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. ... Read more

This thesis examines the occurrence and interpretation of compressive wave phenomena in subsea power cables during normal lay operations, with a focus on addressing a central uncertainty in the field. In current industry practice, the compressive behaviour predicted by numerical cable-lay simulations is frequently regarded as a limiting factor for installation, however, a significant research gap remains regarding whether these compressive waves correspond to actual physical cable responses or are merely artefacts of modelling assumptions. Accordingly, the primary aim of this research is to clarify this gap by evaluating whether the compressive waves observed in OrcaFlex simulations can be substantiated as phenomena that occur during normal lay operations.

To address this question, a comparative modelling, verification, and field-data assessment approach is adopted. First, the theoretical possibility of axial compression is demonstrated using a simplified Kirchhoff rod formulation, showing that time-dependent transverse excitation can induce compressive forces near the touchdown region. Subsequently, a Python-based finite element model is developed to capture both static and dynamic cable responses and is compared with OrcaFlex simulations. The validation strategy proceeds stepwise: starting with analytical catenary theory, followed by static verification across multiple cable types, and finally dynamic comparison using measured operational input data.

This thesis investigates the occurrence and interpretation of compressive wave phenomena in subsea power cables during installation operations. In industry practice, the compressive behaviour predicted by numerical cable-lay simulations is often treated as a limiting factor, although it remains unclear whether these compression waves reflect a physical cable response or are partly due to modelling assumptions. The objective of this research is therefore to assess whether the compressive waves observed in OrcaFlex simulations correspond to phenomena that can occur during normal lay operations.

To address this question, a comparative modelling, verification, and field-data assessment approach is adopted. First, the theoretical possibility of axial compression is demonstrated using a simplified Kirchhoff rod formulation, showing that time-dependent transverse excitation can induce compressive forces near the touchdown region. Subsequently, a Python-based finite element model is developed for both static and dynamic cable response and is compared with OrcaFlex simulations. The verification strategy proceeds stepwise: starting with analytical catenary theory, followed by static validation across multiple cable types, and finally dynamic comparison using measured operational input data.

The simulations are driven by manufacturer cable data and operational data obtained from DEME's cable-lay vessel Living Stone, including vessel motion data from Grafana and onboard sources, as well as ROV observations of the touchdown region. Within the simplified Python model, vessel motions are translated into chute boundary motions, allowing measured operational data to be applied to the cable model in a controlled manner. And the touchdown point is added as an extra boundary condition to verify the catenary shape.

However, an important limitation of this approach is that no sufficiently complete dynamic field dataset was available for the specific severe conditions in which compressive waves are observed in OrcaFlex. As a result, the Python model could only be verified for relatively mild or near-static conditions, and not for a measured case in which the suspected compressive-wave behaviour occurred. Consequently, the combined Python model and vessel data approach is not sufficient on its own to fully answer the main research question.

For that reason, an additional tension analysis is carried out using recorded cable-lay data. Time periods corresponding to normal lay operations are selected and screened based on the availability of reliable tensioner measurements. Particular attention is given to tension peaks, irregularities, and intervals in which negative tension values are observed, as these may indicate conditions relevant to compressive behaviour. These selected cases are subsequently analysed in OrcaFlex using the appropriate vessel motions and cable properties, while the corresponding ROV footage is reviewed in parallel to investigate whether any visually observable compressive behaviour can be identified.


The tensioner-selected cases did not show clear travelling compressive waves in the corresponding OrcaFlex simulations. Although several cases contained irregular or negative tensioner readings, these did not develop into sustained regions of axial compression near the touchdown point. This indicates that negative tensioner values should not be interpreted directly as evidence of compressive-wave behaviour in the suspended cable span, since the tensioner measurement represents a differential force across the onboard tensioner system rather than the true top tension of the free-hanging cable.

Because the project-data-based cases did not reproduce compressive waves, an additional parameter-variation study was conducted. The bending stiffness, touchdown point location, and imposed chute motion were varied to investigate which factors can trigger the phenomenon. The results show that changing the bending stiffness or touchdown point location mainly influences the local cable response but does not, in itself, generate travelling compressive waves. The imposed chute motion, and especially the resulting chute velocity, was found to be the dominant trigger. Compressive waves were only reproduced when the imposed boundary motion became sufficiently dynamic.


The thesis, therefore, combines theoretical analysis, numerical model development, validation, tensioner-based case selection, and parameter variation to investigate compression in subsea power cables from multiple perspectives. The results show that compressive waves were not observed in the analysed normal lay operations. However, the phenomenon cannot be entirely ruled out as physically possible under more severe dynamic excitation. The main conclusion is that the available project data were not dynamic enough to reproduce compressive waves. At the same time, the numerical parameter study indicates that sufficiently high chute velocity can trigger the phenomenon. Further validation requires complete operational datasets from more dynamically severe installation conditions.

