This thesis investigates the feasibility and effectiveness of Acoustic Emission (AE) methods for monitoring fatigue crack growth in metallic materials, with the aim of enhancing predictive capabilities and understanding of crack propagation under cyclic loading. The research specifically examines the correlation between various AE parameters—such as amplitude, count rate, energy rate, and entropy—and fatigue crack growth rates, using a multi-parametric approach.

Experiments were conducted on multiple specimens under different loading conditions, and both time-domain and frequency-domain AE parameters were analyzed. The study found that parameters like energy rate and rise angle were particularly effective in detecting specific stages of fatigue crack growth, while count rate and amplitude provided consistent indicators of crack initiation and progression. However, the study also highlighted limitations in the use of filtering techniques, such as SNR and amplitude filters, which can inadvertently remove crucial AE signals.

The findings suggest that while AE methods have potential for accurately monitoring fatigue crack growth, their effectiveness is influenced by the choice of AE parameters and the management of noise. To improve accuracy, the study recommends further research that includes a broader range of specimens, explores additional AE parameters, integrates complementary techniques such as Digital Image Correlation (DIC), and applies advanced analytical methods like machine learning. Future research should also consider the impact of environmental factors, such as corrosion fatigue, particularly in marine environments where realistic AE data is critical.

Overall, this study contributes to the broader understanding of AE monitoring for fatigue damage, laying a foundation for future research and practical applications, while acknowledging the need for further refinement and validation of AE techniques across diverse materials and conditions.

The demand for offshore wind turbines (OWT) is rising due to the increasing need for renewable energy. This growth is reflected not only in amounts of OWTs but also in their power rating capacity and size. Currently, the L-flange is the most commonly used connection to join the tower to the foundation. However, this connection type has trouble to handle the unprecedentedly high overturning moments that must be transferred between OWT segments. The C1 Wedge Connection (C1-WC) offers a solution by using multiple fasteners to connect a fork-shaped upper flange (UF) to a lower flange (LF), which are both welded to 100+ mm thick tubular steel sections.
The UF is currently manufactured using submerged arc welding to join its three components: the crown (CR), inner web (IW), and outer web (OW). This research investigates the feasibility of electron beam welding (EBW) as an alternative technique. EBW offers increased production speed by utilizing higher power density, allowing the weld to be made in a single pass. However, its full-scale (long-term) performance is unproven. This study explores its feasibility through small-scale experimental tests. The thesis also examines an alternative weld location in the C1-WC UF, comparing the current and alternative weld positions in terms of fatigue and ultimate limit state (ULS) by using finite element analyses.
The main research question is:
To what extent is the welding technique 'Electron Beam Welding' feasible for application in the C1 Wedge Connection™ upper flange manufacturing?
Numerical analyses offer insights into fatigue performance and the use of increased strength in the net cross-section. Although the alternative design, with the weld located in the gross cross-section, demonstrates acceptable fatigue performance under preloaded conditions, it does not outperform the base-case design in service life. Moreover, the increase in net cross-sectional yield strength of the alternative design remains unutilized in the ULS. This suggests that the UF geometry could be designed more efficiently: by reducing the thickness of the inner and outer webs with 19%, an UF mass reduction of 13%, 3.3 tonnes of steel, is achieved.
Experimental tests presented challenges, particularly regarding the precision required for EBW. A slight misalignment in the weld led to incomplete fusion at the weld’s root. Hardness testing revealed a significant increase in hardness in the heat-affected zone (HAZ) and fusion zone (FZ). Charpy impact tests failed to meet the minimum requirement of 31 Joules at -40°C, while tensile tests showed that failure occurred outside the weld. This raised concerns about the weld’s ductility.
Overall, EBW is not yet a feasible option for full-scale production of the C1-WC, mainly due to the needed stringent tolerance requirements and thereby in achieving a consistent welds along the full OWT circumference. Submerged arc welding remains the most reliable method for assembling the UF. Future research should focus on improving EBW’s robustness and reliability, particularly in understanding its fatigue performance and how to account for geometric imperfections.

The current demands of the Offshore and Maritime industry have lead to the design and
production of larger vessels of complex geometries. These vessels are subjected to unique
loading conditions that have not been investigated extensively. Such a case is presented
in wet deck slamming of large catamaran vessels. These vessels have been designed and
produced to conduct offshore platform installation operations and are expected to operate
in severe conditions where wet deck submergence becomes a threat.
Wet deck slamming is a water entry impact phenomenon that can induce extreme loads
when a part of the floating structure pierces the water surface during severe waves. This
event can trigger a transient dynamic response at both local and global levels. The event has
multiple influencing factors that can contribute to the severity and probability of occurrence.
Although the examination of slamming started over a century ago, the criteria concerning
the incident definition, identification and prediction are applied from different perspectives.
Modern methods identify slamming events based on the structural responses, while the sta-
tistical analysis considers correlation between the events, as opposed to older approaches
treating the incidents as independent.
Progress in understanding slamming is achieved through full-scale measurements, exper-
imental tests, and numerical simulations. Each method presents unique advantages and
drawbacks,and all three are used for knowledge development but also verification and vali-
dation. As numerical capabilities increase, it is also important for the experimental method
to evolve and explore new ways of representation of the complex events.
This project aims to investigate wet deck slamming using a rarely used form of model
that could closely mimic real life responses. This research can enhance the experimental
method capabilities and serve as stepping stone for further evolution of similar research.
The experimental test runs for the needs of the project were conducted in the Ship and
Hydromechanics laboratory facilities of TU Delft.

