The structural response and thus the service life of offshore wind turbines is significantly affected by their first natural frequency. However, field measurements from offshore wind farms show that the field response of monopile-founded wind turbines is stiffer than expected, suggesting that improved modelling could lead to optimized cost-efficient structures.

This discrepancy may be associated to conservative foundation modelling. The current industry-standard finite element (FE) approach assumes monopiles as wished-in-place (WIP), thus neglecting installation effects. The goal of this thesis is to calibrate a 3D FE model to better capture the lateral response of impact and vibratory-driven monopiles in predominantly sandy soils under monotonic loading, particularly in the small-strain range that governs the dynamic response of offshore wind turbines, by accounting for installation effects.

PLAXIS 3D is used to model the soil-structure interaction with Hardening Soil elastic-plastic constitutive model that can capture small-strain stiffness (HSsmall). The study has a twofold scope: namely, establishing an interpretation scheme for initial soil properties to model the WIP response and incorporating installation effects through a practical approach that captures the effects of the installation on the soil state, and consequently the lateral capacity, without explicitly simulating the pile installation.

The WIP models, validated against a number of field tests, show that the current modelling approach can accurately predict the lateral response of vibratory driven monopiles, while it underestimates the stiffness of impact driven ones. Therefore, this thesis proposes to artificially incorporate installation effects into the established WIP FE models for impact driven monopiles, by either imposing volumetric strains to the soil plug or by modifying the coefficient of lateral earth pressure at rest. Calibration of these installation parameters is performed against the global monopile response and the post-installation horizontal stress profiles. Both methods lead to increased horizontal stresses around the monopile and result in stiffer global response and improved agreement with the field data.

This thesis offers a comprehensive framework for modelling the lateral response of monotonically loaded monopiles, including installation effects, that could potentially be adopted by industry thanks to its simplicity, computational efficiency and reliance on commonly available data in offshore wind projects.
... Read more

The rapid expansion of offshore wind energy is increasingly pushing projects into seismically active regions, necessitating monopile foundation designs that account not only for typical environmental loads but also for seismic displacement demands. To accurately assess the dynamic response of an offshore wind turbine (OWT) and design its monopile foundation accordingly, it is essential to model the soil-monopile-superstructure interaction with high fidelity. Three-dimensional (3D) finite element (FE) analyses represent the most advanced numerical approach for capturing this complex behaviour. To address this need, this thesis develops a comprehensive FE framework in PLAXIS 3D that integrates free-field site response and soil-structure interaction (SSI) under both linear elastic and nonlinear elastoplastic soil to evaluate the seismic performance of monopile-supported OWTs.
A structured four-step approach is adopted. Step 1 performs one-dimensional (1D), two-dimensional (2D), and 3D site response analyses (SRA) in linear elastic soil, validating the numerical setup against analytical solutions. Step 2 introduces the pile and superstructure, modelled with linear elastic material, in the same medium to assess the dynamic characteristics and their response under steady-state monoharmonic excitation, allowing verification of the SSI model against benchmarks from the literature. To capture more realistic soil behaviour, the framework then incorporates the SANISAND-MS constitutive model, which accounts for nonlinear cyclic sand response, including strain accumulation and stiffness degradation. In Step 3, SRAs with displacement input motions of varying amplitude are performed under both broadband Ormsby and steady-state monoharmonic excitation to evaluate the influence of nonlinearity on free-field response. Finally, Step 4 couples the nonlinear soil with the structure to examine the fully integrated dynamic characteristics under nonlinear SSI conditions.
Overall, the analyses establish best practice boundary conditions and numerical setups for seismic SRA and SSI modelling in PLAXIS 3D for linear elastic soil (Steps 1 and 2), and quantify the transition from inertial to kinematic dominance in the i response of a flexible pile (Step 2). In the nonlinear domain, the results from Step 3 highlight the critical role of loading type in capturing key features of nonlinear free-field response. More specifically, with steady-state monoharmonic excitation of increasing amplitude, the transition from linear to nonlinear behaviour is captured, along with a shift of predominant frequencies to lower values compared to the linear elastic SRA. In contrast, when the same procedure is applied with the broadband impulse load Ormsby wavelet, post-impulse stiffening effects are revealed, expressed as an upshift in resonance frequency. In Step 4, the dynamic characteristics of the monopile-superstructure system are assessed under nonlinear SSI conditions. However, computational cost prevents the execution of full amplitude sweeps, underscoring the practical trade-off between accuracy and run time in high-fidelity 3D nonlinear SSI modelling. Instead, the 1995 Kobe earthquake record is employed to assess the system’s behaviour under real, multi-harmonic excitation. Altogether, the work demonstrates the potential of advanced 3D FE tools to enhance the seismic design of monopiles. ... Read more