... Read more

From the existing literature, no scaled models have been reported that combine slamming on cylindrical bodies with global structural compliance. The absence of validated construction methods hinders experimental validation of slamming predictions, which constitute a design-driving load case for the presented structure. To enable controlled investigation of slamming-induced structural response, this study presents the design, construction, and experimental validation of a scaled flexible model representing a three-floater platform using a beam-based structural idealisation. A coupled FEM–BEM numerical framework was developed to design the model based on scaled wet natural frequencies and global mode shapes. The beam system was fabricated using SLA printing with Tough2000 resin, while the floaters were constructed as rigid elements to concentrate flexibility within the connecting beams. Static and dynamic modal testing was performed in both dry and wet configurations. Following material calibration, the numerically predicted natural frequencies and dominant global mode shapes showed good agreement with experimentally identified modes. Natural frequency deviations remained within 7%, and strain-based measurements confirmed accurate reproduction of the primary global deformation patterns. The results demonstrate that the proposed idealised beam and additive manufacturing approach provides a viable methodology for representing global hydroelastic behaviour of Merganser-like platforms ... Read more

The exploitation of the wind as a renewable energy source has increased over the last years. Transitioning to offshore wind means an increased wind energy potential. Bottom-founded wind turbines are suitable for shallow waters, but floating solutions are needed for greater water depths. Barge-type floating wind turbines can offer advantages compared to other competing alternatives in terms of manufacturability, logistics, and installation in shallower water depths. Combining the barge with a single mooring line layout enables a reduction in material costs, utilizing the weathervane principle, and decreasing the wake effects in the wind farm, therefore increasing power output. The WindBarge from Green Floating Marine Structures (GFMS) was studied in this thesis, which integrates, as a first design approach, a 15 MW upwind turbine on a barge-type structure with vertical side walls, horizontal bottom, and one mooring line including internal redundance.

Few floating wind turbine designs exist with a barge-type floater, and the design of the inner structure has not been investigated thoroughly in academic research. The inner design of the WindBarge is established by the WindBarge project, however, further modifications of the inner structure are proposed in this study. Its structural response is associated with the external loading and eigenfrequencies, providing an insight into the stress field of the inner structure for these types of structures based on ultimate limit state load cases. The simulation procedure using DNV's software suite SESAM was implemented.

Apart from extreme loads, fatigue can be a driving design factor due to the dynamic wind and wave loading. A focus was given on the tower transition region in this study. The external loading and the complex geometry can lead to a multiaxial stress state, and only a few studies in academic research have addressed this topic for floating wind turbines. A multiaxial screening method was utilized based on the 2-dimensional stress space representation and a minimum volume enclosed ellipse (MVEE) to identify the level of non-proportionality. Multiaxial conditions were observed at the deck, and the fatigue damage was calculated based on fatigue limit state load cases.

Three different approaches were implemented and compared for the fatigue assessment: the nominal uniaxial fatigue based on the forces/moments from the beam model of the tower, the hot spot plate fatigue, and the hot spot plate multidirectional fatigue, based on an effective stress according to DNVGL-RP-C203. The multidirectional approach considers different stress directions, therefore, different crack planes to identify the critical plane. For the fatigue damage calculation using the shell model of the floater, the response reconstruction method with SESAM Core was utilized.

Supplementary studies were carried out on how to reduce the observed difference in fatigue damage around the tower base caused by the roll motions, focusing on the cause of these motions, the coupling of the different degrees of freedom of the floater, and the effect of increased damping, roll stiffness, and the yaw controller. A conclusion is made that the design of such a floater is mainly fatigue rather than ultimate limit state driven.

... Read more

Fatigue design for maritime structures currently relies on deterministic models of idealised geometries, which do not account for the substantial variability introduced by misalignments and imperfections. A probabilistic framework is presented that models imperfections as spatially varying random fields or random variables, using the Stochastic Hungry Horse model, Spectral Representation Method, Halton sampling, and Bayesian updating. Applied to a finite element model of a stiffened panel with Non-Watertight details, results show that local plate distortions reduce stresses under water pressure but increase them under global hull girder bending. Stiffener distortions govern the Hot Spots at the Non-Watertight details, producing both tensile and compressive stress concentration factors. Comparison with guidelines indicates that current safety factors are non-conservative, as secondary bending effects are not addressed. The framework captures fabrication- and assemblyinduced variability and supports data-driven refinement of fatigue criteria. ... Read more

