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

The aim of this study is to conclude which parameters are influencing the most the hydrodynamic interaction between the two moored vessels in side-by-side configuration using frequency and time domain simulations. Furthermore to show what is the added value of the hydrodynamic input used in time domain investigations using 2 different integration schemes. Ultimately identify what is the impact on the mooring arrangements and what it needs to be done to ensure safe offloading operations. During the offloading process (18 to 24 hours), the transfer equipment needs to accommodate the relative motions between the two vessels. There are several parameters which might influence the operations, which are dependent on the location (i.e. environmental conditions) and on the mooring system (i.e. vessels size, draft, mooring arrangements, etc.). The side-by-side mooring system analyzed consists of a turret moored FLNG and various size of off-take carriers. A sensitivity analysis is done with respect to the physical parameters (i.e ship dimensions, loading conditions and separation distance) and modelling parameters (i.e parameters dependent on damping factor) in order to determine the effects on the first and second order quantities using frequency domain analysis. This represents the input for time domain numerical investigations in order to assess the relative motions and line tensions under certain environmental conditions. It is known that the diffraction-radiation tools overestimate the results in the gap region. This is an effect of using potential flow which do not count for viscous effects. To overcome this problem and to describe in a realistic mode, multiple methods or numerical techniques has been proposed. With this regard a detailed numerical investigation has been carried out to conclude what is the impact of varying the dissipation factor (i.e between 0 and 0.4) on first and second order quantities. Traditionally, the relative motions are determined using experimental tests which might be time-consuming. In particular for sensitivity analysis, numerical investigations are preferred in order to shape a main conclusion. Ultimately some of the results can be validated thorough experiments. The hydrodynamic interaction revealed important aspects, especially with respect to the off-take carrier. If the carrier is smaller the impact is higher due to the presence of the FLNG. Such that the carrier rolls significantly when there are head or following seas. Furthermore decrease in heave motion of the FLNG at the resonance frequency due to the roll of the carrier has been noticed. The same phenomena occur for heave of the carrier. Thus, this represents a strong coupling between heave-roll motion of the two vessels. Another important aspect revealed is with the respect to the shielding effects under beam conditions. In general FLNG is slightly influenced by the presence of the carrier. In particular, it can be noticed only when it is in ballast condition due to the fact that the draft and thus the displacement of the carrier is considerably higher. The sensitivity analysis points out that the modelling parameters (i.e dissipation factor) does hardly influence the response of the vessels even if it is considered or not. On the other hand drift forces are highly dependent on these parameters as a consequence of sharp changes of the wave elevations between the vessels. Such that if there is no dissipation factor to damp the wave elevation the resulted forces are extremely high compare to the cases where the dissipation factor is different than 0. This occurs only at the wave gap resonance and towards higher frequency region when diffraction effects are important. This is emphasized by the time domain analysis which prove that under the sea-states close to the gap resonance the lines and fenders tension reaches extreme magnitudes. Outside the gap resonance the relative motions and tensions are not influenced by the variation of the dissipation factor. The moored system (i.e FLNG-LNGC or FLNG-LPGC) is governed relatively by long crested waves. Thus it is expected to have higher line loads under these sea-states. The actual mooring design shows that under squall conditions (i.e Tp of 14s) the mooring lines exceeds the safe working limit for the FLNG-LNGC, while for FLNG-LPGC exceeds the minimum breaking load. In terms of relative motions for the same environmental conditions, the criteria is not satisfied. The proposed optimization of the mooring lines does bring reduction in the mooring line tensions, but not enough to drop below safe working limit for FLNG-LPGC configuration. On the other hand for FLNG-LNGC, the line tension is within the limits, but the relative motions still exceeds the criteria. Therefore choosing the optimum mooring configuration is a trade-off between line stiffness and location of the connection points of the mooring lines which sometimes may lead to unpractical solutions.

