A. Tsetas
info
Please Note
<p>This page displays the records of the person named above and is not linked to a unique person identifier. This record may need to be merged to a profile.</p>
11 records found
1
The Influence of Bio-Inspired Soil Improvement on Wave Propagation in Soil
Mitigation of Train-Induced Vibrations
Master thesis
(2026)
-
O. Kirdik, Andrei Faragau, Sjoerd van Gaal, K.N. van Dalen, A. Tsetas, A. Tsouvalas, Eliam Vlijm
This thesis investigates the use of Microbially Induced Calcite Precipitation (MICP) as a sustainable soil improvement technique for mitigating railway-induced vibrations. Field measurements and numerical modeling were combined to characterize the stiffness distribution of an MICP-treated barrier and evaluate its dynamic response. Wave velocities obtained from hammer tests were used to estimate the spatial variation in stiffness, revealing significant longitudinal heterogeneity within the barrier. An inhomogeneous numerical model showed substantially better agreement with the measurements than a homogeneous representation. Building on these findings, the Acoustic Black Hole (ABH) concept was explored as a potential enhancement for vibration mitigation through a numerical parametric study. The results indicate that, under realistic soil stiffness contrasts achievable with MICP, the ABH effect is negligible and attenuation is primarily governed by conventional wave propagation mechanisms. Although the direct application of the ABH concept is not feasible under realistic conditions, the study demonstrates the potential of MICP for creating engineered solutions for future vibration mitigation strategies.
...
This thesis investigates the use of Microbially Induced Calcite Precipitation (MICP) as a sustainable soil improvement technique for mitigating railway-induced vibrations. Field measurements and numerical modeling were combined to characterize the stiffness distribution of an MICP-treated barrier and evaluate its dynamic response. Wave velocities obtained from hammer tests were used to estimate the spatial variation in stiffness, revealing significant longitudinal heterogeneity within the barrier. An inhomogeneous numerical model showed substantially better agreement with the measurements than a homogeneous representation. Building on these findings, the Acoustic Black Hole (ABH) concept was explored as a potential enhancement for vibration mitigation through a numerical parametric study. The results indicate that, under realistic soil stiffness contrasts achievable with MICP, the ABH effect is negligible and attenuation is primarily governed by conventional wave propagation mechanisms. Although the direct application of the ABH concept is not feasible under realistic conditions, the study demonstrates the potential of MICP for creating engineered solutions for future vibration mitigation strategies.
Seismic Response Modelling of a Pile-Supported Wharf with Geometric and Stiffness Irregularities
A Stepwise Evaluation of Seismic Analysis Methods
Pile-supported wharves located in seismic regions exhibit a combined structural and geotechnical response, governed by soil-structure interaction, pile flexibility, and the distribution of mass and lateral stiffness. In geometrically irregular wharves, variations in pile free-standing height, foundation conditions, or local structural stiffness can produce an eccentricity between the centre of mass and the centre of stiffness, so that the structure behaves as a Multi Degree of Freedom (MDoF) system rather than a regular, single-mode one. Such behaviour is difficult to capture reliably using simplified two-dimensional or linear elastic procedures.
This thesis investigates the seismic response of a geometrically irregular pile-supported wharf and examines how and when nonlinear time-history analysis (NLTHA) adds value over modal response spectrum analysis (MRSA) and modal pushover analysis (MPA). A stepwise modelling framework, progressing from modal analysis and MRSA, through MPA, to NLTHA, is applied to a realistic case study on the Albanian coast, where a sloping seabed produces significant variation in pile free-standing height. Soil-structure interaction is represented by a Beam-on-Nonlinear-Winkler-Foundation (BNWF) model with nonlinear p-y springs in a Delta arrangement, with springs spaced at 120 degrees, and structural nonlinearity concentrated in interacting P-M2-M3 plastic hinges. Site-specific seismic demand is derived from probabilistic seismic hazard analysis and one-dimensional site response analysis.
The main findings are as follows. (i) The modal analysis shows that the wharf is governed by several coupled translational and torsional modes rather than a single dominant mode, consistent with its combined soil, plan, and structural irregularity. (ii) For the Contingency Level Earthquake, MRSA predicts a centre of mass displacement demand of 0.122 m against 0.175 m from NLTHA, an apparent underestimation of 30%. Part of this discrepancy stems from the fitted response spectrum used to scale the ground motions, which does not match the intensity of the three most severe records retained for the NLTHA; when the higher design spectrum is used instead, the underestimation reduces to about 18%, showing that part of the gap is attributable to the seismic input rather than to the analysis method itself. (iii) MPA gives a closer global estimate of 0.164 m, within 6% of NLTHA, but this agreement conceals a static procedure whose modal hinge states become ambiguous once several modes contribute, and it does not extend to a reliable representation of how damage develops over time. (iv) The response is ultimately governed by post-yield torsional redistribution: as the shorter, stiffer piles yield, the lateral stiffness distribution changes and the torsional response evolves during the earthquake, a mechanism captured directly by NLTHA, approximated only partially by MPA, and largely missed by MRSA.
NLTHA is found to add the most value precisely where a preliminary modal analysis reveals significant multi-modal or torsional coupling. Since soil, plan, and structural irregularity are each expected, on the same MDoF reasoning, to be capable of producing such coupling on their own, the staged assessment framework developed here is not specific to the combined irregularity of the case study but is expected to generalise to wharves exhibiting any one of these irregularity types. A computationally efficient BNWF-based NLTHA framework is developed that preserves these mechanisms at a cost compatible with standard design practice.
For practice, it is recommended that modal analysis be used first to establish the elastic dynamic characteristics and screen for multi-modal or torsional coupling, proceeding to MRSA. NLTHA is needed where such coupling is significant; MPA is not recommended as a required intermediate step, given the disproportionate modelling effort it demands relative to the limited and ambiguous insight it provides into hinge behaviour. Given the well-known limitations of lumped spring representations of soil-structure interaction, particularly for irregular structures, validation of the BNWF model against a coupled continuum soil-structure model subjected directly to the seismic wavefield is recommended as a priority for future work, together with a larger ground motion set and, for sites with complex or liquefiable soil conditions, a more advanced soil modelling strategy than the one adopted here. ...
This thesis investigates the seismic response of a geometrically irregular pile-supported wharf and examines how and when nonlinear time-history analysis (NLTHA) adds value over modal response spectrum analysis (MRSA) and modal pushover analysis (MPA). A stepwise modelling framework, progressing from modal analysis and MRSA, through MPA, to NLTHA, is applied to a realistic case study on the Albanian coast, where a sloping seabed produces significant variation in pile free-standing height. Soil-structure interaction is represented by a Beam-on-Nonlinear-Winkler-Foundation (BNWF) model with nonlinear p-y springs in a Delta arrangement, with springs spaced at 120 degrees, and structural nonlinearity concentrated in interacting P-M2-M3 plastic hinges. Site-specific seismic demand is derived from probabilistic seismic hazard analysis and one-dimensional site response analysis.
The main findings are as follows. (i) The modal analysis shows that the wharf is governed by several coupled translational and torsional modes rather than a single dominant mode, consistent with its combined soil, plan, and structural irregularity. (ii) For the Contingency Level Earthquake, MRSA predicts a centre of mass displacement demand of 0.122 m against 0.175 m from NLTHA, an apparent underestimation of 30%. Part of this discrepancy stems from the fitted response spectrum used to scale the ground motions, which does not match the intensity of the three most severe records retained for the NLTHA; when the higher design spectrum is used instead, the underestimation reduces to about 18%, showing that part of the gap is attributable to the seismic input rather than to the analysis method itself. (iii) MPA gives a closer global estimate of 0.164 m, within 6% of NLTHA, but this agreement conceals a static procedure whose modal hinge states become ambiguous once several modes contribute, and it does not extend to a reliable representation of how damage develops over time. (iv) The response is ultimately governed by post-yield torsional redistribution: as the shorter, stiffer piles yield, the lateral stiffness distribution changes and the torsional response evolves during the earthquake, a mechanism captured directly by NLTHA, approximated only partially by MPA, and largely missed by MRSA.
NLTHA is found to add the most value precisely where a preliminary modal analysis reveals significant multi-modal or torsional coupling. Since soil, plan, and structural irregularity are each expected, on the same MDoF reasoning, to be capable of producing such coupling on their own, the staged assessment framework developed here is not specific to the combined irregularity of the case study but is expected to generalise to wharves exhibiting any one of these irregularity types. A computationally efficient BNWF-based NLTHA framework is developed that preserves these mechanisms at a cost compatible with standard design practice.
For practice, it is recommended that modal analysis be used first to establish the elastic dynamic characteristics and screen for multi-modal or torsional coupling, proceeding to MRSA. NLTHA is needed where such coupling is significant; MPA is not recommended as a required intermediate step, given the disproportionate modelling effort it demands relative to the limited and ambiguous insight it provides into hinge behaviour. Given the well-known limitations of lumped spring representations of soil-structure interaction, particularly for irregular structures, validation of the BNWF model against a coupled continuum soil-structure model subjected directly to the seismic wavefield is recommended as a priority for future work, together with a larger ground motion set and, for sites with complex or liquefiable soil conditions, a more advanced soil modelling strategy than the one adopted here. ...
Pile-supported wharves located in seismic regions exhibit a combined structural and geotechnical response, governed by soil-structure interaction, pile flexibility, and the distribution of mass and lateral stiffness. In geometrically irregular wharves, variations in pile free-standing height, foundation conditions, or local structural stiffness can produce an eccentricity between the centre of mass and the centre of stiffness, so that the structure behaves as a Multi Degree of Freedom (MDoF) system rather than a regular, single-mode one. Such behaviour is difficult to capture reliably using simplified two-dimensional or linear elastic procedures.
This thesis investigates the seismic response of a geometrically irregular pile-supported wharf and examines how and when nonlinear time-history analysis (NLTHA) adds value over modal response spectrum analysis (MRSA) and modal pushover analysis (MPA). A stepwise modelling framework, progressing from modal analysis and MRSA, through MPA, to NLTHA, is applied to a realistic case study on the Albanian coast, where a sloping seabed produces significant variation in pile free-standing height. Soil-structure interaction is represented by a Beam-on-Nonlinear-Winkler-Foundation (BNWF) model with nonlinear p-y springs in a Delta arrangement, with springs spaced at 120 degrees, and structural nonlinearity concentrated in interacting P-M2-M3 plastic hinges. Site-specific seismic demand is derived from probabilistic seismic hazard analysis and one-dimensional site response analysis.
