R.L.G. Slange
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1
Offshore floating solar structures require lightweight, corrosion-resistant connections that avoid the fatigue drawbacks of conventional welding. SolarDuck has developed a cryogenic pin-lug joint concept for this purpose, in which an interference fit is created by cooling an oversized pin before installation, introducing a beneficial compressive prestress around the connection. This thesis investigates the quasi-static structural performance of this joint by developing and validating a finite element model capable of predicting its load-displacement response and failure behavior.
Existing design standards for pin-loaded joints do not represent the non-standard geometry of this connection or the residual stress introduced during cryogenic assembly, and no validated modeling framework currently exists for this joint type. The thesis therefore addresses how the joint’s structural response and governing failure mode can be predicted numerically, which material model best represents the alloy’s behavior, and how the resulting model can support design-oriented sensitivity analysis.
The methodology combines material characterization from tensile tests, calibration of candidate constitutive models, and development of a full-scale joint finite element model that includes contact behavior and the interference-fit condition. A linear work hardening model was selected over a better-fitting power-law hardening model, since the latter became numerically unstable once local plastic strain exceeded its calibrated range. The model was validated against full-scale experimental results and subsequently used for a sensitivity study on interference level and joint geometry, including a limited
finite element verification of the hole-edge distance.
The full-scale simulation reached a converged load of approximately 285 kN, against an experimental ultimate load of approximately 240 kN. However, the model reached comparable force levels at a much smaller displacement than observed experimentally, indicating that the simulated joint was substantially stiffer than the physical specimen. At the final converged state, the maximum equivalent plastic strain was approximately 30.5% and the maximum equivalent von Mises stress approximately 527 to 531 MPa, both concentrated on the bearing side of the pin-hole contact, with interface opening of up to 1.0 mm on the opposite side. Interference-fit level had limited effect on the global response and critical location. An analytical sensitivity study identified lug shear tear-out as the governing code-based check, with a capacity-to-demand ratio of 0.370, meaning the analytical resistance was well below the reference demand for this mode; this contrasts with the FEM, which consistently indicated bearing-dominated plasticity as the dominant response instead.
The predicted stress and contact patterns agree qualitatively with the pin-lug opening and lug cracking observed experimentally, supporting a bearing-dominated failure tendency. However, the model does not include a fracture criterion and therefore cannot reproduce the post-peak force drop observed experimentally, continuing to rise beyond the experimental ultimate load instead; it also does not reproduce the experimental global stiffness, so the comparison is limited to a qualitative and semi-quantitative validation of the pre-ultimate response rather than a close quantitative match.
The developed approach reproduces the main pre-ultimate deformation mechanisms of the joint and identifies a concrete discrepancy between the analytically governing failure mode and the FEM-indicated dominant mechanism, which future work should resolve. Recommended priorities are incorporating a calibrated fracture criterion, targeted experiments on varying hole-edge distance to reconcile the shear tear-out and bearing discrepancy, and verification of the fitted-state representation and global stiffness mismatch. ...
Existing design standards for pin-loaded joints do not represent the non-standard geometry of this connection or the residual stress introduced during cryogenic assembly, and no validated modeling framework currently exists for this joint type. The thesis therefore addresses how the joint’s structural response and governing failure mode can be predicted numerically, which material model best represents the alloy’s behavior, and how the resulting model can support design-oriented sensitivity analysis.
The methodology combines material characterization from tensile tests, calibration of candidate constitutive models, and development of a full-scale joint finite element model that includes contact behavior and the interference-fit condition. A linear work hardening model was selected over a better-fitting power-law hardening model, since the latter became numerically unstable once local plastic strain exceeded its calibrated range. The model was validated against full-scale experimental results and subsequently used for a sensitivity study on interference level and joint geometry, including a limited
finite element verification of the hole-edge distance.
The full-scale simulation reached a converged load of approximately 285 kN, against an experimental ultimate load of approximately 240 kN. However, the model reached comparable force levels at a much smaller displacement than observed experimentally, indicating that the simulated joint was substantially stiffer than the physical specimen. At the final converged state, the maximum equivalent plastic strain was approximately 30.5% and the maximum equivalent von Mises stress approximately 527 to 531 MPa, both concentrated on the bearing side of the pin-hole contact, with interface opening of up to 1.0 mm on the opposite side. Interference-fit level had limited effect on the global response and critical location. An analytical sensitivity study identified lug shear tear-out as the governing code-based check, with a capacity-to-demand ratio of 0.370, meaning the analytical resistance was well below the reference demand for this mode; this contrasts with the FEM, which consistently indicated bearing-dominated plasticity as the dominant response instead.