Monitoring fatigue damage is complex because ships are designed to crack after 20 years, and typically cracks will only occur after a long time. A novel fatigue damage sensor has been invented to indicate damage in corresponding aluminium welded joints. Fatigue damage is accelerated by introducing a stress concentration in the sensor w.r.t. the stress in the structure. New in this fatigue damage sensor compared to sensors invented in the past is the fact that this sensor is both in geometry and fatigue damage mechanisms similar to aluminium welded joints. Based on the time it takes for the sensor to crack, a prediction can be made of the expected lifetime of corresponding aluminium welded joints. This prediction is based on the in the VOMAS joint industry project developed Total Stress Concept in which fatigue resistance of all aluminum welded joints is combined in a single SN curve. It was found that with the total stress concept, resistance of welded joints is significantly higher than used in current fatigue design methods. Since the fatigue resistance increases with the total stress concept it is also investigated what the effects of implementing the total stress fatigue resistance curves at Damen would be on the fatigue design of high speed craft.

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.

Since the 19th century hopper dredgers have been used for civil operations such as land reclamation and water way maintenance. In the past 25 years a trend of scale enlargement of hopper dredgers driven by economic and productivity reasons is seen. Scaling of the designs has effect on strength and fatigue assessment as well on settling of particles which affect the rate op production. This thesis is focussed on the fatigue assessment of the hopper dredger. Cracked damages in coaming and bottom door areas have been found in relatively large hopper dredgers that are classed by Bureau Veritas. According to Bureau Veritas rules for steel ships fatigue assessment is required if the ship has length greater than 170 m but damages have also been observed in hopper dredgers with a smaller length. This indicates that the hopper dredger design demands for a specific fatigue assessment procedure. The hopper dredger has a unique characteristic load profile consisting of loading and unloading cargo several times a day in combination with small wave heights due to reduced freeboard regulations. Special interest is given on the effect of the loading and unloading cycle in the corners of the bottom discharge openings. It is suspected that the effect of this characteristic cycle may be underestimated and requires an alternative assessment procedure. The main question is what the contribution of this cycle is and how it should be assessed. The relatively low frequency of the cycle raises the suspicion of low-cycle fatigue and this phenomenon is therefore investigated. It is concluded that the loading and unloading cycle has a major contribution to the fatigue damage in the bottom opening corners. In fact 97% of the fatigue damage is induced by the dredging cycle and therefore this cycle could be seen as the sole cause for fatigue cracks in the bottom opening corner structural details. The high stress ranges confirm the suspicion of a low-cycle fatigue phenomenon and therefore further research is done on how to approach this type of fatigue. A method is proposed to take into account bi-axial cyclic plasticity and material hardening based on the use of a maximum principal hot spot stress range. Linear-elastic stresses are corrected to pseudo elastic stresses and effects of SN-curve selection, material selection, bi-axiallity and residual stress are analysed. Based on the found pseudo hot spot stress ranges it is concluded that the linear-elastic stresses in the analysed hopper are not high enough to demand for such a low-cycle fatigue assessment approach. The bi-axial ratio between the two multi-axial plane stress components has no effect on this conclusion.

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.

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

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.

Fatigue failure is a governing limit state for marine structures. Marine structures are composed of the numerous structural members connected by the welded joints which are the typically governing fatigue sensitive locations. In this thesis, attention has been paid to fatigue of welded joints from design and from testing perspective. The double sided longitudinal attachment is a common structural detail in marine structures and available resistance information, SN data, has been used to investigate the life time estimate accuracy in case the total stress is adopted as fatigue damage criterion. The single sided butt joint is a common structural detail in marine structures like steel catenary risers and a specimen has been designed for testing using the hexapod in order to ensure fatigue induced failure at the required location.Fatigue design of double sided welded longitudinal attachments using a Total Stress criterion The recently developed total stress concept considers both the weld notch and far field characteristic contributions. The total through-thickness weld notch stress distribution has been adopted to establish the total stress fatigue damage criterion and corresponding fatigue resistance curve. Thanks to a semi-analytical formulation, it is easy to determine the total weld notch stress distribution. The involved weld load carrying stress component is a characteristic one and unique for each weld notch location. A double weld element beam model has been developed to replace the original single weld finite element beam model in order to capture correct estimates for the double sided longitudinal attachments. Alternatively, a parametric has been established as well. The weld notch angle has turned out to be a governing parameter. Evaluation of the fatigue resistance of the welded double sided longitudinal attachment using the Total Stress concept shows a better accuracy in comparison to the nominal stress, hot spot structural stress and effective notch stress concept results. A governing factor for double sided longitudinal attachments is investigated which is the base plate thickness tb. Although, the IIW classifies the longitudinal attachment based on the attachment length la, the base plate thickness tb is much more dominant. Fatigue testing of single sided butt joints using a Hexapod The riser system forms a significant part of the development costs for floating offshore oil production facilities. The SCReen joint industry project aims to optimise the fatigue design and maintenance costs for Steel Catenary Risers (SCR’s). Generally, published fatigue resistance data of SCR welded joints are performed using resonance bending tests, which have two major limitations: a very high mean stress state and unrealistic variable amplitude loading. In order to obtain realistic fatigue resistance data, the SCReen project plans to do the fatigue tests with the Hexapod. A dedicated specimen containing the critical single sided butt joint has been designed  in order to obtain fatigue failure at the required location, involving two girth welds at the flange-pipe connection and the single sided butt joint at the middle of the specimen. With a post welding improvement, fatigue failure at the intended weld root location can be obtained.

