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

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

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

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

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

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

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

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.

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.

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.

Natural gas accounts for just under a quarter of global energy demand. The gas is mainly delivered through pipelines, but is increasingly shipped overseas by liquefying it to LNG at -162 degrees Celcius. Fluids, containing free surfaces inside ships are likely to start sloshing, as ships move on waves. These fluids induce impact loading on the walls of the tanks. These impacts induce amplified motions in the tanks membranes in the order of millimeters to centimeters,resulting in permanent deformation that could exceed acceptable risk levels. In the industry one case of plastic deformation in the corrugated membranes of the LNG tanks was observed (Gavory and de Seze, 2009), but leakage was prevented. A growing desire for understanding sloshing behaviour raised and action was taken under the name of SLOHEL a JIP. Full scale experiments have been carried out. For investigating detailed structural response on the effect of Fluid-Structure Interaction on a prediction of the plastic deformation of a Mark III membrane, subjected to a local wave impact, a numerical application was carried out. In this thesis, the experiments became input for a numerical Fluid-Structure Interaction model, that is strong two-way coupled. The structural response of a local wave impact is analysed using LS Dyna. Two-way|coupled and one-way|uncoupled results are compared. The pressures of the fluid model have been validated after making simplifications and are within a range of ±10% compared to measured results of the SLOSHEL experiments for an wave impact velocity of 7 m/s. In the simulations three high pressure stages are identified on an impact between two corrugations. First, initial impact. Thereafter, lower corrugation impact ending with upper corrugation impact. At stage 3 large deformations were found in the foam layer behind the stainless steel membrane and for higher impact velocities also in the corrugated membrane. Uncoupled simulations, in which the structure is considered as rigid for the fluid solver, but is able to respond flexible in the structural solver, were performed. In coupled simulations, a full interactive model is considered in which pressures update as a result of structural displacements. This results in dry frequencies for uncoupled simulations and wet frequencies for coupled simulations. It was found that pressures in uncoupled simulations are in often higher. Displacements in the foam show for all cases higher displacements (10-25%) for uncoupled simulations. The plastic strains in the corrugated membrane are higher for high impact velocities (50-100%) for uncoupled simulations. The significance of accounting coupled Fluid-Structure Interaction increases when deformations become non-linear. For this specific case, when yield stress of the corrugations in the membrane is exceeded id est when plastic strain occurs, uncoupled simulations are not able to obtain accurate predictions in structural response.

Research to analyse and/or develop novel materials and techniques is necessary to address competition and safeguard craftmanship in shipbuilding. In this master thesis, two new approaches to reduce the production and maintenance costs are studied: {metal foam core, metal face sheet} sandwich materials and corner adstir fillet stationary shoulder friction stir welding (FSW). The current project is a preparatory analysis for the research in which the two components will be combined. It is anticipated that their combination may generate more added value due to mutual reinforcement.
The study of metal foam based sandwich materials showed that looking at the material level, the {aluminium foam core, steel face sheet} sandwich material is theoretically promising with respect to the conventional steel solid plate in specific situations and applications. When the sandwich material is implemented as part of the stiffened panel, the expected benefit could not be realised. This confirms the search for a more selective use, such in naval vessels when one needs a high resistance to impacts and blasting, and when one wants to conserve the limited internal space.
Compared with the currently existing arc welding technique of double sided T-joints, the corner adstir fillet stationary shoulder FSW scores better on hardness and fatigue resistance of the weld. Since the properties of the weld material correspond to those of the parent material, the weld can no longer be seen as the weakest link of the structure. The fatigue experiment occasionally revealed a new failure type at the undercut, being a sharp corner created by the modified FSW shoulder. This second failure type did not impact on the overall expected failure resistance; however, the limited occurrence does not allow a final statistical interpretation.
Based upon our analyses, there is not yet a place for widespread use of {aluminium foam core, steel face sheet} sandwich materials, but there is an added value for the corner adstir fillet stationary shoulder FSW.

