To decrease vehicle emissions, automotive manufacturers aim to decrease materials usage. Dual-phase (DP) steels are used in safety critical locations of a vehicle and with stronger DP steels, less material is needed. However, DP steels are known to be affected by hydrogen embrittlement (HE), which decreases the ductility and is thus of significant concern.

This work investigates the effect of heat treatment on the microstructure and HE of DP steel. Heat treatments are developed and conducted using dilatometry, with obtained microstructures characterized using scanning electron microscopy (SEM), optical microscopy (OM), electron backscatter diffraction (EBSD) and hardness measurements. Tensile specimens are heat treated using a Gleeble 3800-GTC thermo-mechanical simulator, before evaluating the HE effect by measuring the total absorbed hydrogen content using thermal desorption spectroscopy (TDS), and determining the static mechanical properties using slow strain-rate tensile (SSRT) testing. The fracture micromechanisms are investigated with fractography using SEM and EBSD with interrupted SSRT and fractured specimens.

The critical transformation temperatures are determined at 686±1 ⁰C and 571±10 ⁰C for Ar3 and Ar1 respectively. Intercritical annealing at 590 ⁰C, 630 ⁰C, and 670 ⁰C result in a martensite content of 24.97±6.43 %, 40.41±4.21 %, and 77.63±6.97 %, respectively. Hardness values of 240±31, 300±9, and 380±20 HV1 are obtained for the respective intercritical annealing conditions and are an accurate method for comparing the martensite content and microstructure of dilatometry and Gleeble specimen. The martensite distribution changes from a discontinuous network of lath martensite with small martensite islands at 590 ⁰C, to a continuous network of lath martensite without martensite islands at 670 ⁰C.

TDS yield an increase in the total absorbed hydrogen content of 1.16±0.11 wppm, 1.29±0.13 wppm, and 1.58±0.27 wppm at increasing intercritical annealing temperature, which is attributed to an increase in the ferrite/martensite interphase.

Through SSTT the optimal combination of ultimate tensile strength (UTS) and elongation is found at 630 ⁰C, with UTS of 1025±8 MPa and elongation of 12.6±0.6 %. For all specimens significant HE is observed, with an embrittlement index of 58.9±7.1 %, 89.2±1.1 %, and 85.8±2.5 % for increasing intercritical annealing temperature. The highest embrittlement at 630 ⁰C is attributed to dislocation pile-up around martensite islands, severely increasing embrittlement.

Fractographic investigation revealed the presence of quasi-cleavage fracture bands across the specimen cross-section. This is attributed to preferred crack propagation along martensite bands in the cross-section, with preferred crack nucleation at the martensite surface layer of the specimen short edge. EBSD of the fractured specimen revealed an increase in the kernel average misorientation (KAM) surrounding the ferrite/martensite interface and martensite islands, indicating an increased HE action due to the HEDE and HELP mechanisms.

It is concluded that there is considerable embrittlement of the heat-treated DP steel, which is significantly influenced by the martensite content and distribution. Since DP steels are used in safety critical components, controlling the distribution of martensite in the microstructure is vital, and avoiding surface layer martensite and martensite bands can reduce the HE effect.

