High-strength marine grade steel is often used in naval applications where ballistic impact poses a critical threat to structural integrity. Perforation by a projectile leaves behind a hole surrounded by plastically deformed material, which alters the stress redistribution capacity of the impacted structural component and reduces its residual strength. Understanding and predicting these effects is essential for reliable damage assessment and safe structural design.
This work develops a semi-analytical stress field model for perforated EH36 steel plates that incorporates prestrain effects derived from Vickers hardness measurements around ballistic holes. The model builds upon Stowell’s elastoplastic stress concentration framework, extended with a Swift-type hardening law to capture strain hardening from prior plastic deformation.
The analytical solution results are compared with finite element analysis (FEA) and limited experimental Digital Image Correlation (DIC) data. While the validation dataset is not sufficient for full generalization, the comparisons indicate that incorporating prestrain reduces the stress concentration factor at the edge of the hole compared to analyses without prior strain.

High-strength steels (HSS) are increasingly used in advanced engineering applications due to their exceptional mechanical properties. However, accurately predicting stress fields and localized yielding in welded joints—especially at sharp geometric features such as weld toes and notches—remains a significant challenge, particularly in the inelastic region. This thesis addresses this challenge by developing and validating an analytical framework based on the Strain Energy Density (SED) method for modeling localized structural behavior in HSS welded joints.

The foundational model extends a linear elastic formulation by den Besten to account for post-yield behavior. Three analytical approaches—Neuber’s rule, Stowell’s generalized approach, and Glinka’s SED method—were critically assessed for the suitability of extending den Besten's equation, with Glinka’s method selected for its ability to capture localized plasticity while maintaining computational efficiency. The model assumes localized plasticity surrounded by elastic behavior, and its predictions were verified against two bounding solutions: Irwin’s ideal elastic–perfectly plastic lower bound and an extended linear elastic upper bound. Results consistently fell within these bounds, supporting the model’s physical accuracy.

Numerical validation was also carried out using finite element analysis (FEA) in ANSYS on a Partially Penetrated Double-Sided (PPDS) T-joint configuration. Material modeling incorporated true stress–strain data, realistic weld geometry, and boundary conditions representative of actual welded joints. The simulation results showed strong agreement with the analytical predictions, particularly in the region of localized plasticity near the weld toe where the error was limited to up to about 3%.

This study confirms that the SED-based analytical method offers a practical and accurate approach for assessing welded joints with sharp notches—regions prone to stress singularities. The framework is computationally efficient, physically grounded, and broadly applicable across welded structures. Academically, it supports future research into refining boundary definitions, material models, and mesh optimization techniques. Practically, it enables safer and more efficient designs in HSS applications by providing engineers with reliable tools for evaluating post-yield structural behavior.

Hydrogen infrastructure will play a critical role in meeting the future demand for decarbonization. Existing pipelines can be potentially repurposed, and new pipelines will be installed to transport hydrogen gas. Pipelines, especially offshore pipelines, are subjected to cyclic loading which can lead to potential fatigue damage and failure. The fatigue resistance of steel deteriorates under prolonged exposure to pressurized hydrogen due to the hydrogen embrittlement effect.
Hydrogen assisted fatigue crack growth rate (HA-FCGR) is investigated in this study. The influences of various test parameters including hydrogen gas pressure, cyclic load ratio and frequency, test temperature, and the yield strength of the steel are reviewed and analyzed. Existing models to predict the HA-FCGR are also assessed.
An updated Tri-Linear model is proposed to predict the HA-FCGR of carbon steel. The key values associated with the two knee points, which define the shape of the HA-FCGR curve, are expressed as functions of the test parameters and the yield strength of the steel. Fatigue test results digitized from existing literature are utilized to optimize the experimental constants required for the Tri-Linear model. The modeled HA-FCGR curves are compared against experimental data to demonstrate the agreement with the test results.
The updated Tri-Linear model directly correlates the hydrogen gas pressure, cyclic load ratio and frequency, test temperature, and the yield strength of the steel to the predicted HA-FCGR. Facilitated by the strong correlations, the number of experiments required to qualify the pipeline steel under the design and operating conditions can be reduced.

Understanding the failure behavior of thin-walled steel structures under large-deformation plasticity and fracture is crucial for assessment of accidental loading conditions. This thesis will experimentally determine the crack propagation direction in a thin steel sheet under different combinations of in-plane strain, which is essential for improving simulations. Necking and crack angles in a commercially available ductile steel sheet (DC01) are determined under various stress conditions, ranging from uniaxial to equi-biaxial tension. A state-of-the-art formability testing method (Marciniak) is adapted to meet the specific needs of this project. Through tensile testing, the Lankford Parameters of DC01 steel are determined, with which a theoretical assumption of the crack angle can be made using Hills theory (1952). By varying the specimen and carrier dimensions, the strain ratio range of −1/2 to near 1 is tested. A Digital Image Correlation (DIC) system is used to measure deformations and strains during the whole deforming process. Pictures taken from the DIC system and post-mortem section cuts allow for the determination of the crack angle with respect to the in-plane and through-thickness directions. The resulting crack angles agree with the theoretical angles determined by Hill (1952) when the frame of reference of determining the crack angles are rotated.

