The installation of offshore monopiles using hydraulic impact hammers generates intense impulsive underwater noise, posing risks to marine life and challenges compliance with underwater noise regulations. As monopile sizes continue to increase, associated noise levels also rise , necessitating noise mitigation systems. This study investigates the performance of a double-walled steel noise mitigation screen (NMS) with an integrated air layer and bubble system, designed to reduce underwater noise emissions during pile driving.

A coupled numerical modeling approach was used, combining a finite element model for near-field sound generation with a semi-analytical approach for far-field sound propagation. The model was calibrated and validated against field measurements from an offshore installation campaign. While the unmitigated model showed good agreement with measured sound exposure levels, the numerical representation of the NMS underestimated the observed mitigation performance. While the numerical representation of the NMS captures the primary mitigation mechanism of the NMS rather than its full real-world behavior, it allows for comparing performance across varying soil conditions.

The influence of soil stratification and properties on NMS performance was assessed for uniform and layered sandy soils. Results indicate that soil conditions have a limited influence on unmitigated far-field noise levels (up to 2.2 dB variation) and on mitigated levels when the numerical representation of the NMS is included (up to 1.6 dB variation). However, soil properties significantly affect the distribution of acoustic energy between the seabed and the water column, as well as the amount of energy generated by the pile, thereby influencing the mitigation potential of near-field noise mitigation systems. Dense soils tend to increase absolute mitigation potential due to higher levels of waterborne energy generation, while layered soils facilitate more efficient propagation through upper layers and can promote re-radiation of energy back into the water column.

Due to uncertainties in the numerical representation of the NMS, conclusions regarding optimal soil conditions for its performance remain tentative. Overall, the study demonstrates that soil conditions play a key role in determining the effectiveness of near-field noise mitigation systems and highlights the need for improved modeling approaches to accurately predict NMS performance during offshore pile driving. ... Read more

The growing number and increasing size of Offshore Wind Turbines (OWTs) in Europe have raised significant challenges for monopile foundation design. As turbines become larger and are installed in deeper waters, structural demands and foundation requirements have also increased. Additionally, stricter noise regulations have been established, limiting the use of traditional impact driving techniques and further increasing the complexity of installation methods.

To address these challenges, vibratory methods like Gentle Driving of Piles (GDP) have recently gained traction. However, when powered by hydraulic shakers, they can experience dynamic coupling with the structure, which interferes with both the generation and control of the vibrations needed for effective driving.

To this end, a nonlinear model incorporating the dynamic interactions between the electrical, hydraulic, and structural subsystems has been developed. A parametric study was conducted to determine the minimum pressure required to operate and control the structure when utilizing a PID-based control system.

It was determined that the primary nonlinear dynamic behavior was due to the velocity-pressure coupling and its impact on the nonlinear flow. To address this, a controller was designed that effectively mitigated these effects. Specifically, it was found that the supply pressure could be reduced by up to 25\% while still maintaining effective sinusoidal displacement control. Additionally, the conditions under which synchronization between the shaker-structure could occur were identified, along with ways to avoid this phenomena, as synchronization can lead to system damage and a loss of control authority over the structure.

This study demonstrates the effectiveness of using a PID controller to reduce the coupling between subsystems, thereby enhancing the efficiency of vibratory pile driving methods, promoting their use in monopile installation. ... Read more

In the Netherlands there are many waterways where bridges or locks are situated within the navigation way of inland waterway vessels. To protect these civil structures from potential collision risks, protection structures are placed around them. In this research, a commonly used protection structure, the steel piled fender structure, is analyzed under impact events, followed by a comparison with the design methodology prescribed in the Dutch codes. In the Netherlands, design guidance is provided by the "Richtlijnen Ontwerp Kunstwerken", abbreviated as ROK. The ROK adopts a design formula for the required energy absorption capacity of fender structures from the EAU 2012. By applying the principle of work, the required energy absorption capacity can be converted into an equivalent static load that incorporates dynamic effects. This allows the soil-fender structure system to be designed using static analysis.