Hydrogen-assisted fatigue in pipeline steel has become a major concern in the field of energy transition. In recent decades, growing awareness of environmental issues has significantly increased interest in hydrogen as an alternative energy source. Hydrogen is affordable and can be transported using the existing natural gas infrastructure. However, the structural integrity of pipeline steels, especially under hydrogen-assisted fatigue conditions, poses significant challenges to the durability and safety of modern energy infrastructure. Since fatigue is a common cause of pipeline failure, a deeper understanding of hydrogen’s impact on the fatigue behaviour of these pipeline materials is essential. This project investigates the fatigue crack growth (FCG) behaviour of API X65 pipeline steel and its welding material in hydrogen enriched environments. To achieve this, a modular hydrogen autoclave testing device was designed and implemented to conduct in-situ hydrogen gaseous fatigue tests on eccentrically single-edge notched tension (ESE(T)) specimens. The fatigue crack growth relationship (da/dN vs. ∆K) was measured, and hydrogen-assisted fatigue crack growth was observed and compared to benchmarks in nitrogen and air. The study employs two testing frequencies to examine the influence of loading frequency under constant load and hydrogen pressure. This approach evaluates the combined effects of hydrogen and the microstructures of both base and weld metal. The investigation focuses on material responses in terms of crack propagation rates, measured using direct current potential drop, and fractographic analysis. To determine the required hydrogen pre-charging time, a hydrogen diffusivity FEA (Finite Element Analysis) modelling approach was used to estimate the hydrogen equilibrium concentration in the specimens before testing. The experimental setup involved pre-charging the specimens in a hydrogen environment at 10 MPa, followed by fatigue testing at frequencies of 0.1 Hz and 10 Hz. The findings indicate that the hydrogen environment accelerates fatigue crack growth compared to inert gases and air. The base metal (BM) at 10 Hz showed approximately two times greater susceptibility to hydrogen-assisted cracking compared to the weld metal (WM). At higher ∆K values (∆K > 50 MPa√m), the crack growth rates of base metal (BM) and weld metal (WM) were similar. However, at lower ∆K values (20-30 MPa√m)), the BM exhibited about one order of magnitude higher crack growth rates. The study emphasizes the importance of microstructural characteristics, including grain boundaries, size, and micro-hardness, in affecting fatigue crack growth. The study confirms that loading frequency significantly influences H-AFCG rates, where at loading frequency f = 0.1 Hz, FCGR increases by 2.3 times for BM and 3.9 times for WM compared to results at 10 Hz frequency, with lower frequencies allowing more hydrogen diffusion and promoting the transition from ductile to brittle cracking. Further findings indicated that finer grains in WM enhance hydrogen fatigue resistance due to increased grain boundary densities. However, this advantage diminishes at lower frequencies where hydrogen diffusion is enhanced, increasing susceptibility to hydrogen-assisted fatigue crack growth (HA-FCG). Analysis of fracture surfaces reveals a significant transition from ductile to brittle fracture modes when moving from inert environments (air or nitrogen) to hydrogen. In hydrogen environment, the presence of quasi-cleavage and intergranular facets was prevalent, compared to ductile transgranular features in air or nitrogen. Additionally, in hydrogen environment, secondary microcrack branching a characteristic of ductile fracture is notably absent, especially at lower stress intensity factors (∆K).

Wire arc additive manufacturing (WAAM) can create large parts, due to its high deposition rate. WAAM can be used to create functionally graded materials (FGMs) such as a strong core of high strength low alloy (HSLA) steel and corrosion-resistant cladding of austenitic stainless (AUS) steel. However, HSLA and AUS steel are dissimilar metals with different microstructure and mechanical properties. Joining dissimilar metals with WAAM creates a complex interface with a different set of properties compared to parent materials. The current state of the art covers the microstructure, tensile properties, and hardness of such FGM. However, there is a lack of understanding about the fatigue properties at the interface. This study investigates the fatigue crack growth behaviour at the interface of ER70S-6 (HSLA steel) and ER316L (AUS steel). Initially, a benchmark study of the HSLA and AUS steel was done to obtain Paris parameters, threshold stress intensity range (∆Kth), and hardness. HSLA steel showed 43 to 77% lower Paris slope parameters than AUS steel, which suggests a better fatigue resistance for HSLA steel. However, no significant difference in ∆Kth values was observed. The hardness of HSLA and AUS steel were 247.0±6.1 HV and 227.7±5.5 HV respectively, which suggests a correlation between hardness and fatigue resistance. Further analysis showed a power law relation between the Paris slope parameter and hardness for HSLA steel. This was attributed to the grain size. Smaller grain sizes improve fatigue resistance and increase hardness. Hence, hardness can be an indirect measure of fatigue resistance. Under constant ∆K experiments, AUS steel showed up to a 61.7% difference in fatigue crack growth rate (FCGR). The anisotropy in AUS steels is due to the large grain size in the range of 100 μm and the structure of the weld beads, resulting in heterogeneity. After the benchmark study, the graded materials were tested under similar conditions. The crack in the graded material grows from AUS steel to a 2 mm wide interface and then to HSLA steel. A variation of FCGR at the interface region was measured, notably 1) a decrease of 5.1E-05 to 6.1E-05 mm/cycle before the interface region, 2) a gradual increase of 5.5E-05 to 1.32E-04 mm/cycle within the interface region, and 3) a sudden increase of up to 3E-04 mm/cycle. To understand these behaviour microscopy, fractography, and electron backscatter diffraction (EBSD) analyses were utilised. The sudden increase of FCGR in HSLA steel was attributed to process-induced porous defects. The decrease of FCGR before the interface region was due to the increase in grain boundaries, secondary cracks, an increased crack closure effect, and martensite formation through transformation-induced plasticity (TRIP) in the interface region. The gradual acceleration of FCGR within the interface region was due to the martensite phases formed during solidification. The martensite formed during solidification had twice the hardness in comparison to HSLA steel. The martensite formed by the TRIP effect also resulted in 25-50% higher hardness near the crack flanks compared to the regions away from it. For the application of this FGM, the interface can act as a protective layer by reducing the FCGR. However, the martensite formed during solidification in the interface should be mitigated.