The main findings are as follows. (i) The modal analysis shows that the wharf is governed by several coupled translational and torsional modes rather than a single dominant mode, consistent with its combined soil, plan, and structural irregularity. (ii) For the Contingency Level Earthquake, MRSA predicts a centre of mass displacement demand of 0.122 m against 0.175 m from NLTHA, an apparent underestimation of 30%. Part of this discrepancy stems from the fitted response spectrum used to scale the ground motions, which does not match the intensity of the three most severe records retained for the NLTHA; when the higher design spectrum is used instead, the underestimation reduces to about 18%, showing that part of the gap is attributable to the seismic input rather than to the analysis method itself. (iii) MPA gives a closer global estimate of 0.164 m, within 6% of NLTHA, but this agreement conceals a static procedure whose modal hinge states become ambiguous once several modes contribute, and it does not extend to a reliable representation of how damage develops over time. (iv) The response is ultimately governed by post-yield torsional redistribution: as the shorter, stiffer piles yield, the lateral stiffness distribution changes and the torsional response evolves during the earthquake, a mechanism captured directly by NLTHA, approximated only partially by MPA, and largely missed by MRSA.
NLTHA is found to add the most value precisely where a preliminary modal analysis reveals significant multi-modal or torsional coupling. Since soil, plan, and structural irregularity are each expected, on the same MDoF reasoning, to be capable of producing such coupling on their own, the staged assessment framework developed here is not specific to the combined irregularity of the case study but is expected to generalise to wharves exhibiting any one of these irregularity types. A computationally efficient BNWF-based NLTHA framework is developed that preserves these mechanisms at a cost compatible with standard design practice.
For practice, it is recommended that modal analysis be used first to establish the elastic dynamic characteristics and screen for multi-modal or torsional coupling, proceeding to MRSA. NLTHA is needed where such coupling is significant; MPA is not recommended as a required intermediate step, given the disproportionate modelling effort it demands relative to the limited and ambiguous insight it provides into hinge behaviour. Given the well-known limitations of lumped spring representations of soil-structure interaction, particularly for irregular structures, validation of the BNWF model against a coupled continuum soil-structure model subjected directly to the seismic wavefield is recommended as a priority for future work, together with a larger ground motion set and, for sites with complex or liquefiable soil conditions, a more advanced soil modelling strategy than the one adopted here.
This thesis investigates the seismic response of a geometrically irregular pile-supported wharf and examines how and when nonlinear time-history analysis (NLTHA) adds value over modal response spectrum analysis (MRSA) and modal pushover analysis (MPA). A stepwise modelling framework, progressing from modal analysis and MRSA, through MPA, to NLTHA, is applied to a realistic case study on the Albanian coast, where a sloping seabed produces significant variation in pile free-standing height. Soil-structure interaction is represented by a Beam-on-Nonlinear-Winkler-Foundation (BNWF) model with nonlinear p-y springs in a Delta arrangement, with springs spaced at 120 degrees, and structural nonlinearity concentrated in interacting P-M2-M3 plastic hinges. Site-specific seismic demand is derived from probabilistic seismic hazard analysis and one-dimensional site response analysis.
The main findings are as follows. (i) The modal analysis shows that the wharf is governed by several coupled translational and torsional modes rather than a single dominant mode, consistent with its combined soil, plan, and structural irregularity. (ii) For the Contingency Level Earthquake, MRSA predicts a centre of mass displacement demand of 0.122 m against 0.175 m from NLTHA, an apparent underestimation of 30%. Part of this discrepancy stems from the fitted response spectrum used to scale the ground motions, which does not match the intensity of the three most severe records retained for the NLTHA; when the higher design spectrum is used instead, the underestimation reduces to about 18%, showing that part of the gap is attributable to the seismic input rather than to the analysis method itself. (iii) MPA gives a closer global estimate of 0.164 m, within 6% of NLTHA, but this agreement conceals a static procedure whose modal hinge states become ambiguous once several modes contribute, and it does not extend to a reliable representation of how damage develops over time. (iv) The response is ultimately governed by post-yield torsional redistribution: as the shorter, stiffer piles yield, the lateral stiffness distribution changes and the torsional response evolves during the earthquake, a mechanism captured directly by NLTHA, approximated only partially by MPA, and largely missed by MRSA.
NLTHA is found to add the most value precisely where a preliminary modal analysis reveals significant multi-modal or torsional coupling. Since soil, plan, and structural irregularity are each expected, on the same MDoF reasoning, to be capable of producing such coupling on their own, the staged assessment framework developed here is not specific to the combined irregularity of the case study but is expected to generalise to wharves exhibiting any one of these irregularity types. A computationally efficient BNWF-based NLTHA framework is developed that preserves these mechanisms at a cost compatible with standard design practice.
For practice, it is recommended that modal analysis be used first to establish the elastic dynamic characteristics and screen for multi-modal or torsional coupling, proceeding to MRSA. NLTHA is needed where such coupling is significant; MPA is not recommended as a required intermediate step, given the disproportionate modelling effort it demands relative to the limited and ambiguous insight it provides into hinge behaviour. Given the well-known limitations of lumped spring representations of soil-structure interaction, particularly for irregular structures, validation of the BNWF model against a coupled continuum soil-structure model subjected directly to the seismic wavefield is recommended as a priority for future work, together with a larger ground motion set and, for sites with complex or liquefiable soil conditions, a more advanced soil modelling strategy than the one adopted here.
The Perfectly Matched Layer (PML) has become a powerful tool in computational underwater acoustics and elastodynamics. By employing complex coordinate stretching in the wavenumber–frequency domain, PMLs effectively attenuate outgoing waves from the physical domain and thereby provide an efficient means to truncate the computational domain. Although PMLs have been widely adopted in the finite element and finite difference communities, their use in semi-analytical solutions remains limited. A major challenge is that, when modal analysis is applied to the acousto-elastic domain with PML in a semi-analytical framework, the found modes are not orthogonal. This challenge formulates the main motivation of this research. On the other hand, the modes obtained from the discrete solution of the elastic layer with PML, based on the thin-layer method, do preserve orthogonality. Therefore, this thesis aims to understand the differences between the modal solutions of the semi-analytical and thin-layer methods in the elastic domains with PMLs, which may provide insights into the reasons why modes in the semi-analytical solution are not orthogonal to each other.
The main storyline of this thesis is developed through four cases with increasing system complexity. In the first case, the modes of the elastic domain are computed using both the semi-analytical approach and the thin-layer method (TLM), and the comparison demonstrates the equivalence of the two methods in the absence of PMLs. In the second case, the acoustic domain with PML is investigated using the semi-analytical approach, with emphasis on the polynomial order of the complex-stretching function. Mathematical derivations show that a zero-order polynomial induces discontinuities at the interface, leading to uneliminated boundary terms and perturbing modal orthogonality, while numerical results confirm that higher-order polynomials preserve the cross-orthogonality of modes, as well as the continuous slopes of the potential functions at the interface. In the third case, a quadratic complex-stretching function is employed, and the elastic domain with PML is analyzed using both approaches. The comparison reveals differences in eigenvalues and eigenvectors; finer TLM discretization yields increased matches between the two methods, but excessive discretization results in orthogonality violations. Finally, in the fourth case, the semi-analytical modes of the acousto-elastic domain with PML are studied. Propagating, evanescent, and Bérenger modes are identified, with cross-orthogonality preserved given sufficient integration points. Bérenger modes consistently arise in PML formulations and exhibit anomalous dispersion characteristics.
The main contribution of this thesis lies in revealing the influence of the polynomial order of the complex stretching function on the modes of the acoustic domain with PML. When a quadratic complex-stretching functions are employed, the numerical results suggest that the semi-analytical modes of the elastic or acousto-elastic domains with PML are orthogonal. Therefore, it is suggested that a positive value of polynomial order is recommended when computing normal modes of the acousto-elastic domain with PML. However, in the future, a systematic study on the influence of the polynomial order should be conducted for the elastic layer or acousto-elastic domain with PML.
Furthermore, the comparative study of modal solutions highlights the differences between the semi-analytical approach and the thin-layer method. The nature of modal solutions comes from the different formulations of the eigenvalue problem, leading to different eigenvalues and eigenmodes. For TLM, the over-discretization of the PML domain is not suggested due to the violated orthogonality, although the reasons behind that require further investigations.
Overall, this thesis advances the fundamental understanding of the modal basis of acoustic, elastic, and acousto-elastic layers with PML formulations, providing a foundation for future research in two main directions: (i) the study on modes of the acoustic layers coupled with multiple elastic layers with PML, which better represent realistic ocean environments with geological strata; and (ii) the computation of forced responses of structures in acousto-elastic layers with PMLs to model the pile-water-soil interactions using modal matching techniques. ...
The main storyline of this thesis is developed through four cases with increasing system complexity. In the first case, the modes of the elastic domain are computed using both the semi-analytical approach and the thin-layer method (TLM), and the comparison demonstrates the equivalence of the two methods in the absence of PMLs. In the second case, the acoustic domain with PML is investigated using the semi-analytical approach, with emphasis on the polynomial order of the complex-stretching function. Mathematical derivations show that a zero-order polynomial induces discontinuities at the interface, leading to uneliminated boundary terms and perturbing modal orthogonality, while numerical results confirm that higher-order polynomials preserve the cross-orthogonality of modes, as well as the continuous slopes of the potential functions at the interface. In the third case, a quadratic complex-stretching function is employed, and the elastic domain with PML is analyzed using both approaches. The comparison reveals differences in eigenvalues and eigenvectors; finer TLM discretization yields increased matches between the two methods, but excessive discretization results in orthogonality violations. Finally, in the fourth case, the semi-analytical modes of the acousto-elastic domain with PML are studied. Propagating, evanescent, and Bérenger modes are identified, with cross-orthogonality preserved given sufficient integration points. Bérenger modes consistently arise in PML formulations and exhibit anomalous dispersion characteristics.
The main contribution of this thesis lies in revealing the influence of the polynomial order of the complex stretching function on the modes of the acoustic domain with PML. When a quadratic complex-stretching functions are employed, the numerical results suggest that the semi-analytical modes of the elastic or acousto-elastic domains with PML are orthogonal. Therefore, it is suggested that a positive value of polynomial order is recommended when computing normal modes of the acousto-elastic domain with PML. However, in the future, a systematic study on the influence of the polynomial order should be conducted for the elastic layer or acousto-elastic domain with PML.