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

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.

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.

At Damen’s Research and Development department, continuous improvements are made to existing design practices using recent technological developments and fatigue design is currently on the anvil. High speed craft operating around the globe are subject to millions of load cycles (resulting from ocean waves) and therefore suffer from metal fatigue. As per current practice, such vessels are designed for fatigue using inhouse software with a nominal stress approach supported by static sea-state scatter data and judgement based operational profiles. Recently, new data sources with real time sea-state data over the whole globe have come available, including GPS locations of ships. Together, this enables it to generate real time operational profiles for its ships. A total stress concept has been developed based on results from a JIP called VOMAS. This approach has shown to provide accurate fatigue lifetime estimates for arc-welded aluminium joints. Similar results are expected for steel joints as well. The main goal of this thesis was to explore possible improvements for the fatigue design methodology. A tool called Tanaav was developed for this purpose which makes use of both remote monitoring data and the total stress concept and provides an estimate of the yearly fatigue damage and fatigue lifetime. The tool has been used to predict the fatigue lifetime for three fatigue sensitive details in 46 FCS5009 vessels. Results have been compared to corresponding predictions using current and past fatigue analysis practices. It was observed that using both remote monitoring data as well as the total stress concept provides significant improvements in the predicted fatigue life time. Uncertainties in fatigue influence parameters affecting the predicted fatigue damage can be incorporated using probabilistic. This thesis proposes a method for this and presents two cases as a proof of concept. Work done in this thesis will be validated by Damen in future with the help of full-scale testing on an actual FCS5009 vessel during an offshore measurement campaign. After validation, this method will help in improving fatigue design practice, increasing design flexibility and in optimizing vessel weights in a responsible way.

Marine structures are mostly exposed to mild and moderate sea-states, where the structural response is elastic. Characteristics of environmental loading are highly stochastic in nature, resulting in stress-states that might be multiaxial. This thesis attempts to identify the source and effect of these multiaxial stress-states on fatigue damage in frigate type structures. A stress invariant based method called the Projection-by-Projection approach was adopted to estimate damage in the presence of multiaxial stress-states. This method was tested for its applicability and modified, where necessary, to better cope with the encountered stress-states. The design rules by the International Institute of Welding (IIW) were also used to be able to distinguish physical phenomena from methodological properties. The available data consisted of strain measurements from the United States Coast Guard (USCG) Cutters Bertholf and Stratton. The Bertholf was equipped with the WaMoS wave radar. Measured wave data was reinforced with simulated waves to make the research independent of encountered sea-states. It was concluded that multiaxiality is most relevant at low damage estimates (i.e.: low wave). The governing influence factor is the incoming wave direction. It was also shown that multiaxial stress-states may significantly contribute to fatigue damage (up to a factor 2, depending on the structural detail and location in the vessel) at an estimated life time of 2 · 10^{6} cycles.

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.