The transition to methanol as a promising alternative fuel for maritime applications, presents unique design and material challenges, particularly in retrofitting existing integrated aluminum tanks for methanol storage. Methanol’s toxicity, flammability, and corrosive nature, particularly with respect to aluminum alloys, complicate this process. This research adopts a multi-disciplinary approach, integrating ship design and material science to address these challenges. A complete analysis of regulatory frameworks and material compatibility was conducted to explore viable solutions for converting integrated aluminum tanks, with a focus on incorporating an inner barrier that ensures both compliance with classification societies and the optimization of net volume.

Key methods used in this research include Multi-Criteria Decision-Making (MCDM) analysis and static immersion followed by tensile tests experiments. The MCDM was conducted to ascertain the most viable inner barrier solution for retrofitting integra.

The long-distance maritime transport sector plays one of the most relevant roles in the transport sector. Therefore, this project designs a modular solid hydrogen solution, studying the assumptions made in the literature and including new simplifications that reduce the computational costs of its simulation. Moreover, this project compares the viability of this solution with the different prototypes found on the market.

The energy transition requires us to explore all options for generating non-fossil energy. Companies are starting to invest in technologies such as offshore floating PV systems (OFPV) to avoid congested urban population centres. OFPV structures are likely to consist of many small, simple, flexibly connected floaters. The entire structure must be able to survive extreme offshore conditions. The OFPV response in various sea states is heavily influenced by the connector design.
In this thesis, a 3D boundary element based numerical model is used which was developed by Tuitman [2]. The model is expanded to output the forces and moments experienced by the compliant connectors which have linear stiffness in 6 degrees of freedom. After successful verification and validation, three case studies are presented which are a three floater serially connected model and a 3x3 and 4x4 grid connected model. Various sea states and wave headings are analysed to show the effect on dynamic behaviour of a compliant connector. The time domain-based approach is used to capture nonlinear Froude-Krylov and hydrostatic forces. The remaining hydrodynamic terms are linearised by solving in the frequency domain. The connector response is linearised by using a finite stiffness matrix but the forces are solved in the time domain to include the effects from the nonlinear hydrostatic terms.
The results show that the resonant response of the structure and connectors is critical in determining the floater motions and loads in the connectors. Additionally, the stiffness of the connector influences the natural frequencies of the structure. The forces and moments in the connectors of the grid are much more varied than the serially connected structure because of the complex interaction of the floater hydrodynamics and connector resonance. The floater motions and connector response becomes less evenly distributed for oblique seas and when sea states match the natural frequency of the structure. The grid experiences bending in multiple directions which results in large connector loads at some locations on the structure. When expanding to a larger 4x4 grid there is a different distribution of connector loading than the 3x3 grid because of the extra degrees of freedom. The aft connectors experience lower loads than connectors facing the waves due to a shielding effect.

The increasing demand for renewable energy sources, to achieve ‘green goals’ such as the Paris agreement, and their related projects, are becoming increasingly challenging to accomplish. Nowadays, offshore wind farm projects move into deeper waters and require larger individual Offshore Wind Turbines
(OWT’s), which results in larger monopiles. As a result, heavy lift vessels shipping and installing the monopiles can transport fewer monopiles at a time, as they have outgrown the cargo hold and can only be shipped on the top deck. Considering the stiff nature of the monopile and that it spans almost the entire length of a ship nowadays, companies involved in such projects, such as Jumbo Maritime, have concerns this might heavily affect the ships dynamic behaviour. Therefore, the research presented in this thesis is focused on investigating what effects a lashed monopile on the deck of a heavy lift vessel causes and if friction contact between ship and monopile plays a role. To answer these questions the ship and monopiles are modelled as a coupled system with appropriate boundary conditions. The research is focused on finding lashing & friction contact effects, but also what effects of the number of monopiles introduces onto the system.