The predicted stress and contact patterns agree qualitatively with the pin-lug opening and lug cracking observed experimentally, supporting a bearing-dominated failure tendency. However, the model does not include a fracture criterion and therefore cannot reproduce the post-peak force drop observed experimentally, continuing to rise beyond the experimental ultimate load instead; it also does not reproduce the experimental global stiffness, so the comparison is limited to a qualitative and semi-quantitative validation of the pre-ultimate response rather than a close quantitative match.
The developed approach reproduces the main pre-ultimate deformation mechanisms of the joint and identifies a concrete discrepancy between the analytically governing failure mode and the FEM-indicated dominant mechanism, which future work should resolve. Recommended priorities are incorporating a calibrated fracture criterion, targeted experiments on varying hole-edge distance to reconcile the shear tear-out and bearing discrepancy, and verification of the fitted-state representation and global stiffness mismatch. ...
Offshore floating solar structures require lightweight, corrosion-resistant connections that avoid the fatigue drawbacks of conventional welding. SolarDuck has developed a cryogenic pin-lug joint concept for this purpose, in which an interference fit is created by cooling an oversized pin before installation, introducing a beneficial compressive prestress around the connection. This thesis investigates the quasi-static structural performance of this joint by developing and validating a finite element model capable of predicting its load-displacement response and failure behavior.
Existing design standards for pin-loaded joints do not represent the non-standard geometry of this connection or the residual stress introduced during cryogenic assembly, and no validated modeling framework currently exists for this joint type. The thesis therefore addresses how the joint’s structural response and governing failure mode can be predicted numerically, which material model best represents the alloy’s behavior, and how the resulting model can support design-oriented sensitivity analysis.
The methodology combines material characterization from tensile tests, calibration of candidate constitutive models, and development of a full-scale joint finite element model that includes contact behavior and the interference-fit condition. A linear work hardening model was selected over a better-fitting power-law hardening model, since the latter became numerically unstable once local plastic strain exceeded its calibrated range. The model was validated against full-scale experimental results and subsequently used for a sensitivity study on interference level and joint geometry, including a limited
finite element verification of the hole-edge distance.
The full-scale simulation reached a converged load of approximately 285 kN, against an experimental ultimate load of approximately 240 kN. However, the model reached comparable force levels at a much smaller displacement than observed experimentally, indicating that the simulated joint was substantially stiffer than the physical specimen. At the final converged state, the maximum equivalent plastic strain was approximately 30.5% and the maximum equivalent von Mises stress approximately 527 to 531 MPa, both concentrated on the bearing side of the pin-hole contact, with interface opening of up to 1.0 mm on the opposite side. Interference-fit level had limited effect on the global response and critical location. An analytical sensitivity study identified lug shear tear-out as the governing code-based check, with a capacity-to-demand ratio of 0.370, meaning the analytical resistance was well below the reference demand for this mode; this contrasts with the FEM, which consistently indicated bearing-dominated plasticity as the dominant response instead.
The predicted stress and contact patterns agree qualitatively with the pin-lug opening and lug cracking observed experimentally, supporting a bearing-dominated failure tendency. However, the model does not include a fracture criterion and therefore cannot reproduce the post-peak force drop observed experimentally, continuing to rise beyond the experimental ultimate load instead; it also does not reproduce the experimental global stiffness, so the comparison is limited to a qualitative and semi-quantitative validation of the pre-ultimate response rather than a close quantitative match.
The developed approach reproduces the main pre-ultimate deformation mechanisms of the joint and identifies a concrete discrepancy between the analytically governing failure mode and the FEM-indicated dominant mechanism, which future work should resolve. Recommended priorities are incorporating a calibrated fracture criterion, targeted experiments on varying hole-edge distance to reconcile the shear tear-out and bearing discrepancy, and verification of the fitted-state representation and global stiffness mismatch.