This master’s thesis investigates the fatigue resistance similarity between small-scale test specimens and large-scale structures, specifically focusing on steel welded joints. The primary goal is to improve the understanding of how fatigue data from small-scale tests can be reliably applied to predict the performance of full-scale maritime structures, thereby reducing conservatism in design and optimizing material usage without compromising safety. Fatigue resistance is a critical factor in the design and maintenance of maritime structures. Traditional design methods often use S-N curves derived from small-scale specimen tests, which can be overly conservative when applied to full-scale structures. This conservatism, while ensuring safety, leads to increased material use and associated costs. The aim is to identify and quantify the scaling phenomena that influence the transfer of fatigue data from small-scale specimens to full-scale structures. By proving fatigue resistance similarity and understanding these scaling effects, the research seeks to refine the fatigue design process, thereby enhancing efficiency and reducing environmental impact. The research utilizes various fatigue assessment concepts, including the Nominal Stress Concept (NSC), Hot Spot Structural Stress Concept (HSSSC), and the Effective Notch Stress Concept (ENSC). Finite Element (FE) models of large-scale specimens, created using Abaqus, are used to test these concepts. The study also explores the application of mean stress correction models to improve the fit of large-scale data within small-scale data scatter bands. The methods are evaluated based on their ability to incorporate local geometry information and their effectiveness in reducing scatter in fatigue data. The study demonstrates that incorporating local geometry information is crucial for achieving fatigue resistance similarity between small-scale and large-scale specimens. The HSSSC and ENSC, which account for local geometrical variations, provide better fits for large-scale data within the small-scale data scatter bands compared to the NSC. This confirms the hypothesis that local weld geometry plays a significant role in fatigue resistance similarity. Furthermore, applying a mean stress correction improves the alignment of large-scale data with small-scale data, highlighting the importance of considering residual stresses and load ratios in fatigue assessments.
The findings can have practical implications for the design and maintenance of maritime structures. By improving the accuracy of fatigue life predictions, the research supports the development of more efficient and cost-effective designs. The insights gained from this research help address uncertainties and improve design assumptions for vessel fatigue performance. By incorporating local geometry information and applying mean stress corrections, the research provides a more accurate and less conservative approach to fatigue life prediction. These advancements have the potential to reduce material usage and costs in maritime structure design, while maintaining high safety standards. ... Read more

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.
... Read more

In marine structures, fatigue is typically the dominant limit state. The fatigue resistance of welded joints in steel marine structures is a critical aspect of ensuring the structural integrity and safety of the components. To determine the structural response, finite element method is necessary to evaluate structural stress. For thin-wall structures, shell element model is usually preferred to solid element model, because the former can save time in calculations. In order to study the effect of weld modeling in shell model, the structural stress levels of the shell model in both the as-welded and unwelded cases are investigated respectively, using the solid element model results for reference purposes. The first chapter is the introduction and the second chapter is literature survey of state-of-the-art research. Chapter 3 and chapter 4 focus on the structural stress estimation with respect to frame-stiffener structure and stiffener-plate structure. The traction forces based procedure is clarified and applied to evaluate HS structural stress. In consequence, the preliminary conclusion on the influence of weld modeling on the structural stress of the shell model is obtained.

Apart from that, fatigue resistance data for arc-welded joints are typically derived from small-scale specimens. However, structural aspects can affect fatigue resistance characteristics. Therefore, to validate whether the fatigue resistance observed in small-scale specimens accurately represents that of full-scale structure, similarity research is necessary. In Chapter 5, the effective notch stress of large-scale specimens is evaluated. Fatigue failure criteria are discussed to derive the appropriate lifetime. Finally, the LSS fatigue resistance data evaluated by the effective notch concept are substituted into SSS fatigue resistance scatter band, to validate the similarity between SSS and LSS. ... Read more

Maritime structures, such as offshore support vessels and floating wind turbines, face time-varying loads from environmental elements like wind and waves, as well as operational loads from machinery. This exposure leads to fatigue, a progressive cracking process triggered by cyclic loading, which is a significant concern for structural integrity. Fatigue cracks often originate at stress concentration points, which can occur at different scales due to material defects and structural features. In steel structures, welded joints are particularly vulnerable due to their stress-concentrated geometry, making fatigue analysis crucial for their longevity.

Fatigue in maritime structures is inherently multiaxial due to various external loads and complex structural responses, involving mode-I, mode-II and mode-III loading. In typical maritime applications, mode-I induced damage is inherently governing, but multiaxial loads from torsional moments and out-of-plane forces can also impact fatigue life. Modelling these multiaxial loads accurately is necessary to predict fatigue life in welded joints, which experience both fatigue initiation and crack growth phases.

Fatigue life modelling involves assessing both intact and cracked geometries. For welded joints, fatigue life is typically crack-growth dominated due to inherent defects from welding. Linear elastic models, such as Basquin’s equation, often relate stress to fatigue life, showing a log-log linear relationship. Mode-I and mode-III loading modes differ in their strength and mechanism contributions, which can vary based on whether the load cycles are proportional or non-proportional.

The research focuses on a systematic approach for multiaxial fatigue analysis of arc-welded joints in steel maritime structures. The goal is to develop an accurate, reliable, and simple methodology for assessing fatigue life. This includes selecting appropriate fatigue criteria and developing robust cycle-counting techniques for handling non-proportional multiaxial loads. A von Mises stress-based failure criterion is used, supported by a mode-dependent shear strength coefficient to improve the accuracy of fatigue predictions. Literature-derived coefficients help capture the distinct behaviour of mode-I and mode-III contributions.

To evaluate mode-III response characteristics, new formulations for weld toe notch stress distributions were developed. A linear damage accumulation model offers simplicity in life estimation, while semi-analytical methods improve prediction accuracy. Both intact and cracked geometry parameters show reliable life estimates, though each brings specific strengths – the former for averaging capability and the latter for detailed physics of fatigue.