Fiber Reinforced Composite (FRC) materials are gaining great popularity in marine structures because of their excellent strength-to-weight ratio, low density, and provided freedom in the design process. However, the use of FRC materials comes along with relatively large uncertainties in material properties and structural integrity after manufacturing and during its use. In this research a new in-situ non-destructive stiffness assessment methodology is proposed. This methodology is based on a coupling principle between the laminate structural stiffness properties and the ultrasonic guided wave characteristics of FRC materials. In the methodology, a range of possible stiffness properties is defined based on the structural information available for a structure of interest. The average relation between this stiffness range of interest and corresponding wave characteristics is described using a set of coupling coefficients which are determined using numerical simulations. For this, a batch of reference laminates is constructed that covers the entire stiffness range of interest. Input for the system are the group velocities of the zeroth-order symmetric and antisymmetric guided wave modes, measured on the structure of interest. The potential of the proposed methodology is evaluated using a numerical feasibility study based on numerical simulations. Good results were obtained for different test scenarios, varying in the amount of structural information available on the structure of interest. Thereafter, the in-situ application of the methodology has been examined in an experimental setup. Good measurement and analysis times were achieved by using a compact measuring device that is capable of recording the wave signal in five directions simultaneously. A reliable accuracy assessment of the in-situ application of the methodology was, however, difficult to obtain due to the lack of reliable reference studies. Therefore, in future research reliable reference information should be gathered. Additionally, a next step for future research should focus on decreasing the amount of information available on the structure of interest.

For floating photovoltaic systems, an uncommon type of offshore structure is considered, which is very flexible and deforms with the motion of the waves. In this research, the bending characteristics of a drop-stitch floater is analysed, which is an inflatable panel. By inflating the drop-stitch floater to a low air pressure, it obtains a flattened shape and the ability to support the flexible solar panels, while still retaining flexibility to deform with the motion of the waves. The bending characteristics of a drop-stitch floater are more complex than common offshore structures due to different non-linearities: wrinkling, hyperelastic material behaviour and internal pressure-volume work. Getting a better understanding in the bending response is important to eventually determine the response and limit states in offshore conditions. A finite element and experimental analysis has been performed of the response in a three point bending load case. Also, an experimental uniaxial tensile test has been performed to evaluate the hyperelastic anisotropic material behaviour. Different yarns spacings, internal air pressures and face sheet thicknesses are evaluated. This showed that there are two types of failure modes with distinct behaviour for an uniaxial pure bending load case: a global wrinkling and folding failure mode.

Highly-loaded low-speed roller bearings are frequently used in equipment offshore e.g. slew bearings in crane, sheave bearings and FPSO turret bearings. Reducing maintenance costs, increasing productivity, ensuring safety of people and structural integrity and longevity assurance of equipment can be achieved by the condition monitoring of highly-loaded low-speed roller bearings.

Currently, very limited research is performed in the field of condition monitoring with acoustic emission (AE) monitoring for highly-loaded low-speed (<10 rpm) bearings with naturally developed damage. Acoustic emission monitoring has shown promising results in early stage and real time identification of defects according to literature. Furthermore, contamination of wear particles in lubricant shows an increase in AE activity according to literature.

In this research, the applicability of AE monitoring for damage assessment of highly-loaded low-speed roller bearings is investigated in a full scale duration test. Furthermore, the influence of contaminated lubricant with steel particles is investigated in the contamination test to study the feasibility of real-time detection of contaminated lubricant during operations and as a supporting technique for wear debris analysis in lubricant.

From the results, an increase in AE activity expressed in hit-rates is observed for a wide frequency band of 40 - 580 kHz during the duration test where the basic rating lifetime (L10) is consumed from 40% to 50%. Wear development in the bearing is observed during visual inspection and has been quantified with lubrication analysis.
A safety limit and a limit if the bearing is at risk for the AE activity has been defined with the results of the baseline and duration test. This research suggests that AE monitoring can be applied on highly-loaded low-speed bearings to assess the damage for representative operational conditions.