Furthermore, the comparative study of modal solutions highlights the differences between the semi-analytical approach and the thin-layer method. The nature of modal solutions comes from the different formulations of the eigenvalue problem, leading to different eigenvalues and eigenmodes. For TLM, the over-discretization of the PML domain is not suggested due to the violated orthogonality, although the reasons behind that require further investigations.
Overall, this thesis advances the fundamental understanding of the modal basis of acoustic, elastic, and acousto-elastic layers with PML formulations, providing a foundation for future research in two main directions: (i) the study on modes of the acoustic layers coupled with multiple elastic layers with PML, which better represent realistic ocean environments with geological strata; and (ii) the computation of forced responses of structures in acousto-elastic layers with PMLs to model the pile-water-soil interactions using modal matching techniques. ...
The Perfectly Matched Layer (PML) has become a powerful tool in computational underwater acoustics and elastodynamics. By employing complex coordinate stretching in the wavenumber–frequency domain, PMLs effectively attenuate outgoing waves from the physical domain and thereby provide an efficient means to truncate the computational domain. Although PMLs have been widely adopted in the finite element and finite difference communities, their use in semi-analytical solutions remains limited. A major challenge is that, when modal analysis is applied to the acousto-elastic domain with PML in a semi-analytical framework, the found modes are not orthogonal. This challenge formulates the main motivation of this research. On the other hand, the modes obtained from the discrete solution of the elastic layer with PML, based on the thin-layer method, do preserve orthogonality. Therefore, this thesis aims to understand the differences between the modal solutions of the semi-analytical and thin-layer methods in the elastic domains with PMLs, which may provide insights into the reasons why modes in the semi-analytical solution are not orthogonal to each other.
The main storyline of this thesis is developed through four cases with increasing system complexity. In the first case, the modes of the elastic domain are computed using both the semi-analytical approach and the thin-layer method (TLM), and the comparison demonstrates the equivalence of the two methods in the absence of PMLs. In the second case, the acoustic domain with PML is investigated using the semi-analytical approach, with emphasis on the polynomial order of the complex-stretching function. Mathematical derivations show that a zero-order polynomial induces discontinuities at the interface, leading to uneliminated boundary terms and perturbing modal orthogonality, while numerical results confirm that higher-order polynomials preserve the cross-orthogonality of modes, as well as the continuous slopes of the potential functions at the interface. In the third case, a quadratic complex-stretching function is employed, and the elastic domain with PML is analyzed using both approaches. The comparison reveals differences in eigenvalues and eigenvectors; finer TLM discretization yields increased matches between the two methods, but excessive discretization results in orthogonality violations. Finally, in the fourth case, the semi-analytical modes of the acousto-elastic domain with PML are studied. Propagating, evanescent, and Bérenger modes are identified, with cross-orthogonality preserved given sufficient integration points. Bérenger modes consistently arise in PML formulations and exhibit anomalous dispersion characteristics.
The main contribution of this thesis lies in revealing the influence of the polynomial order of the complex stretching function on the modes of the acoustic domain with PML. When a quadratic complex-stretching functions are employed, the numerical results suggest that the semi-analytical modes of the elastic or acousto-elastic domains with PML are orthogonal. Therefore, it is suggested that a positive value of polynomial order is recommended when computing normal modes of the acousto-elastic domain with PML. However, in the future, a systematic study on the influence of the polynomial order should be conducted for the elastic layer or acousto-elastic domain with PML.
Furthermore, the comparative study of modal solutions highlights the differences between the semi-analytical approach and the thin-layer method. The nature of modal solutions comes from the different formulations of the eigenvalue problem, leading to different eigenvalues and eigenmodes. For TLM, the over-discretization of the PML domain is not suggested due to the violated orthogonality, although the reasons behind that require further investigations.
Overall, this thesis advances the fundamental understanding of the modal basis of acoustic, elastic, and acousto-elastic layers with PML formulations, providing a foundation for future research in two main directions: (i) the study on modes of the acoustic layers coupled with multiple elastic layers with PML, which better represent realistic ocean environments with geological strata; and (ii) the computation of forced responses of structures in acousto-elastic layers with PMLs to model the pile-water-soil interactions using modal matching techniques.
The main storyline of this thesis is developed through four cases with increasing system complexity. In the first case, the modes of the elastic domain are computed using both the semi-analytical approach and the thin-layer method (TLM), and the comparison demonstrates the equivalence of the two methods in the absence of PMLs. In the second case, the acoustic domain with PML is investigated using the semi-analytical approach, with emphasis on the polynomial order of the complex-stretching function. Mathematical derivations show that a zero-order polynomial induces discontinuities at the interface, leading to uneliminated boundary terms and perturbing modal orthogonality, while numerical results confirm that higher-order polynomials preserve the cross-orthogonality of modes, as well as the continuous slopes of the potential functions at the interface. In the third case, a quadratic complex-stretching function is employed, and the elastic domain with PML is analyzed using both approaches. The comparison reveals differences in eigenvalues and eigenvectors; finer TLM discretization yields increased matches between the two methods, but excessive discretization results in orthogonality violations. Finally, in the fourth case, the semi-analytical modes of the acousto-elastic domain with PML are studied. Propagating, evanescent, and Bérenger modes are identified, with cross-orthogonality preserved given sufficient integration points. Bérenger modes consistently arise in PML formulations and exhibit anomalous dispersion characteristics.
The main contribution of this thesis lies in revealing the influence of the polynomial order of the complex stretching function on the modes of the acoustic domain with PML. When a quadratic complex-stretching functions are employed, the numerical results suggest that the semi-analytical modes of the elastic or acousto-elastic domains with PML are orthogonal. Therefore, it is suggested that a positive value of polynomial order is recommended when computing normal modes of the acousto-elastic domain with PML. However, in the future, a systematic study on the influence of the polynomial order should be conducted for the elastic layer or acousto-elastic domain with PML.
Furthermore, the comparative study of modal solutions highlights the differences between the semi-analytical approach and the thin-layer method. The nature of modal solutions comes from the different formulations of the eigenvalue problem, leading to different eigenvalues and eigenmodes. For TLM, the over-discretization of the PML domain is not suggested due to the violated orthogonality, although the reasons behind that require further investigations.
Overall, this thesis advances the fundamental understanding of the modal basis of acoustic, elastic, and acousto-elastic layers with PML formulations, providing a foundation for future research in two main directions: (i) the study on modes of the acoustic layers coupled with multiple elastic layers with PML, which better represent realistic ocean environments with geological strata; and (ii) the computation of forced responses of structures in acousto-elastic layers with PMLs to model the pile-water-soil interactions using modal matching techniques.
Vibratory Driveability Analysis of Offshore Monopiles
A Study on Soil Resistance Models Using GRLWEAP and SIMOX Field Data
Master thesis
(2025)
-
B.N. Posma, E. Kementzetzidis, A. Metrikine, A. Tsouvalas, A. Tsetas, S.I. Moghadam, N Buket Yenigül
Investigation of Response Amplification and Dynamic Stability of a Hyperloop System
Analysing the Interaction of the Vehicle with a Tubular Shell
Master thesis
(2024)
-
L. Huber, K.N. van Dalen, Andrei Faragau, A. Tsetas, J. Paul, J.S. Hoving, Sascha Lamme
Hyperloop is a high-speed transportation mode that operates through magnetic levitation of so-called pods which are then transported through a tunnel in which a low-pressure environment is created. Due to reduced air resistance and friction, high velocities are achievable. This thesis addresses the behaviour of such a system operating at high velocities, focusing mainly on dynamic stability. The novelty of this study lies hereby in the implementation of a more realistic guideway model in the analysis. This was achieved by modelling the Hyperloop tube as an infinitely long cylindrical shell on a viscoelastic foundation from which the vehicle is suspended through an electromagnetic force, including an active control system.
This study aims to evaluate the system’s stability and its sensitivity to changes in the model by using mainly analytical methods which are supplemented by numerical computations where needed. There are two fundamentally different instability mechanisms studied, namely the electromagnetic instability caused by the inherent unstable nature of the suspension system and the wave-induced instability which originates from the energy feedback of radiated anomalous Doppler waves excited by the vehicle moving at large velocities. The research is motivated by two central questions: (1) What is the influence on the steady-state response when employing the more realistic guideway model? (2) What is the influence on the stability when employing such a model?
To address the first research question, only the steady-state of the guideway is analysed. Hereby, the electromagnetic force as well as the vehicle are disregarded and replaced by a constant moving load. To solve the system, the governing equations are projected on circumferential modes and transformed from space-time to the Laplace-wavenumber domain. This study analyses various scenarios based on their dispersion curves and/or steady-state response compared to a reference case.
For the second research question, the study analyses stability phenomena whereby the response of the guideway is expressed using a convolution of its Green’s function and the unknown electromagnetic force. Linearising the system around its steady-state equilibrium allows a computation and analysis of its eigenvalues which describe its stability. To complement and verify the analysis of the linearised system, a non-linear transient model is solved by using a numerical time-stepping algorithm.
The study highlights that the cylindrical shell model significantly affects the critical velocity, compared to an Euler Bernoulli beam model. Analysing the vehicle-structure interaction through the linearised stability analysis reveals stability concerns primarily driven by wave-induced instability when operating at supercritical velocities. In the subcritical regime, instability is only caused by the electromagnetic suspension, which can be overcome when choosing the right control parameters. To reduce the risk of instability it is advisable to choose the structural dimension such that the system operates at subcritical velocities to avoid the interaction between the two instability mechanisms.
This research advances the understanding of Hyperloop dynamics and its stability, providing a foundation for future studies and practical implementations aimed at developing the novel transportation mode. ...
This study aims to evaluate the system’s stability and its sensitivity to changes in the model by using mainly analytical methods which are supplemented by numerical computations where needed. There are two fundamentally different instability mechanisms studied, namely the electromagnetic instability caused by the inherent unstable nature of the suspension system and the wave-induced instability which originates from the energy feedback of radiated anomalous Doppler waves excited by the vehicle moving at large velocities. The research is motivated by two central questions: (1) What is the influence on the steady-state response when employing the more realistic guideway model? (2) What is the influence on the stability when employing such a model?
To address the first research question, only the steady-state of the guideway is analysed. Hereby, the electromagnetic force as well as the vehicle are disregarded and replaced by a constant moving load. To solve the system, the governing equations are projected on circumferential modes and transformed from space-time to the Laplace-wavenumber domain. This study analyses various scenarios based on their dispersion curves and/or steady-state response compared to a reference case.