High-strength steels offer the benefit of weight, size and cost reductions in structural applications, but their use has seen limitations owing to concerns about their higher yield-to-tensile strength ratio (σ_y/σ_u) relative to lower-strength steels. A maximum limit on the σ_y/σ_u ratio is imposed as a material requirement by various steel structural design standards, product specifications and material certification rules, due to the σ_y/σ_u ratio's connection to strain hardening and the amount of plastic deformation that can be sustained without the loss of strength at locally collapsing or locally straining zones. However, the σ_y/σ_u ratio acts only as an indirect indicator of structural ductility, being one of the many factors that affect the plastic deformation capacity of a structure and whose effect and relevance vary with the structural context. This thesis hence investigates the role and applicability of the σ_y/σ_u ratio as a ductility requirement in steel design, in view of other important material and geometrical properties, with a focus on improving the assessment of ductility in steel structures, towards more efficient and confident use of steels with high σ_y/σ_u. Two mechanisms of plastic localisation that affect the structural ductility are identified as being critical to the present stipulation of σ_y/σ_u limits in design practice: local buckling in plastic hinges in stocky beams subject to bending and ultimate ductile fracture of cracked structural details.

For the plastic hinge rotation capacity of stocky beams, welded I-section high-strength steel beams are considered in this thesis, due to their widespread use in steel structures. A parametric study using a finite-element modelling approach validated by experiments in the literature, taking into account welding residual stresses, geometrical imperfections and plastic strain hardening behaviour, is performed to investigate the role of the σ_y/σ_u ratio in the plastic buckling response. Not only does the σ_y/σ_u ratio play an important role in this context, the maximum allowable σ_y/σ_u for ensuring a certain rotation capacity at a plastic hinge depends on the yield strength σ_y, the slenderness of the flange, the slenderness of the web, and the relative slenderness between the flange and web. The local ductility requirements relating to plastic hinge rotation can be made more efficient if they are made to be dependent on the σ_y, σ_y/σ_u and cross-sectional slenderness.

The maximum σ_y/σ_u ratio has also been considered to function as an indirect provision for ductility where localised tensile plastic straining is expected to occur (such as notched and cracked details). This was initially based on the concept of delaying the onset of necking instability in the structure but has subsequently come to be seen as part of a set of substitute upper-shelf toughness requirements for ensuring that the ultimate strength of structural details susceptible to cracking assumed in design can be achieved. This σ_y/σ_u requirement is implemented alongside a minimum tensile test fracture elongation (Α) and a minimum Charpy value for the avoidance of brittle fracture behaviour. However, the mechanical basis for the use of σ_y/σ_u and Α in this capacity is lacking. Recently, there has been a growing trend towards the use of minimum upper-shelf Charpy energies C_v as a better upper-shelf ductility indicator, but a clear consensus on the implications of this for the use of σ_y/σ_u and Α has yet to emerge. Furthermore, even C_v is not a direct measure of fracture toughness. The most direct way to assess the upper-shelf toughness is to perform fracture toughness tests, such as the pre-cracked single-edge notched bending tests, but these tests are deemed too costly and complicated to implement for general material certification purposes. Often, correlations between the upper-shelf notch toughness in terms of C_v and the fracture toughness in terms of the J-integral are used to estimate the material’s fracture toughness from Charpy tests. The existing correlations, however, are predominantly based on empirical findings and have not systematically considered if and how variations in the σ_y/σ_u and Α might affect the correlation between the notch toughness and the fracture toughness.

In light of this, a parametric damage-mechanics approach is used to investigate how the correlation between the notch toughness C_v from Charpy impact tests and the critical ductile fracture initiation toughness J_Q from quasi-static fracture tests is affected by changes in the fracture elongation Α, yield strength σ_y and yield-to-tensile strength ratio σ_y/σ_u obtained from tensile testing. To this end, a phenomenological rate-dependent plasticity model coupled with damage and temperature effects is developed. The validity of the modelling approach is shown by its ability to simultaneously model the tensile test, the Charpy V-notch test and the precracked single-edge notched bending test. This is demonstrated for two steels, AH36 and S690QL, capturing the force-displacement responses and the characteristic ductile fracture mechanism of slant fracture in all three tests. The applicability of simplifying assumptions used to reduce the damage model complexity while maintaining its ability to simulate the mechanical response in key tests covering an important range of stress states is exemplified. The insights gained are used to reduce the calibration effort needed for the subsequent parametric study on the correlation between material certificate data (σ_y/σ_u, Α, C_v), damage mechanics parameters and fracture toughness.