From the results of the lubricant contamination test, an increased AE activity with respect to higher lubricant contamination levels were observed. Furthermore, with a proper choice of the SNR-level, there is no masking effect of acoustic emission signals due to contaminated lubricant. The results of the contamination tests, indicates the possibility of detecting contaminated lubricant with AE monitoring.

The objective of this research is to evaluate what is required to make the operation of multiple robots feasible for large scale seafloor imaging campaigns. Three blocking issues were identified; high cost of subsea robots, limited handling efficiency and insufficiently performant localization technology. These are solved by applying, in order: novel engineering solutions, automation, and solutions from literature. A proof of concept system design and a model are developed to analyze how the physical parameters of the subsea robots relate to the performance of the subsea mapping system. A case study shows that for large areas, the proposed solution is 80 times faster and less costly than conventional approaches. In conclusion, there is a technically feasible approach that can be used photograph the seafloor in a more cost-effective manner on a large scale.

Reducing the levelised cost of energy is crucial to accelerating the energy transition. To develop offshore wind solutions in greater water depths, a floating solution is required. The time-domain simulations of these Floating Offshore Wind Turbines (FOWTs) under wind-wave misalignment used in research and industry projects are computationally intensive and limits researchers and industry in their developments. To better understand the sensitivities of the fatigue loads of FOWTs to different parameters and environmental conditions, a computationally efficient method is needed. The aim of this research is to develop a frequency-domain method to quantify the effects of misaligned wind and waves on the response of a semi-submersible floating offshore wind turbine. Therefore, the following research question is defined: What is the effect of misaligned wind, windsea waves, and swells on the loads at the tower base of a semi-submersible type floating offshore wind turbine?

Several sensitivity studies are conducted to quantify the contribution of yaw-roll coupling effects and aerodynamic damping to the responses and loads. From these studies, it appears that the yaw-roll coupling can increase the response when excited at wind/wave directions in which the structure is asymmetric.
The magnitude of this effect is related to the wave peak period (and the resulting wavelength), the angle of misalignment with respect to the structure, and the apparent length of the structure. Also, the lack of aerodynamic damping in the direction of the rotor plane (side-side direction) leads to a noticeable
increase in the response, directly or through coupling effects. Finally, the frequency-domain method is compared with the time-domain simulations (BHawC-OrcaFlex) carried out by Siemens Gamesa. Although reasonable agreement is found for the load driving rigid body modes, significant differences in the tower bottom loads are found for the lowest and highest production wind speeds.

These results show that misaligned wind and waves can increase the response for headings where the structure is asymmetric due to coupling effects. Wind-wave misalignment leads to an increased response in the direction of the rotor plane due to the lack of aerodynamic damping. In general, the wind-wave misalignment can also have a mitigating effect on the maximum equivalent moment at the tower base, as the aerodynamic damping also reduces the response in the wave frequency range. Furthermore, the comparison shows the need to extend the frequency-domain method with the first tower bending modes and improvement of aerodynamic/mooring property estimation. Based on the findings and the conclusions, the recommendation is to investigate the floater specific sensitivities at an early stage of the design. Future research should focus on: the implementation of tower flexibility, improvement of the quasi-static estimation of mooring stiffness, frequency dependent aerodynamic properties, and implementation of second-order wave forcing.

In this 3D fracture-based topology optimization framework, the author extended the 2D framework on tailoring fracture resistance for brittle materials (Zhang et al., 2022) to 3D. In the optimization, the topology is described by radial basis functions interpolated level set function, and the problem is solved by the Interface-enriched Generalized Finite Element Method (IGFEM). Cracks are assumed to exist on enriched nodes that are added on the boundary of the geometry to increase the accuracy of approximation. The first part of the work assumes cracks to be semi-circular with the crack plane perpendicular to the boundary, and the crack opening direction to be either parallel to the KM- or LM-plane of the global coordinates. An extended framework that assumes crack opening direction perpendicular to the surface first principal stress is also developed at the end. The energy release rates (ERRs) of the cracks are evaluated with the topological derivative method, which requires only a stress field of the geometry and weight functions that relate the stress and stress intensity factors (SIFs). The weight functions are found by a finite element analysis on a cuboid with a crack. This approach is computationally efficient because it eliminates the need of actually modeling and meshing the crack planes in the geometry during optimization. Moreover, a 3D stress recovery technique (or stress improvement procedure, SIP) is used to recover the nodal stress non-locally to improve the accuracy. The objective function is then established with the ]-mean aggregation of the ERRs. Finally, Numerical examples in 3D, including the famous L-bracket benchmark problem, are performed to prove the correctness and capacity of the framework. In conclusion, this extended framework shows more flexibility and provides more information to the optimized design than the 2D framework by considering an added dimension both in analyzed geometry and crack shape, i.e., the effect of the anisotropy of cracks can be captured.