For the second research question, the study analyses stability phenomena whereby the response of the guideway is expressed using a convolution of its Green’s function and the unknown electromagnetic force. Linearising the system around its steady-state equilibrium allows a computation and analysis of its eigenvalues which describe its stability. To complement and verify the analysis of the linearised system, a non-linear transient model is solved by using a numerical time-stepping algorithm.
The study highlights that the cylindrical shell model significantly affects the critical velocity, compared to an Euler Bernoulli beam model. Analysing the vehicle-structure interaction through the linearised stability analysis reveals stability concerns primarily driven by wave-induced instability when operating at supercritical velocities. In the subcritical regime, instability is only caused by the electromagnetic suspension, which can be overcome when choosing the right control parameters. To reduce the risk of instability it is advisable to choose the structural dimension such that the system operates at subcritical velocities to avoid the interaction between the two instability mechanisms.
This research advances the understanding of Hyperloop dynamics and its stability, providing a foundation for future studies and practical implementations aimed at developing the novel transportation mode. ...
Hyperloop is a high-speed transportation mode that operates through magnetic levitation of so-called pods which are then transported through a tunnel in which a low-pressure environment is created. Due to reduced air resistance and friction, high velocities are achievable. This thesis addresses the behaviour of such a system operating at high velocities, focusing mainly on dynamic stability. The novelty of this study lies hereby in the implementation of a more realistic guideway model in the analysis. This was achieved by modelling the Hyperloop tube as an infinitely long cylindrical shell on a viscoelastic foundation from which the vehicle is suspended through an electromagnetic force, including an active control system.
This study aims to evaluate the system’s stability and its sensitivity to changes in the model by using mainly analytical methods which are supplemented by numerical computations where needed. There are two fundamentally different instability mechanisms studied, namely the electromagnetic instability caused by the inherent unstable nature of the suspension system and the wave-induced instability which originates from the energy feedback of radiated anomalous Doppler waves excited by the vehicle moving at large velocities. The research is motivated by two central questions: (1) What is the influence on the steady-state response when employing the more realistic guideway model? (2) What is the influence on the stability when employing such a model?
To address the first research question, only the steady-state of the guideway is analysed. Hereby, the electromagnetic force as well as the vehicle are disregarded and replaced by a constant moving load. To solve the system, the governing equations are projected on circumferential modes and transformed from space-time to the Laplace-wavenumber domain. This study analyses various scenarios based on their dispersion curves and/or steady-state response compared to a reference case.
For the second research question, the study analyses stability phenomena whereby the response of the guideway is expressed using a convolution of its Green’s function and the unknown electromagnetic force. Linearising the system around its steady-state equilibrium allows a computation and analysis of its eigenvalues which describe its stability. To complement and verify the analysis of the linearised system, a non-linear transient model is solved by using a numerical time-stepping algorithm.
The study highlights that the cylindrical shell model significantly affects the critical velocity, compared to an Euler Bernoulli beam model. Analysing the vehicle-structure interaction through the linearised stability analysis reveals stability concerns primarily driven by wave-induced instability when operating at supercritical velocities. In the subcritical regime, instability is only caused by the electromagnetic suspension, which can be overcome when choosing the right control parameters. To reduce the risk of instability it is advisable to choose the structural dimension such that the system operates at subcritical velocities to avoid the interaction between the two instability mechanisms.
This research advances the understanding of Hyperloop dynamics and its stability, providing a foundation for future studies and practical implementations aimed at developing the novel transportation mode.
This study aims to evaluate the system’s stability and its sensitivity to changes in the model by using mainly analytical methods which are supplemented by numerical computations where needed. There are two fundamentally different instability mechanisms studied, namely the electromagnetic instability caused by the inherent unstable nature of the suspension system and the wave-induced instability which originates from the energy feedback of radiated anomalous Doppler waves excited by the vehicle moving at large velocities. The research is motivated by two central questions: (1) What is the influence on the steady-state response when employing the more realistic guideway model? (2) What is the influence on the stability when employing such a model?
To address the first research question, only the steady-state of the guideway is analysed. Hereby, the electromagnetic force as well as the vehicle are disregarded and replaced by a constant moving load. To solve the system, the governing equations are projected on circumferential modes and transformed from space-time to the Laplace-wavenumber domain. This study analyses various scenarios based on their dispersion curves and/or steady-state response compared to a reference case.
For the second research question, the study analyses stability phenomena whereby the response of the guideway is expressed using a convolution of its Green’s function and the unknown electromagnetic force. Linearising the system around its steady-state equilibrium allows a computation and analysis of its eigenvalues which describe its stability. To complement and verify the analysis of the linearised system, a non-linear transient model is solved by using a numerical time-stepping algorithm.
The study highlights that the cylindrical shell model significantly affects the critical velocity, compared to an Euler Bernoulli beam model. Analysing the vehicle-structure interaction through the linearised stability analysis reveals stability concerns primarily driven by wave-induced instability when operating at supercritical velocities. In the subcritical regime, instability is only caused by the electromagnetic suspension, which can be overcome when choosing the right control parameters. To reduce the risk of instability it is advisable to choose the structural dimension such that the system operates at subcritical velocities to avoid the interaction between the two instability mechanisms.
This research advances the understanding of Hyperloop dynamics and its stability, providing a foundation for future studies and practical implementations aimed at developing the novel transportation mode.
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. ...
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. ...
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.
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.
Master thesis
(2024)
-
M. Diez Amon, A. Tsouvalas, A. Tsetas, J.O. (Oriol) Colomes Gene, P. Robbert I. Onwuachu
Pile run refers to the phenomenon that occurs during the installation of pile and monopile foundations in soils with low bearing capacity, where soil resistance is insufficient for the controlled driving of the monopile. The increasing frequency and associated risks of such occurrences in offshore installations underscore the necessity for effective pile run mitigation strategies.
This thesis investigates the feasibility of utilizing a mitigation structure to reduce pile run during the impact driving of monopile foundations. The primary objective is to model and analyze the dynamics of the hammer-pile-soil system under impact driving and pile run, with a focus on the impact of a mitigation tool in monopile response.
The research adopts a structured methodology, beginning with a qualitative study to identify effective mitigation measures. A 1-D finite element (FE) model is developed to simulate pile run, while a 2-D FE multiphysics model characterizes the fluid-structure interaction (FSI). The study explores the effects of various geometric parameters of the mitigation structure on forces and stresses experienced during pile run.
Key findings indicate that the implementation of a mitigation structure can significantly reduce pile run velocity, with its performance being influenced by the geometry of the structure and the velocity of the pile. A trade-off exists between velocity reduction and maintaining structural integrity. The research also highlights the critical role of FSI in determining the structural response of the mitigation tool and its interaction with the monopile.
Conclusions drawn from the results suggest practical applications for the mitigation measure, emphasizing the need for further investigation into the design and deployment mechanisms of the structure. Recommendations for future research directions are provided, aiming to enhance the understanding and application of pile run mitigation strategies in offshore engineering practices. ...
This thesis investigates the feasibility of utilizing a mitigation structure to reduce pile run during the impact driving of monopile foundations. The primary objective is to model and analyze the dynamics of the hammer-pile-soil system under impact driving and pile run, with a focus on the impact of a mitigation tool in monopile response.
The research adopts a structured methodology, beginning with a qualitative study to identify effective mitigation measures. A 1-D finite element (FE) model is developed to simulate pile run, while a 2-D FE multiphysics model characterizes the fluid-structure interaction (FSI). The study explores the effects of various geometric parameters of the mitigation structure on forces and stresses experienced during pile run.
Key findings indicate that the implementation of a mitigation structure can significantly reduce pile run velocity, with its performance being influenced by the geometry of the structure and the velocity of the pile. A trade-off exists between velocity reduction and maintaining structural integrity. The research also highlights the critical role of FSI in determining the structural response of the mitigation tool and its interaction with the monopile.
Conclusions drawn from the results suggest practical applications for the mitigation measure, emphasizing the need for further investigation into the design and deployment mechanisms of the structure. Recommendations for future research directions are provided, aiming to enhance the understanding and application of pile run mitigation strategies in offshore engineering practices. ...
Pile run refers to the phenomenon that occurs during the installation of pile and monopile foundations in soils with low bearing capacity, where soil resistance is insufficient for the controlled driving of the monopile. The increasing frequency and associated risks of such occurrences in offshore installations underscore the necessity for effective pile run mitigation strategies.
This thesis investigates the feasibility of utilizing a mitigation structure to reduce pile run during the impact driving of monopile foundations. The primary objective is to model and analyze the dynamics of the hammer-pile-soil system under impact driving and pile run, with a focus on the impact of a mitigation tool in monopile response.
The research adopts a structured methodology, beginning with a qualitative study to identify effective mitigation measures. A 1-D finite element (FE) model is developed to simulate pile run, while a 2-D FE multiphysics model characterizes the fluid-structure interaction (FSI). The study explores the effects of various geometric parameters of the mitigation structure on forces and stresses experienced during pile run.
Key findings indicate that the implementation of a mitigation structure can significantly reduce pile run velocity, with its performance being influenced by the geometry of the structure and the velocity of the pile. A trade-off exists between velocity reduction and maintaining structural integrity. The research also highlights the critical role of FSI in determining the structural response of the mitigation tool and its interaction with the monopile.
Conclusions drawn from the results suggest practical applications for the mitigation measure, emphasizing the need for further investigation into the design and deployment mechanisms of the structure. Recommendations for future research directions are provided, aiming to enhance the understanding and application of pile run mitigation strategies in offshore engineering practices.
This thesis investigates the feasibility of utilizing a mitigation structure to reduce pile run during the impact driving of monopile foundations. The primary objective is to model and analyze the dynamics of the hammer-pile-soil system under impact driving and pile run, with a focus on the impact of a mitigation tool in monopile response.
The research adopts a structured methodology, beginning with a qualitative study to identify effective mitigation measures. A 1-D finite element (FE) model is developed to simulate pile run, while a 2-D FE multiphysics model characterizes the fluid-structure interaction (FSI). The study explores the effects of various geometric parameters of the mitigation structure on forces and stresses experienced during pile run.