Existing design standards for pin-loaded joints do not represent the non-standard geometry of this connection or the residual stress introduced during cryogenic assembly, and no validated modeling framework currently exists for this joint type. The thesis therefore addresses how the joint’s structural response and governing failure mode can be predicted numerically, which material model best represents the alloy’s behavior, and how the resulting model can support design-oriented sensitivity analysis.
The methodology combines material characterization from tensile tests, calibration of candidate constitutive models, and development of a full-scale joint finite element model that includes contact behavior and the interference-fit condition. A linear work hardening model was selected over a better-fitting power-law hardening model, since the latter became numerically unstable once local plastic strain exceeded its calibrated range. The model was validated against full-scale experimental results and subsequently used for a sensitivity study on interference level and joint geometry, including a limited
finite element verification of the hole-edge distance.
The full-scale simulation reached a converged load of approximately 285 kN, against an experimental ultimate load of approximately 240 kN. However, the model reached comparable force levels at a much smaller displacement than observed experimentally, indicating that the simulated joint was substantially stiffer than the physical specimen. At the final converged state, the maximum equivalent plastic strain was approximately 30.5% and the maximum equivalent von Mises stress approximately 527 to 531 MPa, both concentrated on the bearing side of the pin-hole contact, with interface opening of up to 1.0 mm on the opposite side. Interference-fit level had limited effect on the global response and critical location. An analytical sensitivity study identified lug shear tear-out as the governing code-based check, with a capacity-to-demand ratio of 0.370, meaning the analytical resistance was well below the reference demand for this mode; this contrasts with the FEM, which consistently indicated bearing-dominated plasticity as the dominant response instead.
The predicted stress and contact patterns agree qualitatively with the pin-lug opening and lug cracking observed experimentally, supporting a bearing-dominated failure tendency. However, the model does not include a fracture criterion and therefore cannot reproduce the post-peak force drop observed experimentally, continuing to rise beyond the experimental ultimate load instead; it also does not reproduce the experimental global stiffness, so the comparison is limited to a qualitative and semi-quantitative validation of the pre-ultimate response rather than a close quantitative match.
The developed approach reproduces the main pre-ultimate deformation mechanisms of the joint and identifies a concrete discrepancy between the analytically governing failure mode and the FEM-indicated dominant mechanism, which future work should resolve. Recommended priorities are incorporating a calibrated fracture criterion, targeted experiments on varying hole-edge distance to reconcile the shear tear-out and bearing discrepancy, and verification of the fitted-state representation and global stiffness mismatch.
Multiaxial stress response in TLP-type FOWT substructures
An Investigation into Fatigue Assessment of Critical Weld Seams in the Time Domain
In an era where global energy consumption has been rising steadily, the demand and supply of renewable energy sources have become increasingly important. Offshore wind is a promising renewable energy source, which has been actively developed in the last decades in the form of bottom-founded offshore wind, bound to shallow water. Floating Offshore Wind Turbines (FOWT) offer a solution for countries without shallow coastal waters with an abundance of wind potential. In a competitive market with different solutions, Bluewater Energy Services (Bluewater) has developed a tension leg platform (TLP) type substructure to support a 15MW turbine.
These FOWT installations are subject to the random nature of environmental loading which is highly stochastic in terms of amplitude, frequency and phase angle and is hence considered multiaxial non-proportional (random) variable amplitude loading. Multiaxial loading does not, however, guarantee a multiaxial stress response. One of the considerations for offshore structures subject to repetitive loading is fatigue lifetime estimation, which is predominantly done using uniaxial assessment methods. This is not an issue if the stress response is dominantly uniaxial, however, literature has demonstrated that evaluating a specimen subject mode-{I,III} multiaxial stress response using uniaxial (mode-{I}) assessment methods can lead to a significant overestimation of the fatigue lifetime.
To investigate the (multiaxial) stress response of fatigue-critical weld seams in TLP-type FOWT substructures, a new simulation methodology needed to be developed, which allowed for the structure's rigid body motions and elastic deformations to be captured. This methodology was built on an existing hybrid rigid-flexible modelling approach. It was improved upon by considering local (panel-based) hydrodynamic pressure and kinematic structural boundary conditions in order to conduct coupled (aerodynamic-hydrodynamic-mooring) analysis with integrated structural analysis. This approach allowed for the system motion and structural response to a dynamic metocean environment to be simulated using a range of commonly occurring seastates. The integrated coupled analysis was completely conducted within the Ansys environment.
