The increasing demand for renewable energy sources is driving a significant rise in the adoption of offshore wind energy. Offshore wind turbines are typically supported by large foundation piles driven into the seabed. The primary method of installation is by hydraulic impact hammers. However, this method generates excessive underwater noise among other drawbacks. Consequently, alternative techniques are being explored by both researchers and industry. One promising alternative is vibratory driving, which theoretically produces less underwater noise compared to impact driving. Nevertheless, there remains a substantial amount of energy transmitted into the water column, particularly from the higher harmonics of the driving frequency. While energy at the fundamental frequency is essential for efficient driving, energy associated with higher harmonics does not contribute to this efficiency and can significantly increase radiated underwater noise. To address this issue, this study proposes a mitigation strategy to block energy transfer at super-harmonic frequencies in the pile-water-soil system. To selectively target these frequencies without affecting the fundamental one, a mitigation approach utilizing locally-resonant metamaterials is proposed. This involves integrating a transition piece between the vibratory hammer and the pile, that incorporates periodically inserted multiple-degrees-of-freedom systems. By manipulating the natural frequencies of these periodic inclusions, the transition piece forms band-gaps at the relevant super-harmonics. Initial findings indicate that this design has the potential to effectively mitigate noise and vibration at targeted frequencies. Nonetheless, further investigations employing more sophisticated models are necessary to validate these outcomes.@en

Offshore wind energy holds significant promise as a solution in the energy transition. However, installing offshore pile foundations can generate substantial levels of underwater noise, posing potential risks to marine life. This paper examines the influence of asymmetric impact forces and pile inclination on producing underwater noise and seabed vibrations based on cases of a small- and large-diameter monopile. The study focuses on scenarios involving inclined and eccentric forces and tilted piles. The analysis reveals that non-symmetrical conditions significantly impact the sound pressure levels around the ring frequency of the pile due to various noise generation mechanisms. However, it is observed that the vertical component of the impact force predominantly contributes to the generation of underwater noise, primarily due to its considerably higher amplitude.

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Periodic fluid-solid layered media exhibit distinctive features that can be utilized in various engineering disciplines, such as selective transmission of guided waves, omnidirectional band-gaps and Fano resonances, depending on the spatial configuration and the material properties of the fluid and solid layers involved. This work utilizes the Thin-Layer Method (TLM) in the study of layered fluid-solid media, by extending the original normal modes-based method to acousto-elastic problems. By means of this development, the band-gap structure of these periodic systems can be analysed and their effect when incorporated in fluid or solid full-/half-spaces can be investigated seamlessly for different periodic arrangements. Conclusively, this study presents a framework to analyse these systems, serving as a basis for non-local continuum theories, as well as unveiling desirable dispersion characteristics for the design of metamaterials.

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In recent times, railway transportation has received increasing attention, particularly for its ability to operate entirely on electricity sourced from renewable sources. However, the growing demand for railway services has transformed previously acceptable issues into significant challenges, disrupting normal traffic operations. One such issue is ground-borne vibration especially in urban and inter-urban locations. This study explores the efficacy of a novel mitigation technique, termed a "metawedge," in reducing ground-borne vibration at the receiving end. The metawedge consists of a series of periodically arranged barriers that act as resonators. Unlike traditional metamaterials, each resonator within the metawedge possesses slightly different natural frequencies compared to its neighbours. With an appropriate choice of this variation, incoming Rayleigh (surface) waves are converted into body waves, redirecting energy deeper into the ground. Simulation results demonstrate that the metawedge can significantly diminish vibration levels with just a few resonators. Additionally, unlike conventional single trenches, which effectively mitigate vibrations only at specific angles of incoming waves (outside the critical cone), the metawedge remains efficient within this cone. While a theoretical proof-of-concept has been previously presented by the authors, this study makes a step forward by proposing a realizable design. Consequently, this work showcases the potential and feasibility of metamaterials to address present and future challenges in railway transportation.@en