Highly-loaded low-speed roller bearings are commonly used in the offshore machinery, for example in turret moorings systems of Floating Production Storage and Offloading (FPSO) units and in slew bearings in heavy-lifting cranes. Safe and reliable operation and structural integrity can be ensured by monitoring the condition of the bearing. Condition monitoring based on acoustic emission (AE) has shown good potential for monitoring the health of highly-loaded low-speed bearings. With the increase in potential and proven condition monitoring methods, the need for lifetime predicting methods is increasing.
In this thesis, the feasibility of a digital twin (DT) concept is tested to make lifetime predictions of a highly-loaded low-speed roller bearing. The developed DT uses a combination of the Paris model and an AE-based model presenting the current state of the bearing. In the remaining useful life (RUL) prediction, the final crack size and total number of load cycles are determined based on an integrated form of the Paris model. Three different forms were tested for the RUL prediction.
The Paris model and the AE-based model showed good agreement in a benchmark case, based on which the two models were combined. The AE-based prediction was tested for hits and counts, with the counts yielding into better results due to a better fitting of the model constants. The linear RUL based on the number of load cycles resulted in the best prediction. The developed DT showed good potential in making lifetime predictions of highly-loaded low-speed roller bearings and enrichment of future DTs with prognostic information.

Numerous traffic bridges have been constructed over the last 150 years. Because bridges are commonly built based on an expected lifetime of 75 to 100 years, a large quantity of bridges are reaching the end of their design lifetime, or even are far overdue. To ensure these bridges remain operational without experiencing catastrophic failures, they have to be recalculated, and if need be, repaired, strengthened or replaced. Particularly older bridges are commonly constructed utilizing rivets. Their often overly complex geometries, the fact that riveting has become largely obsolete as a construction process, and old bridges are commonly not design for fatigue loading, engineers regularly face significant challenges when reassessing such bridges.

While fatigue phenomena have been extensively investigated throughout the years, studies pertaining to the fatigue of riveted connections are relatively limited. The Eurocode on fatigue, EN-1993-1-9 includes only two detail categories. Additional guidelines, like RBK Steel expand upon these detail categories, but focus primarily on built-up beam cross-sections in riveted structures, rather than riveted connections. In order to attempt to more accurately assess the complex joints present in ancient steel bridges, this thesis attempts to answer the following question: What would be a suitable approach to model complex riveted joints and assess their fatigue life considering a balance between the level of complexity and applicability in design practice?

A literature study is performed to identify the different factors that affect the fatigue resistance of riveted connections, as well as to highlight several of the available methods to perform a fatigue assessment. Through the investigation of experimental studies complemented with Finite Element (FE) Analyses, a design methodology for a full scale riveted model is drawn up, and finally a FE model of a joint of the John S. Thompsonbridge is constructed. A critical location within the joint is identified. On this critical locations, several stress- and strain-based fatigue life analyses are performed, namely the use of Stress Concentration Factors (SCF), Smith-Watson-Topper’s (SWT) strain-life equation, and the multiaxial shear strain criterion (MSSC) method, to investigate the effects of incorporating mean stress effects and multiaxiality.

From these analyses, it is concluded that SCF appears to provide overly conservative fatigue life estimates, whereas SWT and MSSC provide more probable results. The increased life estimate through MSSC suggests a limited degree of multiaxiality present in the critical location. All three methods require a detailed FE model, complicating the fatigue assessment of the joint. While the SCF method is slightly simpler to use than SWT and MSSC, it does not weigh up to the conservativity of its life estimation. SWT is deemed the most suitable approach for the fatigue assessment of riveted joints, given that it is more widely applicable and relevant than MSSC.

Fiber reinforced composites have been increasingly used in the aerospace and automotive industry, due to their potential advantages for designing flexible, strong and lightweight structures. More recently they are also being considered for the manufacturing of marine propellers. Since they have the potential to lower the weight, lower the maintenance costs, increase the efficiency at off-design conditions, improve cavitation inception speed and minimize acoustic signatures. Exploiting the full potential of composite propellers however requires that they need to be cost-effective and to do so it is required that the lifetime of the blade is sufficient. To determine the lifetime of the blade it is
crucial to determine how the composite blade will respond to a wide variety of environmental and loading conditions that it will experience over its life. Basically one needs to determine what effects fatigue will have on the material properties of the blade. The main objective of this thesis is to estimate the fatigue lifetime of composite marine propellers subjected to a pressure distribution determined by a Fluid-Structure Interaction
(FSI)model with the use of a progressive damage model integrated into a Finite
Element Model (FEM). The fluid structure interaction model to determine the pressure distribution has already been created Maljaars (2019). Currently the mostly used and most reliable approach is to use an experimental approach in order to estimate the fatigue life. However, for custom designs such as marine propellers this is a long and costly process. Modelling fatigue the fatigue performance in an earlier stage of the design cycle will result in significant cost and time savings.