Next, time-varying mode-{I,III} stress signals were reconstructed from the structural analysis model using a traction-based structural stress assessment method. In order to assess the extent to which fatigue-critical weld seams are subject to multiaxial stress response, a damage accumulation calculation should be conducted. However, given the added computational expense of such a calculation, a screening method was first considered in order to better understand the characteristics of stress response behaviour. The screening method considered was based on a Minimum-Circumscribed Ellipse method.
From this screening method, it was found that stress response behaviour is predictable and can be related to specific loading & response frequencies. Furthermore, it was found that parts of the considered structure were subject to significant levels of multiaxial non-proportional stress response and that applying uniaxial fatigue assessment criteria risks overestimating lifetime estimation and misplacing the governing fatigue hotspot. Finally, it was concluded that fatigue-critical weld seams in a FOWT TLP-type substructure are subject to multiaxial stress to the extent that multiaxial damage accumulation models should be considered. If this is not possible, a multiaxial stress response screening should be conducted to identify fatigue governing locations in the welds, as well as locations subject to multiaxial stress.
...
These FOWT installations are subject to the random nature of environmental loading which is highly stochastic in terms of amplitude, frequency and phase angle and is hence considered multiaxial non-proportional (random) variable amplitude loading. Multiaxial loading does not, however, guarantee a multiaxial stress response. One of the considerations for offshore structures subject to repetitive loading is fatigue lifetime estimation, which is predominantly done using uniaxial assessment methods. This is not an issue if the stress response is dominantly uniaxial, however, literature has demonstrated that evaluating a specimen subject mode-{I,III} multiaxial stress response using uniaxial (mode-{I}) assessment methods can lead to a significant overestimation of the fatigue lifetime.
To investigate the (multiaxial) stress response of fatigue-critical weld seams in TLP-type FOWT substructures, a new simulation methodology needed to be developed, which allowed for the structure's rigid body motions and elastic deformations to be captured. This methodology was built on an existing hybrid rigid-flexible modelling approach. It was improved upon by considering local (panel-based) hydrodynamic pressure and kinematic structural boundary conditions in order to conduct coupled (aerodynamic-hydrodynamic-mooring) analysis with integrated structural analysis. This approach allowed for the system motion and structural response to a dynamic metocean environment to be simulated using a range of commonly occurring seastates. The integrated coupled analysis was completely conducted within the Ansys environment.
Next, time-varying mode-{I,III} stress signals were reconstructed from the structural analysis model using a traction-based structural stress assessment method. In order to assess the extent to which fatigue-critical weld seams are subject to multiaxial stress response, a damage accumulation calculation should be conducted. However, given the added computational expense of such a calculation, a screening method was first considered in order to better understand the characteristics of stress response behaviour. The screening method considered was based on a Minimum-Circumscribed Ellipse method.
From this screening method, it was found that stress response behaviour is predictable and can be related to specific loading & response frequencies. Furthermore, it was found that parts of the considered structure were subject to significant levels of multiaxial non-proportional stress response and that applying uniaxial fatigue assessment criteria risks overestimating lifetime estimation and misplacing the governing fatigue hotspot. Finally, it was concluded that fatigue-critical weld seams in a FOWT TLP-type substructure are subject to multiaxial stress to the extent that multiaxial damage accumulation models should be considered. If this is not possible, a multiaxial stress response screening should be conducted to identify fatigue governing locations in the welds, as well as locations subject to multiaxial stress.
...
In an era where global energy consumption has been rising steadily, the demand and supply of renewable energy sources have become increasingly important. Offshore wind is a promising renewable energy source, which has been actively developed in the last decades in the form of bottom-founded offshore wind, bound to shallow water. Floating Offshore Wind Turbines (FOWT) offer a solution for countries without shallow coastal waters with an abundance of wind potential. In a competitive market with different solutions, Bluewater Energy Services (Bluewater) has developed a tension leg platform (TLP) type substructure to support a 15MW turbine.
These FOWT installations are subject to the random nature of environmental loading which is highly stochastic in terms of amplitude, frequency and phase angle and is hence considered multiaxial non-proportional (random) variable amplitude loading. Multiaxial loading does not, however, guarantee a multiaxial stress response. One of the considerations for offshore structures subject to repetitive loading is fatigue lifetime estimation, which is predominantly done using uniaxial assessment methods. This is not an issue if the stress response is dominantly uniaxial, however, literature has demonstrated that evaluating a specimen subject mode-{I,III} multiaxial stress response using uniaxial (mode-{I}) assessment methods can lead to a significant overestimation of the fatigue lifetime.