Correctly predicting the structural behavior of the Pioneering Spirit is vital for ensuring the structural integrity of the ship and the cargo. A detailed finite element model is used to predict the structural behavior of the Pioneering Spirit. The finite element method is based on fundamental principles in solid mechanics. However, when using finite element models considerable differences between the predicted and observed behavior of a structure can occur, even when best industry practices are used to create such models. Because of these
differences there is a need to validate the detailed finite element model of the Pioneering
Spirit.
Finite element model updating is a method that can validate finite element models. In this method the discrepancy between the measured behavior and the observed behavior is minimized by modifying model assumptions and parameters. Currently a number of sensors is installed on the Pioneering Spirit, which can be used to find the measured behavior. Whether or not the measured behavior is detailed enough to be used in the validation of the finite element model is the subject of this research. To investigate this a simplified finite element model of the Pioneering Spirit was created using beam elements, this model provides the predicted behavior. Then sensitivity-based finite element model updating was implemented and applied to the beam model.
Simulated
measurements were used to show that the beam model can be updated using the current sensor setup. When actual measurements were used to update the beam model it was found that the beam model does not correlate with the measured behavior, making it impossible to update the beam model in a meaningful way.
The detailed model does correlate with measured behavior. By assuming that the method will work similarly for the detailed model as it did for the beam model, it can be concluded that the detailed model can be validated using the current sensor setup for a static case. For a dynamic case this is not possible.

In the field of fatigue testing in laboratory conditions, the common practice is uniaxial testing (i.e. in tension). The real-life loads on structures are usually not limited to one direction but contain multiaxial components. For this kind of testing, the ship and offshore structures department of Delft University of Technology developed a machine that is capable of exciting stresses in a specimen that resembles real life. A preliminary study has been conducted for the feasibility of using the acoustic emission method for monitoring fatigue on this multiaxial testing machine and in specific on a tubular double-side welded T-joint in offshore applications. This method is a tool to help to understand the fatigue progress in the material during testing.
Fatigue is a process by which damage is caused under cyclic loading below the yield stress. This phenomenon initiates on a microscale were dislocations accumulate and grow into a crack. The crack can grow until the structural integrity fails in complete rupture. The fatigue lifetime may be predicted; this is done with the help of uniaxial fatigue models. For multiaxial fatigue these models are not accurate and therefore a multi axial fatigue model should be developed. The crux here is to do these experiments to gain knowledge of the multiaxial fatigue to develop an accurate model for multiaxial fatigue.
The acoustic emission method is a non-destructive evaluation method that can help to determine the condition of a structural component. The acoustic emission signals have features that contain information on what is going on in the material. Feature extraction can also give insight to the fatigue lifetime. In addition, the acoustic emission signals can be used for localization of the damage. The localization calculation is done by comparing different times of arrivals and with this information the source can be localized. If this localization and tracking of the fatigue crack can be achieved on the multiaxial testing machine the new prediction models that are developed at the university can be more rigorously evaluated.
Combining the knowledge of fatigue and acoustic emission method, the following main research question is formulated: “Is it possible to detect a fatigue crack with the acoustic emission method, and how can the crack length and position be estimated in the tubular welded T-joint specimen multiaxial loading?”
Before the main question could be tackled, the background noise of the testing machine is measured for possible interference for future test. The second sub-question is: What features of the acoustic emission signal can be acquired and processed robustly for the tubular specimen? The last sub-question is: What are the essential features of the acoustic emission signals released by fatigue cracks under multiaxial loading? This last question could not be answered because there were no multiaxial experiments during the duration of this project. However, the methodology presented here as shown on a uniaxial case is believed to be directly applicable for multiaxial loading cases as well
Three experiments were performed of which one uniaxial test was used for detailed acoustic emission analysis. The data was analyzed with localization of the fatigue crack in the specimen. The localization of the crack and tracking its position is shown to be possible when the data is of sufficient quality. The difference in the estimate of the crack size by acoustic emission and the measured crack length turned out to be smaller than 10% when the crack reached its maximum length.

When a grounding incident occurs, the officer of the watch is usually unable to assess the severity of the damage. Most ships do have damage response procedures in place. In case of military ships these are well developed, where a well-trained crew, in the form of damage parties, is available to respond adequately. For non-navy ships, this is hardly possible, mostly due to crew restrictions. But even for navy vessels, underwater damage is hard to assess because of accessibility issues of flooded compartments. This is undesirable because it needs to be known which compartments have flooded in order to assess the situation with respect to residual buoyancy and stability. Based on the outcome, the officer in charge can decide whether to evacuate or to stay on board and control the situation. For establishing which compartments will flood the extent of the raking damage must be known. It is proposed that the accelerations (or rather, decelerations) measured on board during a grounding incident can be used to make a prediction of the extent of raking damage.

A three degree of freedom external dynamics calculation model for ship grounding has been made, which is used to predict the extent of bottom raking damage and the grounding force with associated decelerations of the ship in terms of surge, sway and yaw. When the ship decelerates, caused by a grounding event, these motions can be logged through continuous ship motion measurements. From these deceleration time traces it is possible to calculate the bottom damage path through a double integration with respect to time. Moreover, with these decelerations, the location of the raking damage along the ship’s bottom can be calculated as well. A straight forward method is proposed, which is validated against data on ship grounding external dynamics from earlier research and data from large-scale grounding experiments, both carried out in the nineties of the previous century.