Fatigue lifetime has proven to be a leading design parameter for naval vessels. In the field of fatigue analysis and design of welded joints, two overall methods are distinguished: the temporal and spectral approach. Using empirical models, the spectral approach is typically preferred due to its efficiency, even though damage estimates are conservative. With the temporal approach, nonlinear effects can be captured, providing higher accuracy at the cost of additional computation time. Within this framework, an improvement of the spectral approach was pursued, focusing on incorporating the mean stress effect. Past efforts to include mean stress corrections were continued by focusing on applying the Walker correction. Nonlinear effects on the mean stress response were investigated and were found to be quite small for the case of a pontoon vessel. This enabled the use of the still water bending moment as a single input to the spectral mean stress model. By using Walker in the spectral mean stress correction, an overall reduction of conservatism of 21% was achieved. Future research should focus on testing new wave cases, applying additional fatigue models and investigating the effects for hull shapes of naval vessels.

With increasing attention to climate change,renewable energy generation has become a major topic for research anddevelopment. Wind and solar energy are generated on land, whereas wave, windand tidal energy generators are getting attention in the offshore domain. Anovel extension of onshore solar energy is the concept of offshore solar energyusing floating platforms. As little research has been performed on the conceptof offshore solar energy generations, main challenges in the field are relatedto consequences to the marine ecology, economics and production and structuraldesign of the platforms. In this thesis, a numerical model to assess wrinklingbehaviour of thin, floating sheets with application to the structural design ofoffshore solar platforms is developed. Since wrinkling of thin sheets, in general, isinitiated by a structural instability (i.e. buckling), the developed modelconsists of an arc-length method that is capable to deal with bifurcationpoints and to switch to bifurcation branches. In this way, buckling andpost-buckling behaviour of thin sheets are modelled and wrinkled shapes can beassessed without imposing a priori definition of unbalancingimperfections or loads. The computational model is developed using a shelldiscretization with Isogeometric rotation-free Kirchhoff-Love elements, whichare higher-order elements with a B-spline or NURBS basis with global supportand global higher-order continuity of the solution. For the illustrativepurpose and future use, a similar Euler-Bernoulli beam model was developed andnumerical solvers for static, dynamic, modal and linear buckling analysis wereimplemented. The model was verified using various benchmarkstudies for static, modal, (post-)buckling and dynamic analysis. In particular,the post-buckling solver was assessed by modelling the collapse of a sphericalroof and using buckling (post-)bucking of a cantilever strip. Both benchmarkshave shown excellent agreement with previous publications. Additionalverification was done on the approaching accuracy and prediction of bifurcationpoints. It was found that this accuracy showed the accurate prediction of thebifurcation point, although slightly underpredicted for finer meshes and higherorders. Additionally, the model wasapplied to three cases where wrinkling is involved. In these cases, sheets withlow bending stiffness were modelled such that their post-buckling shapes showmultiple half-waves and thus wrinkles. Based on the model of a floating sheetsubject to surface traction (e.g. wind or current), design parameters werevaried. From this case, it follows a decrease in foundation stiffness or anincrease in flexural rigidity (either by varying Young's modulus or thickness)implies the number of wrinkles to decrease and the wrinkling instability tooccur for lower loads. Thirdly, based on the wrinkling geometries of a quarterdisk, design consideration for VLFTSs for offshore solar energy generation weregiven. These are: (i) adding reinforcement to arrest wrinkles and to introducestructural hierarchy for structural reliability; (ii) consider the effect ofdifferent mooring system connections to the (reinforced) platform; and (iii)investigate the effect of holes and point loads on local wrinkling behaviour. Basedon the results of the study, it is concluded that the isogeometric thin shellformulation is suitable for different structural analyses and that in particularthat robustness and accuracy on a per-degree of freedom basis is observed inthe isogeometric post-buckling analysis. This adds post-buckling analysis tothe seamless integration of Computer Aided Design (CAD) and Analysis ofIsogeometric Analysis. Suggestions for further studies include severalimprovements of the current implementation (patch coupling, boundary conditionimplementation), utilization of nonlinear material models for modelling ofrubber-like materials, adaptive re-meshing using THB-splines to capture localwrinkling phenomena and Fluid-Structure Interaction computations with anonlinear structural and fluid description of VLTFSs in large waves.