In this research, a triangular-shaped fender structure with fixed geometrical properties is examined. Additionally, the bow structure of a CEMT class IV ship is included to capture the interaction between the ship and the fender structure during impact events. The aim of this study is to obtain a transient response of the system, investigate the interaction between the ship, soil, and fender structure during dynamic impact events, and evaluate this against the design methodology prescribed in the Dutch codes.

To determine the transient response, two different models are developed. A simplified dynamic model based on linear beam theory is constructed. In this model, the problem is reduced to two-dimensional space, where the fender structure is modeled as a 2D frame partially supported by linear elastic foundations representing the surrounding soil. The ship's bow structure is simplified as a mass-spring system attached to the frame structure. Furthermore, a non-linear finite element model (NLFE) is developed in three-dimensional space. The NLFE model includes the full geometry of the fender structure of interest, with the ship's bow structure modeled up to the most frontal bulkhead, and the soil is represented by a series of non-linear discrete springs.

Before performing NLFE impact simulations that account for the full interaction between the ship, soil, and fender structure, an extensive sensitivity study was conducted on the individual models, showing expected behavior. Impact simulations performed during the sensitivity study were used to set up the impact configuration for the final head-on and oblique impact simulations, in which the ship, soil, and fender structure behave flexible simultaneously, allowing full interaction.

A comparison between the simplified model and the head-on impact simulation from the NLFE model revealed different peak responses. The response of the NLFE model was dominated by significant plastic deformations, resulting in larger peak displacements compared to the simplified model. However, during the initial stage of impact loading, dominated by elastic deformations, both models showed good agreement.

Finally, NLFE simulation results are compared to the design methodology prescribed in the Dutch codes. The energy absorption observed in the fender-soil system from the NLFE simulations is compared to the required energy absorption capacity according to the codes. For the head-on impact scenario, significant crushing of the ship's bow led to reduced energy absorption by the fender-soil system. In the oblique impact simulation, the NLFE model predicted higher energy absorption by the fender-soil system compared to the code requirements. There are numerous differences in modelling assumptions between the NLFE model and the method prescribed in the codes. Moreover, the NLFE model includes simplifications of the real-world problem that may lead to an overestimation of the absorbed energy by the fender-soil system. Therefore, extending the NLFE model by removing simplifying assumptions remains highly recommended.
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The study explores different system identification techniques that utilize machine learning, including the Restoring Force Surface (RFS) method and Sparse Identification of Nonlinear Dynamics (PySINDy).
These methods have potential to help uncover physical models for soil-pile interaction.
To test these methods , the research first applies them to well-known benchmark systems—simple mechanical models with known nonlinear behaviours. This ensures that the identification techniques work correctly before applying them to real pile-driving experiments.
The experimental data comes from lab-scale vibratory pile-driving tests using strain gauges and accelerometers. The study analyses how forces acting on the pile change over time, focusing on both the tip and shaft resistance. Various mathematical models are tested to see which best captures the nonlinear behaviour. ... Read more