Climate change is one of the top global issues that the United Nations has identified that can adversely impact people all around the globe. Moving towards a hydrogen economy can reduce greenhouse emissions produced from burning fossil fuels which is one of the biggest contributors to global warming and climate change. Hydrogen fuel has a high gravimetric density and being a clean fuel it has the potential to become a sustainable energy source for the growing market. However, its low volumetric density makes it a difficult fuel to store, thus making storage technologies in the hydrogen supply chain an important part. The current storage technologies, however, are impeded by shortcomings such as low hydrogen densities, extreme pressure and temperature operating conditions and inefficiencies during the storage process. Combining existing hydrogen storage technologies such as compressed hydrogen gas and metal hydrides with 2D materials comes across as an excellent option as they can complement each other in their functioning. In this regard, borophene is considered a viable material for hydrogen storage due to its lightweight, good thermal, mechanical and electrical properties. Most of the studies performed so far on hydrogen storage in borophene were on hydrogen physisorption via weak van der Waals forces. In this thesis work, the chemisorption of hydrogen on borophene via strong covalent bonds is studied. This is because borophene with chemisorbed hydrogen is more energetically stable than with physisorbed hydrogen. Also, chemisorbed hydrogen on borophene performs better than physisorbed hydrogen in terms of safety and long-term storage. Hydrogen chemisorption on pristine, defective (i.e. with single and double vacancies) and metal decorated borophene are studied in this work, using density functional theory (DFT) and nudged elastic band (NEB) calculations to compute quantities such as enthalpy and activation energy barriers of chemisorption. Using Bader charge analysis and partial density of states analysis, it was observed that the addition of charge on the hydrogen bond weakens the bond, expediting the chemisorption reaction. The charge transfer from borophene to the chemisorbed H atoms was found to stabilize the final chemisorbed state. In the case of metal decorations, there is an additional steric factor that influences the activation barrier height for chemisorption. In general, metal decorations performed better than pristine and defective borophene systems in terms of the desorption barrier height, among which double decorated K atoms on borophene substrate had the best performance, improving the barrier height by 20% compared to the corresponding value for the pristine system.

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.

Due to the need for more renewable energy, interest in floating wind turbines has greatly increased over the last years. These turbines have to withstand harsh environmental conditions, such as extreme waves called rogue waves. These rogue waves (or freak waves) are exceptionally large waves compared to the surrounding sea state and are defined by two different size criteria, one relating to the crest height and the other to the wave height. Research has shown that these rogue waves occur more frequently than expected, from which the question arose whether these rogue waves could be dangerous for floating wind turbines.
This thesis aims to answer that question by evaluating the probability of such a wave interacting with a floating offshore wind turbine and the subsequent induced motion response. A location off shore California is evaluated for a spar-type floating wind turbine (SFWT). In this research, different time-extreme (TE) and space-time-extreme (STE) statistical wave crest and crest-to-trough models are used with historical wave data to estimate rogue wave occurrence probabilities. The STE models approximate the probability of a wave occurring within an area in time, as opposed to a point in time (TE), and are considered state-of-the-art. Wave data from 1998-2020 is used, which is a significant amount compared to the life of a typical offshore structure. A frequently chosen model for research into a SFWT is the OC3-Hywind concept, which consists of the National Renewable Energy Laboratory (NREL) 5MW reference turbine and a substructure based on the Hywind spar.