Key findings indicate that the implementation of a mitigation structure can significantly reduce pile run velocity, with its performance being influenced by the geometry of the structure and the velocity of the pile. A trade-off exists between velocity reduction and maintaining structural integrity. The research also highlights the critical role of FSI in determining the structural response of the mitigation tool and its interaction with the monopile.
Conclusions drawn from the results suggest practical applications for the mitigation measure, emphasizing the need for further investigation into the design and deployment mechanisms of the structure. Recommendations for future research directions are provided, aiming to enhance the understanding and application of pile run mitigation strategies in offshore engineering practices.
Motion Compensated Drilling From a Floating Vessel
Performance-based Motion Compensation selection for Floating Large Diameter Drilling
When drag anchors are no longer suitable for anchoring floating wind turbines to the seabed because of unfavorable soil conditions, it is necessary to use drilled and grouted anchor piles instead. For the installation of such anchor piles it is necessary to drill large diameter holes, with diameters ranging between 1 and 5 meters. The drilling method used to construct such boreholes is the reversed circulation drilling (RCD) method. Floating application of this method has only been done up until now in mild wave conditions. To make this operation more economically feasible, it is necessary to increase the workability of this operation in more challenging environmental conditions. A motion compensation device is a vital aspect in this.
A systematic approach was used to determine which type of motion compensation device is best suited for this operation. Numerical models were set up in the commercial software OrcaFlex, where a wide range of different motion compensation types were tested. These motion compensation systems vary in the amount of degrees of freedom compensated and the actuation method. The actuation methods are passively actuated gas springs and position-based motion control using hydraulic actuators.
First, it was identified what combination of degrees of freedom and actuation method is best suitable. This was determined by performing a regular wave analysis and comparing the performances of the proposed motion compensation systems. The performance was indicated by the ability of the motion compensation system to control the weight on bit, reduce the maximum stress as well as the fatigue damage accumulation in the drill string. From this analysis it was concluded that a passive actuated concept that compensates for the heave, roll, and pitch of the vessel is the most effective scheme.
The optimal type of motion compensation system was then pursued by designing two different systems capable of compensating for the heave, roll, and pitch motion of the vessel. The first, relatively simple, type consists of a vertical gas spring and a universal joint where the drill rig can rotate freely. The other type is a passive variant of the Barge Master platform motion compensation system. This second concept was designed using a multi-objective optimization algorithm, the NSGA-II algorithm. The difference between these two systems is that the platform concept has rotational stiffness, while the simple type is free in this degree of freedom.
It was finally concluded that both concepts are viable solutions, and that both systems represent a motion compensation system that compensates for the heave, roll, and pitch motion of the vessel. The performance regarding the indicators is very similar, therefore choosing one of the concepts would come down to the other advantages and disadvantages of the concepts.
...
A systematic approach was used to determine which type of motion compensation device is best suited for this operation. Numerical models were set up in the commercial software OrcaFlex, where a wide range of different motion compensation types were tested. These motion compensation systems vary in the amount of degrees of freedom compensated and the actuation method. The actuation methods are passively actuated gas springs and position-based motion control using hydraulic actuators.
First, it was identified what combination of degrees of freedom and actuation method is best suitable. This was determined by performing a regular wave analysis and comparing the performances of the proposed motion compensation systems. The performance was indicated by the ability of the motion compensation system to control the weight on bit, reduce the maximum stress as well as the fatigue damage accumulation in the drill string. From this analysis it was concluded that a passive actuated concept that compensates for the heave, roll, and pitch of the vessel is the most effective scheme.
The optimal type of motion compensation system was then pursued by designing two different systems capable of compensating for the heave, roll, and pitch motion of the vessel. The first, relatively simple, type consists of a vertical gas spring and a universal joint where the drill rig can rotate freely. The other type is a passive variant of the Barge Master platform motion compensation system. This second concept was designed using a multi-objective optimization algorithm, the NSGA-II algorithm. The difference between these two systems is that the platform concept has rotational stiffness, while the simple type is free in this degree of freedom.
It was finally concluded that both concepts are viable solutions, and that both systems represent a motion compensation system that compensates for the heave, roll, and pitch motion of the vessel. The performance regarding the indicators is very similar, therefore choosing one of the concepts would come down to the other advantages and disadvantages of the concepts.
...
When drag anchors are no longer suitable for anchoring floating wind turbines to the seabed because of unfavorable soil conditions, it is necessary to use drilled and grouted anchor piles instead. For the installation of such anchor piles it is necessary to drill large diameter holes, with diameters ranging between 1 and 5 meters. The drilling method used to construct such boreholes is the reversed circulation drilling (RCD) method. Floating application of this method has only been done up until now in mild wave conditions. To make this operation more economically feasible, it is necessary to increase the workability of this operation in more challenging environmental conditions. A motion compensation device is a vital aspect in this.
A systematic approach was used to determine which type of motion compensation device is best suited for this operation. Numerical models were set up in the commercial software OrcaFlex, where a wide range of different motion compensation types were tested. These motion compensation systems vary in the amount of degrees of freedom compensated and the actuation method. The actuation methods are passively actuated gas springs and position-based motion control using hydraulic actuators.
First, it was identified what combination of degrees of freedom and actuation method is best suitable. This was determined by performing a regular wave analysis and comparing the performances of the proposed motion compensation systems. The performance was indicated by the ability of the motion compensation system to control the weight on bit, reduce the maximum stress as well as the fatigue damage accumulation in the drill string. From this analysis it was concluded that a passive actuated concept that compensates for the heave, roll, and pitch of the vessel is the most effective scheme.
The optimal type of motion compensation system was then pursued by designing two different systems capable of compensating for the heave, roll, and pitch motion of the vessel. The first, relatively simple, type consists of a vertical gas spring and a universal joint where the drill rig can rotate freely. The other type is a passive variant of the Barge Master platform motion compensation system. This second concept was designed using a multi-objective optimization algorithm, the NSGA-II algorithm. The difference between these two systems is that the platform concept has rotational stiffness, while the simple type is free in this degree of freedom.
It was finally concluded that both concepts are viable solutions, and that both systems represent a motion compensation system that compensates for the heave, roll, and pitch motion of the vessel. The performance regarding the indicators is very similar, therefore choosing one of the concepts would come down to the other advantages and disadvantages of the concepts.
A systematic approach was used to determine which type of motion compensation device is best suited for this operation. Numerical models were set up in the commercial software OrcaFlex, where a wide range of different motion compensation types were tested. These motion compensation systems vary in the amount of degrees of freedom compensated and the actuation method. The actuation methods are passively actuated gas springs and position-based motion control using hydraulic actuators.
First, it was identified what combination of degrees of freedom and actuation method is best suitable. This was determined by performing a regular wave analysis and comparing the performances of the proposed motion compensation systems. The performance was indicated by the ability of the motion compensation system to control the weight on bit, reduce the maximum stress as well as the fatigue damage accumulation in the drill string. From this analysis it was concluded that a passive actuated concept that compensates for the heave, roll, and pitch of the vessel is the most effective scheme.
The optimal type of motion compensation system was then pursued by designing two different systems capable of compensating for the heave, roll, and pitch motion of the vessel. The first, relatively simple, type consists of a vertical gas spring and a universal joint where the drill rig can rotate freely. The other type is a passive variant of the Barge Master platform motion compensation system. This second concept was designed using a multi-objective optimization algorithm, the NSGA-II algorithm. The difference between these two systems is that the platform concept has rotational stiffness, while the simple type is free in this degree of freedom.
It was finally concluded that both concepts are viable solutions, and that both systems represent a motion compensation system that compensates for the heave, roll, and pitch motion of the vessel. The performance regarding the indicators is very similar, therefore choosing one of the concepts would come down to the other advantages and disadvantages of the concepts.
Flexibly formed concrete
Exploiting the deformation behaviour of weft-knitted formworks caused by concrete pressure
To reduce the construction industry’s negative influence on global climate, emissions related to concrete consumption need to be addressed. This implies reducing the amount concrete used, by creating material-efficient structures. One of concrete’s main advantages is that it can be moulded into virtually any shape. Despite the fact that modern digital design tools enable the effortless design and calculation of lightweight and graceful structures, this potential often goes unrealised. This can be attributed to the challenges associated with constructing intricate and custom geometries using conventional formwork techniques that depend on single-use cut timber or milled foam. Not only do these methods make the construction of these types of structures labour and cost intensive, they also cause them to be wasteful.
KnitCrete, which uses knitted technical textiles as stay-in-place moulds for concrete structures, has proven to be a solution for building doubly curved structures, eliminating the need for time-consuming, costly, and wasteful moulds. However, due to its inherent high flexibility and the challenges of predicting and controlling the geometry during the casting process, the technology relies on coating procedures using high-strength cement paste coating to stiffen the geometry before concrete can be poured.
This research addresses both issues and proposes a design approach, which models the deformation behaviour of the uncoated knitted formwork under concrete pressure to determine the final geometry of flexibly formed concrete structures, hence gaining better understanding on the deformation behaviour of knitted textile formworks and bypassing the stiffening steps during fabrication.
Developing a method to predict the final geometry of flexibly formed concrete structures involves various research disciplines, including material science, and structural mechanics. The research approach is divided into three parts. The first part investigates the stress-strain relationship of various textiles with different knitting patterns, alongside the rheological and mechanical strength properties of different cementitious mixtures. The second stage focuses on developing (semi-)analytical models to predict the deflection behaviour of membranes subjected to varying boundaries, loads, and material properties. Finally, the accuracy of the models are validated by the construction of multiple prototypes.
In conclusion, this thesis introduces a fabrication system that exploits the deflection behaviour of flexible formworks to create funicular shell structures and lays the foundation for implementing (semi-)analytical approaches to model these deformations.
...
KnitCrete, which uses knitted technical textiles as stay-in-place moulds for concrete structures, has proven to be a solution for building doubly curved structures, eliminating the need for time-consuming, costly, and wasteful moulds. However, due to its inherent high flexibility and the challenges of predicting and controlling the geometry during the casting process, the technology relies on coating procedures using high-strength cement paste coating to stiffen the geometry before concrete can be poured.
This research addresses both issues and proposes a design approach, which models the deformation behaviour of the uncoated knitted formwork under concrete pressure to determine the final geometry of flexibly formed concrete structures, hence gaining better understanding on the deformation behaviour of knitted textile formworks and bypassing the stiffening steps during fabrication.