Electromagnetic actuators are an interesting option for inducing vibrations in structures. However, their performance when attached to a flexible structure is relatively unknown. Therefore, this thesis aims to investigate the coupled dynamic behaviour of an electromagnetic actuator and a flexible structure. The goal is to see if the actuator can induce large displacements in the structure, in a controllable manner, and using a small input power.
Physical experiments were performed with an actuator placed on top of a flexible beam. In addition, a computer model was made that could simulate the system and predict its response. In the experiments, multiple input settings for the actuator were tested using frequency sweeps. Two different control settings were compared: the open-loop setting, which controls the current that is sent through the actuator, and the closed-loop setting, which controls the motion of the moving cylinder in the actuator. For both settings, an input signal is sent to the system. Respectively the current or the cylinder motion, relative to the tip of the beam, has to follow that input signal. The amplitude and frequency of the signal can be adjusted.
The experiments showed that there is no perfect input setting. Each setting has its advantages and disadvantages. Therefore, the best input setting to use depends on the situation. Using the open-loop setting at the resonance frequency of the system resulted in large beam tip displacements, a high effectiveness. However, this coincided with a low predictability of the displacements. On the other hand, the closed-loop setting gave a high predictability with a low effectiveness.
Looking at the efficiency of the system, the beam tip displacements normalised by the electrical input power, also did not give an ideal input setting. This was a result of the dynamic behaviour of the beam and the actuator counteracting each other a little. The beam vibrated most efficiently at its resonance frequency. However, this coincided with large relative displacements of the moving cylinder in the actuator. This generated a large Back EMF, causing the actuator to use significantly more electrical power. Thereby, the Back EMF cancels out the efficiency of the resonance.
In the closed-loop setting, the relative displacement is being controlled, and therefore it cannot increase to large values. This kills the resonance in the system, preventing the beam tip displacement from increasing.
The computer model was reasonably capable of predicting the response of the system. Due to a few inaccuracies in the model, it often slightly overestimated the displacements of the beam. However, the model showed patterns comparable to the results of the experiments, with resonance peaks at the same frequencies.
The model was also used to simulate the system response to closed-loop settings where either the beam tip displacement or the absolute motion of the cylinder was controlled, instead of the relative motion. These control settings were not possible in the physical experiments, due to limitations in the test setup. However, the model results were promising. Both of these settings could give large beam tip displacements, a high effectiveness, in combination with a high predictability. More research with physical experiments on these settings is recommended.
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This thesis presents the development and validation of a Semi-Analytical Finite Element (SAFE) model for conical shells. The motivation behind this research is to provide a computationally efficient and accurate numerical model for analysing conical shells. Starting with general equations of motion, kinematic equations, constitutive equations, and boundary conditions, the SAFE method is used to construct a numerical framework for conical shells.

Validation of the SAFE method is performed through comparative analyses with a detailed COMSOL model. The comparison focuses on the natural frequencies, mode shapes, and responses to both uniform and non-uniform harmonic loading. The results demonstrate that the SAFE method achieves accurate predictions in all analyses.

To demonstrate the robustness of the SAFE model, the analysis is extended to include a coupled conical-cylindrical shell system. Similar analyses are performed, and the model continues to provide accurate predictions.

These findings highlight the capability of the SAFE method in delivering both computationally efficient and accurate solutions for the analysis of conical shells. ... Read more

It is well-established that the effect of the soil significantly influences the overall behaviour of structures, particularly in the presence of vibrations. The majority of the developed approaches can be categorised into two groups: the direct approach and the substructure method. The first one is based on detailed modelling of the soil, foundation and structure using FEM and is not commonly used because it is computationally expensive and thus not feasible for standard engineering practice. The second method uses the dynamic stiffness matrix of the substructure to include it in the analysis of the general structure. Due to the impracticality of the first method, various methodologies have been investigated to better address SSI through the dynamic stiffness matrix, also known as impedance functions. They provide an approximate representation of the SSI that facilitates quicker analysis and sufficiently accurate results.

These approximation techniques typically involve intricate systems composed of lumped masses, dashpots, and springs. The characteristics of these elements vary based on soil and foundation types. For instance, a circular plate laying on homogeneous soil can be modelled as a mass, spring, dashpot single-degree-of-freedom system, where the coefficients of the elements represent the SSI of the disc. Similarly, a pile embedded in soil can be modelled with springs on the side that represent the effect of the friction between the structure and the soil. Some of the existing models for these two cases will be studied in this work.

Four mechanical analogues will be computed and compared for the first SSI problem: Lysmer's, Kausel's, and two of Wolf's models. These models define the mass, stiffness, and damping coefficients differently, but yield similar responses. Additionally, the results will be compared to a finite element (FE) model with the same soil and foundation characteristics to assess their applicability. For the second SSI problem, Novak's solution for piles under vertical vibrations will be studied and compared to the response of an FE model. The relevance of these SSI problems lies in the simplicity of reproducing the analogues to use as a reference. They are suitable benchmark problems to evaluate an alternative computational approach that is proposed in this work.