With the growing demand for renewable energy, an increased number of offshore wind farms are planned to be constructed in the coming decades. The monopile is the main foundation of offshore wind turbines in shallow waters while the installation process itself takes place with large hydraulic impact hammers. This process is accompanied by significant underwater noise pollution which can hinder the life of mammals and fish. To protect the marine ecosystem, strict sound thresholds are imposed by regulators in many countries. Among the various noise mitigation systems available, the air-bubble curtain is the most widely applied one. While several models exist which aim to describe the mitigation performance of air-bubble curtains, they all assume a cylindrically symmetric wave field. However, it is well known that the performance of the air-bubble curtains can vary significantly in azimuth due to the inherent variations in the airflow circulation through the perforated pipes positioned on the seabed surface. This paper presents a new model which is based on a multi-physics approach and considers the three-dimensional behavior of the air-bubble curtain system. The complete model consists of three modules: (i) a hydrodynamic model for capturing the characteristics of bubble clouds in varying development phases through depth; (ii) an acoustic model for predicting the sound insertion loss of the air-bubble curtain; and (iii) a vibroacoustic model for the prediction of underwater noise from pile driving which is coupled to the acoustic model in (iii) through a three-dimensional boundary integral formulation. The boundary integral model is validated against a finite element model. The model allows for a comparison of various mitigation scenarios including the perfectly deployed air bubble curtain system, i.e.no azimuth-dependent field, and an imperfect system due to possible leakage in the bubbly sound barrier along the circumference of the hose.

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The installation of foundation piles for offshore wind turbines using traditional hydraulic impact hammers raises concerns about the impact of underwater noise on marine life. To address this issue, the offshore wind industry investigates the use of alternative driving techniques, such as vibratory pile installation, to reduce sound levels and expedite installation. This paper discusses a method for modelling underwater sound generated in vibratory piling and presents sound maps of broadband sound levels. The complete model comprises sub-models, including the generation of the source field and the propagation of the sound in range-dependent shallow water environments. The sound source model utilizes a non-linear three-dimensional pile–soil-water modelling framework tailored for vibratory pile installation in layered media, capturing the coupled pile-soil-water interaction at the source. The sound propagation model employed for generating sound maps is a normal mode model, designed to simulate propagation loss in range-dependent acousto-elastic half-spaces of varying bathymetry. The paper concludes with the theoretical case study of underwater noise emission from vibratory pile installation in the North Sea. Numerical simulations with the adopted modelling framework can be used by marine biologists to assess the environmental impact of underwater sound on marine species.@en

The Gentle Driving of Piles (GDP) is a new technology for vibratory (mono)pile installation that is based on simultaneous application of low-frequency/axial and high-frequency/torsional vibrations. In this paper, a numerical modelling framework, that has been developed and successfully applied to axial vibratory driving, is employed to study GDP. In that manner, the major driving mechanism of this method is comprehended on the basis of field observations and numerical analyses. As regards the numerical model, the pile is described as a thin cylindrical shell and the soil medium is treated as a linear elastic layered half-space. The pile-soil coupling is realized via a history-dependent frictional interface, that accounts for friction force degradation due to the accumulation of loading cycles at the soil material points. The redirection of the friction force vector due to the high-frequency torsion manifests as the main driving mechanism of GDP. Finally, the soil disturbance during installation is compared for the cases of GDP and axial vibratory driving, showcasing the dissimilar characteristics of the induced soil motion.

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Vibratory offshore pile driving offers a potential solution for reducing the underwater noise generated during the installation of foundation piles compared to using impact hammers. Existing noise prediction models are specifically tailored to impact pile driving scenarios. This paper introduces a novel methodology for underwater noise predictions during vibratory pile driving. A non-linear driveability model is utilised to derive realistic non-linear interface friction forces, which are then incorporated into a noise prediction model. The study emphasises the significance of integrating a driveability analysis, revealing substantial differences from traditional models that assume perfect contact between the pile and soil. The authors argue that the proposed model provides more realistic outcomes when considering smooth driving without refusal, in contrast to traditional models designed for impact piling. The results illustrate noticeable deviations in pressure levels and seabed vibrations between the linear and presented methods at the driving frequency and its superharmonics. Furthermore, the research demonstrates that the noise field is highly sensitive to variations in system dynamics and excitation spectrum during driving, using both small- and large-diameter monopiles as examples. This research contributes to developing more effective driving techniques to reduce underwater noise pollution and facilitate sustainable offshore wind turbine installations.