The area size which leads to the STE models estimating a higher rogue wave occurrence probability than the TE models is investigated for the first time. This is done by comparing the TE and STE probability models to each other on the basis of their maximum estimated exceedance probability of the rogue threshold, for different area sizes. Only on rare occasions, with a very small area, did the most conservative TE wave crest model estimate a higher probability than the STE2 (wave crest STE) model. Therefore, the STE2 model appears most conservative when evaluating rogue crests for a SFWT.
The Rayleigh model was the most conservative among TE crest-to-trough models that were considered, and estimated higher probabilities than the STE1QD (wave crest-to-trough STE) model in several sea states up until an area size of 12x12m2.
The influence of the shape of the wave spectrum was investigated by two spectral bandwidth parameters. All four wave crest models showed an increase in rogue crest probability as the wave spectrum became narrower. Regarding the crest-to-trough models, no clear preference was observed.
Long-term rogue wave probabilities were calculated for four sizes of rogue crests and waves, where the squared area of the STE models was based on the waterline diameter of the OC3-Hywind spar (6.5x6.5m2). Probabilities were averaged per sea state bin, and it was observed that for this area the STE1QD model was more conservative than the Rayleigh model. When single maximum exceedance probabilities are compared, the Rayleigh model can be more conservative in several sea states until an area of 12x12m2, but when overall occurrence probabilities are considered, this area appears to be roughly 6.5x6.5m2.
Following the long-term rogue wave probability analysis, four rogue waves were each embedded in random wave series, based on their most likely wave spectrum, using a deterministic extreme wave model. With these wave time histories, simulations were run in OrcaFlex using a publicly available OC3-Hywind model. During the simulation the turbine was assumed to be in parked condition and only the wave loading was considered. Mostly the pitch angles and nacelle accelerations are limiting for safety, due to stability criteria and sensitive components in the nacelle. These limits are imposed by turbine manufacturers but are never shared publicly. Therefore, the results were compared with reference values from the literature. The dynamic response did not exceed these maximum reference values, but they can be considered quite serious as some did exceed lower thresholds.

Based on the long return period of the considered rogue waves, together with their induced dynamic response which does not exceed maximum reference values, it appears that these rogue waves alone are not dangerous for the considered SFWT. However, ultimately this will depend on the sensitivity of the components inside the turbine, as well as the desired risk profile of the wind farm owner.

Structural health monitoring of maritime and offshore structures can contribute to the reduction of the amount of uncertainty in assessment of the operational loads and the structural integrity. Such a survey can also reduce the large safety margins during the design and optimize the conservative inspection and maintenance schedules. One of the emerging variants of structural health monitoring are the so-called digital twin systems that not only allow the monitoring of the structures, but can also simulate the past or the future state of the structural integrity. In the context of this thesis, the feasibility of a digital twin system for the structural health assessment of marine structures based on machine learning algorithms and utilization of big data is examined. The main research question is if the digital twin technology can be enhanced with machine learning to accurately monitor the structural health of marine structures in terms of loading and fatigue damage accumulation. In order to answer this question four modules have been developed and tested. The first module uses artificial neural networks trained on operational data to predict the fatigue damage accumulation and the frequency that corresponds to the maximum stress power density. The error of the developed networks turned out to be negligible in the investigated cases, with standard deviation of less than 4% in the predictions. The peak frequency of the stress power density is predicted using a random forest regression algorithm. The optimized version of the algorithm has led to an accuracy of 93% with standard deviation of less than 1% in the predictions.
The second module is used to recalibrate the design response amplitude operators of the structure using the predictions of the first module or the operational data, if available. The design response amplitude operators are scaled and shifted in order to minimize the deviation between the spectral fatigue calculations and the predicted (or measured) data.The third module is a static/quasi-static load-reconstruction module. Using on-board strain measurements it is able to calibrate the loading of the structure based on a reformulation of the conventional finite element problem for static and quasi-static loading. A sensitivity analysis of the algorithm effectiveness have been performed using multiple load cases in which the reconstruction error turned out negligible. The fourth and final module relates to structural reliability analysis. This module is based on the estimation of the Hansofer-Lind reliability index as a minimization problem. The employed optimization engine is a variation of the particle swarm optimization algorithm using chaotic system behaviour to iteratively calculate the user-defined parameters. The limit state equation is formulated in a way that it takes the uncertainties related to the fatigue damage accumulation prediction model and Miner's Rule into consideration. Uncertainties related to the material and the fabrication process are not taken into account. The developed modules have been tested on structural details of a vessel monitored in the scope of Monitas Joint Industry Project (JIP). About a year's worth of data has been used to train the machine learning algorithms.