Fatigue resistance data available in the literature were used to create a general S-N design curve for typical weld quality. For high-quality welds, new fatigue tests conducted using a specially designed hexapod platform revealed higher resistance than standard predictions. This high-capacity, six-degree-of-freedom testing apparatus allows rigorous multiaxial fatigue assessments, filling gaps in non-proportional load data and providing a foundation for improved life predictions.

Ultimately, by combining theoretical modelling, experimental data, and novel testing methods, this research advances multiaxial fatigue assessment in maritime steel structures, with findings that emphasize the critical need for mode-specific contributions and the potential benefits of enhanced testing for accurate fatigue life predictions. ... Read more

Ships at sea encounter wave-induced cyclic loading, meaning fatigue is a governing limit state. The structural response introduces stress concentrations, hotspots, at notch geometry like welded joints locations. At the same time, the welding process typically introduces defects, meaning welded joints are typically the governing fatigue-sensitive details. Accurate predictions of the fatigue lifetime are paramount for ensuring safety and economic viability in the maritime sector.

Because of the welding-induced defects, the fatigue lifetime of welded joints is growth rather than initiation-dominated; adopting a cracked geometry-based fatigue damage criterion like the stress intensity factor seems straightforward. Weld notch characteristic crack growth behaviour, however, becomes crucial. Since both short and long crack growth are considered important, a typical one-stage Paris relation-based long crack model must be extended, particularly since most of the welded joint fatigue lifetime is spent in the notch-affected short crack growth region. The short crack growth behaviour can be both non-monotonic as well as monotonically increasing, depending on the elastoplastic response conditions. A two-stage crack growth model, incorporating both short crack growth behaviour and the long crack growth characteristic, has already been proposed in a different project based on compact tension (CT) specimen crack growth testing results obtained using potential drop measurements. However, the corresponding strain/stress field possibly explaining the crack growth behaviour in terms of elastoplasticity is not part of the output. Images have been captured, which can be analyzed with Digital Image Correlation (DIC) to provide the field measurements.
\noindent The first aim of the research has been to explore dedicated displacement field formulations to capture the crack tip location and corresponding stress intensity factor simultaneously by developing a one-step DIC approach. The expected benefits of this approach over other DIC methods consist of an expected increase in accuracy and the elimination of a post-processing step. Current DIC methods generally exist out of a two-step approach. First, a DIC procedure is employed with a generalized kinematic basis. Secondly, a post-processing step is employed to determine the crack tip location and stress intensity factor (SIF). Specifically for the crack tip, the post-processing step generally assumes a dedicated kinematic basis function describing the field around the crack. Directly incorporating the dedicated function into the DIC procedure allows for the intermediate generalized kinematic assumptions to be eliminated.
The resulting crack size and SIF from this approach can be used to determine the crack growth relationship. This relationship can be used for the second aim of this thesis, which has been to determine the material parameters of the proposed two-stage crack growth model and validate its ability to capture the short crack growth behaviour.
However, the SIF values obtained with the developed one-step DIC do not align with the analytically obtained values. Regarding the crack growth relationship, this difference in loading introduces a mean shift. Nevertheless, the observed crack growth behaviour aligns with the trends established in prior investigations, which combined potential drop crack size results with analytically determined SIF values.
A three-step approach has been adopted to investigate the accuracy of the SIF determination:
Step 1: A direct approach using a finite element formulation based on global rather than local DIC, enforcing displacement field continuity as required for a crack path-independent SIF calculation. \par
Step 2: An indirect approach using the global DIC-based displacement field (step 1) and Williams' crack tip displacement field formulation to obtain the crack tip location and SIF at the same time.
Step 3: A direct approach using Williams' crack tip displacement field formulation to obtain the crack tip location and the SIF simultaneously.
Analyses showed that involving Williams' asymptotic solution (steps 2 and 3) for long cracks in a simple far-field stress condition is beneficial. Accurate crack tip location and SIF estimates have been obtained. However, for complex stress fields at notched geometries additionally containing geometry boundaries in the same region, higher order terms are required to obtain a converged displacement field. An accuracy improvement has been observed when adopting a direct approach (step 3) rather than an indirect one (step 2) approach. A global FEM-based direct approach (step 1) allows for describing complex displacement fields and obtaining SIF estimates independent of the Williams' formulation. The resulting estimates from this approach agree with the Williams-based estimations (steps 2 and 3). From this, it can be concluded that the obtained difference in the SIF follows from a fundamental difference in the load obtained by analytical determination and the DIC approaches. This conclusion is supported by the difference in the structural stress obtained from the DIC approaches compared to the analytical estimate.
The second aim of this thesis requires further investigation into the loading discrepancy before the total life model can undoubtedly be validated. Nevertheless, preliminary usage remains possible with the usage of consistent SIF determination. Resulting in the establishment of model parameters using likelihood regression for the CT and as-welded joint specimens. \par
For the final aim of this thesis, the total life fatigue strength criterion is obtained by integrating the two-stage crack growth model. This criterion is applied in a case study to estimate the total fatigue lifetime of a critical welded joint in a general cargo carrier. In order to determine the estimated lifetime of the critical joint, the fatigue loading is required. While knowledge of the cargo load is available, the impact of wave-induced loading requires investigation. Two distinct approaches have been used to determine the wave loading. Initially, Four representative wave loading scenarios established by Bureau Veritas regulations are considered. The cumulative fatigue damage resulting from these scenarios is then extrapolated to encompass the entirety of loading conditions experienced by the ship.
Secondly, a wave spectrum approach is undertaken to address uncertainties in loading analysis. A hydro-structural solver is utilized to calculate loading across various sea states. The total fatigue damage is based on a weighted average of a representative sea spectrum. The fatigue lifetimes derived from these methodologies are compared to the hot spot structural stress concept as typically adopted in industry for reference purposes. A comparison of the results shows that the hot spot structural stress concept underestimates the fatigue lifetime due to the increased conservatism required in this model. ... Read more

