring this problem could be solved. A Jet-ring is a tool to reduce the sand induced inside shaft friction by fluidizing the inside of the monopile during installation. To make this concept a viable option, the Jet-ring should be retrievable as a reduction in cost but also in materials. The aim of this thesis is to design a retrievable Jet-ring for a variable-diameter monopile without changing the diameter of the Jet-ring. For this goal to succeed the focus will be on the possibility of using inside-out jetting. This design would be an addition to the Vibro-jet® of GBM Works, which is not retrievable yet. Keeping the diameter of the Jet-ring constant keeps the design simple, reducing the chance of failure inside the monopile where the Jet-ring has to withstand harsh conditions under a lot of water and sand.

After creating a complete design of the retrievable Jet-ring, an experiment was designed to prove the concept. The designed concept is a Jet-ring which is small enough to fit through all sections of the monopile. In order to reduce the shaft friction from a distance, the nozzles are pointed outwards. This way the nozzles first have to erode a layer of sand before reaching the monopile wall. For comparison, 4 different water inlet concepts were tested, including the designed Jet-ring. All additional concepts were chosen to confirm and compare certain
aspects of the Jet-ring. The test setup included several cameras, a flow and a pressure meter. The concepts were placed at the bottom of the tank with a sand bed above it during the experiment. A repeatable experiment was created by keeping the relative density equal which was done by keeping the starting bed height constant combined with an equal weight of sand at every experiment. At the top of the sand bed, a bolt was placed which would fall through when the sand was fluidized. After execution, the height of the sand bed was measured
when the bolt fell through. In addition to the bed height, the duration until fluidization was measured. For all concepts, three different flow rates were executed minimally three times per flow rate to reduce outliers.

The processed results can be divided into two categories: visual and numerical results. The visual results contained the flow patterns and other mechanisms helping to fluidize the sand bed. By looking at the flow patterns, explanations were found for the numerical results. The numerical results are the bed height and duration until fluidization. Looking at the results, the importance of having pressure behind the flow was shown. Comparing the base case, with a homogeneous water inlet, to the other concepts jetting with ten nozzles showed that fewer nozzles with a higher pressure resulted in faster fluidization. Furthermore, the importance of prolonging the erosion mechanism is shown. A concept directly jetting into the side of the tank lost a lot of energy in the
flow resulting in almost double the fluidization time. While the energy dissipation against the wall of the tank is substantial, energy dissipation due to erosion seems relatively small. This was concluded by comparing a concept that jetted from the centre of the tank and a concept that jetted from the side. The last conclusion makes the designed tool using inside-out jetting a viable option for the retrievable Jet-ring.

While the results are promising, some remarks should be made on the experiment and the design. While congestion by sand could occur during the experiments the results still give a relevant view on the capability of the concept to reduce the inside shaft friction of the monopile because the same mechanisms such as erosion and fluidization are used to penetrate the sand during the congestion as during monopile installation. Furthermore, because of the smaller diameter of the tank, the complete inside was fluidized. This could be not the case during monopile installation as sand could accumulate at the centre of the monopile. However, this possibly accumulating sand is not sure to influence the shaft friction as the sand against the monopile wall that induces the shaft friction is fluidized or weakened. These additional studies could strengthen and prove the concept of inside-out jetting further in the future. ... Read more

This thesis explores the feasibility of using a helical pile foundation with a tension leg mooring system to support a 15MW floating offshore wind turbine and assesses its financial viability compared to alternative anchor concepts.