The expected fatigue lifetime of the composite marine propeller is concluded to be at least 1012 cycles under the considered loading condition. If the propeller would rotate at 600 rpm for 24 hours per day this would translate to over 3000 years. The reason behind this large number is that the combination of the applied pressure load and the strength of the carbon fiber propeller is such that the stresses in each ply are very low. These low stresses create almost no damage throughout the propeller blade even for ultra high cycles.

Over the past few decades, the Netherlands has built numerous steel railway bridges to improve its infrastructure. The repeated loading from railway traffic makes steel bridges susceptible to fatigue damage. The stress ranges at fatigue-prone locations are generally higher in railway bridges than in road traffic bridges. Due to the limited space available due to road and rail alignment, the connection between the main girder and cross-girder is often identified as a critical fatigue location.

The main- to cross-girder connection can be designed for fatigue using different design principles. Designing the connection with a certain rotational stiffness leads to higher fatigue stresses at this location. Meeting the fatigue requirements at the connection can often be achieved by local adjustments, such as welding extra steel plates to the fatigue-induced location to evenly distribute stresses from the cross-girder to the main girder. This can be a costly solution. Designing a connection that is flexible could reduce stresses at the main- to cross-girder connection. Solving fatigue issues can be done by making global adaptations to the structure. The question arises: which aspects can be adjusted such that slight changes can notably improve fatigue resistance in a cost-effective manner?

A literature study and a finite element investigation of the main- to cross-girder connection are performed. A reference model is made after which parameters are altered to investigate their impact on the fatigue response of the connection. The reference model is based on the design of the bridges from project Oostertoegang. However, instead of considering a connection with a certain rotational stiffness, the design is made more flexible for this study. Ansys 2022 R2 is used to create a finite element model of the bridge with shell elements. The hot spot stress method is used to conduct the fatigue assessment. In total four critical fatigue locations are researched for six parameters. The parameters researched are: the center-to-center distance between the cross-girders, the height of the cross-girder and main girder, the thickness of the inner web plate of the main girder, the diaphragm, and the steel deck plate.

From the analysis, it can be concluded that for one detail (M3) local measures should be applied to meet fatigue criteria. The three other details can satisfy requirements within feasible limits. The most cost-effective and realistic way is to increase the thickness of the inner web of the main girder. Other cost-effective but less feasible solutions to optimize the flexible design for fatigue are: decreasing the thickness of the diaphragm and increasing the height of the cross-girder. These parameters show the best ratio between the costs needed to alter the parameter and the total fatigue damage change of the critical detail.

Furthermore, it is determined that the fatigue assessment using finite element analysis with shell elements can be optimized by using the hot spot stress method in combination with modeling the weld using an increased thickness.

Catamarans are popular in the offshore sector as they combine good transverse stability and ample deck space with low wave resistance. However, their slender hull shape results in low restoring qualities in heave and pitch motions. The large motions in rough weather can often result in water impacting the underside of the deck connecting the two hulls, a phenomenon called cross-deck slamming. The impulse excitation from cross-deck slamming can then produce a transient hydroelastic response of the structure called whipping. Whipping excites mode shapes that would not normally be present in the response, as their natural frequencies are significantly higher than the wave encounter frequency. This results in detrimental contributions to fatigue life through high-amplitude cyclical bending moments. Both the calculation of slamming loads and the prediction of resulting structural responses have been a challenge for several decades. The highly nonlinear and three-dimensional character of the phenomenon, combined with the strongly coupled fluid-structure interaction means that it is unpredictable, and even the definition of slamming events has been a matter of disagreement among researchers.\\

Experiments are still a vital part of these investigations, for validating ever-improving numerical techniques. An essential issue with experiments is the extent to which mode shapes and natural frequencies can be emulated in model scale. Traditional hydroelastic models are segmented and use either a flexible backbone or flexible joints to introduce stiffness. This often results in an excellent description of the 2-node bending mode, but an increasing error for higher modes leads to stress inaccuracies. In this investigation, a fully elastic model of a catamaran is designed and produced for hydroelastic experiments. The advantages and limitations of the concept are identified, and the verification against structural models is presented.