This work's objective is to determine the feasibility of analysing the response of these systems using a hybrid computational approach. This method involves generating responses using an FE model with a range of soil and foundation parameters as a first step. Secondly, these results are input into a Neural Network (NN). The NN is trained to reproduce the response of any system by knowing only the geometric characteristics of the foundation, and the shear modulus, Poisson's ratio, and density of the soil. The network's output is the frequency response function (FRF) of said system, which is subsequently used in an optimisation process. The output of this second process is the values for the system's mass, stiffness, and damping characteristics. Ultimately, these values can be used to construct the dynamic stiffness matrix necessary for solving the soil-structure interaction of a particular structure.

The central question addressed in this research is: What constitutes an efficient data-driven process for translating large soil-foundation datasets into simplified mechanical analogues that provide a reduced and accurate representation of the system? After assessing its applicability, it can be concluded that a hybrid two-step computational approach is an efficient and generic method for solving SSI problems. ... Read more

This thesis investigates the estimation of period lengthening (T˜/T) in structures using model updating, which assesses frequency shifts due to changes in boundary conditions, such as flexible bases. A 2D finite element (FE) model based on Timoshenko beam elements was developed and optimized through Sequential Least Squares Quadratic Programming (SLSQP) to align simulated dynamics with measured natural frequencies and mode shapes of a steel structure.

The research was divided into two phases. Phase one utilized synthetic modal data to validate the model updating process in an error-free environment, showing that the algorithm effectively resolves stiffness-to-mass ratios rather than individual properties. Period lengthening predictions proved more accurate and stable with stiff springs, while soft springs demonstrated higher sensitivity and discrepancy. Phase two applied real measurement data, yielding results consistent with synthetic data but revealing a “plateau” in the cost function, where optimal parameter determination was challenging due to model and measurement uncertainties.

The findings indicate that model updating is a feasible method for estimating period lengthening, especially for flexible foundations. However, results are sensitive to modeling and measurement uncertainties, necessitating careful evaluation to ensure optimization convergence and parameter precision. The study recommends further research to mitigate uncertainties and enhance the method’s applicability in high-rise building design. ... Read more

The offshore monopile decommissioning demand will become definite in the coming years. Our responsibility is to ensure the rights and duties of other legitimate uses by completely removing the ageing monopile from the seabed to continuously redeveloping offshore wind farms within the same location. The growing number of past, present, and future monopile installations opens up the challenges and opportunities to be responsible and lead the decommissioning market. With the goal of complete removal, a novel GDP technique can be the win-win solution for offshore wind operators and contractors to extract the monopiles completely from the seabed using torsional and axial vibration
This thesis seeks to understand the torque and normal force to safely clamp a monopile during a torsional vibration so that the monopile continuously slips over the soil. Gradual soil failure along the pile-soil interface's full depth due to the monopile's torsional motion is a possible theory to explain the failure mechanism. When an upper part of the pile successfully moves relative to the soil, kinetic friction occurs until the soil resistance is larger than the shearing at one point. If more shearing is added by adding more torque, more layers below will be broken while the upper part keeps sliding due to lower friction than static friction. While the linear elastic theory of solid and thin shell bodies is used within a 3D FE modelling in Ansys to couple the soil and pile, the clamping force due to the GDP shaker is decoupled from the analysis. Failure criterion is defined outside the simulation so that the gradual soil failure is done through several simulations assuming discrete soil layers.
The FE model is constructed and verified by analytical calculation through the semi-infinite cavity-pile-soil, wave reflection, and finite cavity-pile-soil-spring-dashpot problems. Several cases of gradual soil failure are simulated and show that the torque amplitudes form a distribution. Firstly, a probabilistic sense is proposed to interpret the torque amplitude and search for the optimum depth of the soil failure. Secondly, a convergence check is made with the help of an analytical shell-spring by considering more soil elements by virtue of good correlation of the shear stress between the analytical and FE model. It eventually suggests that a convergence of the torque amplitude can be achieved, which reinforces the theory of gradual soil failure. The interpretation suggests that the current GDP shaker is one step closer for a monopile extraction test with typical monopile dimensions that correspond to a typical 1 m diameter. A first approximation of the required torque and clamping force is then proposed to benefit the analytical model for larger diameters up to 6 m. ... Read more