Developing a method to predict the final geometry of flexibly formed concrete structures involves various research disciplines, including material science, and structural mechanics. The research approach is divided into three parts. The first part investigates the stress-strain relationship of various textiles with different knitting patterns, alongside the rheological and mechanical strength properties of different cementitious mixtures. The second stage focuses on developing (semi-)analytical models to predict the deflection behaviour of membranes subjected to varying boundaries, loads, and material properties. Finally, the accuracy of the models are validated by the construction of multiple prototypes.
In conclusion, this thesis introduces a fabrication system that exploits the deflection behaviour of flexible formworks to create funicular shell structures and lays the foundation for implementing (semi-)analytical approaches to model these deformations.
...
To reduce the construction industry’s negative influence on global climate, emissions related to concrete consumption need to be addressed. This implies reducing the amount concrete used, by creating material-efficient structures. One of concrete’s main advantages is that it can be moulded into virtually any shape. Despite the fact that modern digital design tools enable the effortless design and calculation of lightweight and graceful structures, this potential often goes unrealised. This can be attributed to the challenges associated with constructing intricate and custom geometries using conventional formwork techniques that depend on single-use cut timber or milled foam. Not only do these methods make the construction of these types of structures labour and cost intensive, they also cause them to be wasteful.
KnitCrete, which uses knitted technical textiles as stay-in-place moulds for concrete structures, has proven to be a solution for building doubly curved structures, eliminating the need for time-consuming, costly, and wasteful moulds. However, due to its inherent high flexibility and the challenges of predicting and controlling the geometry during the casting process, the technology relies on coating procedures using high-strength cement paste coating to stiffen the geometry before concrete can be poured.
This research addresses both issues and proposes a design approach, which models the deformation behaviour of the uncoated knitted formwork under concrete pressure to determine the final geometry of flexibly formed concrete structures, hence gaining better understanding on the deformation behaviour of knitted textile formworks and bypassing the stiffening steps during fabrication.
Developing a method to predict the final geometry of flexibly formed concrete structures involves various research disciplines, including material science, and structural mechanics. The research approach is divided into three parts. The first part investigates the stress-strain relationship of various textiles with different knitting patterns, alongside the rheological and mechanical strength properties of different cementitious mixtures. The second stage focuses on developing (semi-)analytical models to predict the deflection behaviour of membranes subjected to varying boundaries, loads, and material properties. Finally, the accuracy of the models are validated by the construction of multiple prototypes.
In conclusion, this thesis introduces a fabrication system that exploits the deflection behaviour of flexible formworks to create funicular shell structures and lays the foundation for implementing (semi-)analytical approaches to model these deformations.
KnitCrete, which uses knitted technical textiles as stay-in-place moulds for concrete structures, has proven to be a solution for building doubly curved structures, eliminating the need for time-consuming, costly, and wasteful moulds. However, due to its inherent high flexibility and the challenges of predicting and controlling the geometry during the casting process, the technology relies on coating procedures using high-strength cement paste coating to stiffen the geometry before concrete can be poured.
This research addresses both issues and proposes a design approach, which models the deformation behaviour of the uncoated knitted formwork under concrete pressure to determine the final geometry of flexibly formed concrete structures, hence gaining better understanding on the deformation behaviour of knitted textile formworks and bypassing the stiffening steps during fabrication.
Developing a method to predict the final geometry of flexibly formed concrete structures involves various research disciplines, including material science, and structural mechanics. The research approach is divided into three parts. The first part investigates the stress-strain relationship of various textiles with different knitting patterns, alongside the rheological and mechanical strength properties of different cementitious mixtures. The second stage focuses on developing (semi-)analytical models to predict the deflection behaviour of membranes subjected to varying boundaries, loads, and material properties. Finally, the accuracy of the models are validated by the construction of multiple prototypes.
In conclusion, this thesis introduces a fabrication system that exploits the deflection behaviour of flexible formworks to create funicular shell structures and lays the foundation for implementing (semi-)analytical approaches to model these deformations.
Master thesis
(2022)
-
T.R. Speelman, A. Tsouvalas, K.N. van Dalen, H.O. Sertlek, A. Tsetas, P.T. Nobel
In the underwater environment, it is all but silent. In most parts of the oceans, sunlight is barely available and thus marine animals have evolved to rely on sound for navigation, foraging and communication. Marine animals are not the only sources of underwater sound. Other natural sources such as earthquakes, waves and rain cause ambient noise, but also loud impulses. Furthermore, human activities consisting of dredging, surveying, construction and shipping cause loud noise in a wide frequency range. Underwater radiated noise (URN) from shipping has severe negative consequences on marine animals. The negative effects include auditory masking, stress, and behavioural and acoustic responses, possibly leading to collision with ships. It is evident that the mitigation of the underwater radiated noise of a ship is advantageous and worth researching.
In the past, plenty of research has been done into the modelling of the acoustic characteristics of vessels. A research gap was identified in the modelling of mitigation methods of the underwater radiated noise from ships with a focus on marine mammals. Up until now, most research has focused on the attenuation of vibrations on board a ship or radiated noise due to the propellers. At all times, the focus was either on human comfort or the radiated noise in general, however, for marine mammals certain frequency bands are of greater importance. It is valuable to assess the URN of a ship in the design phase, such that adjustments can be made to decrease the URN without excessive costs. The research goal of this thesis is:
Predict and mitigate the structure-borne underwater radiated noise of a ship in the design phase caused by onboard machinery.
The research goal and corresponding research questions are answered by first setting up the framework for the models. This framework sets the frequency analysis range to 20 - 200 Hz, formulates a reference ship case for validation of the data and gives the inputs and boundary conditions for the models. The acoustic metric of interest is set to the source level (SL) in dB re 1 μPa2m2.
Secondly, a simplified model is setup in Ansys 2021R2. This model is a 3D solid element model shaped like a beam. The equivalent beam (EB) model has similar global properties as the reference ship case. Around the EB model an acoustic domain is located that is modelled to represent an infinite domain. No physical boundary effects are included as the source level is per definition not dependent on this.
Subsequently, mitigation methods for machinery URN are researched and the resilient mount was found to be the most promising. A resilient mount is applied to the EB model in Ansys 2021R2 and source level spectra for different mount parameters are investigated.
Lastly, a two degree of freedom (2-DOF) schematisation is made that incorporates the Ansys model using a dynamic stiffness. The 2-DOF schematisation allows for faster computations of the complete model and thus a more extensive parameter study of the resilient mount is possible.
Over the frequency analysis range of 20 - 200 Hz, the results show that machinery structure-borne URN can be reduced by 45 - 65 dB re 1 μPa2m2. The reduction oscillates over the frequencies at lower frequencies. A linear SL reduction was observed from 60 Hz and above, which gradually lessened for higher frequencies. The SL reduction increased from 45 dB re 1 μPa2m2 to 60 dB re 1 μPa2m2 when the resilient mount damping ratio was changed from 0.18 to 0.02. In addition, the normalised resilient mount parameter study showed the system's parameter sensitivities and responses. It became clear that the resilient mount does not respond to a change in resilient mount damping as expected. The accuracy of the absolute results is subject to assumptions and limitations, which introduce uncertainties.
The absolute decrease in URN with the resilient mount was computed using acceleration input rather than force input. The acceleration input was found to have overestimated the 'no-mount' case, which was used to compare cases with the resilient mount. The total URN reduction with the resilient mount could thus have been overestimated. Furthermore, an effect of the model boundaries and the model domain size on the results was present in the models. The magnitudes of the results were influenced by this effect, which could not be eliminated due to the computational limitations reached. Finally, there was a scarcity of model input data and reference data. The magnitudes of the results were obtained and compared to limited data in order to determine the accuracy of the results.
Taking these limitations into consideration, the findings of this study should be interpreted with caution. The findings support literature claims that a resilient mount can reduce structure-borne machinery URN by 20 - 40 dB re 1 μPa2m2, with more reduction at higher frequencies.
The effect of the application of the resilient mount on marine mammals was hard to quantify. The structure-borne machinery URN is a part of the total URN of a ship. Due to the logarithmic relation of the noise, the reduction of one part could have very limited effects. Furthermore, the total soundscape in the ocean is formed by the combined noise of many ships. Moreover, the relation between the perceived nuisance of marine mammals and the URN levels is hard to indicate. The effect is undoubtedly positive but could be negligible in the bigger picture. At low speeds and close distances, the machinery URN is governing and the influence of URN from other ships is reduced. In those cases, the reduction of structure-borne machinery URN with resilient mounts could be expected to be the most positive for marine mammals.
...
In the past, plenty of research has been done into the modelling of the acoustic characteristics of vessels. A research gap was identified in the modelling of mitigation methods of the underwater radiated noise from ships with a focus on marine mammals. Up until now, most research has focused on the attenuation of vibrations on board a ship or radiated noise due to the propellers. At all times, the focus was either on human comfort or the radiated noise in general, however, for marine mammals certain frequency bands are of greater importance. It is valuable to assess the URN of a ship in the design phase, such that adjustments can be made to decrease the URN without excessive costs. The research goal of this thesis is:
Predict and mitigate the structure-borne underwater radiated noise of a ship in the design phase caused by onboard machinery.
The research goal and corresponding research questions are answered by first setting up the framework for the models. This framework sets the frequency analysis range to 20 - 200 Hz, formulates a reference ship case for validation of the data and gives the inputs and boundary conditions for the models. The acoustic metric of interest is set to the source level (SL) in dB re 1 μPa2m2.
Secondly, a simplified model is setup in Ansys 2021R2. This model is a 3D solid element model shaped like a beam. The equivalent beam (EB) model has similar global properties as the reference ship case. Around the EB model an acoustic domain is located that is modelled to represent an infinite domain. No physical boundary effects are included as the source level is per definition not dependent on this.
Subsequently, mitigation methods for machinery URN are researched and the resilient mount was found to be the most promising. A resilient mount is applied to the EB model in Ansys 2021R2 and source level spectra for different mount parameters are investigated.
Lastly, a two degree of freedom (2-DOF) schematisation is made that incorporates the Ansys model using a dynamic stiffness. The 2-DOF schematisation allows for faster computations of the complete model and thus a more extensive parameter study of the resilient mount is possible.
Over the frequency analysis range of 20 - 200 Hz, the results show that machinery structure-borne URN can be reduced by 45 - 65 dB re 1 μPa2m2. The reduction oscillates over the frequencies at lower frequencies. A linear SL reduction was observed from 60 Hz and above, which gradually lessened for higher frequencies. The SL reduction increased from 45 dB re 1 μPa2m2 to 60 dB re 1 μPa2m2 when the resilient mount damping ratio was changed from 0.18 to 0.02. In addition, the normalised resilient mount parameter study showed the system's parameter sensitivities and responses. It became clear that the resilient mount does not respond to a change in resilient mount damping as expected. The accuracy of the absolute results is subject to assumptions and limitations, which introduce uncertainties.