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Current offshore wind turbine installation and positioning methods require mechanical equipment attached on the lifted components and human intervention. The present paper studies the development of a contactless motion compensation technique by investigating a magnetically controlled pendulum. The technique involves the interaction of a magnetic pendulum with an electromagnetic actuator. Two control modes are considered: the imposition of a desired motion to the mass and the motion attenuation of a prescribed pivot excitation. The numerical model is validated and calibrated against experiments and demonstrates excellent predictive capabilities. The control exerted is effective for a broad range of excitation frequencies and amplitudes. Important parameters associated with the performance of the technique such as the separation distance of the magnets and the saturation of the controller are identified. The controllability regions for effective control depending on the characteristics of the excitation are derived. The force amplitude of the contactless actuator is comparable to currently-used active tugger line control systems, but with the additional advantage of both attractive and repulsive forces. The findings of this paper illuminate the path for the further development of a non-contact control technique which has the potential to increase the efficiency of offshore wind installations.

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The successful deployment of offshore wind turbines hinges on the installation process, particularly the temporary suspension of the turbine components during assembly. External factors or imbalances in control forces can induce vibrations, emphasizing the need for precise control, especially in the torsional mode, to ensure the delicate alignment required for bolted connections. This paper introduces a contactless technique to control the torsional vibrations of a rigid cylinder using electromagnetic interaction between two magnets, incorporating magnetically-imposed damping and active control algorithms. The magnetically-imposed dissipation is achieved by introducing nonlinear damping into the system, i.e. by controlling the orientation of the field exerted by the electromagnetic actuator. Leveraging the nonlinear coupling of the interaction between the magnets and the modification of the stable equilibrium position, the results show a satisfactory active control performance (low residual error and swift response). The key parameters for control efficiency are identified as the separation distance between the magnets, the fluctuation step of the actuator’s magnetic field, and the magnetically-induced stiffness relative to the inherent stiffness of the system. Consequently, the proposed method lays a promising foundation for a non-contact control technique, particularly valuable in offshore wind turbine installations.

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This paper studies the mechanism that leads to the reduction of frictional soil reaction forces during pile driving, termed friction fatigue. We focus on axial vibratory driving, an environmentally friendly monopile installation method, and examine two friction fatigue formulations, i.e. a penetration-based and a cyclic memory mechanism. Friction fatigue plays a pivotal role in pile drivability and post-installation bearing capacity for piles installed via axial vibratory driving. Through numerical analyses and validation against field data from onshore experiments, the efficacy of these memory mechanisms is assessed. The results reveal that the proposed cyclic memory mechanism provides consistently more accurate predictions than the corresponding penetration-based approach, offering a promising option for modelling friction fatigue in vibratory driving. This study advances our understanding of friction fatigue in the context of vibratory driving for offshore monopile installation, emphasizing the need for further numerical and experimental works in this topic.

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This paper presents a computationally efficient model for vibratory pile installation. A semi-analytical finite element (SAFE) model for thin cylindrical shells is derived to represent the pile. The linear dynamic response of the soil medium is described by means of Green's functions via the Thin-Layer Method (TLM) coupled with Perfectly Matched Layers (PMLs) to account for the underlying elastic half-space. Furthermore, the non-linear pile–soil interaction is addressed through a history-dependent frictional interface and a visco-elasto-plastic tip reaction model that can be characterized on the basis of standard geotechnical in-situ measurements. The solution to the non-linear dynamic pile–soil interaction problem is based on the sequential application of the Harmonic Balance Method (HBM). The constituent components of the model are first benchmarked against established numerical schemes. Subsequently, model predictions are compared with experimental data collected from field tests. It is demonstrated that the proposed model amalgamates rigorous theoretical elements and promising prediction capabilities in a computationally efficient framework, applicable to engineering practice.