Welded details in orthotropic steel decks (OSD) are susceptible to fatigue cracks which are caused by local weld defects, geometric stress concentrations and residual tensile stresses. Residual tensile stresses are formed during the welding process and the distribution depends on many factors. Knowledge about the residual stress distribution is required for accurate fatigue assessment. The rib-to-crossbeam detail in the design of the new Schipholbridge does not meet the fatigue requirements according to Eurocode 3 when using a reasonable geometry. Therefore, high frequency mechanical impact (HFMI) post weld treatment will be used to improve fatigue resistance. Incorporating the residual stress distribution in fatigue life prediction models will improve their accuracy, which can lead to more favorable results which can avoid the use of HFMI. The main objective of this master thesis is to determine the accuracy of a subsequently coupled thermo-mechanical finite-element model which predicts welding induced residual stresses in a segment of an OSD. The fatigue life prediction is not included in the scope of this thesis. The accuracy of the model was determined by comparing temperature
and distortions with experimental data from three specimens consisting of a 900x400mm deck plate, a 350mm deep trapezoidal stiffener and a 600x15mm crossbeam web with a Haibach cope hole. During the welding, temperatures were recorded by a FLIR© E96 thermal camera. Welding distortions were obtained by subtracting the geometry before welding from the geometry after welding. The 3D
geometry was obtained by the Artec© LEO scanner. The scans were post processed in Artec© studio software and data was extracted by the NumPy-stl package in Python. The predicted temperature distribution and deformations corresponded well to experimental results. The maximum deviation in the
temperature distribution 1.8s after welding was 67 °C and occurred at the last position of the welding torch. The maximum deviation between predicted and measured upward displacements of the deck plate 10mm from the edge was 0.2mm, with smaller deviations on average. Due to the good correspondence
of experimental and numerical results, residual stresses were presented. On the deck plate in longitudinal direction, tensile stresses of yield strength magnitude were obtained after unclamping. At the rib-to-crossbeam connection, the stress in the direction perpendicular to the weld toe at the location of the weld toe was equal to the yield strength after unclamping. The stresses quasi-linearly go to zero through the thickness of the rib.

The safety of offshore assets such as wind turbines, offshore cranes, and turret mooring systems partly relies on the integrity of heavy-duty bolted joints. These critical connections can be of different dimensions and exposed to monotonic and cyclic loading. Despite being small components of a structure, bolts need to be maintained appropriately to ensure structural safety and reliability. The integrity of bolted joints can be assured by checking for adequate preload. In addition, fatigue failure resulting from preload loss is of primary concern, as it can occur suddenly without any visible changes. Hence, bolts should be re-tightened periodically to ensure sufficient preload. However, in offshore conditions, this countermeasure leads to high maintenance costs while exposing the crew to unfavourable conditions and risks.

This research focuses on the feasibility of condition-based maintenance of bolts using ultrasonic waves. An energy attenuation method has been implemented for this feasibility research. The main objective is to establish a proper methodology and hypotheses for preload detection. Furthermore, along with preload detection, the feasibility of crack detection was investigated. Finally, the proposed methodology and hypotheses were validated by performing experiments and numerical simulations.

The experimental and numerical results verify the proposed methodology by showing an increasing trend in the energy and the power of the transmitted ultrasonic wave for increasing preload. Also, the feasibility of crack detection using the same setup has been positively evaluated.

The obtained results suggest that the ultrasonic waves can be employed to monitor bolts for condition-based maintenance. Additionally, a number of relevant research activities are recommended based on this study.

Semi-submersibles are common vessels in the offshore sector. These vessels usually have a bracing structure to restrain floater movement and to support the deckbox structure. The bracing configuration influences strength, fatigue and other semi-submersible parameters. The first assignment objective was to study the structural design impact on a twin-pontoon semi-submersible when applying different bracing configurations. The influence of bracings on characteristic responses, defined as loading and accelerations governing for the strength and fatigue of semi-submersibles, was studied first. Generally, an increase in bracing diameter results in an increase in characteristic response. However, bracings do not affect characteristic responses much, as differences below 11% are observed. Global strength assessments in the ultimate and accidental limit states were performed using finite element analyses (FEA) for different bracing configurations. Each bracing configuration differentiates itself being beneficial for certain load cases, or is beneficial regarding fatigue sensitive locations. The structural design of semi-submersibles with different bracing configurations were modified to have similar structural performance compared to a reference semi-submersible. The bracing configurations were evaluated based on payload, structural centre of gravity, structural redundancy and fatigue. Generally, the presence of transverse horizontal bracings affects structural performance most significantly, since a payload reduction of 22% is observed when not present due to the dominant splitting force load case and ineffective load path. Adding diagonal bracings in the horizontal or vertical plane, reduces column and deckbox loading for the longitudinal shear, torsion moment and inertia load cases, resulting in a payload increase up to 5%. Omitting braces results in the lowest amount of fatigue sensitive locations. However, since the columns and deckbox structure needs strengthening, welding volume at other fatigue sensitive locations increases, which affects fatigue negatively. The bracing configuration selection should be merely based on the semi-submersible’s requirements. Therefore, the designer should first rank the requirements after which a bracing configuration can be designed.