The study commenced with an extensive literature review, drawing from a diverse range of sources, including reviews, offshore guidelines, research papers, and expert interviews. The literature review was conducted to enhance the understanding of the use of helical piles in the offshore industry, encompassing their fundamental characteristics, current applications, and potential contributions. For this investigation, a Tension Leg Platform (TLP) featuring a 15 MW wind turbine, engineered by Heerema Engineering Solutions (HES), was employed. Time-domain simulations, accounting for environmental conditions, were carried out using the OrcaFlex software. These simulations led to the determination of the mooring line tensions, with the maximum value identified as the design load case. During the helical pile geometry optimization process, which focused on maximizing uplift capacity, it was established that a single helical pile in dense sand can achieve a maximum uplift capacity of 7.91 MN, while taking into account geotechnical, structural, and installation constraints. As a result, considering the significant uplift capacity demands of TLPs, it is essential to employ helical pile grouping, necessitating a minimum of four helical piles. Consequently, the design of the helical pile group anchors confirms their ability to meet the uplift and lateral capacity requirements of TLPs. Furthermore, the exploration of variations in equipment, steel strength, helical pile geometry, and soil conditions has yielded promising results. It is important to note that achieving the required installation depth for these group anchors involves a significant force and torque during the installation process, presenting potential challenges. Consequently, the development of equipment capable of meeting these installation requirements is vital for ensuring the feasibility of these helical pile group anchors.

The performance of helical pile group anchors was analysed from economic, technical, and environmental perspectives, with a comparison to suction and driven pile anchor concepts. The economic evaluation showed that the estimated total costs for the 4-pile helical group anchor, utilizing a singlepile installation method, are notably higher when factoring in the costs related to developing helical pile equipment, making it less financially attractive compared to the developed suction and driven pile anchor concepts. However, when excluding these equipment costs, the 4-pile helical group anchor ranked as the second-most financially attractive option, regardless of the installation technique, with only the single driven pile anchor being less expensive. Notably, it was discovered that, in a scenario where structural, geotechnical, and installation constraints are disregarded, the single pile helical anchor emerges as the most financially attractive among all anchor types, even surpassing the single pile driven anchor. This suggests that overcoming challenges related to scaling up helical pile dimensions, like enhancing their structural integrity, and reducing installation requirements through innovative designs, could make the use of fewer piles in a helical pile group anchor a feasible choice. In addition to these quantifiable factors, helical piles offer the advantage of a low-noise installation method, which becomes increasingly important due to the growing noise disturbance legislation in certain regions. Furthermore, their adaptability to various ground conditions through flexible installation techniques makes them a convenient choice for sites with limited access or specific inclination requirements...

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Heerema Marine Contractors is assigned to execute the installation of 64 monopiles for the He Dreiht Offshore Wind Farm provided by Energie Baden-Württemberg (EnBW). The installation of the monopiles is done by means of a template structure. This template is specifically designed for this project and is installed on the seabed temporarily. The initial design of the template includes large mudmats to ensure the required stability during the full monopile installation sequence. Analysis shows that this initial template design does not perform well with regard to the template installation sequence. Additionally, when lifting the template through the wave zone, the large horizontal surface area of the mudmats yields significant hydrodynamic forces acting on the template in vertical direction. .
Therefore, the main objective of this thesis is to generate and develop alternative foundation concepts to optimize the template installation sequence. The foundation concepts considered in this research are a mudmat, suction bucket, push-in pile and helical pile foundation concept. Using the mudmat as a foundation concept yields a foundation area of 924 m^2 in order to ensure stability on the seabed, independent of the soil type. The suction bucket foundation concepts results in a horizontal foundation area of 390 m^2 for sand, or 117 m^2 for a clay seabed. The push-in pile and helical pile foundation concepts developed for a sand seabed, result in an area of 306 m^2 and 319 m^2. For a clay seabed respectively a foundation area of 75 m^2 and 88 m^2 is required.
The operability of each foundation concept is determined by means of a hydrodynamic analysis of the template when lifting it through the wave zone. The operability is the amount of time the template can be lowered through the wave zone, expressed in percentage based on 24 hours. A simplified method stated by DNV is used to determine the hydrodynamic loads acting on the foundation in heave direction. For the considered homogeneous sand and clay seabed, the mudmat foundation concept proved to have the lowest operability. The suction bucket concept scored second, and the push-in pile and helical pile foundation concept showed the best performance on operability for both types of seabed.
To evaluate which concept yields the most optimal template installation sequence, a multi-criteria analysis (MCA) is performed. This MCA also takes the installation time, construction costs and the risk of damage into account as criteria. Taking all these criteria into account, it is obtained that the mudmat foundation concept does provide the best performance when considering the template installation sequence, based on the assumptions stated throughout this thesis. Even though the mudmat foundation concept has the lowest performance of all foundation concepts when lifting the template through the wave zone, this concept overall performs better than the alternative foundation concept due to a short installation time on the seabed, low construction costs and low damage risks. The suction bucket foundation concept shows an average score on all criteria. This resulted in the second preferred option. The push-in pile and helical pile foundation concepts show a good operability, however, these concepts score low on the construction costs, installation time and damage sensitivity. As a result, these concepts show the lowest performance considering the template installation sequence.
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New offshore wind farms consisting of monopile-founded Offshore Wind Turbines (OWTs) are to be built in earthquake-prone areas. To design the monopile foundation and to accurately define the dynamic response of an OWT, the soil-monopile-superstructure interaction should be modelled properly. Due to the fact that the Three-Dimensional (3D) Finite Element (FE) analyses are complex and computationally expensive, research is focused on One-Dimensional (1D) FE models, in which the soil-monopile interaction is traditionally described via distributed translational springs representing the soil lateral load. Nevertheless, the increase of monopile diameter, followed by the monopile length-to-diameter ratio (L/D) decrease, implies the contribution of additional resistant components such as distributed moment, base shear and moment.