Understanding the dynamic behaviour of soil is crucial for the design of geotechnical structures, particularly for offshore wind turbine foundations. This study evaluates the performance of a nonlinear numerical model in accurately capturing soil response under dynamic shear loading, comparing its simulated response to experimental data obtained from Resonant Column Tests (RCT). The research systematically investigates the role of higher-order odd harmonics in the numerical solution, assesses the differences between linear and nonlinear models, and validates the nonlinear model against experimental results.

The findings reveal that higher-order harmonics contribute negligibly to the numerical response, simplifying the model's implementation. Furthermore, the nonlinear model effectively captures the resonance shift observed in experimental data, outperforming the linear model, particularly at higher strain levels where soil softening effects become significant. While the model demonstrates a strong correlation with experimental results, discrepancies arise in high-strain scenarios, primarily due to limitations in damping representation. To address this, corrective approaches such as a damping correction factor and strain radius adjustments were explored, improving model accuracy but failing to eliminate errors entirely. The study highlights the necessity of incorporating a strain-dependent damping formulation to enhance predictive capabilities for high-strain soil behaviour.

The results confirm that nonlinear numerical modelling is a valuable tool for capturing essential soil dynamics, but further refinements in damping representation are required to improve alignment with experimental data. Future research should focus on advanced damping models to ensure greater accuracy in high-strain dynamic soil simulations.
... Read more

The renewable energy sector is experiencing rapid growth, with offshore wind energy gaining significant global importance. Designing bottom-fixed offshore wind turbines (OWTs) with monopile (MP) foundations in seismic-prone regions, particularly in coarse-grained soils, presents challenges due to the risk of soil liquefaction during earthquake excitations. Current design practices for soil-structure interaction modelling address seismic pore pressure effects by accounting for the net decrease in soil shear modulus resulting from the development of excess pore water pressure.
This thesis examines the cyclic contour diagrams framework (CDF) for its capacity to predict excess pore water pressure (EPP) build-up in coarse-grained soils under seismic conditions. The framework is assessed by employing PM4Sand in PLAXIS2D to produce the cyclic contour diagrams for a coarse-grained material. This material is then used to perform site response analysis (SRA) on a soil column in PLAXIS2D. Subsequently, a total stress SRA is conducted using DEEPSOIL V7.0 to obtain the equivalent cyclic stress ratio (CSR) and the number of cycles by employing a best-match soil material similar to the one utilized in the PLAXIS analysis. By analyzing the cyclic stress history obtained from the SRA, the equivalent number of cycles and cyclic stress ratios are applied to the material's cyclic contour diagrams. Finally, the predictive capabilities of the method to predict the EPP calculated in the PLAXIS model are assessed.
Two case studies illustrate the framework's practical application. The first case focuses on Akita Port, Japan, which experienced significant damage during the 1983 Nihonkai-Chubu earthquake. The CDF was able to accurately predict liquefaction severity, validating the method. The second case study involves an offshore wind farm in the Netherlands, examining how liquefaction impact monopile foundation designs. The study highlights the importance of incorporating soil degradation because of liquefaction into the design of offshore wind turbines.
The findings indicate that the CDF is a viable tool for evaluating liquefaction potential in coarse-grained soils under seismic loading in engineering practice.
... Read more