Fatigue of semi-submersibles remains a challenge in today’s practice, since service cracks are frequently found during inspections. Fatigue resistance is usually determined by S-N curves based on tested small-scale specimens (SSS). Large-scale specimen (LSS) and full-scale specimen (FSS) fatigue tests are performed less frequent. Semi-submersible tubular brace-column and brace brace fatigue resistance is typically determined by LSS tubular joint tests. The fatigue resistance similarity between SSS planar joints and LSS tubular joints was examined by the hot spot stress and averaged effective notch stress concepts. Five different LSS tubular joint fatigue tests, derived from literature, were studied by shell FEA and volume FEA. Weld geometry in tubular joint shell FEA can affect results significantly. When not present, bending stresses are overestimated up to 208%. For most LSS tubular joints, similarity with respect to SSS planar joints is increased for the average effective notch stress concept, compared to the hot spot stress concept. Most LSS tubular joints fit inside the average effective notch
stress SSS planar joint fatigue resistance data. Dissimilarity for divergent LSS tubular joints can be related to different modelling assumptions compared to actual conditions, where differences in boundary conditions and weld geometries is most likely. Fatigue resistance similarity, expressed as the strength scatter index and intercept log₁₀(𝐶), of LSS tubular joints is increased compared to hot spot fatigue resistance. Differences in slope 𝑚 are similar. A SSS planar joint based design S-N curve is therefore applicable for tubular fatigue sensitive locations of semi-submersibles and other structures. Moreover, this study increases the applicability of the average effective notch stress as fatigue assessment concept. More LSS tubular joints should be studied to demonstrate similarity with higher confidence. 
To accurately estimate fatigue lifetime, a detailed fatigue assessment is performed of a tubular brace-brace connection. Multi-axial loading was studied at global and local level, however at the critical saddle location, mode-I stress dominates. Therefore, multi-axial fatigue was not considered. The structural stress,𝑆𝑠, average effective notch stress,𝑆𝑒, and hot spot stress,𝑆ℎ, were applied as fatigue assessment concepts. The fatigue assessment of the simple tubular joint concluded insufficient fatigue lifetime. Stress intensities at the critical saddle location were reduced by implementing internal ring-stiffeners, classifying the connection as a complex tubular joint. Stress reduction factors of 3.9 and 4.1 were achieved, resulting in fatigue damages below 1, thus acceptable. Compared to common fatigue assessment concepts, the detailed 𝑆𝑠 and 𝑆𝑒 fatigue assessments reduce the possibility of service cracks and maintenance and inspection work can be planned more precise. However, DNV-GL and IIW guidelines state a fatigue resistance slope change is present above 10⁷ cycles (N), which is not accounted for in 𝑆𝑠 and 𝑆𝑒. To study the presence of a slope change and to possibly establish a more accurate fatigue damage estimation for 𝑆𝑠 and 𝑆𝑒. a recommendation for further research is to include
more fatigue tests for 𝑁 > 10⁷, from which a design S-N curve can be derived.

On board of High Speed marine Craft (HSC), the crew and the passengers are exposed to high levels of Whole Body Vibrations (WBV) and large magnitude Repeated mechanical Shocks (RS) caused by the motions of the craft. The HSCs are typically 10 meters long, capable of reaching a maximum speed up to 50 knots and widely used by various maritime organizations. However, the operators and crew suffer from fatigue and injuries, leading to a reduced effectiveness and operational capacity of the marine craft. In an attempt to reduce the physical loads, passive Shock Mitigating Seats (SMS) can be installed. Numerous research has shown that an improperly designed SMS may amplify the wave impacts forces through phenomena such as bottoming out and dynamic amplification. Therefore, it is necessary to ensure that a suspension design works properly by testing the performance during wave impact events, before the seat is manufactured and installed onboard the craft. The problem is that it is difficult to determine the performance of the seats in both the design and off-design conditions with either sea trials or laboratory tests. This research focuses on the prevention of injuries and adverse health effects due to repetitive wave impacts by incorporating an injury model in the analyses of various suspension principles in the design of SMS. In the current analyses of SMS either simplistic or specific models are used which restrict the application of these models to other suspension designs. Therefore, a computer program based on the finite element method is developed that allows realistic input accelerations in the surge, heave and pitch direction. The program incorporates highly non-linear elements, including the effect of bottoming. Additionally, the validity of the half-sine approximation for the wave impact excitation pulse was reviewed and concluded to be inappropriate for design purposes as it underestimates the probability of bottoming. Furthermore, the modified evaluation methods of ISO 2631 Part 5 using an optimized age-dependent coefficient based on gender in combination with a Weibull injury risk model were implemented to evaluate the resulting seat level accelerations. A case study was conducted on a Fast Raiding Interception and Special forces Craft (FRISC) of the Royal Netherlands Navy (RNLN). A design based on a parallelogram of pinned truss elements in combination with a coil spring element was altered by replacing the coil spring with a gas-spring element. The design was analysed with dynamic simulations of full-scale measurements of wave impacts on a lifeboat of the Royal Sea Rescue Institution (KNRM). For the most severe wave impact of the acceleration record, the seat level acceleration was reduced from 17.7 [g] to 2.8 [g]. An operator of the age of 24 years who is exposed to the accelerations for half an hour a day, 30 days a year for two consecutive years was assumed. The probability of spinal injury was reduced for a male operator from 99.5% to 16.3% and for a female operator from 100.0% to 42.0%. These results illustrate the high risk of injury to which the HSC operators are exposed. By incorporating highly non-linear elements and spinal injury models, the program and evaluation methods are capable of modelling various SMS suspension designs and analyse the performance. This can assist the seat designer, engineer or researcher with investigating suitable suspension designs before sea trials, experimental tests and prototype iterations. Therefore, the method offers the possibility to save time and reduce costs. Furthermore, the method can assist in defining new regulations in order to limit the exposure of the operators to physical loads and reduce the risk of spinal injury to an acceptable level.