This thesis examines the 3D mechanisms to be accounted for in the 1D FE modelling of the soil-monopile-superstructure seismic response in case of a single-phased, linear visco-elastic soil layer. Both 3D and 1D FE analyses are conducted with the FE software OpenSees. The 1D analyses are simulated in two consecutive steps: first a Site Response Analysis is performed, next the recorded displacements over the soil layer depth are applied to the spring supports and the dynamic interaction of the system is simulated. Three different monopiles are considered with L/D equal to 26, 9 and 5. Two superstructures are examined, which are modelled as Single-Degree-of-Freedom systems. Distributed translational springs are assigned to the slender monopile (L/D=26), while for the stubbier monopiles the contribution of distributed rotational springs is examined as well. Lastly, the effect of considering the base moment and shear is also examined.

The stiffness of the soil reaction curves is calibrated by applying a monotonic lateral load and moment at the pile head, in case of the translational and rotational springs, respectively. The spring stiffness values are assumed uniform along the monopile length. As a next step, the dynamic response of the calibrated 1D models is examined in steady-state conditions, under the action of mono-harmonic excitation, and compared to the 3D results. Ultimately, the seismic response of the 1D models is examined in case of two earthquake excitations with different frequency contents.

In case of the monopiles with L/D=9 and 5, it is concluded that the use of monotonically-calibrated distributed translational and rotational springs provides a good match between 3D and 1D regarding the monopile head and superstructure response under seismic loading. Nevertheless, these 1D FE models cannot predict the base moment, for which a base rotational spring should be employed. In case of the stubbier monopile, with L/D=5, the base shear seems to positively affect the moment profile as well. Lastly, regarding the monopile with L/D=26, the employment of translational springs alone seems sufficient for the accurate prediction of the seismic response; however, the hereby monotonically-calibrated distributed translational springs result in a mismatch between 3D and 1D.
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Offshore structures are often founded on mud-mat foundations which prevent the structure from settling excessively. Pressure differences are generated under mud-mats during removal from the seabed. These differences lead to a force that resists the uplift. A literature study on breakout resulted in a hypothesized distinction of four mechanisms that occur during the uplift of offshore shallow foundations. An experimental program was designed to study these mechanisms. Small-scale tests on clay and sand were executed. The effectiveness of selected mitigation measures was studied, as well as the influence of the measures on the mechanisms. Existing calculation methods that model suction or quantify the breakout force were evaluated. It is believed that the results from the experimental program led to a comprehensive list of parameters that should be included in a design method to model the total resisting force to uplift, taking the mechanisms into account. ... Read more