The transition to more sustainable energy has led to a growing demand for offshore wind energy, necessitating larger structures and heavier foundation piles. During pile installation this increases the risks of uncontrolled pile run, which can have fatal outcomes and project delays. Offshore superstructures predominantly rely on largediameter open-ended driven piles. Intermediate soils pose significant challenges in determining soil resistance during driving and pile run, with calculations typically performed using Static Soil Resistance to Driving (SRD) methods. The Alm & Hamre method is preferred for deep foundation piles in mixed soils due to its inclusion of friction fatigue.
The goal of this research with corresponding research objective is to improve pile run velocity and trajectory predictions for offshore open-ended pile installation in intermediate soils. This objective is reached through research on drainage state, identifying soils susceptible to a shift in this drainage state, and analyzing velocitydependent resistance for CPT and pile velocities. The findings are then incorporated into a modified SRD model. The results are compared to a case study using CPT and borehole data as input, and installation video’s and driving data as validation material.
Key findings indicate that with increased pile velocity the drainage state of several soils can shift towards the more undrained spectrum and therefore the soil will have a smaller soil resistance. These soils with a lower soil resistance during pile installation velocities then predicted include intermediate soils such as silt, sandy silt, and silty clay. Thin alternating layers of sand, clay, and silt are also likely to experience a shift in drainage state. Later silty sand is identified as a soil with a high possibility of being prone to such drainage state shifts.
The SRD method, incorporating velocity-dependent resistance, predicts pile run 31% more accurate than models without this consideration. By including velocity-dependent resistance drops, the model accounts for the changes in soil resistance that occur during pile run, leading to more accurate predictions compared to the standard SRD model. The model used in this research uses a single SRD update for velocity dependent resistance. However, in scenarios with large pile runs trajectories and high pile velocities, or when a substantial portion of the soil is prone to a drainage shift, performing a single update for velocity-dependent resistance will not result in a converged solution. As such for a correct solution, multiple iterations are necessary.
When the model predicts a deeper Self weight penetration depth than observed, the predictions for pile run initiation are not reliable. Given that pile run initiation can be very delicate, further research is needed for locations with CPT and borehole data directly beneath the pile. Additionally, incorporating hammering parameters, such as the added weight due to hammer momentum, should be explored to improve these predictions. ... Read more

In the current energy market, where increasing demand for renewable energy is met with larger turbines which require larger installation vessels, understanding operational limits of such vessels becomes invaluable. For the installation of wind turbine generators at great heights, a stable working platform is offered by jack-up (JU) vessels, which use legs to elevate the hull and thus crane of the vessel above the waves. The operation taken into consideration is the preloading process where load is increased on the legs to ensure enough foundation capacity for later operations such as lifting of wind turbine generators. Traditionally, vessels have only been positioned above flat seabeds, but with the increased demand in the wind sector, unfavorable locations with sloped seabeds also need to be considered.
In order to capture global behaviour of a JU on a sloped seabed, two modelling methods are combined to create a framework; soil-structure interaction and structural finite element model. This framework implements a sub structuring approach by cutting and simulating a single leg on a sloped seabed using soil-structure interaction modelling. The results of the first method are foundation stiffness values and reaction loads during preloading of the JU while the second method takes those results as input to calculate internal loads of the JU.
The results show that the proposed framework is able to fulfill the task of combining two models and obtain meaningful results that can inform the JU during preloading phase. Then with this framework, the results show that sloped seabed induces an extra set of moments and lateral loads due to the asymmetric seabed causing an eccentric reaction load. These moments are then redistributed through the JU to other legs and their footings.
These results imply that for a JU on a sloped seabed, a framework combining geotechnical and structural domains can be applied to show the increased load due to the sloped seabed. Additional to the results, a sensitivity study is implemented for a variation of soil type and soil-structure geometry, the latter providing a more realistic seabed slope and larger reaction loads. ... Read more

In response to the urgent need for sustainable energy sources to combat climate change, offshore wind power has emerged as a promising solution. However, the installation process of offshore wind turbines, particularly the driving of monopile foundations, presents challenges, notably concerning underwater noise pollution and its environmental impacts. This research studies the efficacy of an alternative approach to traditional installation methods: the vibratory pile driving, renowned for its minimized noise impact. It focuses on its effects on the long-term performance of monopiles under cyclic lateral loading, through numerical simulations. By addressing certain uncertainties, the aim of this work is to contribute to optimizing offshore wind turbine installation practices and ensuring the stability and performance of monopile foundations in challenging marine environments.