Fatigue might be the governing limit state in marine structures mainly induced by the waves and wind. Therefore, using a lap joint for the shell plating to replicate the appearance of a riveted joint for a retro yacht is critical due to the limited fatigue resistance. For this joint type cracks initiated at the root, which should always be avoided due to hard detectability, might appear. Arc- and laser-welded lap joints are researched to estimate fatigue resistance and reduce the risk of root failure.
The hot spot peak stress of weld toe and weld root notches is insufficient to reflect the fatigue strength, explaining why an effective notch stress parameter is used, able to deal with both weld toe and weld root notch induced fatigue. Semi-analytical weld notch stress formulations are established in order to avoid solid finite element modelling to capture the effective notch stress. Particular attention is paid to the lap joint characteristic extreme weld notch load carrying level. A second-order weld load carrying stress formulation is introduced. Since the (laser) weld is typically small in comparison to the plate thickness, a dedicated way of weld modelling is proposed, assuming a shell finite element type of formulation, allowing to capture the far field stress for both the weld toe and weld root notch accurately.
Fatigue tests are performed to obtain the laser-welded lap joint mid-cycle fatigue resistance information, including the material characteristic length as effective notch stress coefficient. Particular attention is paid to mean (residual) stress effects in comparison to the arc-welded joint equivalent to explain the fatigue performance.
Comparing the lap joint fatigue resistance in arc-welded and laser-welded configurations to the resistance of other joint types, the results turned out to be aligned. Investigating different material characteristic parameters, either an effective point or an effective line is used. A line based parameter shows a better performance. The main reason is that the actual notch stress gradient is explicitly considered. The mean (residual) stress seems to be larger for arc-welded joints. Furthermore, the mean (residual) stress correction seems at the same time to be one reason for toe induced- rather than expected root induced failure. For arc-welded joints, the change in failure location induced by stress level dependent secondary bending moments is another reason. The fatigue resistance parameter confidence for laser-weld weld joints is relatively low in comparison to the confidence for arc-welded ones, as a result of data size: respectively ≈190 and ≈3000.
When not considering the change in secondary bending moments in the fit of a Se-N curve, the overall fit parameters and quality do not change significantly for arc-welded lap joints. However, the influence on the effective stress range is substantial and changed the value on average by 4.7 % and up to 14.7 %.
Considering only the mid-cycle fatigue resistance, laser welding is superior to arc welding when exposed to low load and response ratios resulting from the lower residual stress level. The FAT class design curve of arc-welded joints for the hot spot structural stress concept with a load and response ratio of 0.5 can be used with a mean stress correction to estimate the fatigue resistance of laser welds. ... Read more

With the growing demand for high productivity and fabrication efficiency in building marine structures at European shipyards, the Robotic Survey, Repair and Agile Manufacture (RESURGAM) H2020 project has been initiated. The aim is to develop Friction Stir Welding (FSW) for the industry: an efficient solid-state joining process. Originally developed for the welding of aluminumand magnesium, FSW involves a fast-rotating tool being pushed into the material, moving along the interface of the structural members to be joined. The rotating tool introduces friction-induced heat to soften and mix the material in order to establish a joint. Given the many technical, environmental, and economic advantages of the FSW process, the aim is to introduce the process to the welding of steel. However, FSW locally modifies the material, introducing different zones, meaning the distribution of local mechanical properties like the Youngs modulus E is highly inhomogeneous and may vary from one location to another. In order to establish the distribution, a field measurement technique like Digital Image Correlation (DIC) can be adopted. This thesis has aimed to obtain distinguished elastic material properties for the parent material and nugget material zone, including the most likely transition boundaries using DIC. First, Texture quality analysis was implemented to determine the feasible subset size. Following the experiment and simulation setup 2D DIC algorithms like Augmented Lagrangian DIC (ALDIC) and Reliability-Guided Digital Image Correlation (RG-DIC) have been compared for formulation performance evaluation and practical measurement in a further stage. For the comparison reference, the unfiltered measurement results from global DIC algorithm Correli and results from 2D FEM analysis were employed. A Brutal Force method was adopted in order to identify the weld nugget zone boundary lines.
Finally, the local elastic mechanical property parameters like Young’s Modulus and Poisson’s Ratio were calculated for the specified material region. And conclusions were made that plastic material properties would have appeared under the load level of 400MPa. Future outlooks based on the research results and failures were discussed in the last chapter. ... Read more

Fatigue lifetime has proven to be a leading design parameter for naval vessels. In the field of fatigue analysis and design of welded joints, two overall methods are distinguished: the temporal and spectral approach. Using empirical models, the spectral approach is typically preferred due to its efficiency, even though damage estimates are conservative. With the temporal approach, nonlinear effects can be captured, providing higher accuracy at the cost of additional computation time.