First, a correlation based on regression between the damage parameters and the mechanical properties from mill test certificates is found by calibrating the damage parameters assuming a range of material certiciate properties typical of S690Q steels. Then, the correlation between C_v and J_Q is assessed by simulating the single-edge notched bending test for the varying mill test certificate properties (σ_y/σ_u, Α, C_v), taking into account how the damage parameters change with these mechanical properties. It is seen that although varying σ_y/σ_u and Α has some effect on how the total notch energy C_v is correlated to J_Q, it does not reflect a significant effect on the ductile fracture initiation toughness but is rather associated with the fact that the C_v includes a significant portion of energy for stable ductile propagation and fracture occurring at the specimen's free surface, while J_Q primarily concerns the early, onset stage of stable ductile tunnelling behaviour at the centre of the specimen. The σ_y/σ_u and Α are seen to have an even smaller effect on the correlation between J_Q and the energy (C_vm) dissipated up to the occurrence of the peak force in the instrumented Charpy test, in comparison with the C_v--to--J_Q correlation, especially for low C_vm.

As a result of the investigation of these two key mechanisms, a shift towards more context-specific ductility criteria, accounting for the structure's failure mechanism and geometry as well as properties obtained from material testing, is recommended. Additionally, the current use of σ_y/σ_u and Α as part of a set of substitute upper-shelf toughness requirements has little basis, with σ_y/σ_u and Α showing weak correlations to ductile fracture resistance. Instead, the use of upper-shelf notch toughness parameters like C_v and C_vm, which better correlate with ductile fracture initiation toughness, is recommended as a more suitable proxy for direct fracture toughness testing. Along the way, a valuable modelling approach for damage mechanics whose relevance extends beyond the present research objectives has been demonstrated, involving a calibration process that is centred around a small number of easily accessible engineering properties and realising complex fracture simulations with reduced calibration effort.

Cylindrical structures are often used in the offshore industry for their hydrodynamic efficiency, which minimizes drag from waves and currents. Their cylindrical shape provides structural integrity and stability, evenly distributing stress to withstand marine environments. These structures are commonly used in oil rigs, wind turbines, and underwater pipelines.

This thesis investigates the different structural effects of cut-outs on thin-walled cylindrical structures, particularly in the context of pile driving. During the pile driving process, stress waves are created, which create stress concentrations around the cut-outs. A critical review of the literature has identified that the impact of these stress waves on various hole shapes within cylindrical structures is a relevant topic, particularly concerning fatigue.

To find out the impact of different hole shapes on the other limit states of a cylindrical foundation with an emphasis on the fatigue limit state multiple finite element, multiple finite element analysis (FEA) models are made, and the impact of the hole shape is measured.

First, the feasibility of the hole design is tested for the ultimate limit state (ULS) and the static fatigue limit state (FLS) using calculations performed in Ansys. Multiple models are also created in Ansys to calculate the eigenfrequencies of the cylindrical structure with different hole shapes. LS-Dyna is used for stress wave analysis. Initially, the cylindrical structure is given a large length to ensure the stress wave doesn’t reflect during the simulation, allowing the response of the hole to be measured. Finally, the cylindrical structure is placed on a realistic soil model to account for the reflecting stress wave.

The findings of this research have significant practical implications for the design and analysis of cylindrical structures in the offshore industry. They demonstrate that different hole geometries can be successfully implemented in cylindrical structures while satisfying ULS and FLS design requirements. The eigenfrequencies of the holes are excited by the stress wave that occurs during pile driving. When a soil-like structure is added, the reflecting stress wave diminishes this effect. The results suggest that elongated holes may offer superior performance compared to round holes in terms of fatigue damage resistance during pile driving.

Multiple methods exist for simulating crack propagation in the finite element method (FEM). Among these, the extended finite element method (XFEM) shows great potential as it allows for the use of larger elements while maintaining accuracy and practicality. Within XFEM, the employment of traction separation laws (TSLs) is a promising approach to capture post-necking effects, which are typically predicted poorly with larger elements. These TSLs are often assumed to be constant along the crack length and are determined by fitting to experimental data.

However, it has been proven that necking and fracture both depend on the state of stress, expressed as stress triaxiality. It is also known that the stress triaxiality within the crack region varies as the crack progresses through a plate due to changing boundary conditions and crack blunting. Consequently, the necking behaviour ahead of the crack tip changes, necessitating multiple stress state-dependent TSLs along the crack length to capture this effect accurately.

In this thesis, a subroutine is developed to enable the use of multiple TSLs along an extending crack in Abaqus based on the stress triaxiality of each element. Additionally, the relationship between TSL parameters and stress triaxiality was determined by minimizing the difference between simulations and experimental data of a large-scale CCT experiment. With this relationship known, it is possible to account for the effect of the changing stress state when determining the TSLs.

The study finds that stress triaxiality-dependent TSLs produce a significantly better match with experimental results compared to a stress triaxiality invariant TSL. This underscores the importance of addressing stress-state dependency in TSL determination. The study also highlights that mesh size and material dependency on TSLs must be addressed so that these TSLs can be applied universally across different conditions.