Two models are integrated and merged to address the previous objectives. The first model simulates the dynamic behaviour of the soil after vibratory installation effects. Meanwhile, the second model analyzes monopile response to lateral loading induced by environmental factors like wind and waves. The OpenSees software is employed for the computation of 3D finite element analyses, and the soil, represented as dry, initially dense, Karlsruhe fine sand, is modeled using the SANISAND constitutive model, which relies on the Critical State Soil Mechanics framework, to accurately capture stress and state-dependent behaviour. Only half of the monopile's embedment depth is evaluated, due to computational constraints.

Both the behaviour of the soil after the vibro-installation process and after the lateral loading are evaluated. Significant vertical and radial displacement occurs during pile driving, leading to settlement around the pile shaft and mudline as soil densify. Horizontal displacement patterns indicate an initial outward movement followed by lateral drawing-in towards the pile shaft, driven by soil compaction and rearrangement induced by installation vibrations. Notably, post-installation, there is a marked increase in relative density around the pile shaft, enhancing soil strength and friction, particularly near the pile tip. This densification, along with changes in mean effective stress, significantly affects soil behaviour and sets the stage for subsequent lateral loading.

After the lateral loading stage, the influence of installation on pile response becomes apparent. Post-installation soil conditions profoundly impact lateral displacement patterns, with vibro-installed piles exhibiting larger displacements during initial loading cycles compared to wished-in-place piles. Throughout lateral loading cycles, localized soil densification and remoulding further influence stiffness and displacement patterns. Notably, the relative density changes reflect these alterations, showing the intricate interplay between installation effects and lateral loading response. Overall, the results emphasize the necessity of considering installation processes in predicting pile behaviour accurately.

While this study provides valuable insights into the behaviour of piles in dry sand conditions, it also underscores several limitations that necessitate further research. Future investigations should address these limitations to provide more robust insights into the behaviour of offshore wind monopiles and inform more effective design and installation practices in the renewable energy sector. ... Read more

Offshore wind energy is a pivotal element in the renewable energy transition and achieving net-zero emissions by 2050. Most offshore wind turbines require the installation of a sturdy foundation called a monopile. The current method of installing monopiles offshore is known as piling, which involves driving large steel cylinders into the seabed with a hydraulic hammer. One of the major problems with this method is that it generates high levels of noise, which travel through the water and are harmful to marine life. Therefore, several European governments have introduced laws that regulate the amount of noise that may be emitted.
Multiple solutions have been developed to reduce the amount of noise that is generated but they are costly or negatively impact operational speed. Therefore, GBM Works, a startup in the offshore wind industry, is developing an innovative method for installing monopiles known as the Vibrojet®. The Vibrojet® combines both a Vibrohammer on top of the monopile with a jet at the bottom. The jet aims to fluidise the sand inside the monopile to reduce the friction on the inner shaft. This not only reduces the amount of noise emitted but also increases the installation speed of the monopile and may reduce the required pile dimensions.
This research aims to optimise the Vibrojet® performance during offshore installation by reducing shaft friction as much as possible while minimising jet flow. This approach maximises the installation speed of the monopile while reducing the capacity requirements for all parts of the Vibrojet® system. When installing a monopile while jetting, a soil skeleton forms (soil plug) in the middle of the pile while all sand particles near the inner shaft gets fluidised. The displacement of the surface of the soil plug has been assessed in this research to estimate the plug shape and optimise the performance of the Vibrojet®.
At Deltares a test setup of a soil container was constructed to imitated a 2D version of the inside of a monopile during Vibrojet® installation. The front of the container was made of see-though Perspex to enable analysis of the processes inside the pile. Inside the container holding 1,500 kg of sand, 0.3\% of the particles were ultraviolet (UV) coated to enable tracking. Visual software was written to track these particles with a camera, allowing for the measurement of flow velocities and visualise the surface of the plug shape.
It was discovered that the equation for plug surface displacement overestimated the displacement observed in the laboratory tests. By analysing the results from various installation settings, discrepancies in the equation were identified. The following improvements were recommended to enhance the accuracy of the equation:
• Addition of a seepage force term to the equation
• Addition of separate term for sedimentation effect
• Factor for the return flow of particles
• Influence of the slope angle on flow erosion
A relationship was discovered indicating that the optimal flow rate for Vibrojet® installations can be determined by the total volume of sand that needs to be fluidised. A model was proposed to optimise the performance of the Vibrojet® by assessing the displacement of the plug and utilising this relationship to calculate the connection between the flow rate and the volume of the fluidised zone.
... Read more