To investigate the (multiaxial) stress response of fatigue-critical weld seams in TLP-type FOWT substructures, a new simulation methodology needed to be developed, which allowed for the structure's rigid body motions and elastic deformations to be captured. This methodology was built on an existing hybrid rigid-flexible modelling approach. It was improved upon by considering local (panel-based) hydrodynamic pressure and kinematic structural boundary conditions in order to conduct coupled (aerodynamic-hydrodynamic-mooring) analysis with integrated structural analysis. This approach allowed for the system motion and structural response to a dynamic metocean environment to be simulated using a range of commonly occurring seastates. The integrated coupled analysis was completely conducted within the Ansys environment.
Next, time-varying mode-{I,III} stress signals were reconstructed from the structural analysis model using a traction-based structural stress assessment method. In order to assess the extent to which fatigue-critical weld seams are subject to multiaxial stress response, a damage accumulation calculation should be conducted. However, given the added computational expense of such a calculation, a screening method was first considered in order to better understand the characteristics of stress response behaviour. The screening method considered was based on a Minimum-Circumscribed Ellipse method.
From this screening method, it was found that stress response behaviour is predictable and can be related to specific loading & response frequencies. Furthermore, it was found that parts of the considered structure were subject to significant levels of multiaxial non-proportional stress response and that applying uniaxial fatigue assessment criteria risks overestimating lifetime estimation and misplacing the governing fatigue hotspot. Finally, it was concluded that fatigue-critical weld seams in a FOWT TLP-type substructure are subject to multiaxial stress to the extent that multiaxial damage accumulation models should be considered. If this is not possible, a multiaxial stress response screening should be conducted to identify fatigue governing locations in the welds, as well as locations subject to multiaxial stress.
These FOWT installations are subject to the random nature of environmental loading which is highly stochastic in terms of amplitude, frequency and phase angle and is hence considered multiaxial non-proportional (random) variable amplitude loading. Multiaxial loading does not, however, guarantee a multiaxial stress response. One of the considerations for offshore structures subject to repetitive loading is fatigue lifetime estimation, which is predominantly done using uniaxial assessment methods. This is not an issue if the stress response is dominantly uniaxial, however, literature has demonstrated that evaluating a specimen subject mode-{I,III} multiaxial stress response using uniaxial (mode-{I}) assessment methods can lead to a significant overestimation of the fatigue lifetime.
To investigate the (multiaxial) stress response of fatigue-critical weld seams in TLP-type FOWT substructures, a new simulation methodology needed to be developed, which allowed for the structure's rigid body motions and elastic deformations to be captured. This methodology was built on an existing hybrid rigid-flexible modelling approach. It was improved upon by considering local (panel-based) hydrodynamic pressure and kinematic structural boundary conditions in order to conduct coupled (aerodynamic-hydrodynamic-mooring) analysis with integrated structural analysis. This approach allowed for the system motion and structural response to a dynamic metocean environment to be simulated using a range of commonly occurring seastates. The integrated coupled analysis was completely conducted within the Ansys environment.
Next, time-varying mode-{I,III} stress signals were reconstructed from the structural analysis model using a traction-based structural stress assessment method. In order to assess the extent to which fatigue-critical weld seams are subject to multiaxial stress response, a damage accumulation calculation should be conducted. However, given the added computational expense of such a calculation, a screening method was first considered in order to better understand the characteristics of stress response behaviour. The screening method considered was based on a Minimum-Circumscribed Ellipse method.
From this screening method, it was found that stress response behaviour is predictable and can be related to specific loading & response frequencies. Furthermore, it was found that parts of the considered structure were subject to significant levels of multiaxial non-proportional stress response and that applying uniaxial fatigue assessment criteria risks overestimating lifetime estimation and misplacing the governing fatigue hotspot. Finally, it was concluded that fatigue-critical weld seams in a FOWT TLP-type substructure are subject to multiaxial stress to the extent that multiaxial damage accumulation models should be considered. If this is not possible, a multiaxial stress response screening should be conducted to identify fatigue governing locations in the welds, as well as locations subject to multiaxial stress.