It is becoming increasingly clear that the effects of climate change should be decreased or even mitigated. Green alternative sources of energy are being explored, and wind energy emerges as an important option that can be exploited on a large scale. Wind turbines are placed more and more often offshore due to larger and more stable wind resources. The most common foundations for these turbines are monopiles. The future outlook for these turbines and their foundations is that they will become bigger. An important design characteristic of monopiles is the natural frequency of the pile. In order to keep dynamic effects to a minimum, excitation frequencies should not coincide with the natural frequency. The primary source of excitation of monopiles are bending moments. Therefore, this thesis will take a closer look at the dynamic bending capability of monopiles.

There is a lack of knowledge on dynamic effects of bending moments on steel cylindrical shells (monopiles). While the dynamic axial buckling capacity of cylindrical shells has been researched extensively, there is little known on the dynamic bending buckling capacity of such shells. This thesis investigates the influence of loading rate on the dynamic buckling capacity of monopiles subjected to bending moments. For this, a finite element model is constructed which is validated by analytical models. Later on, the finite element model is used to conduct a parametric study to investigate the effect of loading rate on buckling capacity.

The scope of this study excludes factors such as soil dynamics, fluid structure interactions, residual stresses, and lateral forces. The focus is solely on the cylindrical shell of monopiles, excluding secondary steel from consideration. Initial geometric imperfections are considered as local perturbations necessary to initiate buckling, while other factors that may affect lateral forces or overall structural capacity are excluded.

This study will show that different parameters play part in the dynamic bending buckling behavior of cylindrical shells. The natural frequency of the cylinder, as well as the non-dimensional length together with the yield stress of the material play an important role in the dynamic buckling capacity. This research concludes that cylindrical shells with higher natural periods are more influenced by dynamic bending moments than cylinders with shorter periods. Next, shorter, stocky cylinders exhibit higher dynamic buckling capacities than slender cylinders. Also, imperfections are found to decrease the buckling capacity of cylindrical shells, but this effect diminishes for increasing loading rates.

Keywords: Offshore wind energy; Cylindrical shells; Dynamic buckling; Loading rate; Imperfections; FEM;

Floating Offshore Wind Turbines (FOWTs) have emerged as a promising technology for generating clean energy in deep water locations. Bluewater Energy Services proposes a Tension Leg Platform (TLP) as the floating support structure. An effective Structural Health Monitoring (SHM) system (of which there are various types) can facilitate timely interventions and optimize inspection and maintenance activities by providing continuous insights into their structural condition. Fatigue damage is especially critical for the support structures of FOWTs, as they are subject to cyclic loads that can cause structural damage.

This research proposes a fatigue monitoring system for a TLP supporting FOWTs. The methodology used is Modal Decomposition and Expansion (MDE). Due to the complexity of the studied structure in terms of structural dynamics, MDE is selected for its ability to capture dynamic behaviour. The main objective of the fatigue monitoring system is to perform the full-field strain estimation based on a limited number of sensors. This could also allow for the verification of the design considerations. With the presented response reconstruction approach, the stress in the locations prone to fatigue of the platform can be monitored and therefore, enabling estimation of the remaining lifetime of the structure and optimize maintenance planning.

Different analytical and numerical models are used in this investigation. The application of MDE in a simple structure (i.e. a cantilever beam) is first assessed to verify the performance and to gain insights of the methodology. Later, MDE is applied to a simplified TLP model to validate the response reconstruction approach and demonstrate its applicability for TLP-like structures.

Finally, a FOWT numerical model is used to design the fatigue monitoring system. The proposed system consists of two layouts of two strain gauges in the upper column of the TLP, and two layouts of two strain gauges on each pontoon. This system can provide full-field strain estimations with over 88.9% accuracy and predict fatigue damage accumulation with errors of less than 0.01%. Also, the system presented in this thesis only predicts global responses. However, the fatigue of the structure is also influenced by local structural responses due to sea pressures. Therefore, future research to include local effects in the predictions is recommended.

The increasing focus on reducing greenhouse gas emissions has led to the attractiveness of offshore hydrogen pipelines in achieving sustainable energy goals. Hydrogen, as a transport medium for energy, offers a viable alternative for transmitting large amounts of energy from offshore facilities to the shore. By storing hydrogen and subsequently converting it back into electricity during periods of peak demand, this approach aligns with the goal of establishing a green, net-zero economy by 2050. However, maritime activities in the proximity of offshore pipelines introduce a serious risk of damaging pipelines by accidental or emergency anchoring scenarios. Damage from dropped or dragged anchors can displace and harm pipelines, leading to environmental risks, safety hazards, and costly repair operations. Comparing hydrogen pipelines to existing oil and gas pipelines, there are significant differences. While oil leakage from anchor hooking poses risks to the environment and marine ecosystems, hydrogen imposes new risk factors which must be taken into account. Hydrogen negatively affects the structural integrity of pipelines, and anchor hooking leads to elevated stress levels within the pipeline material, accelerating fatigue crack growth, and reducing the operational lifespan of the pipeline. This leads to the following research question: ”What is the post-hooking lifetime of a hydrogen pipeline damaged by an anchor?”