This thesis investigates the soil-structure interaction and corresponding stability of pneumatic caisson foundations compared to monopile foundations for offshore wind turbines. The monopile foundation has been the industry standard for years, yet pneumatic caisson foundations are being investigated as a possible alternative. This research applies finite element analysis with the software Plaxis 3D to explore the monotonic and cyclic behaviour of the two foundation types, analysing important properties including soil deformation, energy dissipation, and cyclic stability.

The results of the monotonic behaviour analysis show that caissons are experiencing non-linear soil behaviour at lower forces than monopiles. However, their elastic capacity is sufficient to support the environmental loads, resulting in lower displacements under these loads. Monopiles exhibit greater flexibility, energy dissipation, and a tendency to increase soil stiffness over cycles. At the same time, caissons maintain structural stability with small permanent deformation and low energy dissipation, as demonstrated by cyclic loading tests. Pneumatic caissons demonstrate potential as viable alternatives to monopiles, as they offer increased initial rigidity and reduced displacements. Additionally, the pneumatic caisson foundation can provide a solution in environmentally sensitive areas because the pneumatic caissons can be installed with low noise and vibration impacts on the environment. As the load increases to its maximum capacity, the caisson shows a more abrupt failure compared to the monopile. Beyond the load tipping point, where the caisson shows non-linear behaviour, it undergoes significantly more deformations as force increases. The monopile shows a more gradual increase in deformations with an increase in force. As a result, the monopile shows a more gradual failure behaviour.

Practical challenges in the caisson’s production, transportation, and installation need to be overcome. For the production of the big caisson structures, specialised production facilities are required with direct access to the sea. Due to the caisson dimensions used in this thesis, it is too heavy to float and must be transferred using large cranes and barges or semi-submersible vessels. Although this study did not look into it, dimension optimisation or the use of different materials to construct the caisson might save money on transport, particularly if the caisson can float. In order to install the caisson at the correct depth, water must be pumped into its hollow chamber to produce enough downward force to counteract buoyant forces and wall friction.

In summary, the pneumatic caisson foundation offers a viable alternative with advantages in terms of stiffness and lower displacements compared to monopiles. Pneumatic caissons are a promising foundation solution for offshore wind turbines. However, the economic feasibility of the pneumatic caisson method in the offshore environment remains to be examined. ... Read more