Shock mounts are crucial components in reducing the transmissibility of a shock or impact, thereby protecting the objective system. Such shock mounts are application-specific and need a redesign for specific uses. This study establishes a transient nonlinear finite element optimization procedure to design a shock mount for withstanding an underwater blast wave. The work showcases the analysis of two design concepts. The first design demonstrates average acceleration transmissibility of 5.51e-3. The second design exploits the phenomenon of reflection of waves at an interface. This design is unique, promising a factor 3.263 lower average transmissibility than the first design which is similar without the metal interface. The study also details the modeling and analysis of both shock mount designs using 2D axisymmetric and 3D transient nonlinear finite element analyses. The results of both these analyses were comparable, making a 2D study efficient and reliable in the analysis and optimization of shock mounts.

The phenomenon of fatigue in orthotropic steel deck (OSD) bridges is a predominant problem because of complexity of the prediction methods. In the past, many researchers have studied the fatigue behaviour of various details in OSDs via both experiments and Finite Element Modelling (FEM). In the present research, the connection of open stiffener to crossbeam at the location of cope hole in OSDs has been be studied. Structural hot-spot stress method using surface stress extrapolation has been used to investigate the cracks in stiffener in the longitudinal direction and cracks in crossbeam.

FEM is extensively used for analysing OSDs. In engineering applications, 2D shell elements are widely used instead of 3D solid elements for analysis due to less computational cost. The welds are generally not modelled with shell elements for fatigue assessment of welded structures. In this study, large difference of SHSS is obtained by shell and solid elements for both simple and complex fillet welded details and also for the OSD. This difference in structural hot-spot stress (SHSS) is reduced by the application of three weld modelling techniques with shell elements: (i) the IIW approach, (ii) the Eriksson’s approach and (iii) a combination of IIW and Eriksson’s approaches. All the three methods are based on increasing the thickness of shell elements at the weld region which are easy to be applied in practice. The dependence of SHSS on mesh size and element type is also investigated in this thesis.

A parametric study is performed first on simple and then on complex fillet welded details to check whether the weld modelling technique can be applied to different geometries, loading and boundary conditions. The solid element model of the complex detail is first validated with experimental strain measurements. Then, SHSS values from other numerical models are compared with the solid model. Representative load cases are investigated initially followed by load combinations. The weld modelling method with shell elements gives good consistency in the ratio of hot-spot stress compared to the solid element model for these details. The deformations are also investigated for all load cases and load combinations. The combined weld modelling technique (iii) with shell elements replicated the weld stiffness of the solid model for both in-plane and out-of-plane load cases.

As a final step in checking the consistency of SHSS ratios between shell elements with welds and solid elements in the application of OSD, a parametric investigation is performed. This study involved two geometric variants of OSD with different load positions. These two variants were based on the design of existing bridges in The Netherlands with relatively thin plate and newly designed ones with thicker plates. The parametric study is divided into two parts. The first part is based on representative load cases. The second part is based on SHSS influence lines for determination of critical loading positions having maximum and minimum hot-spot stress. For both these studies, the weld modelling approaches with shell elements gave a good match of SHSS compared to the solid models. The SHSS results from the shell model with weld are more consistent compared to the regular shell model without weld. From the preliminary parametric study on OSD, it is found that after weld modelling with shell elements using the combined approach of IIW and Eriksson, less scatter is observed in the SHSS ratios. The coefficient of variation (CV) in SHSS ratio for crossbeam is 6.8% and that for stiffener is around 5.1% which is low. The SHSS values are computed based on the stress perpendicular to weld toe. From the detailed parametric study, the mean value of SHSS ratio is 1.07 (range: 0.99-1.15) for the crossbeam and 1.02 (range: 0.98-1.10) for the stiffener. The CV of SHSS ratio is 5.4% for the crossbeam and 4% for the stiffener. The stress profiles are also investigated at the critical locations of OSD. The shell model with the combined weld modelling approach is in good agreement with not only SHSS but also with the stress at a distance far away from the stress concentration when compared to the solid model. The deformations are also very similar for both the numerical models. Thus, it is concluded that the combined weld modelling technique using the IIW and the Eriksson’s approach with shell elements could be used for accurate fatigue life assessment using hot-spot stress method where the measure of accuracy is with respect to the solid element model.