To address this research question, the methodology applied in this thesis consists of two main approaches: numerical simulations and a fatigue crack growth model. The simulations specifically consider an incident where an 8-inch pipeline was damaged by an AC-14 High Holding Power (HHP) anchor. The pipeline is internally pressurised at maximum gauge pressure, and simulations are conducted considering daily and yearly variations in loading cycles, specifically at 10% and 50% pressure reductions. Through these simulations, the stress distribution and variation within the pipeline material resulting from the anchor impact are investigated, providing insights into the behaviour of the pipeline under such conditions. The fatigue crack growth model used in this study is based on the Paris law, which describes the relationship between crack depth and the number of cycles required for crack propagation under cyclic loading conditions. The presence of hydrogen significantly accelerates the rate of fatigue crack growth. As a result, adjustments are made to the Paris law to account for this effect, particularly in determining the range in which the law remains applicable. The crack growth analysis focuses on determining a critical crack depth, which could possibly lead to pipeline failure. A Failure Assessment Diagram (FAD) is used to determine the maximum allowable crack size, ensuring the safety of the pipeline. The FAD, along with the wall thickness of the pipeline, serves as a critical criterion for assessing structural integrity. The remaining lifetime of the pipeline following an accident depends on which criterion, either the FAD or the wall thickness, indicates failure first.

The crack growth analysis conducted in this research reveals that as the crack depth progresses under the influence of hydrogen, it eventually reaches a critical depth that introduces a potential risk of pipeline failure. Specifically, when considering yearly pressure variations, the crack reaches this critical depth in slightly over 8 years. Although the attained crack depth at this point is not yet through-thickness, the crack growth rate experiences a significant increase after 8 years, ultimately resulting in a through-thickness crack 9 years after the initial impact. The findings of this study have significant implications for the future development and maintenance of offshore hydrogen pipelines. By understanding the consequences of anchor hooking incidents and their impact on the operational lifespan of hydrogen pipelines, this research contributes to the development of robust and resilient infrastructure for a sustainable energy future.

The process of salvaging wrecks can be costly and time-consuming, making it crucial for salvagers to select the appropriate technique(s) and accurately estimate the total costs to minimize expenses and optimize efficiency. Salvagers typically use their expertise, experience, and data from previous projects to make these estimates. Diamond wire cutting is a material-cutting technique with much potential for use in wreck salvaging. However, a previously conducted literature review revealed a lack of sufficient information in the existing literature to determine the cutting rates of diamond wire used in wreck removal, indicating a gap in current knowledge. A deeper understanding of the technique is needed to evaluate salvagers' capability of cutting diamond wire in wreck removal. Therefore, the primary focus of this paper is to provide a method to estimate the cutting rate of diamond wire when used in wreck removal. The effect of wire speed, normal load on the workpiece, various materials encountered in wreck cutting, different geometries, and environments on cutting rate have been investigated. Filling this knowledge gap is of particular interest to Boskalis, as they currently have limited knowledge and experience with diamond wire cutting, making it challenging to estimate costs. Additionally, research into wreck removal techniques is socially significant, as the ability to remove wrecks efficiently and with minimal environmental impact is crucial. Therefore, this thesis aims to provide a deeper understanding of diamond wire cutting in its application in wreck removal.


Main factors determining the potential of a technique for removal include the total duration of removal, the required assets, and the associated risks. The primary focus of this thesis is to develop a method to estimate the cutting rate of diamond wire when used in wreck removal. The cutting rate will be predicted using the Archard wear equation, which requires an unknown coefficient ($K$) that can be determined through experiments. A real-world set-up with diamond wire was used to obtain data.

The results show that for the steels and cast iron in this survey, Archard's assumption that the wear volume is inversely proportional to hardness is not supported. Consistent with Archard’s equations, the wear volume has been found to be proportional to normal load, sliding distance but independent of speed within the range of tested conditions. As such, the Archard wear equation can be applied to the materials tested, but its applicability to materials that differ significantly from those tested is uncertain. Furthermore, results show that cooling significantly impacts both wire life and cutting rate. Specifically, fully submerging the workpiece resulted in a
XX\% improvement in the cutting rate as compared to spraying with coolant. It should be noted that these results were obtained under controlled conditions where normal load, wire speed, and cutting angle were kept constant, and proper cooling was possible. These operational parameters may vary in real-world scenarios, such as cutting through a shipwreck, leading to deviation from the predicted cutting rates.

The state-of-the-art design approach of composite structures in Formula One (F1) rely on expensive full-scale crash tests and simple models that cannot describe the underlying physical phenomena during crushing. Although the models can describe the crush forces and resulting accelerations, their predictive capabilities are poor, and several iterations are usually required to arrive at a satisfactory design, causing the teams money and resources. Reductions in costs could be possible by reducing the number of full-scale experiments and increasing the amount of analysis.