Within this framework, an improvement of the spectral approach was pursued, focusing on incorporating the mean stress effect. Past efforts to include mean stress corrections were continued by focusing on applying the Walker correction.
Nonlinear effects on the mean stress response were investigated and were found to be quite small for the case of a pontoon vessel. This enabled the use of the still water bending moment as a single input to the spectral mean stress model. By using Walker in the spectral mean stress correction, an overall reduction of conservatism of 21% was achieved.

Future research should focus on testing new wave cases, applying additional fatigue models and investigating the effects for hull shapes of naval vessels. ... Read more

Due to the growing quest for renewable energy, wind turbines grow in size. This means that higher demands are posed on the transporting structures, also called tower grillages, which brings its challenges for their design. The fact that the structures considered in this thesis are placed on a ship makes the design even more complicated. Therefore, a procedure is developed in this thesis that is able to optimize a tower grillage, while keeping the underlying ship intact. This tool is based on a combination of size and shape optimization.

The optimization is performed with the dual annealing algorithm. The objective is mass reduction, with the stress in the grillage, and stress and buckling in the ship as constraints. These constraints are taken into account by a penalty function. The design variables are the angles at which the radial supports attach and the thicknesses of the radial supports and the other elements of the grillage. The main question that is answered in this thesis is how this optimization procedure can help in the design of a tower grillage, where the underlying ship structure is implemented as a boundary condition.

First, a single layout is optimized with the ship modelled as a rigid boundary condition. Next, a model of a section of the ship is made and used as the boundary condition on the tower grillage. A comparison between these models showed that the main differences in stress between the two optimization results
are seen in the largest parts of the grillage: the sides, the flanges and the can. The maximum stress in the latter two elements is larger, which also results in larger thicknesses in the optimization with the ship. This affects the average mass in the ship, which is increased with 5.7% when compared to the grillage on the rigid constraint.
After that, a full grillage is optimized. A first attempt demonstrated that the stress in the ship could only be decreased a little with the initial settings of the optimization. That required a slightly different model, to make sure that the stress in the ship remains acceptable. With that model, a mass decrease
of 32 tons could be obtained, which is a decrease of 9.1 % compared to the original design by Vuyk Engineering.

From the different optimization steps became clear that the main factors influencing the optimal distribution of radial supports are the lengths of the supports, and the location of the outer brackets. The compliance of the ship changes the stresses in the grillage, and therefore the optimization result. The
stress in the ship can be reduced only to some extent, so the effect of a stress violation in the initial design is that a new bracket design needs to be made. The results show that the stress is governing for the current optimization, and buckling is not.
The conclusion is that the current procedure can help in the design by providing a global image of lighter layouts which avoid stress and buckling constraint violations in the ship. However, the limitation of the research is that the result is only a global image. It is therefore considered not worth the effort and
time to apply the current method in an engineering environment. It does however show the potential of this method, so future enhancements can make the procedure suitable for engineering. ... Read more

Fatigue is typically a governing limit state for maritime structures, and weld joints are the most critical locations. Various fatigue damage criteria have been developed involving either an intact or cracked geometry parameter, incorporating local or global information. In this thesis, the effective notch stress based total life fatigue damage criterion has been used in order to improve the accuracy of fatigue lifetime prediction and reduce the workload of engineers in the early design stage. First, the Battelle structural stress calculation procedure is clarified and examined with three models. A hot spot type A, B and C mesh convergence study are conducted. In order to quantify the influence of mesh quality, several ugly mesh models are tested with their good mesh counterparts. For validation purposes of the total life model, a large scale specimen fatigue test result has been used and the mesh quality investigations are included. The DNVGL based life time estimate is provided for the sake of comparison. Finally, several conclusions and recommendations are provided for future applications. ... Read more

Solar energy is considered to be one of the most promising energy alternatives among many renewable energy sources. On the pathway towards achieving climate goals, dramatic scale up of clean technologies is required, however the installation of solar energy has the burden of intense land requirement. Therefore, the onset of floating solar technology can be the turning point to give incentive for more substantial deployment of solar energy, avoiding land occupancy concerns. Besides the potential of installing bodies on water, floating solar brings the benefit of an efficiency increase superior to 10 percent in relation to land based solar, depending on the solar panel technology and environmental conditions. This efficiency increase can be attributed to the natural cooling effect of the seawater, located below the solar platform. ... Read more