The demand for wind energy has led to the need for larger offshore wind turbines, which require larger monopiles for support. Vibratory pile driving is a common technique for installing these monopiles, but current engineering models are designed for smaller diameter monopiles and may not be applicable to larger ones. Additionally, the vibratory action produced by counter-rotating eccentric masses in the hammer can cause unintended bending motions in the pile if there is a slight misalignment. This study aimed to investigate the effects of gear misalignment on the response of monopiles under vibratory action, including these imperfections.

To achieve this, a hammer model was created and coupled with a pile model and a simplified soil reaction. A parametric study was conducted to evaluate the pile response to the modeled vibratory hammer with imperfections over a range of specified variables. The transfer of force from the hammer to the pile and the range of driving frequencies that activate the isolation springs in the vibratory hammer were examined, as well as the effect of misalignments on the power consumption of the vibratory hammer and the bending displacements caused by these misalignments.

The study found that there was no significant impact on axial vibrations as a result of misalignment of rotating masses.

The force transfer ratio from hammer to pile is 0.5 to 0.7 for smaller diameter piles, and it is important to include force transfer ratios in simple engineering models during the initial design phase of pile drivability prediction models when the decision is made not to model the hammer.

The study also showed that modeling the hammer-pile-soil system with shell elements instead of 1D rod elements yields similar trends in the responses, but the absolute response values of large diameter piles with simple 1D rod elements leads to an underestimation of the response in the lower driving frequency range and an overestimation in the higher driving frequency range.

Even if as much as 50% of the eccentric masses are misaligned, the impact of bending vibrations on the power consumption of the vibratory hammer is insignificant. However, bending displacement can become substantial when 6% of the eccentric masses are misaligned. Smaller diameter piles are more susceptible to vibrations at lower frequency ranges, while larger diameters are impacted in higher frequency ranges.

Only large diameter monopiles are susceptible to axial vibrations near their natural frequencies, while all diameter piles are affected by bending vibrations near their natural frequencies.

Isolator springs are effectively activated in the region of higher driving frequencies (> 28 Hz) for large diameter monopiles. In the case of small diameter piles (< 3 m), the relative motion between suppressor housing and pile head remained above unity for all driving frequencies, meaning poor activation of the isolator springs.
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Pile tip damage due to impact with an object (boulder) during the driving process of monopiles is of main concern in the offshore wind sector. Numerical modelling could accurately describe this failure mechanism and used as a tool in practice but also for the development of appropriate guidelines. In this research project, dynamic Finite Element Analysis is conducted in order to achieve modelling the driving and impact process and properly capture the response of the monopile.

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Jack-up designers, owners and operators are expanding their activities into seismically active areas such as Japan and Taiwan. Further development of expertise in seismic analysis of jack-ups is required to improve safety and reduce costs. At present, a linear elastic foundation model is used in seismic assessments. Improved understanding of foundation behaviour can lead to a significant reduction in conservatism. In this thesis a brief comparison of available foundation models is made. The radiation damping model has been improved, which is found to have minor effect on jack-up response. To capture non-linear foundation behaviour, a hypoplastic macro-element has been implemented in seismic simulation software OpenSees. This results in reduced non-linear resonance and an amplitude-dependent resonance-shift. The modelled loads at critical locations in the structure consistently decrease more than 25\% as a result. Plastic displacements and hardening are found to be significant for highly non-linear soils and severe earthquakes. This thesis demonstrates significant conservatism in the linear foundation model. Implementation of the proposed hypoplastic macro-element can thus reduce costs and increase demonstrated capabilities of jack-ups significantly. ... Read more