This thesis aims to evaluate the capabilities of a commercially available material model at two distinct modelling scales by conducting physical and virtual experiments on two different specimen geometries; tubular coupons and a new open-section channel specimen. Firstly, the new open-section specimen with a geometry representing the typical F1 crash structures was designed, manufactured and tested. The newly proposed specimen showed controlled failure modes, and displayed typical characteristics of composite structures under crushing loading. Secondly, the two specimen geometries were numerically analyzed on the macro-scale (laminate-level) and meso-scale (ply-level) using a commercially available material model implemented in the Abaqus finite element software. Mesh-dependency of the macro-scale approach was discussed and numerical aspects required to perform crush simulations on the meso-scale were identified and discussed. Lastly, calibration of the numerical models was performed to understand the effects of model inputs on the output responses, to evaluate the validity of the input parameters, and to assess two different calibration approaches; the first method is based on macro-scale modeling, while the second method is based on surrogate-based inverse parameter identification at the meso-scale.

The numerical models could not reproduce the experimental crush responses of either geometry when using experimentally characterized material data. Following the second calibration approach, the qualitative and quantitative crush responses of the tubular specimen could be accurately described, but the channel specimen showed no improvements in their response. The macro-scale modelling approach with the current material model was judged to be impractical for large-scale analysis, and the meso-scale approach showed need for further developments.

Impacts on offshore installations and ships can cause strains at high rates. It is known that mild steel lower yield stress and hardening are strain rate dependent. Measuring this material behavior is not easy, as test results from many researchers, show heavy vibration. Vibration is normally referred to as ringing. This thesis aims at measuring stress-strain behavior of mild steel at intermediate strain rates between 100 𝑠 −1 and 500 𝑠 −1 with minimal vibration. Two major characteristics from stress-strain behavior in this research, are the lower yield stress and plastic tangent modulus. A comparison of public test results, showed a steel characteristic for transferring quasi-static stresses, to stresses at a desired strain rate. This characteristic has been used to transfer a quasi-static stressstrain curve to stress-strain curves at intermediate strain rates. This strain rate dependent material model is used in calculations. The test is modelled by means of an explicit finite element analysis. The analysis showed that adding a plastic hinge in the specimen, can reduce bending and vibration. The test setup is a drop tower, where the drop weight falls into a U-shaped specimen. Strains are measured in an elastically and plastically deforming area on the specimen. Stresses in the plastic area can be obtained from strains in the elastically deforming area. Strains are measured by means of digital image correlation. Stresses are compared with loads obtained from drop weight positions and consequent accelerations. Measured plastic strains compare well to predicted plastic strains. However, stresses from first tests show vibration in a range of 20 percent of the average stress or more. If such bending occurs, it is not exactly known where the average stress is. However, the amount and direction of bending can be obtained from the 3D position measurements. The bending investigation is only used to identify causes of ringing. With insight in bending and loads from two separate measurements, this test is considered to produce results that represent the material behavior of mild steel. A raw test result with a vibration range of 20 percent of the average stress, in the first test setup, shows the potential for obtaining raw measurements from a U-shaped specimen in a drop test.

Bipolar half plates (BPPs) are important parts of Proton Exchange Membrane (PEM) hydrogen fuel cells. Metallic BPPs are generally produced by stamping or hydro forming. However, these processes are not continuous and relatively slow compared to rotary forming. Rotary forming is a continuous process and could potentially be a faster BPP production method. The BPPs are plastically deformed in bending and membrane modes. It is necessary to determine the bending and membrane strain because differentiating them can aid in the design of future BPP rotary forming production methods. Bending strain predominantly could lead to fractures on the outside of the bend. Membrane strain causes material thinning and could potentially lead to necking. These deformation modes need to be well understood so that the highest forming levels are achieved with a high level of geometrical consistency without exceeding material failure limits. The goal of this research is to determine the bending and membrane strain of 0.1 mm thick SS316L sheet material formed by rotary forming. This is done by experimental testing and analysis. From the experimentally deformed material, cross-sections are made and analysed. The thickness and bending strain are measured from these cross-sections. The experimental results are compared to FEA simulations. An important assumption is plane strain and it is experimentally validated. Plane strain FEA models are used to further understand the problem. The FEA simulations are validated, using the single channel strain experiments. Simulations with channels aligned with the cylinder axis are validated with a maximum thickness error of 3%. However, channels perpendicular to the axis are validated with an error of 8.9%. The FEA simulations are then used to further analyse equivalent plastic strain, stress, springback and final shape. The maximum equivalent plastic strain is 0.34. The maximum VonMises stress is 1200 MPa and maximum springback is 0.075 mm. The final shapes along the cylinder axis meet the experimental results the best. The bending and membrane strain are experimentally measured and simulated in Abaqus CAE. The bending and membrane strain highly depend on channel orientation and indentation depth. However, the bending strain is greater than membrane strain for both experiments and simulations. This research should be extended to investigate more complex geometries and designs. This is of course a more realistic example for BPPs.

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.

Over the last two decades, the size and capacity of (container) vessels calling port at the Port of Rotterdam have increased considerably. To moor these huge and heavy ships safely at the quay, fenders are frequently installed. The present guidelines for the “Design of Fender Systems”, which were established in 2002, are due to be updated in 2023. Part of the update of these new guidelines for the design of fenders by working group 211 of the Permanent International Commission for Navigation congresses (PIANC) consists of the verification and validation of the hull pressure criterion, taking into account the recent growth of (container) vessels. Obtaining a generic criterion is challenging due to the enormous diversity in vessel sizes and structural layouts. In addition, fender dimensions and types may also have a significant influence on the fender-induced load. This leads to the following research question: “How can critical fender-induced loads acting on the parallel side hull be quantified, accounting for the diversity of vessels and fenders?”