The current fatigue assessment methods for naval ships are intentionally conservative. This is in part due to uncertainties in the fatigue capacity (i.e. fatigue resistance) of the ship: the nominal and hot spot structural stress incorporate implicit conservatism due to the inherent data-scatter and the Linear Damage Accumulation Model (LDAM) by Palmgren and Miner is not sufficiently accurate to assess random variable amplitude loading histories. The safe-life engineering solution to this is to employ a safety factor. This typically reduces the critical damage from 1.0 to 0.5, or even 0.2, based on engineering judgement. The main flaw of the Linear Damage Accumulation Model is that it does not properly account for the effects of the loading history. To address this flaw the non-linear probabilistic fatigue damage accumulation model by Leonetti is researched and thoroughly validated with variable amplitude fatigue data from the literature. The combination between the effective notch stress and the model from Leonetti poses significant improvements in the quality of the fatigue life predictions. The effective notch stress is used to formulate a generalised fatigue resistance curve (reducing scatter due to different geometries, hot spot types and mean stress ratios), after which the non-linear probabilistic fatigue damage accumulation model is used to work towards a generalised damage accumulation model. For the considered variable amplitude fatigue databases from literature, the Effective Notch Stress Concept (ENSC) - Non-Linear Damage Accumulation Model (NLDAM) combination proves to reduce the lifetime ratio scatter index (1:1.13, instead of 1:1.28) the most in relation to the DNV method (Hot Spot Structural Stress concept with the LDAM), allowing to reduce the implicit conservatism in the fatigue-life model. ... Read more

Fatigue strength is typically a governing limit state for marine structures. Predicting an accurate fatigue lifetime is important for structures (e.g. wind turbines) that are supposed to be out at sea for decades. It is valuable for all type of structures that encounter stochastic sea loadings to optimise the design so that fatigue failure is predicted for, or material is saved in preventing over conservative estimates. This research contributes to the investigation of multiaxial fatigue concepts by getting insight in the mode-III loading & response conditions for the effective notch stress concept. The mode-III loading & response will in this research be represented by a tubular welded joint loaded under torsion. A semi-analytical formulation is developed to describe the weld notch shear stress distribution analytically. This formulation can be used as input for the effective notch stress concept. The formulation is based on three components, the notch stress, weld load carrying stress and far field stress component. Furthermore, the static force and moment equilibrium is satisfied. The weld load carrying stress component is based on a weld load carrying stress estimate. This coefficient is used because the weld geometry causes a local change in stiffness, a shift in neutral axis, meaning the weld becomes load carrying up to some extent. The distribution must be corrected for this phenomenon which is done by this estimated term. The geometry is a tubular welded joint. This specimen can be loaded with torsion which in the case of the tubular joint cause mode-III shear in the through thickness direction at the weld notch level. The weld load carrying stress estimate is based on the geometry ratios of the assessed specimens. It is individually fitted for all 6,300 geometries by comparing the results to stress distribution obtained with 2D finite element analysis. The results of the individual fitted load carrying stress estimates are used as input of a multi variable polynomial regression analysis. The regression analysis led to a polynomial function that describe the load carrying stress estimate which uses geometry ratios as input. The far field information and geometry dimensions are the input for the semi-analytical formulation to obtain the weld notch shear stress distributions analytically. Accurate results are obtained and therefore the formulation can be used as input for the effective notch stress concept. The effective notch stress concept is also based on a material characteristic length, ρ*, over which the stress distribution can be integrated to obtain the effective notch stress. This parameter is obtained using a log likelihood regression analysis and led to a value of ~1 [mm]. Although this is a good result, the confidence regarding the value is low due to low variety in the experimental data set and little data in general. The findings of this study will be used in further research regarding the 4D fatigue project and the effective notch stress concept in general at the Delft University of Technology. ... Read more