Waves impacting on maritime structures can cause damage which endangers the environment around these structures. Due to high hydrodynamic loading, structures can enter the plastic regime, resulting possibly in structural failure. The understanding of the principles related to this structural failure due to waves is a key concept in this research. Due to a gap in existing literature, a simple approximation for the influence of plasticity on fluid-structure interaction is the first required step towards a better comprehension of structural failure of ship and offshore structures due to hydrodynamic loading. The choice is made to analyse a pendulum because of its ability to show clear and easy to interpret results, due to its few degrees of freedom. In an experiment, three different double pendulums are subjected to breaking waves created by dispersive focusing at three different locations in front of the pendulum. The top hinge of the pendulum is a normal hinge and the bottom hinge is a friction hinge that approximates plastic behaviour in a mechanical manner. The energy dissipated in the friction hinge, modelling plastic energy, and the energy transferred from the wave to the structure, called `total absorbed energy', decrease with increasing frictional torque. This means that when a structure allows for more plastic deformation, the energy that will be absorbed in total by the pendulum is larger. The energy transferred to the double pendulum shows more variability for the wave focused furthest away from the pendulum. A reduced-order model is constructed that can help explain the behaviour of the different pendulums.

The goal of this thesis is to propose a stress state dependent ductile fracture criterion for shell elements that can be used for crash simulations for example collisions of ships with offshore structures. Fracture has been extensively researched for solid elements for finite element analysis in Bai & Wierzbicki [2009], among others. Shells have also been investigated as a special case of the findings of solid elements. These elements are mostly used in large, thin-walled structures such as ships, as they reduce the minimum required elements in a model, which reduces computation time. Dunand & Mohr [2010] reported that these elements are unable to capture through-thickness necking. This necking localizes strain and creates stress concentrations that decrease the validity of plane stress assumption of shells. In contrast to prior research, this thesis focuses on shell elements that are in the range of 0.5 to 5 times the material thickness by proposing a phenomenological correction method. This method estimates the effective strain perpendicular to the neck at the instance of fracture based on the assumption that the stress state does not change at the onset of necking and a simplified analytical model of the neck itself. The preference for a correction method is validated from a numerical plate model based on analysis proposed by Marciniak et al. [1973]. From comparison with the numerical results, it can be concluded that the proposed method performed better than existing methods for smaller shell elements lengths and similar for larger shell elements. The improvement is most dependent on the accuracy of the necking strain. A complete simplified approach for testing and effective fracture strain prediction is proposed and validated with calibration and raking experiments from Haag et al. [2017] and Bijleveld [2018].

In recent years, Wire and Arc Additive Manufacturing (WAAM), is gaining attractions both from academia and industry, because it has the potential to substitute or complement the traditional manufacturing methods. Traditional materials and methods are reaching their limitation when demanding and special applications are considered.

The current growth in WAAM applications will boost the WAAM consumable material development in the coming future. WAAM applications mostly refer to small batch production or prototypes, which often requires special wire compositions or can benefit from tailoring the consumable composition for the desired components. On one hand, solid wire production is only economically viable when large volumes are involved. On the other hand, metal-cored wires are particularly suitable to produce tailored or small-batch consumable compositions, which is very attractive for WAAM. However, only limited research has been carried out in the use of metal-cored wires in additive manufacturing. In this research, the focus is to explore the use of metal-cored wire in WAAM applications. Two chemical compositions, one ferrous, medium carbon low steel alloy (AM-XC-45), and one non-ferrous, Stellite 6 (cobalt-based superalloy), metal-cored wires were investigated based on industrial interests at RAMLAB.

The microstructure of the WAAM AM-XC-45 thin wall was characterized using optical microscopy and scanning electron microscopy. Pearlite, ferrite, bainite, and martensite are present in the deposited wall. Columnar grains are found near the fusion line. The repeated thermal cycles cause the grains to become finer from the top to bottom layers. The mechanical properties including microhardness and tensile strength were tested and compared with the traditional processing methods like casting, milling, and forging. It showed a comparable or superior microhardness and tensile strength to the traditional process whereas the relative lower elongation of the deposited AM-XC-45 thin wall indicates that further post heat treatment is needed to improve the ductility of the part.

Stellite 6 is a cobalt-based superalloy, which has good wear and corrosion resistance and retains these properties at high temperatures. In this study, Stellite 6 metal-cored wire (WEARTECH® WT-6 GMAW-C, AWS A5.21 ERCCoCr-A) is selected and optimized to obtain results comparable to the commonly employed and more costly laser deposition, in terms of microstructure and mechanical properties. The optimal deposition parameters were developed using the S355 steel substrate, and then adopted on depositing on the AISI 420 stainless steel substrate. Characterization showed that Stellite 6 layer mainly contains Co-Cr-Fe solid solution with FCC crystal structure as the matrix and the Cr7C3 and Cr3C2 were identified in the microstructure through SEM, EDS, and XRD. These carbides can contribute to the strength. In addition, the identified Co4W2C carbides can contribute to the wear properties of the Stellite 6 WAAM deposits. The dilution and hardness of the WAAM deposit can reach the same level as laser deposition.

