22 research outputs found
Dynamic Response Of Two Interacting Extensible Barsin Frictional Contact
No full textIn this paper, a new model is developed to describe the nonlinear dynamics of twoaxially deformable bars sliding relative to each other in which the interaction is governed byfriction. The first bar is fixed at one end and is subjected to a distributed normal force perpen-dicular to its axis to activate friction at the common interface, while the second bar is allowed toslide relative to the fixed one. A semi-analytical solution method is developed in which only thenonlinear interaction is addressed numerically. The dynamic behaviour of the bars is expressedas a summation of vibration modes including the necessary rigid body mode to allow for thepermanent sliding of one bar relative to the other. This results in a computationally efficientscheme without compromising the accuracy of the solutions. The developed model can be usedin pile driveability studies. In this case the fixed bar resembles the soil column while the secondbar describes the dynamics of the driven pile.Offshore EngineeringDynamics of StructuresEngineering Structure
A unified modelling framework for vibratory pile driving methods
No full textThe ambitious goals towards the decarbonization of the global energy sector have amplified the demand for renewable energy resources. Amongst the renewables, offshore wind possesses a pivotal role in this endeavour, showcasing remarkable growth in recent years. However, this rapid expansion has been accompanied by a series of technical challenges. Foundation installation comprises one of the most critical phases in the construction of an offshore wind farm and engineering advancements in this topic are vital to accommodate this developmental pace. Bottom-fixed foundations are primarily used to support offshore wind turbines and amongst the available concepts, the monopile is the foremost one. The installation of these substructures is most commonly performed via impact hammering. Notwithstanding the robustness and efficacy of this technique, major environmental concerns have been raised due to the significant levels of underwater noise pollution during driving. In view of this alarming issue, alternative and sustainable pile installation techniques have been progressively drawing attention during the last decade and an increasing number of research projects focus on their investigation and development. At present, the offshore wind industry is increasingly adopting vibratory pile driving. The previous method has been successfully employed in onshore projects for decades, albeit its wider use in the offshore environment is hindered due to the incompleteness of available field observations. To boost the improvement of vibratory installation methods, a new technology has been recently proposed by the Delft University of Technology, namely the Gentle Driving of Piles (GDP). The preceding method aims to enhance the installation performance of vibratory driving for tubular (mono)piles and to reduce the associated noise emissions, via the simultaneous application of low-frequency/axial and high-frequency/torsional vibrations. Naturally, the shift to these technologies is accompanied by emerging research questions pertaining to pile installation, vibro-acoustic and post-installation performances. In this thesis, the development of an engineering-oriented modelling framework for axial vibratory driving and GDP is the primary objective, thereby focusing on the topic of sustainable monopile installation.Dynamics of Structure
Installation of large-diameter monopiles: Introducing wave dispersion and non-local soil reaction
No full textDuring the last decade the offshore wind industry grew ceaselessly and engineering challenges continuously arose in that area. Installation of foundation piles, known as monopiles, is one of the most critical phases in the construction of offshore wind farms. Prior to installation a drivability study is performed, by means of pile driving models. Since the latter have been developed for small-diameter piles, their applicability for the analysis of large-diameter monopiles is questionable. In this paper, a three-dimensional axisymmetric pile driving model with non-local soil reaction is presented. This new model aims to capture properly the propagation of elastic waves excited by impact piling and address non-local soil reaction. These effects are not addressed in the available approaches to predict drivability and are deemed critical for large-diameter monopiles. Predictions of the new model are compared to those of a one-dimensional model typically used nowadays. A numerical study is performed to showcase the disparities between the two models, stemming from the effect of wave dispersion and non-local soil reaction. The findings of this numerical study affirmed the significance of both mechanisms and the need for further developments in drivability modeling, notably for large-diameter monopiles.Dynamics of StructuresOffshore Engineerin
An alternating frequency-time harmonic balance method for fast-slow dynamical systems
No full textThe Alternating Frequency-Time (AFT) Harmonic Balance method has been widely applied in the analysis of non-linear mechanical systems under periodic excitation. Customarily, a periodic displacement is considered as ansatz in a harmonic balance analysis. In the present work, a deviation from the latter ansatz is realized and the periodicity is assumed in the velocity, leading to a linear term in the displacement of the system. The latter approach aims to facilitate the analysis of a certain class of systems, which are characterized by a fast periodic motion and a slow non-periodic motion. The motivation of this study originates in the area of offshore engineering and more specifically in the topic of monopile installation. During vibratory pile installation, the pile is forced into the soil under the combined action of a periodic excitation at the pile top and the self-weight of the pile and the vibratory device. As a result, the pile simultaneously penetrates into the soil as a rigid body (slow motion) and vibrates in the driving frequency and its super-harmonics both as a rigid and a flexible body (fast motion). In this study, the AFT harmonic balance with the ansatz of periodic velocity is implemented in different problem cases. A set of non-linear mechanical systems are analysed, ranging from a single-degree-of-freedom to a continuum, to showcase the potential application of the method and to verify its accuracy.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Dynamics of StructuresOffshore EngineeringEngineering Structure
A non-linear three-dimensional pile–soil model for vibratory pile installation in layered media
No full textThis 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.</p
Dynamic Pile Response During Vibratory Driving and Modal-Based Strain Field Mapping
No full textFor 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.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Dynamics of StructuresOffshore EngineeringEngineering Structure
The mechanics of the Gentle Driving of Piles
No full textGentle 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.Dynamics of StructuresOffshore EngineeringEngineering Structure
Correction to: A mode-matching method for the prediction of stick-slip relative motion of two elastic rods in frictional contact (Acta Mechanica, (2022), 233, 2, (753-773), 10.1007/s00707-021-03132-z)
No full text In the original publication, Eqs. (11) and (17) are published incorrectly, and this has been corrected as follows: (Formula presented.) The original article has been revised.Dynamics of StructuresOffshore EngineeringEngineering Structure
Ground motion reduction in vibratory pile driving via axial and torsional vibrations
No full textIn 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.Dynamics of StructuresOffshore EngineeringEngineering Structure
Axial vibratory driving installation effects on the monopile response to cyclic lateral loading
No full textIn 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.Applied Earth Sciences | Geo-Engineerin