The absolute decrease in URN with the resilient mount was computed using acceleration input rather than force input. The acceleration input was found to have overestimated the 'no-mount' case, which was used to compare cases with the resilient mount. The total URN reduction with the resilient mount could thus have been overestimated. Furthermore, an effect of the model boundaries and the model domain size on the results was present in the models. The magnitudes of the results were influenced by this effect, which could not be eliminated due to the computational limitations reached. Finally, there was a scarcity of model input data and reference data. The magnitudes of the results were obtained and compared to limited data in order to determine the accuracy of the results.
Taking these limitations into consideration, the findings of this study should be interpreted with caution. The findings support literature claims that a resilient mount can reduce structure-borne machinery URN by 20 - 40 dB re 1 μPa2m2, with more reduction at higher frequencies.
The effect of the application of the resilient mount on marine mammals was hard to quantify. The structure-borne machinery URN is a part of the total URN of a ship. Due to the logarithmic relation of the noise, the reduction of one part could have very limited effects. Furthermore, the total soundscape in the ocean is formed by the combined noise of many ships. Moreover, the relation between the perceived nuisance of marine mammals and the URN levels is hard to indicate. The effect is undoubtedly positive but could be negligible in the bigger picture. At low speeds and close distances, the machinery URN is governing and the influence of URN from other ships is reduced. In those cases, the reduction of structure-borne machinery URN with resilient mounts could be expected to be the most positive for marine mammals.
...
In the underwater environment, it is all but silent. In most parts of the oceans, sunlight is barely available and thus marine animals have evolved to rely on sound for navigation, foraging and communication. Marine animals are not the only sources of underwater sound. Other natural sources such as earthquakes, waves and rain cause ambient noise, but also loud impulses. Furthermore, human activities consisting of dredging, surveying, construction and shipping cause loud noise in a wide frequency range. Underwater radiated noise (URN) from shipping has severe negative consequences on marine animals. The negative effects include auditory masking, stress, and behavioural and acoustic responses, possibly leading to collision with ships. It is evident that the mitigation of the underwater radiated noise of a ship is advantageous and worth researching.
In the past, plenty of research has been done into the modelling of the acoustic characteristics of vessels. A research gap was identified in the modelling of mitigation methods of the underwater radiated noise from ships with a focus on marine mammals. Up until now, most research has focused on the attenuation of vibrations on board a ship or radiated noise due to the propellers. At all times, the focus was either on human comfort or the radiated noise in general, however, for marine mammals certain frequency bands are of greater importance. It is valuable to assess the URN of a ship in the design phase, such that adjustments can be made to decrease the URN without excessive costs. The research goal of this thesis is:
Predict and mitigate the structure-borne underwater radiated noise of a ship in the design phase caused by onboard machinery.
The research goal and corresponding research questions are answered by first setting up the framework for the models. This framework sets the frequency analysis range to 20 - 200 Hz, formulates a reference ship case for validation of the data and gives the inputs and boundary conditions for the models. The acoustic metric of interest is set to the source level (SL) in dB re 1 μPa2m2.
Secondly, a simplified model is setup in Ansys 2021R2. This model is a 3D solid element model shaped like a beam. The equivalent beam (EB) model has similar global properties as the reference ship case. Around the EB model an acoustic domain is located that is modelled to represent an infinite domain. No physical boundary effects are included as the source level is per definition not dependent on this.
Subsequently, mitigation methods for machinery URN are researched and the resilient mount was found to be the most promising. A resilient mount is applied to the EB model in Ansys 2021R2 and source level spectra for different mount parameters are investigated.
Lastly, a two degree of freedom (2-DOF) schematisation is made that incorporates the Ansys model using a dynamic stiffness. The 2-DOF schematisation allows for faster computations of the complete model and thus a more extensive parameter study of the resilient mount is possible.
Over the frequency analysis range of 20 - 200 Hz, the results show that machinery structure-borne URN can be reduced by 45 - 65 dB re 1 μPa2m2. The reduction oscillates over the frequencies at lower frequencies. A linear SL reduction was observed from 60 Hz and above, which gradually lessened for higher frequencies. The SL reduction increased from 45 dB re 1 μPa2m2 to 60 dB re 1 μPa2m2 when the resilient mount damping ratio was changed from 0.18 to 0.02. In addition, the normalised resilient mount parameter study showed the system's parameter sensitivities and responses. It became clear that the resilient mount does not respond to a change in resilient mount damping as expected. The accuracy of the absolute results is subject to assumptions and limitations, which introduce uncertainties.
The absolute decrease in URN with the resilient mount was computed using acceleration input rather than force input. The acceleration input was found to have overestimated the 'no-mount' case, which was used to compare cases with the resilient mount. The total URN reduction with the resilient mount could thus have been overestimated. Furthermore, an effect of the model boundaries and the model domain size on the results was present in the models. The magnitudes of the results were influenced by this effect, which could not be eliminated due to the computational limitations reached. Finally, there was a scarcity of model input data and reference data. The magnitudes of the results were obtained and compared to limited data in order to determine the accuracy of the results.
Taking these limitations into consideration, the findings of this study should be interpreted with caution. The findings support literature claims that a resilient mount can reduce structure-borne machinery URN by 20 - 40 dB re 1 μPa2m2, with more reduction at higher frequencies.
The effect of the application of the resilient mount on marine mammals was hard to quantify. The structure-borne machinery URN is a part of the total URN of a ship. Due to the logarithmic relation of the noise, the reduction of one part could have very limited effects. Furthermore, the total soundscape in the ocean is formed by the combined noise of many ships. Moreover, the relation between the perceived nuisance of marine mammals and the URN levels is hard to indicate. The effect is undoubtedly positive but could be negligible in the bigger picture. At low speeds and close distances, the machinery URN is governing and the influence of URN from other ships is reduced. In those cases, the reduction of structure-borne machinery URN with resilient mounts could be expected to be the most positive for marine mammals.
In the past, plenty of research has been done into the modelling of the acoustic characteristics of vessels. A research gap was identified in the modelling of mitigation methods of the underwater radiated noise from ships with a focus on marine mammals. Up until now, most research has focused on the attenuation of vibrations on board a ship or radiated noise due to the propellers. At all times, the focus was either on human comfort or the radiated noise in general, however, for marine mammals certain frequency bands are of greater importance. It is valuable to assess the URN of a ship in the design phase, such that adjustments can be made to decrease the URN without excessive costs. The research goal of this thesis is:
Predict and mitigate the structure-borne underwater radiated noise of a ship in the design phase caused by onboard machinery.
The research goal and corresponding research questions are answered by first setting up the framework for the models. This framework sets the frequency analysis range to 20 - 200 Hz, formulates a reference ship case for validation of the data and gives the inputs and boundary conditions for the models. The acoustic metric of interest is set to the source level (SL) in dB re 1 μPa2m2.
Secondly, a simplified model is setup in Ansys 2021R2. This model is a 3D solid element model shaped like a beam. The equivalent beam (EB) model has similar global properties as the reference ship case. Around the EB model an acoustic domain is located that is modelled to represent an infinite domain. No physical boundary effects are included as the source level is per definition not dependent on this.
Subsequently, mitigation methods for machinery URN are researched and the resilient mount was found to be the most promising. A resilient mount is applied to the EB model in Ansys 2021R2 and source level spectra for different mount parameters are investigated.
Lastly, a two degree of freedom (2-DOF) schematisation is made that incorporates the Ansys model using a dynamic stiffness. The 2-DOF schematisation allows for faster computations of the complete model and thus a more extensive parameter study of the resilient mount is possible.
Over the frequency analysis range of 20 - 200 Hz, the results show that machinery structure-borne URN can be reduced by 45 - 65 dB re 1 μPa2m2. The reduction oscillates over the frequencies at lower frequencies. A linear SL reduction was observed from 60 Hz and above, which gradually lessened for higher frequencies. The SL reduction increased from 45 dB re 1 μPa2m2 to 60 dB re 1 μPa2m2 when the resilient mount damping ratio was changed from 0.18 to 0.02. In addition, the normalised resilient mount parameter study showed the system's parameter sensitivities and responses. It became clear that the resilient mount does not respond to a change in resilient mount damping as expected. The accuracy of the absolute results is subject to assumptions and limitations, which introduce uncertainties.
The absolute decrease in URN with the resilient mount was computed using acceleration input rather than force input. The acceleration input was found to have overestimated the 'no-mount' case, which was used to compare cases with the resilient mount. The total URN reduction with the resilient mount could thus have been overestimated. Furthermore, an effect of the model boundaries and the model domain size on the results was present in the models. The magnitudes of the results were influenced by this effect, which could not be eliminated due to the computational limitations reached. Finally, there was a scarcity of model input data and reference data. The magnitudes of the results were obtained and compared to limited data in order to determine the accuracy of the results.
Taking these limitations into consideration, the findings of this study should be interpreted with caution. The findings support literature claims that a resilient mount can reduce structure-borne machinery URN by 20 - 40 dB re 1 μPa2m2, with more reduction at higher frequencies.
The effect of the application of the resilient mount on marine mammals was hard to quantify. The structure-borne machinery URN is a part of the total URN of a ship. Due to the logarithmic relation of the noise, the reduction of one part could have very limited effects. Furthermore, the total soundscape in the ocean is formed by the combined noise of many ships. Moreover, the relation between the perceived nuisance of marine mammals and the URN levels is hard to indicate. The effect is undoubtedly positive but could be negligible in the bigger picture. At low speeds and close distances, the machinery URN is governing and the influence of URN from other ships is reduced. In those cases, the reduction of structure-borne machinery URN with resilient mounts could be expected to be the most positive for marine mammals.
A parameter sensitivity study for a Soft Yoke Mooring System (SYMS)
Developing a design tool
Floating production units require to be permanently moored offshore. For shallow water field developments, typical designs include soft yoke mooring systems which is a single point mooring technology. Produced fluid is transported via a geostatic tower along the transfer hoses to the production unit. Single point mooring systems will weathervane into the offshore environment (wind, waves and current) and can often be considered as surge dominant induced by low frequency wave drift forces. Due to low damping at these low frequencies, an oscillation close to the natural surge frequency appears causing dominant mooring loads. By connecting the floating structure to the tower with various rigid steel frame components, including multiple hinges, unconstrained motions are preserved. This results in dynamically complex systems requiring advanced numerical methods for time domain simulations, in order to deliver the mooring design loads. By selecting mass and dimensions of the soft yoke mooring system, the stiffness of the mooring system can be adapted to influence the response of the floater.