Fatigue crack monitoring is essential to Ship and Offshore structures, in order to guarantee structural integrity and safety on board. Regular inspections, which are currently performed manually are hazardous, costly and infrequent. The CrackGuard JIP has the goal of developing a system for reliable monitoring of fatigue cracks based on the characteristic magnetic pattern that occurs around a fatigue crack as a result of magnetization by the Earth magnetic field. This method, which is called the Self Magnetic Flux Leakage method, makes use of the disturbance caused by a fatigue crack on the magnetic flux inside the material and can be detected by a leakage pattern of this magnetic flux around the fatigue crack.

The application of the self magnetic flux leakage method for fatigue crack monitoring on a full-scale structures demonstrates that the method is very suitable for monitoring fatigue cracks in ship and offshore structures. The amplitude of the observed flux leakage pattern is increased by the large amount of ferromagnetic materials in these structures, which improves crack monitoring and even allows for the tracking of a propagating fatigue crack on the bases of the obtained measurement results.

The Ocean Cleanup plans to deploy a barrier array, to concentrate and capture plastic in the North Pacific gyre by 2020. Both shape and behaviour of the barrier are quite different from the most marine structures.
To get an estimation of the behaviour and loads on the system, the system has been modelled by The Ocean Cleanup in Orcaflex. This software uses the Morison equation.
The hydrodynamic coefficients used in this equation are obtained from experiments with cylinders far from the free surface. Using these coefficients with the Morison equation is a generally accepted method to estimate the loads on submerged cylinders such as piles. However, the barrier is in the free surface and free to move.
This research focuses on the determination of the hydrodynamic coefficients of an unconstrained floating barrier. A model has been set up to evaluate the hydrodynamic coefficients of a floating barrier in regular waves. The barrier was simulated using a numerical model as wave tank. This numerical wave tank has been preliminarily verified with the linear wave theory, in order to optimise the mesh resolution. The numerical model uses fluid-structure interaction to model the flow around a rigid body, the barrier. The hydrodynamic coefficients were in post-processing determined from the response of the numerical model, with the Morison equation, by means of a least squares method.
The barrier with 2 degrees of freedom, sway and heave, has been compared to model tests performed at MARIN with 3 degrees of freedom, sway, heave and roll. The model with 2 degrees of freedom shows reasonable comparison in waves.
These results found in this thesis indicate that the mass coefficient, C_m of the barrier in both the horizontal and vertical plane is close to zero. The drag coefficient, C_d in the vertical plane is close to zero, in the horizontal plane C_d is close to 0.5, both show resemblance to results found in literature.

A method has been developed with which the rigid-body motions, stress and fatigue damage response of a TLP-type FOWT are determined in time domain, with the aim of assessing the influence of non-linear hydrodynamic response on the fatigue damage. The dynamics are described by means of a system of coupled 6DOF-EoMs and solved in Matlab, after which the validity of the method is confirmed by means of validation and verification using OrcaFlex models.
Subsequently, a method has been proposed to linearize the dynamics of the developed model. For linearization of quadratic damping, three different methods have been proposed, after which the best performing method is determined by means of a comparative study. Subsequently, using the fully linearized model, the difference in long-term damage w.r.t. the nonlinear model is calculated and the influence of multiaxiality is investigated.
The results have shown that the linear approach is suitable for approximation of long-term fatigue damage in hotspots that are located in the connections between the central column and the pontoons of the TLP. However, in hotspots with relatively low stress response, the long-term damage is underestimated by 48% due to the high sensitivity of the damage to differences in the stress response. Finally, the presence of multiaxiality and non-proportionality has been demonstrated for a number of hotspots and during both operational and extreme conditions.

Fatigue cracks could occur in the material of offshore and marine structures that are cyclically loaded. These cracks occur due to the cyclic stresses induced by the waves causing premature failure. Monitoring these fatigue cracks using a wireless system gives the opportunity to guarantee the integrity of the structure without manual inspection, making this a safer way of inspection. In addition, inspection is very costly due to the man hours and because of the fact that the locations of fatigue cracks are mostly not easily accessible. Also, visual evaluation of the crack length can be difficult to assess, which could result in early retirement of structures or unnecessary and costly maintenance.