Fatigue is one of the driving limit states in the design of wind turbines, as they are subject to varying loadsfrom winds, waves and gusts. One such governing area is the butt-welded connection from the shell to thering flange. To optimize the tower design and to help focus future research, more insight on the influence ofmaterial properties and geometrical features on the fatigue life of these butt welds is needed. Due to thecosts associated with testing, SGRE stated the need for a model that could assess the total fatigue life of buttwelds subject to Mode I cracking in a rather quick way. After literature study, it was proposed to use theUniGrow model.The UniGrow method is a total fatigue life model in which the material is considered to consist of elementaryblocks. Fatigue is modelled as a process of continuous crack initiation of the elementary block ahead of thecrack tip. In this model, the plastic compressive residual stress ahead of the crack tip plays an importantrole in the determination of the crack growth speed. These plastic compressive residual stresses ahead ofthe crack tip can either be determined analytically (analytical UniGrow) or by using numerical methodssuch as FEA (numerical UniGrow). The plastic compressive residual stresses ahead of the crack tip areconverted into a plastic residual SIF that is used to reduce the applied residual stress intensity factor (i.e.plasticity decreases crack growth speed). Due to the application of a number of methods such as theCreager-Paris equations, the current UniGrow implementation is solely valid for pure Mode I crack growth.Validation of the UniGrow method in the crack propagation range has been performed with results fromexperiments on CT specimens made of S355 steel. Good correspondence between the experimental resultsand predicted crack growth rates were found for all stress ratios except R = 0.25. Both Analytical andNumerical Unigrow methods have been analysed. Based upon the results of the validation it wasrecommended to use a numerical method (elastoplastic FEA) to determine the plastic residual stressesahead of the crack tip as this provides more physically accurate results. The analytical method can be usedto obtain an initial guess of the elementary block size, as it is a much faster method. Furthermore, using theMorrow method for the determination of the fatigue life of the elementary block ahead of the crack tip wasfound to provide more accurate results than the SWT damage parameter. This is due to the fact that in theelementary block ahead of the crack tip, cyclic stress relaxation occurs and, therefore, mean stress effectsneed not be accounted for. For short cracks, the fatigue life of the elementary block is usually outside theLCF range (Nf > 5*104 cycles). Here, the use of the Morrow method without considering residual stresses isnot recommended since cyclic stress relaxation does not occur due to limited plasticity.The performance of the UniGrow method as total fatigue life was studied using literature research and acomparison to another total fatigue life model: Two-Stage-Model. In the studied literature, researchers tendto use the weight function method to determine the SIF in welded geometries. Analysis comparing theweight function method to FEA results showed that the WF method generally overpredicts the SIFscompared to FEA. For a flat plate, the difference was found to be a maximum of 3.5%, whereas for the weldedgeometry, the difference was up to 12%.From the literature and the comparison of the UniGrow model to the Two-Stage-Model, it was concludedthat the UniGrow model is incapable of predicting proper fatigue crack initiation lives as it is unable tocapture the complexity of short crack growth. The model provides satisfactory results for cracks wherecrack propagation is a dominant part of the fatigue life (such as welded geometries and notched specimensin the LCF regime). It was proposed to use the UniGrow model to determine fatigue lives of notchedspecimens up to 105 cycles. It was found that this limit also depends on the SCF present in the specimen.For welded geometries, available results in literature showed good correspondence. However, the weldsused by SGRE are of better quality (lower SCF) than the ones used in UniGrow research. This means thatrelatively more of the fatigue life will be spent in fatigue crack initiation. More research is therefore neededto examine whether this affects the functioning of the UniGrow method for welds.The UniGrow method was lastly used to determine which of the parameters controlling the geometry of theweld excess (the weld flank angle, weld toe radius and weld excess height) is of greatest influence on thefatigue performance of a butt weld. From this analysis it was concluded that changing the weld excessheight, followed by the weld flank angle has the most beneficial effect on the fatigue life. The results fromthis parametric study should be used with great care: the usage of the UniGrow method combined withneglecting the residual stresses could lead to incorrect predictions of fatigue crack initiation life.Simplifications of the material and crack type could also have had an influence on the results. ... Read more

Offshore wind turbines are designed to persist harsh environmental conditions, including corrosion. Pitting corrosion is one of the most dangerous forms of corrosion. In combination with a fatigue loading, a corrosion pit could result into the failure of a structure, due to a minimal weight reduction. Of the corrosion fatigue process, the transition of a corrosion pit to a fatigue crack is one of the less understood topics. In this thesis, a combined analytical/numerical model is presented that implements mechanisms of both the fatigue, and the corrosion process to determine the moment in time and location in the corrosion pit of the pit-to-crack transition. The hypothesis used to build the model, is that the pit-to-crack transition occurs at the moment that a fatigue crack initiates. Assumptions are made to be able to predict the level of stress and strain at the surface and below the surface of a corrosion pit using a FEM model. Using the stress and strain levels, damage is accumulated until the failure criterion is reached, i.e. when a fatigue crack will initiate. In this research study, elliptical pit shapes are considered, with and without a certain roughness at the surface of the corrosion pit. From the results it is found that a more sharp shaped corrosion pit is a more favourable location for fatigue crack initiation, than a more blunt shaped corrosion pit. The number of cycles to fatigue crack initiation resulting from the developed model are higher, than the number of cycles to failure found by the DNV GL B1 SN-curve for free corrosion conditions. Also, the DNV GL SN-curve is more steep, than a fitted curve through the data points resulting from the developed model. Through a sensitivity study and by considering the number of assumptions and simplifications made in this research study, it is concluded that the spread and uncertainty of the outcome of the developed model is too large to be able to draw conclusions based on the comparison between the results and the DNV GL SN-curves. More research on the input parameters of the developed model will give more certainty and a better validity to the model. It is found that the effect of surface roughness to the fatigue crack initiation life is not substantial, because it affects the stress and strain levels only up to a relatively small depth. The location of fatigue crack initiation is in the pit bottom for blunt shaped corrosion pits, and in the pit wall for sharp shaped corrosion pits. For circular shaped corrosion pits, the difference between the fatigue crack initiation life at the pit wall and the pit bottom is negligible. It is concluded that the model gives a comprehensive basis for the prediction of the pit-to-crack transition, but more (experimental) research on the input parameters will be necessary to be able to give a substantiated estimate of structural life of offshore wind turbine foundations. ... Read more