Currently, mass and dimensions of new soft yoke mooring designs are established using proven mooring solutions by conducting small adjustments of mooring components in time consuming dynamic simulation software. Improvement in efficiency of the design process is accomplished by developing a computational efficient design tool, to be used before dynamic modelling, to provide the designer with an optimized set of mooring design parameters in a given environment. The developed design tool contains an integrated single degree of freedom model including given vessel properties and extreme collinear offshore environment, in order to produce estimates of the maximum surge response based on the given mooring characteristics. Eventually, by adapting the mooring stiffness accordingly, the design tool is able to provide an optimum set of mooring design parameters which minimizes the maximum mooring load.
The mooring characteristics are implemented using a linear and non-linear approach. A validation of surge response results has been performed for three collinear extreme offshore environments including wind, waves and current using state-of-the art dynamic modelling software OrcaFlex. Results showed that the linear model can capture the sensitivity of the surge response related to mooring design parameter variation. It was demonstrated with the Runge-Kutta method that the achieved accuracy with the proposed non-linear model will be similar or even worse than using the linear model without the relevant computational costs. Therefore, the linear model was implemented in the design tool and accomplishes surge response calculations of 10,000 sets of mooring design parameters within a few minutes.
Results showed that the consequence of the linear mooring force and single degree of freedom assumptions in the design tool leads to an underestimation of the surge response in an extreme collinear environment. Despite the surge dominance, especially heave and pitch motions induce high mooring loads at maximum surge offset. Analyses of two moderate collinear environments in OrcaFlex revealed less underestimation and even overestimation of the surge response by the design tool, which indicates the high dynamic complexity of soft yoke mooring systems. However, the surge response validation procedure using OrcaFlex affirmed that the design tool provides the correct set of mooring design parameters resulting in a minimum mooring load within 2~10% accuracy for all three environments. By implementing the proposed set in OrcaFlex and obtaining the design loads, a 25% decrease in absolute maximum mooring load is demonstrated when compared to the benchmark mooring design parameters. This indicates the increased efficiency of the design process by using the developed design tool.
An additional graphical user interface is programmed where the designer can import their own vessel properties, environmental conditions and can vary desired mooring design parameters. Conclusively, a design tool has been developed for preliminary estimations of the set of mooring design parameters that have been shown to minimize the mooring loads and to limit the number of time domain analyses in future projects.
...
Currently, mass and dimensions of new soft yoke mooring designs are established using proven mooring solutions by conducting small adjustments of mooring components in time consuming dynamic simulation software. Improvement in efficiency of the design process is accomplished by developing a computational efficient design tool, to be used before dynamic modelling, to provide the designer with an optimized set of mooring design parameters in a given environment. The developed design tool contains an integrated single degree of freedom model including given vessel properties and extreme collinear offshore environment, in order to produce estimates of the maximum surge response based on the given mooring characteristics. Eventually, by adapting the mooring stiffness accordingly, the design tool is able to provide an optimum set of mooring design parameters which minimizes the maximum mooring load.
The mooring characteristics are implemented using a linear and non-linear approach. A validation of surge response results has been performed for three collinear extreme offshore environments including wind, waves and current using state-of-the art dynamic modelling software OrcaFlex. Results showed that the linear model can capture the sensitivity of the surge response related to mooring design parameter variation. It was demonstrated with the Runge-Kutta method that the achieved accuracy with the proposed non-linear model will be similar or even worse than using the linear model without the relevant computational costs. Therefore, the linear model was implemented in the design tool and accomplishes surge response calculations of 10,000 sets of mooring design parameters within a few minutes.
Results showed that the consequence of the linear mooring force and single degree of freedom assumptions in the design tool leads to an underestimation of the surge response in an extreme collinear environment. Despite the surge dominance, especially heave and pitch motions induce high mooring loads at maximum surge offset. Analyses of two moderate collinear environments in OrcaFlex revealed less underestimation and even overestimation of the surge response by the design tool, which indicates the high dynamic complexity of soft yoke mooring systems. However, the surge response validation procedure using OrcaFlex affirmed that the design tool provides the correct set of mooring design parameters resulting in a minimum mooring load within 2~10% accuracy for all three environments. By implementing the proposed set in OrcaFlex and obtaining the design loads, a 25% decrease in absolute maximum mooring load is demonstrated when compared to the benchmark mooring design parameters. This indicates the increased efficiency of the design process by using the developed design tool.
An additional graphical user interface is programmed where the designer can import their own vessel properties, environmental conditions and can vary desired mooring design parameters. Conclusively, a design tool has been developed for preliminary estimations of the set of mooring design parameters that have been shown to minimize the mooring loads and to limit the number of time domain analyses in future projects.
...
Floating production units require to be permanently moored offshore. For shallow water field developments, typical designs include soft yoke mooring systems which is a single point mooring technology. Produced fluid is transported via a geostatic tower along the transfer hoses to the production unit. Single point mooring systems will weathervane into the offshore environment (wind, waves and current) and can often be considered as surge dominant induced by low frequency wave drift forces. Due to low damping at these low frequencies, an oscillation close to the natural surge frequency appears causing dominant mooring loads. By connecting the floating structure to the tower with various rigid steel frame components, including multiple hinges, unconstrained motions are preserved. This results in dynamically complex systems requiring advanced numerical methods for time domain simulations, in order to deliver the mooring design loads. By selecting mass and dimensions of the soft yoke mooring system, the stiffness of the mooring system can be adapted to influence the response of the floater.
Currently, mass and dimensions of new soft yoke mooring designs are established using proven mooring solutions by conducting small adjustments of mooring components in time consuming dynamic simulation software. Improvement in efficiency of the design process is accomplished by developing a computational efficient design tool, to be used before dynamic modelling, to provide the designer with an optimized set of mooring design parameters in a given environment. The developed design tool contains an integrated single degree of freedom model including given vessel properties and extreme collinear offshore environment, in order to produce estimates of the maximum surge response based on the given mooring characteristics. Eventually, by adapting the mooring stiffness accordingly, the design tool is able to provide an optimum set of mooring design parameters which minimizes the maximum mooring load.
The mooring characteristics are implemented using a linear and non-linear approach. A validation of surge response results has been performed for three collinear extreme offshore environments including wind, waves and current using state-of-the art dynamic modelling software OrcaFlex. Results showed that the linear model can capture the sensitivity of the surge response related to mooring design parameter variation. It was demonstrated with the Runge-Kutta method that the achieved accuracy with the proposed non-linear model will be similar or even worse than using the linear model without the relevant computational costs. Therefore, the linear model was implemented in the design tool and accomplishes surge response calculations of 10,000 sets of mooring design parameters within a few minutes.
Results showed that the consequence of the linear mooring force and single degree of freedom assumptions in the design tool leads to an underestimation of the surge response in an extreme collinear environment. Despite the surge dominance, especially heave and pitch motions induce high mooring loads at maximum surge offset. Analyses of two moderate collinear environments in OrcaFlex revealed less underestimation and even overestimation of the surge response by the design tool, which indicates the high dynamic complexity of soft yoke mooring systems. However, the surge response validation procedure using OrcaFlex affirmed that the design tool provides the correct set of mooring design parameters resulting in a minimum mooring load within 2~10% accuracy for all three environments. By implementing the proposed set in OrcaFlex and obtaining the design loads, a 25% decrease in absolute maximum mooring load is demonstrated when compared to the benchmark mooring design parameters. This indicates the increased efficiency of the design process by using the developed design tool.
An additional graphical user interface is programmed where the designer can import their own vessel properties, environmental conditions and can vary desired mooring design parameters. Conclusively, a design tool has been developed for preliminary estimations of the set of mooring design parameters that have been shown to minimize the mooring loads and to limit the number of time domain analyses in future projects.
Currently, mass and dimensions of new soft yoke mooring designs are established using proven mooring solutions by conducting small adjustments of mooring components in time consuming dynamic simulation software. Improvement in efficiency of the design process is accomplished by developing a computational efficient design tool, to be used before dynamic modelling, to provide the designer with an optimized set of mooring design parameters in a given environment. The developed design tool contains an integrated single degree of freedom model including given vessel properties and extreme collinear offshore environment, in order to produce estimates of the maximum surge response based on the given mooring characteristics. Eventually, by adapting the mooring stiffness accordingly, the design tool is able to provide an optimum set of mooring design parameters which minimizes the maximum mooring load.
The mooring characteristics are implemented using a linear and non-linear approach. A validation of surge response results has been performed for three collinear extreme offshore environments including wind, waves and current using state-of-the art dynamic modelling software OrcaFlex. Results showed that the linear model can capture the sensitivity of the surge response related to mooring design parameter variation. It was demonstrated with the Runge-Kutta method that the achieved accuracy with the proposed non-linear model will be similar or even worse than using the linear model without the relevant computational costs. Therefore, the linear model was implemented in the design tool and accomplishes surge response calculations of 10,000 sets of mooring design parameters within a few minutes.
Results showed that the consequence of the linear mooring force and single degree of freedom assumptions in the design tool leads to an underestimation of the surge response in an extreme collinear environment. Despite the surge dominance, especially heave and pitch motions induce high mooring loads at maximum surge offset. Analyses of two moderate collinear environments in OrcaFlex revealed less underestimation and even overestimation of the surge response by the design tool, which indicates the high dynamic complexity of soft yoke mooring systems. However, the surge response validation procedure using OrcaFlex affirmed that the design tool provides the correct set of mooring design parameters resulting in a minimum mooring load within 2~10% accuracy for all three environments. By implementing the proposed set in OrcaFlex and obtaining the design loads, a 25% decrease in absolute maximum mooring load is demonstrated when compared to the benchmark mooring design parameters. This indicates the increased efficiency of the design process by using the developed design tool.
An additional graphical user interface is programmed where the designer can import their own vessel properties, environmental conditions and can vary desired mooring design parameters. Conclusively, a design tool has been developed for preliminary estimations of the set of mooring design parameters that have been shown to minimize the mooring loads and to limit the number of time domain analyses in future projects.