In this research, parallel hull sections are used in numerical simulations to investigate the allowable load of fenders and to derive the influence of panel size and dimensions (tall or wide). Including detailed parallel hull sections for a representative group of vessels, makes it possible to look beyond simplified geometries, such as stiffened panels, and specific case studies. First, the structural response and corresponding governing failure modes were studied. In addition to existing failure modes described in fender-induced loads, tripping of stiffeners as a possible governing failure mode was included. A modification to available analytical formulations was made to describe the critical tripping pressure of stiffeners with a flange under patch loads more accurately. The proposed critical tripping pressure induced by a fender is underestimated by the analytical model in comparison to the numerical simulations of the parallel sections. When the rotational restraint of the web frame attached to the tripping stiffener is considered, a closer correlation between the analytical results and the numerical simulations of the parallel hull is foreseen. For the numerical simulations, a parametric approach was adopted, where different impact locations and contact areas were applied for several vessel types and sizes. The lowest steel grade of vessels currently applied in shipbuilding was implemented to obtain the lower limit of allowable fender-induced loads.

The key finding of this study is that allowable fender-induced loads are largely influenced by the vessel's structural dimensions, such as web frame spacing, and the size of the fender panel with respect to the ship's geometry. The constant hull pressure criterion currently used by PIANC can be maintained but should be limited to a total allowable reaction force, because, for large panels, it overestimates the capacity. Furthermore, it has been shown that for large ships, wide panels outperform tall panels because they activate web frame(s). Making panels much wider does not necessarily yield more capacity because the stress concentration remains in the web frames. For small vessels, the trend is less clear, as the web frame is activated at an earlier stage (less far apart) and the capacity does not increase exponentially with the width. In addition, high panels on small vessels sometimes lead to the activation of a deck and thus increase the allowable load. The overall conclusion of this research is that the PIANC criterion should be limited to a total reaction force. Furthermore, by correctly sizing fender panels, more efficient use of the vessel's capacity can be ensured, as web frames provide more capacity. The findings of this research can be used to allow small and large vessels to safely berth onto existing facilities.

Marine structures are frequently equipped with rubber fender systems, which absorb the berthing en­ergy in order to protect both the marine structure and the berthing vessel. These fender systems absorb the kinetic berthing energy by elastic deflection and the associated reaction fender force introduces a berthing impact load acting on the vessel’s side hull. In guidelines and rules on ship design, recommendations regarding the structural resistance due to external fenders are not present. On the other hand, special requirements state minimal strengthening for tug resistance, which results in marked areas on a vessel’s side hull at which tug contact is allowed. Also for ships equipped with integrated steel fenders in their side hull, also known as beltings, minimal strengthening is required. Since the use of fender systems in ports is common, and all ships require to berth in a port the maximum hull loading due to fender contact is an important factor to take into account from the vessel’s perspective. PIANC WG33 published design recommendations for the maximum allowable hull pressure in kN/m2 for different types of vessels. The size of the fender contact area is in practice determined by dividing the design reaction force by the maximum allowable hull pressure. However, the hull pres­sures in this recommendation are based on the pressure on the keel of a fully laden vessel. Based on this background information, not all values are reliable, since some pressures correspond to a draft of 70 meters. Besides that, the pressure formulation does not contain information on the specific geom­etry of the contact area, i.e. height and width. This thesis systematically analyses the strength of the vessels’ parallel side hull for different failure modes, e.g. yielding in the stiffeners due to excessive bending­ or shear stresses. The structural ge­ometry of various vessel types is represented by various grillages. Two different pressure distributions were considered: a soft contact area, and a rigid contact area, to cover the most extreme behaviour of a fender panel. The results of this study show that the allowable load is largely influenced by the geometry of the contact area. The fender panels designed using the PIANC WG33 fender design are not always opti­mal. Especially for fender panels having large widths, the current guidelines seem to be too optimistic. Consequently, it is necessary to define a maximum width to what extent the current guidelines are allowable. This study shows that a specific allowable force in kN for a specific geometry is preferred over the current guidelines. In this study, a general formulation for the maximum allowable hull loading is proposed. This for­mulation requires the specific structural lay­out of the vessel that encounters the berthing facility. If the specific ship’s particulars are not known, recommendations are provided for different vessel types. These recommendations consist of a new acceptance criterion for each vessel type, which can be used to optimise the geometry of fender panels. The findings of this study can be used in the design of fender systems and the new design criterion has been submitted to the members of PIANC WG211.