Additionally, a 3D temperature distribution model is developed by adopting the moving mesh technique to numerically study the WAAM process. The model helps to gain a better understanding of the physical phenomena (heat and mass transfer) during the WAAM deposition. The AM-XC-45 was used to validate the developed model. The simulated and experimental results were compared. The dilution and HAZ of WAAM deposited single bead was used as validation criteria. The A1 line (1100K) simulation is in a good agreement with the experimental result. The fusion line (1800K) shows some deviation. Compared with laser deposition, the main differences between the two processes lie on the heat source, the boundary conditions, and the material responses to the process.

In steel structures, a lot of attention is paid to lightweight structure, i.e. reduction of dead load without compromising structural safety, integrity and performance as well as cost-effectiveness. Thanks to modern steel aluminium sandwich panel manufacturing technology a new possibility became available for lightweight structural design.The objective of this thesis is to evaluate the application of sandwich panels in the construction of steel structures with the aim of weight reduction without affecting other parameters like safety, performance, cost, etc. In this thesis, both column and plate buckling theories are considered and applied to the sandwich panel to evaluate its behaviour under in-plane compressive load. Effects of various material models and imperfections on buckling strength of sandwich panel are evaluated. Stiffened plate and sandwich panel is compared in terms of buckling resistance and self-weight. Three different sandwich panels made from faceplates of steel grade, S355, S690 & S1100, are used for replacement of S355 stiffened plate. Efforts made to understand the effect of various physical parameters on buckling resistance of sandwich panel in both column and plate buckling theories.Finally, as a case study, sandwich panel technology is used to redesign the Huisman structure. The objective is to investigate whether applying sandwich panels in redesign makes it possible to obtain a sufficient weight reduction without losing its performance. For this case study, sandwich panels with faceplates of steel grade S355, S690 and S1100 are used. Static and buckling strength of the new design is evaluated. Also, the cost of new design and original design is evaluated and results are compared. Cost analysis is done to evaluate whether a sandwich panel is an economical solution.Findings of this thesis are that in future it is possible to use sandwich panels in offshore structures to save a significant amount of weight while taking considerations into account. Sandwich panels can be successfully used to replace stiffened plates. Sandwich panels with faceplates made from extra high strength steel can give significant weight reduction. But the use of sandwich panels also results in an increase in the overall cost of the structure. So in terms of costs, it is questioned whether or not using sandwich panels is economically beneficial for offshore equipment.

The International Maritime Organization (IMO) issued on the 13th of April 2018 a press release titled: “UN body adopts climate change strategy for shipping”. [1] This document illustrates the commitment of the IMO to significantly reduce the emission of greenhouse gasses (GHG) from international shipping in coming decades. However, the question arises whether the ambitious goals, set out in this press release, are feasible to be met.

This aim of this thesis is to investigate the economic and technological feasibility of powering a vessel while emitting minimal greenhouse gasses in 2050. The focus will be on power plant configuration consisting of an energy storage medium and energy converter. A case study, concerning a representative ROPAX ferry built in 2050, is performed to illustrate the feasibility of using such a power plant configuration.

Energy storage media considered are hydrogen, ammonia and batteries. The use of internal combustion engines, fuel cells and an ammonia reformer are also investigated. The performance of power plant configurations with respect to their estimated greenhouse gas emission, overall weight, required volume, initial investment cost and cost of the required energy per day of operation is evaluated and based on the results of this study several conclusions are drawn.

• First and foremost is the need to reduce the price difference of hydrogen and ammonia compared to LNG. Without this reduction, no single synthetic fuel can be considered economically feasible. Three options to reduce the price difference are identified:
- a high LNG price including high emission tax;
- a low commercial electric energy price; or
- synthetic fuel production using excess renewable energy.
Especially the last of these three is considered to be a feasible measure due to the expected presence of excess energy from solar and wind farms.
• The second conclusion is that using fuel cells or batteries are the most environmentally friendly option due to the emission of N2O when using internal combustion engines.
• Thirdly, all sustainable configurations require a considerable amount of space.
• Furthermore, the combination of relative low electrical efficiencies and high hydrogen price result in high daily costs for hydrogen fueled internal combustion engines and PEMFCs.
• The initial investment cost and system weight of batteries-based configurations are high.

Following these conclusions, only three power plant configurations remain. Based on expected safety and logistical concerns and the expected potential for improvements, a single configuration was determined to be the most suitable to be used on a ROPAX ferry in 2050. This power plant configuration is composed of a SOFC directly fueled by ammonia.

After assessing the safety concerns associated with handling ammonia and the implications the established power plant configuration has on the general arrangement of the ship, it is concluded that the use of such a power plant configuration is highly likely to be feasible in 2050. However, it is recommended to perform a more in-depth analysis of required safety measures and operating characteristics of fuel cells in future research.