A wireless crack monitoring system based on the Self Magnetic Flux Leakage (SMFL) method could be a solution for this problem. This method suggests that the ferromagnetic material is passively magnetized by the Earth’s magnetic field and the magnetic signal changes when a crack appears in the material. The measured data is sent wirelessly to the control room of the marine structure. Here, the data can be assessed to determine the size of the fatigue crack. With this information, it can be decided what course of action should be taken to ensure the integrity of the marine structure as long as possible. This will result in a safer way of working and being economically more efficient.

For accurate crack sizing, the SMFL must be interpreted correctly. During cyclic loading the stresses in the material change, affecting the magnetic signal. This is called the stress-induced magnetization. The aim of this research is to investigate the effect of stress-induced magnetization on the SMFL in the stress concentration zone of a structural steel plate, and its implications for crack monitoring by the SMFL method.

The measured stress magnetization curves are obtained by means of an experiment. In this experiment, a steel plate with an elliptical hole is cyclically loaded up to the yield stress. The magnetic signal is measured in a grid around the hole. The results show a maximum variation of 25 µT. Depending on the application, the stress-induced magnetization may need to be considered for the interpretation of the measured signals for crack monitoring using the SMFL method.

In the past decades the understanding of fracture of ductile material has increased substantially. Research has shown that the onset of fracture highly depends on the full state of stress inside the material. Better understanding of fracture resulted in more advanced and complex fracture models being conceived, allowing researchers to predict fracture in ductile solids with improved accuracy.


Nonlinear finite element analysis is a powerful tool at the disposal of researchers to predict the response of ship and offshore structures. When it comes to simulating accidental loads, such as collisions, often basic criteria are applied to include fracture in a finite element model. However, accurate fracture prediction is of great importance to determine the ice resilience of vessels, or to obtain reliable estimates of the sustained damage due to maritime collision. This research focuses on the latter.


This thesis is concerned with bridging the gap between the recent developments in fracture prediction and the application of failure criteria in finite element analysis of ship collision. A selection of recently published fracture models has been made and experiments have been conducted on S235 structural steel for calibration and validation of these models. Four small scale experiments have been conducted. These experiments serve a dual purpose: first, to gather information on the material behaviour during deformation. Second, to obtain information on the effect of different stress conditions on fracture. An iterative method has been employed to accurately model the material behaviour. For the calibration of the fracture models a method has been conceived and applied that takes into account the full histories of stress and strain during the deformation process up to fracture.

Before application as failure criteria for finite elements, the calibrated fracture models require a correction based on the size of the elements: a modification to an already existing theoretical framework has been proposed and applied to obtain element-size dependent failure criteria.


A large scale drop tower experiment has been designed to simulate a so called raking damage scenario. This experiment has been conducted on the same material as the small scale experiments and serves as a validation for a finite element model that has been created using the information on the material behaviour obtained from the small scale experiments. The different failure criteria have been implemented into the commercial finite element package LS-DYNA and have been applied to the finite element model. The results have been compared to the results of the raking damage experiment.


It was concluded that the application of complex multi-parameter failure models in analysis of maritime collision does not necessarily provide an improvement over conventional fracture prediction methods. The inability of shell elements to accurately describe strain concentrations and the effect of the element size introduce uncertainties that overrule the benefits of stress-state dependent prediction of element failure.

This report, which from now and onwards will be called the main body of the thesis, covers the core of the
problem that has to be solved. Generally, the aim of this thesis is to investigate a laminated composite’s
structural response, made of FRP, when its material properties are assumed stochastic instead of
deterministic. The materials shown in this thesis are investigated as part of a bigger project that concerns the
construction of several Mine Counter Measure Vessels.
In this document, the main focus is given on the results of experiments and/or analytical calculations whereas
the theories/methods used to extract these results are presented in the supporting document. As a result,
readers that are familiar with statistics, classical lamination theory, FEA modeling among others, can easily
read this report without coming across knowledge that they already have. In general, this document covers
the writer’s contribution to academics whereas the supporting document gives an extensive explanation on
what was used in this document.

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.

Fixed support structures for offshore wind turbines are commonly used for shallow water (till 45 meters). In many countries shallow-waters are rare. Floating support structures may be the solution for these areas. Many concepts have been developed but three concepts have been analyzed (spar, semi-submergible and the tension leg platform (TLP)) in the literature. This study focuses on the TLP, which has the lowest weight of these concepts but the dynamic system is complex and has significant more risk than the other support structures, for example the risk of resonance of structural elements.