The computational modeling of crack propagation is important in aerospace and automotive industries because cracks play a significant role in structural failure. In comparison with the quasi-static case, inertial effects are significant in dynamic crack propagation. The classical numerical tool for modeling dynamic crack propagation is the standard finite element method (FEM). However, the method suffers from problem of mesh-dependency. Enriched finite element procedures such as the eXtended/Generalized finite element method (X/GFEM) are able to solve this issue by adding enriched degrees of freedom (DOFs) to nodes of the original mesh, thereby providing full decoupling between the mesh and cracks. Yet, X/GFEM suffers from other issues that are inherent to the formulation such as the need for more complicated methods for applying non-zero Dirichlet boundary condition, and an intricate computer implementation. In the thesis, the Discontinuity-Enriched Finite Element Method (DE-FEM) is used to simulate dynamic crack propagation as an alternative to X/GFEM. DE-FEM also decouples cracks from the background mesh while solving X/GFEM’s aforementioned issues. This is achieved by adding enriched DOFs along cracks. Prescribing non-zero essential boundary conditions is as straightforward as in standard FEM. Moreover, DE-FEM’s enrichment strategy provides simplicity on crack propagation, as the crack tip node can be transformed into a crack segment node directly with little effort. Also, fewer enriched DOFs than in X/GFEM are appended while advancing cracks. In the numerical examples of either stationary crack with dynamic loading or dynamically propagating cracks, DE-FEM is able to reproduce analytical and/or experimental results. Both Bathe’s time integration method and the classical Newmark constant average acceleration method are applied and compared. Results show DE-FEM is a suitable candidate to solve dynamic fracture problems.

Higher strength steels (HSS) hollow sections with steel grades from S460 up to and including S700 are becoming increasingly competitive in long span structures. Reduction of self weight is accomplished with the smaller wall thickness of the hollow section, leading to fabrication, transport and execution benefits. The use of HSS has a positive effect on the CO2 equivalent emission, which is aligned with the European Green Deal for the future development of the construction sector. Lack of experimental evidence is identified as one of the main reasons to propose rather pessimistic material factors in the revised version of EN1993-1-8 [5] for design in tubular joints. The revised version, published in 2020, recommends material reduction factors for the design of joints made of steel with yield strength larger than 460 and up to 700 MPa, in the range from 0.9 to 0.8.
In this thesis, the behaviour of the X-joints made of steel grade S355, S500 and S700 are investigated experimentally and numerically, and the proposed material reduction factors are discussed. Five full-scale welded X-joints in tension with rectangular hollow section were tested Stevin Lab II, TUD. Additionally, base material and butt-welded coupon specimens are tested to obtain the engineering stress-strain relationship of the base material, weld and heat affected zone (HAZ).
Based on the results of coupon tests, finite element software ABAQUS is used to model X-joint and to supplement the limited number of experiments. Furthermore, result of a parametric study is presented in which effects of three parameters: the yield strength, the parameter β (ratio of the width of the brace member to that of the chord member) and the thickness of the chord are investigated to analyse their influence on the structural performance of X-joints. Following properties are thoroughly examined: static strength, stiffness, ductility and failure mode. Finally, conclusions are drawn on the validity of the material reduction factors.

An increasing trend is occurring in the use of high strength steel (HSS) in tubular joints. Reason for this trend is the high yield strength of HSS which leads to reduced weights and costs. Characteristic for HSS is the reduced ductility in comparison to normal steels. Therefore, Eurocode 3 part 1-8 (2020) recommends material reduction factors (Cf) for the design of joints made of steel with a yield strength higher than 355 MPa. The standard includes steel grades up to S700. The material reduction factors need to be validated to ensure efficient design of tubular HSS joints. This research intents to investigate the behaviour of X joints made from different kind of steel grades. The goal is to investigate if the material reduction factors for HSS, proposed in the new version of the Eurocode, are justified. Coupon experiments are conducted to determine the material properties of the X joints. Two geometrically identical X joints with varying steel grades of S355 and S500 are experimentally tested. The conducted X joint experiments, which are evaluated by digital image correlation (DIC), provide the necessary evidence to understand the behaviour of the failure mode and the fracture strain. Finite element analysis confirms the experimental evidence. The results from both the experiments and finite element analysis are used to validate the Eurocode design resistance. Furthermore, a parametric study is executed to investigate the influence of the material properties and the brace width over chord width ratio on the numerical resistance of X joints. This results in a total of 39 different X joint models that are included in this research. Lastly, the obtained numerical resistances are compared to the predicted design resistance according to Eurocode.