Currently, most ships are designed on the basis of rules and reference ships for which often only the critical structural parts are calculated and designed in detail. This process can result in an over-dimensioned ship with the standard structural outcome of longitudinal stiffeners, transverse stiffeners, and bulkheads with a fixed distance due to ease of manufacturing. With the use of finite element analysis (FEA), the complete structure of a ship is analyzed against prescribed loads, which facilitates the determination of the detailed dimensions of all stiffeners and plates within a reasonable lead time and could result in better engineering in the form of a lighter ship. In addition, the most common structural forms could be optimized by replacing them with unique and optimal shapes. Topology optimization (TO) uses FEA, and it facilitates unique structural shapes. TO generates an optimized material distribution for a set of loads and constraints within a given design domain. The result can be used to inform the design of an improved part. Although the results provide helpful insight, they often cannot be used literally, as they are organic and cannot be manufactured with typical steel shipbuilding methods. The objective of this study is to research the possibility to design the structure of a steel midship with TO where the resulting structural form is manufacturable using steel-cut plates and cost-effective from a shipbuilding perspective. However, constraints that result in a manufacturable structure that can be made cost-effectively from steel-cut plates have not been developed and implemented in TO. To meet the objective, this project was initiated in cooperation with C-Job and the University TU Delft. The methodology was established based on a software comparison followed by an extensive trial and errortesting process. The study was executed in a case study for which the domain concerned the midsection of a 203m offshore vessel named Orion, as TO could result in substantial computational time such that analyzing a hull section is more efficient. The optimization was performed in multiple iterations with different design objectives using the method of Solid Isotropic Material with Penalization (SIMP). After a baseline comparison, the manufacturing constraints were implemented and developed. Despite the availability of manufacturability constraints, it is currently not possible with the software used in this study to design the complete structure of a steel midship. However, it can be very useful to employ TO as a suggestion early in the design process, as this can result in manufacturable structures (see Figure 77). The TO software used in this study can help designers with structural suggestions in the basic design phase when there are fewer design limitations. This case study resulted in unusual 45-degree, X-shape components that are highly efficient for sustaining shear loads and which resulted in a weight reduction of the mid-section of 2.4%. In addition, the result shows that unique structural shapes under various angles can result in an optimal strength-weight design rather than in orthogonal structural parts with a fixed span.

The aim of this thesis is to determine the influence of the Y/T, yield to tensile strength, ratio on the stress fields of high strength steels. This is relevant to the offshore industry as regulations restrict the use of steels with a high yield to tensile strength ratio, which is a characteristic of high­-strength and
ultra high­-strength steels. Identifying the influence of these parameters on behavior could lead to a scientific underpinning and review of the rules. The focus of this thesis is on stress concentrations at circular cutouts under uniform tension. The local stress and strain was found, which could be used to
assess the risk of strain localization and fracture.
Analytical models of Stowell, Neuber and Irwin are used to establish new relationships between the Y/T ratio and the stress field. The analytical methods of Stowell and Neuber were able to account for strain hardening, while the Irwin analytical relationship only provides a elastic-­perfectly plastic solution. Besides the stress field, an analytical formulation for the local strain based on Stowell’s theory is also established.
Numerical simulations were carried out for S690, S960, and S1100 materials. The numerical material models are later used in the research to validate analytical approaches. Also, an infinite plate approach is applied to the specimens by scaling the ligaments of the specimens by a factor of 6.
A parametric study has been carried out to gain insights into the influence of the governing parameters in relation to local strain. The parameters studied are the Y/T, yield strength, strain hardening exponent, and strength coefficient. Each parameter is varied separately.
The main conclusions are that a high Y/T ratio causes a slight decrease in plastic stress concentration factor, resulting in a slight increase in local strain. The strain hardening exponent shows the same trend, but the changes are even less significant. The yield strength and strength coefficient have no effect on the stress concentration factor or local strain. None of the parameters show any influence on the extent of the plastic zone of the material.

The forming of double curvature shipbuilding steel plates is performed by experienced craftsmen at IHC Metalix. Their experience will get lost when they retire; therefore, it can be valuable for future metal workers to capture the forming process of a double curved plate theoretically. An analytical description and understanding of the forming process will help craftsmen. In the future, it could even lead to the development of automatic forming machines.
This research investigates plastic deformation initiated by a rolling and a three point bending machine forming a saddle shaped plate. A 2Doptimization for the three point bending process is performed using beam theory. A single bend is validated by a 2D Finite Element Analysis (FEA). The rolling process is analytically described by equations obtained from literature. These equations are used to find a relation between the material stretching/membrane strains and the rolling forces. Furthermore, an elastic perfectly plastic stress strain curve is used. Material hardening and residual stresses due to repeated bending or rolling operations are not taken into account.
The bending optimization resulted in the least number of bending operations needed over a cross section, assuming the cross section behaves like a beam. The rolling equations resulted in required rolling forces for a desired membrane strain. The craftsmen know by experience that bending operations should be performed first to initiate a first curvature. Thereafter, the plate is rolled to initiate the second curvature and form the saddle shape.
In this thesis, the first steps are taken to capture and predict the manual forming process of double curvature steel plates. Future work on this topic should take the 3D-effects of the three point bending into account which lead to more accurate analytical descriptions. Furthermore, other assumptions could be analyzed to get better understanding of their contribution.