In the years to come, the Netherlands will face a substantial challenge as over 1,500 kilometers of aging quay walls and sheet pile walls approach the end of their technical lifespan. Infrastructure managers anticipate that the necessary replacements will necessitate investments amounting to billions of euros. Moreover, this task carries a significant environmental footprint, notably in terms of CO2 emissions. The construction work required for these replacements will also result in disruptions and reduced accessibility, inconveniencing users.
This study addresses two pivotal aspects. Firstly, it focuses on enhancing the design aspects of new structures and optimizing costs, with a specific focus exploring how these enhancements can ease the financial challenges faced by infrastructure managers. Secondly, it investigates the safety of existing structures and explores ways to maximize their loadbearing capacity while maintaining safety standards. The expected outcomes of this study promise improved design aspects, cost-efficiency, and enhanced safety measures.
Quay walls can fail due to various mechanisms. This research investigates three primary causes: yielding of soil, yielding of quay wall and anchor yielding. Quay walls illustrate the complexities of soil-structure interaction. To address this, models were developed in both Plaxis and D-Sheet Piling. D-Sheet Piling was the preferred choice due to its computational speed. The reliability analysis was conducted with Probabilistic Toolkit. Considering the calculation methods, First Order Reliability Method (FORM) was employed, emphasizing in efficient computational results in contrast to the Monte-Carlo approach.
In the first aspect, the partial factors were recalculated and compared them with the existing EC partial factor approach. To optimize the current design methodology, the retaining height of the structure was adjusted based on its reliability index. Additionally, the maximum anchor force required was re-evaluated for the structure. This procedure has been conducted for two scenarios, considering and not considering model uncertainty.
Furthermore, an analysis was conducted to understand how altering the retaining height can lead to reduced steel usage, subsequently impacting costs and CO2 emissions. In the second aspect, it was pursued to enhance the structure’s performance by introducing a factor "n" across four distinct scenarios: 1. Simultaneously increasing all loads. 2. Increasing the surcharge loads on the terrain. 3. Increasing the bollard load. 4. Raising the final excavation level in front of the quay wall. While this study aligns with the extensive body of research in the field of civil engineering, It seeks to offer a new and sustainable approach on understanding quay wall design, focusing specifically on the designers’ viewpoint. Through the exploration of innovative design frameworks and approaches, this research seeks to make a valuable contribution to the long-term sustainability of quay wall structures. It aims to redefine our approach to accessibility and safety in these crucial structures. The comprehensive investigations conducted throughout this study provide an enhanced comprehension of quay wall design, reliability, and the optimization of performance. ... Read more

In the past decade, suction caissons have emerged as a preferred offshore foundation solution for wind turbines due to their silent installation process and potential for recyclability. However, there has been growing speculation regarding the necessity of under base filling, which involves filling the gap between the top plate of the suction caisson and the seabed. Some experts have suggested that under certain conditions, this under base filling may not be required at all. Furthermore, concerns have been raised about the efficacy of under base filling in achieving full contact between the top plate and the seabed, as it has been observed that gaps may persist even after the filling is applied. Consequently, doubts have been cast on the overall need for under base filling. However, there is limited research focused on understanding the behavior of water plugs in the absence of under base filling, at different loading conditions ( Compression , tension , cyclic etc.). This knowledge gap motivates this thesis study, which aims to investigate the behavior of water plugs specifically in dense sand samples, as sand is considered more critical compared to clay in terms of its variability in drainage conditions that can influence the foundation’s performance. To achieve this, a series of centrifuge tests were conducted on suction caissons that were partially installed and some fully installed. The results of the experiments shed light on the role of under base filling in different loading scenarios. Under monotonic compressive loading at higher rates, it was observed that under base filling played no significant role in the load transfer . Both the caissons with and without under base filling exhibited similar load transfer mechanisms, indicating that filling the gap may not be necessary in such loading conditions. Additionally, under tension loading, it was found that under base filling had little to no effect on the development of tensile capacity. By expanding our understanding of the necessity and effectiveness of under base filling, this study contributes to the ongoing discussion surrounding suction caisson design and installation practices for offshore wind turbine foundations. ... Read more