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Underwater noise from offshore pile driving has raised significant concerns over its ecological impact on marine life. To protect the marine environment and maintain the sustainable development of wind energy, strict governmental regulations are imposed. Assessment and mitigation of underwater noise are usually required to ensure that sound levels stay within the noise thresholds. The air-bubble curtain system is one of the most widely applied noise mitigation techniques. This paper presents a multi-physics approach for modeling an air-bubble curtain system in application to offshore pile driving. The complete model consists of four modules: (i) a compressible flow model to account for the transport of compressed air from the offshore vessel to the perforated hose located in the seabed; (ii) a hydrodynamic model for capturing the characteristics of bubble clouds in varying development phases through depth; (iii) an acoustic model for predicting the sound insertion loss of the air-bubble curtain; and (iv) a vibroacoustic model for the prediction of underwater noise from pile driving which is coupled to the acoustic model in (iii) through a boundary integral formulation. The waterborne and soilborne noise transmission paths are examined separately, allowing us to explore the amount of energy channeled through the seabed and through the bubble curtain in the water column. A parametric study is performed to examine the optimal configuration of the double bubble curtain system for various soil conditions and pile configurations. Model predictions are compared with measured data. The model allows for a large number of simulations to examine different configurations of a single bubble curtain and a double big bubble curtain@en

Gentle Driving of Piles (GDP) is a new vibratory installation technology for tubular (mono)piles. It is characterized by the simultaneous application of low-frequency axial and high-frequency torsional vibrations, envisaged to achieve both high installation performance and reduced underwater noise emissions. The concept of GDP has been demonstrated experimentally in a medium-scale onshore field campaign, showcasing the potential of the method in terms of installation and post-installation performances. To further comprehend the mechanics of the GDP method, the driving process is studied by means of a novel pile–soil model; this framework has been recently developed and successfully applied to the problem of axial vibratory driving. In particular, the pile is treated as a thin cylindrical shell via a Semi-analytical Finite Element (SAFE) approach and a linear elastic layered soil half-space is considered via the Thin-Layer Method (TLM) coupled with Perfectly Matched Layers (PMLs). The pile–soil coupling is realized through a hereditary frictional interface and an elasto-plastic tip formulation, both characterized by standard geotechnical in-situ measurements. The comparison of numerical results with field data is favourable for drivability purposes, showcasing the potential of the numerical framework for the analysis of GDP. Conclusively, the mechanics of the installation process are deciphered and the redirection of the friction force vector – induced by high-frequency torsion – is identified as the main driving mechanism of GDP.

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Gentle Driving of Piles (GDP) is a new technology for the vibratory installation of tubular (mono)piles. Its founding principle is that both efficient installation and low noise emission can be achieved by applying to the pile a combination of axial and torsional vibrations. Preliminary development and demonstration of the proposed technology are the main objectives of the GDP research programme. To this end, onshore medium-scale tests in sand have been performed on piles installed using both impact and vibratory driving methods (including GDP). While the results of the installation tests are presented by Tsetas et al. (2023), this work focuses on the post-installation performance of GDP-driven piles under a sequence of slow/large-amplitude (cyclic) and faster/low-amplitude (dynamic) load parcels. The field data point out the influence of onshore unsaturated soil conditions, which result in complex cyclic pile stiffness trends due to the interplay of pile–soil gapping and soil's fabric changes. The pile stiffness under small-amplitude vibrations is strongly correlated with the previous response to large load cycles, and noticeably frequency-dependent for load cycles with a period lower than 1 s. Overall, the post-installation performance of GDP-driven piles appears to be satisfactory, which encourages further development and demonstration at full scale.