The offshore wind sector in Europe has grown significantly over the past 20 years. Following this surge in number of installations, the first generation of turbines is reaching its end-of-life (EOL) phase and thought needs to be put into end-of-life strategies. As the market for lifetime extension is still in its infancy and re- powering is economically unappealing, decommissioning is the most obvious solution. The tower, Rotor Nacelle Assembly (RNA) and electrical cables will most likely be removed by reverse-installation methods, as well as the Transition Piece (TP). Removal of the monopile foundation however, is considerably more diffi- cult due to the large embedded length of the pile in the soil. The common way of decommissioning such piles in industry (mostly the Oil & Gas (O&G) industry) is to cut them at or below the seabed and leave the remaining stumps in place. This research investigates more sustainable methods that remove the foundation completely, as this is more in line with the modern circular economy and regulations, i.e., OSPAR, are getting more stringent, focusing on complete removal of all offshore structures in the future. To remove the entire monopile foundation using only a blunt uplift force, tremendous amounts of force are required. It is thus desired to lower this extraction force by some technique to reduce soil resistance. This research investigates the possibility of reducing soil resistance surrounding monopiles by utilizing pitch and heave vessel motions. Such motions can induce an harmonic force on the monopile during the first mo- ments of lifting. The hypothesis is that low frequency oscillations of about 0.1 Hz can induce plastic strain accumulation of the soil, resulting in permanent displacement of the pile. This hypothesis is tested by con- structing two models, one describing vessel motion in MATLAB, and the other describing the pile-soil inter- action under cyclic loading, which is modelled in the open source software OpenSees. An extensive market study is performed to find a representative maximum set of dimensions of the first generation of monopiles installed offshore. Monopiles with a grouted connection to the transition piece are considered as first gener- ation monopiles, and it is these monopiles that are of interest to the study because they need to be removed the first. The vessel considered in this research is the Pioneering Spirit and the crane its Jacket Lift System (JLS), due its large lifting capacity. After delineating to the monopile size and the vessel, assumptions are made regarding the extraction process. A cranemaster is added to the system to avoid slack in the lines and a connection tool between the crane wire cables and the monopile is selected, assuming a rigid connection between the two. The results indicate that applying a low frequency cyclic load to a monopile can result in the accumulation of plastic strain of the soil and, consequently, to larger monopile displacements than in monotonic load cases of the same magnitude. In the simulations performed in this research, this effect, called cyclic degradation, is observed to be more intense in sands than in clays. It is observed that plastic strain is largest when large forces are applied to the model, hence it is desired to maximize the combination of tension and amplitude of the applied cyclic force. The effect of cyclic degradation however, is stronger if the forcing frequency is increased to a value near the resonance frequency of the pile-soil system. This value lies in the region of 4 to 8 Hz, depending on the force input characteristics because of the non-linear behavior of soils. Achieving fre- quency multiplication of the input force has been considered in both active and passive ways. It is concluded that only active frequency multiplication may deliver the needed cyclic degradation, since passive frequency multiplication diminishes the amplitude of the cyclic force, driving up the resonance frequency of the pile- soil system even further. The applicability of the pile-soil model, and thus these conclusions, is only valid during the first moments of lifting, when displacements are relatively small. The results obtained in this thesis are the result of a relatively conservative soil dynamics model and a non- conservative hydrodynamical model. The reliability of the results could be further improved if the two models are combined in a co-simulation, where at every time-step the pile displacement and vessel motion are deter- mined. Additionally, expanding the OpenSees model so that accurate representations of large pile displace- ments are included, will allow for calculation of longer simulations, where the pile is completely removed from the soil. The hydrodynamic model can be improved by using an entire sea-state as input. ... Read more