This thesis focuses on the going-on-location (GoL) operation of jack-up vessels in the offshore wind energy industry. The GoL process involves the transition of a jack-up vessel from a free-floating state to a state where it’s elevated above the waterline and pinned to the seabed. The workability of these vessels is significantly affected by the impact loads experienced during the GoL process, particularly when interacting with stiff seabeds and non-negligible sea states. In current methodologies, certain parameters like wave height and wave period set the limitations to when GoL process can proceed. For safety reasons, the GoL process is halted if these conditions are exceeded. Instead of solely depending on external factors like wave height an wave period, the focus transitions towards how the vessel dynamics relate to the impact forces it encounters during the GoL. To address this limitation, a comprehensive framework has been developed that combines hydrodynamic, structural, and soil-spudcan interaction elements to evaluate impact forces during the GoL process. The framework offers a flexible and case-specific configuration. It allows for easy modifications, integration, and
replacement of components and input parameters. This case-specific arrangement is advantageous due to the wide range of jack-up vessels and environmental variations. In model implementations adhering to the framework, the jack-up vessel is represented as a multi-body structure, in contrast to the conventional rigid-body representation often employed. Within the multibody approach the spudcans, the legs, and the vessel are described by separate bodies each with its own properties. The primary focus of this research is on the dynamic soil-spudcan interaction process, which has not been extensively covered in existing standards. The soil-spudcan interaction model is to determine the instantaneous force acting on the spudcan as it contacts the seabed during GoL. By integrating elasto-plastic soil behavior into the
soil-spudcan interaction element, the model encompasses descriptions of soil resistance to spudcan penetration and lateral displacement, taking into account memory and potentially stateful characteristics. A simulation model, adhering to the framework, has been developed, integrating hydromechanical, structural,
and soil-spudcan interaction submodels within the Orcaflex environment. Three distinct simulation scenarios are examined: an undisturbed vessel (free-floating), a disturbed vessel (full GoL), and a pinned vessel (elevated
jack-up). The disturbed vessel scenario, which includes a full GoL process, has exhibited consistency in both undisturbed vessel simulations, where the vessel is the free-floating stage, and in pinned vessel simulations,
where the vessel is in the pinned stage. The impact phase is situated between these two boundary cases, and the framework effectively represents simulation models within its scope. In addition, simulations with varying sea states are performed for regular and irregular sea states. Simulations involving varying regular wave patterns suggest that the maximum downward spudcan velocity (DSV) is a critical parameter influencing the magnitude of impact forces on the spudcans. For irregular waves, the simulations indicate that the maximum impact forces are more closely related to the pinned vessel scenario, as
the maximum impact occurs towards the end of the impact phase.
In conclusion, this thesis has effectively described the behavior of jack-up vessels during the impact phase of the GoL process. For any model utilizing the framework, the GoL process can be simulated, and the results
can be analyzed to assess workability. Furthermore, the study proposes a potential correlation between vessel dynamics and maximal impact forces, a relationship that could potentially guide on-board decision-making
processes. The enhanced understanding of the interaction between the spudcan and the seabed, along with the comprehensive framework, contributes to improving the decision-making process for executing the GoL
operation of jack-up vessels in the offshore wind energy industry. ... Read more

Technology improvements and growing maturity of the offshore wind industry have resulted in significant cost reductions and rise in demand. More can be achieved by focusing on improving the understanding of key design areas of an offshore wind turbine (OWT). Since fatigue is one of the main design criteria for offshore structures and little is known about fatigue cumulative development in time under operating conditions, it formed the basis of this thesis. However, fatigue is a complex phenomenon that is dependent on numerous interlinked parameters, such as damping, loading type, and stiffness of support structure (SUS). For the purpose, data analysis was performed on measurements which were gathered over a period of four years from a monopile-supported OWT site. The measurement data included information about the environmental and operational conditions of the OWT; and strain, acceleration and inclinometer readings from its SUS. Use of different data acquisition systems during the measurement campaign inherently required significant efforts to synchronise and preprocess raw data to appropriate state for data analysis. Time-dependent lag was identified via computation of cross-correlation sequences between data segment pairings, and the lag was successfully corrected. With the use of a standardized rainflow cycle counting algorithm, strain-derived moment time series were converted to constant amplitude events, which could be used to derive damage equivalent bending moments, M-N curves and fatigue damage accumulation frequency spectra. Short-term damage equivalent bending moment (STEL) was revealed to have a linear dependency with turbulence intensity when the OWT was in the run-up operational state. Additionally, the fatigue damage was demonstrated to be higher for downstream turbines due to wake effects. Also, fatigue life consumption of OWT SUS increases above rated wind speed conditions despite decrease in load magnitude due to increase in SUS response frequency. The latter was confirmed with fatigue damage accumulation frequency spectra, which revealed that the importance of high frequency band (1 – 5 Hz) increases in conjunction with wind speed at above rated wind speed conditions. However, most of the overall fatigue damage is accumulated at the low frequency band, which associated with wind and wave loading, first SUS bending modes, 1P and 3P rotor harmonics. ... Read more