The structural integrity of the total structure is important for the tension leg platform wind turbine (TLPWT). This study investigates the modelling techniques of the flexible TLPWT, with the aim to model the dynamics of floating wind turbine correctly. An Aero-hydro-elastic-servo model is implemented in Matlab, which includes aerodynamics of the wind turbine, hydrodynamic loads on the floating structure and mooring system, the flexibility of the total structure and the control system of the wind turbine. This model solves the equation of motion with the Houbolt numerical time integration method. In addition, the validity of the model is confirmed by validation using an Orcaflex model. The model is used to analyze the effect of the gyroscopic moments and the non-harmonic periodic load oscillations on the motion responses.

Steel structures are vulnerable to cyclic loading. Small cracks may initiate and grow in the structure, this is called fatigue. Fatigue is stress driven and resonance drives stresses. The fatigue performance can be improved by avoiding resonance of structural elements. A method has been developed to find a design with the natural frequencies outside the wind, wave and passing blade frequencies. The method consists of two algorithms, mode tracking algorithm and the selection algorithm. The method is used for a North-Sea site and the result of this an improved design, which has the natural frequencies outside the frequencies where wave and wind have energy. This design has better dynamic characteristics, which indicate better fatigue performance, in comparison of the reference TLPWT, which is predominantly designed to prevent slack tendons. The approach has shown to be successful but the method can only assist in the preliminary design phase of a TLPWT for any given site.

In order to monitor elliptical fatigue crack growth in ferromagnetic steel using magnetic methods, analternative to the self magnetic flux leakage method must be derived as elliptical cracks can grow to significant sizes before they reach through the thickness of the plate material. An approach is sought by translating subtle changes in magnetisation back to the Villari effect, a phenomenon which depicts how applied stress induces changes in magnetisation in ferromagnetic objects. Since the magnitude of these changes in magnetisation is small, other nonlinear effects of similar order such as magnetic relaxation and hysteresis are identified, measured and quantified preliminarily.

The magnetic behaviour in this project is assumed to be quasi-static, and derivations of the expressions for the magnetic field around simple geometric shapes are provided in order to understand magnetic behaviour and verify the outcome of the numerical simulations. It is shown that the numerical simulations
produce identical magnetostatic induction fields as the analytically derived expressions when using a sufficiently refined mesh.

An attempt is made to measure long-term magnetic relaxation by subjecting a solid prolate spheroid to a continuous uniform background field for periods of an hour while trying to measure differences in the induction field at a fixed distance. Short-term relaxation, the time it takes for an object to reach a certain
magnetisation when the background field is abruptly changed, is also investigated. It is concluded that both effects could not be successfully measured using the current setup. In order to draw proper conclusions, further research into this topic should be conducted using more accurate equipment for extended periods of time.

Upon investigation it is discovered that it can not be assumed that the steel specimens exhibit a uniform permanent magnetisation. A self-developed method is introduced through which non-uniform magnetisation in three directions can be calculated by means of inversion using a set of magnetic induction field measurements in a plane below the specimen when the background field is zero. These measurements are translated to magnetisation using a set of higher order square Gaussian distribution functions that are spaced in a grid over the domain of the test specimen in order to vary the magnetisation locally.

Literature that shows comparable results regarding description of non-uniform permanent magnetisation using an array of induction field measurements has not been found. The concept of hysteresis is introduced and a method is presented through which the parameters of the Jiles-Atherton hysteresis model can be determined using parameter fitting in combination with a forward numerical model created in COMSOL. Closure of minor loops require modifications to the original JA equations which are implemented in the forward model. The numerical model is encapsulated within the Shuffled Leaping Frog parameter optimisation algorithm in order to compute the correct hysteresis
parameters. It is found that it is possible to successfully determine the parameters of multiple specimens using weak magnetic fields, and therefore minor loops, which is unparalleled in literature.

Eventually, the Villari effect is introduced and an attempt is made to measure and model the effect usingan extension of the Jiles-Atherton model proposed by Naus. Experiments have shown that using this methodology the magnetostriction parameters can be succesfully obtained. A recommendation is
provided into how these results can be implemented in crack-propagation models in future research.

Floating offshore solar platforms (FOSPs) are envisaged to become Ultra Large Floating Structures (ULFS) with horizontal dimensions of several kilometers. The payload of these structures comprises mainly the photovoltaic panels as well as cabling and limited electrical equipment. Thus the payload per unit area of these FOSPs is very low. Therefore it is likely that these platforms will become lightweight and flexible ULFS. Such a lightweight flexible ULFS is expected to act as a damping lid on the water surface influencing the ocean waves. The problem is to find analytical or semi-analytical solution for highly simplified modelled structure with varied stiffness interacting with surface waves. One possible way to solve the problem is to treat the structure as the boundary condition of the surface waves and apply Green’s function method to determine velocity potential and structure response.