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For offshore wind turbines (OWTs), the monopile comprises the most common type of foundation and vibratory driving is one of the main techniques for monopile installation (and decommissioning). In practice, prior to pile installation, a pile driving analysis is performed to select the appropriate installation device and the relevant settings. However, pile penetration results from a complicated vibrator-pile-soil interaction and better understanding of the latter is necessary for an efficient installation process. During the course of installation, the interface and boundary conditions of the pile continuously alter due to the soil layering and the non-linearity of the soil reaction. In this paper, a set of experimental data from an onshore experimental campaign are employed in a numerical scheme to identify the pile strain field based on in vacuo modes of simpler yet related systems. By mapping the pile strain field onto physically-based shape functions, the evolution of the soil reaction during pile installation can be studied, in order to facilitate the back-analysis of driving records and, by extension, improve pile drivability and vibro-acoustics predictions.

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The present study introduces a modified version of PD Control for the case of a magnetically controlled pendulum. The response was observed in both experimental and numerical simulations taking into consideration the non-linearity posed by the system. The modified PD controller was compared to the simple counterpart for further concrete justification of its superiority. The results attained highlight the benefits of the modified PD control in all facets of control performance, namely the efficiency, the accuracy of the representation of the interaction, the sensitivity on alterations of the control gains as well as the prediction of the experimental response by the numerical simulation. Thus, the control method proposed can serve as a promising foundation for the further development of a non-contact position control technique for offshore wind turbine installation purposes. @en

In this paper, the characteristics of the induced ground motion are studied for two pile installation methods. Specifically, the classical axial vibratory driving is compared with the Gentle Driving of Piles (GDP) method, to investigate the effect of high-frequency torsional excitation in the soil response. For that purpose, a non-linear 3-D axisymmetric pile-soil interaction model - benchmarked against field data for both methods - is used to perform the numerical study. The friction redirection mechanism, that is mobilized due to the torsional excitation in GDP, leads to a different wavefield in the soil medium compared to axial vibro-driving. In the latter only SV-P wave motions are elicited, whereas torsion introduces SH wave motions as well. For the numerical study, the model is comprised by a thin cylindrical shell coupled with a linear elastic layered half-space through a history-dependent frictional interface. The Thin-Layer Method (TLM) coupled with Perfectly Matched Layers (PMLs) is employed to accurately describe the wave motion in the soil medium. Comparisons in terms of the peak particle velocities (PPVs) and soil particle trajectories showcase significant motion reduction due to redirection of the soil friction forces, which elicits high-frequency SH waves and reduces the SV-P wave motion.

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Gentle Driving of Piles (GDP) is a new technology for the vibratory installation of tubular (mono)piles. Its founding principle is that both efficient installation and low noise emission can be achieved by applying to the pile a combination of axial and torsional vibrations. Preliminary development and demonstration of the proposed technology are the main objectives of the GDP research programme. To this end, onshore medium-scale tests in sand have been performed on piles installed using both impact and vibratory driving methods (including GDP). After presenting the development of a purpose-built GDP driving device and the geotechnical characterisation of the site, this paper covers the execution of GDP installation tests. Focus is on the installation performance of GDP-driven piles, which is discussed with the aid of structural and ground monitoring data. The comparison between piling data associated with GDP and standard axial vibro-driving points out the potential of the proposed installation technology, particularly with regard to the beneficial effect of the torsional vibration component. The findings of this study encourage further development of the GDP method and its future extension to offshore full-scale conditions.

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The growing interest in offshore wind leads to an increasing number of wind farms planned to be constructed in the coming years. Installation of these piles often causes high underwater noise levels that harm aquatic life. State-of-the-art models have problems predicting the noise and seabed vibrations from vibratory pile driving. A significant reason for that is the modeling of the sediment and its interaction with the driven pile. In principle, linear vibroacoustic models assume perfect contact between pile and soil, i.e., no pile slip. In this study, this pile-soil interface condition is relaxed, and a slip condition is implemented that allows vertical motion of the pile relative to the soil. First, a model is developed which employs contact spring elements between the pile and the soil, allowing the former to move relative to the latter in the vertical direction. The developed model is then verified against a finite element software. Second, a parametric study is conducted to investigate the effect of the interface conditions on the emitted wave field. The results show that the noise generation mechanism depends strongly on the interface conditions. Third, this study concludes that models developed to predict noise emission from impact pile driving are not directly suitable for vibratory pile driving since the pile-soil interaction becomes essential for noise generation in the latter case.@en