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

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

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.

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.

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

During fatigue crack growth of metallic structures, acoustic stress waves are introduced into the material. These acoustic emission waves are the result of the sudden stress release during the nucleation of the crack surface. Using dedicated acoustic emission monitoring equipment, these waves can be recorded. When multiple sensors are able to pick up the waves of a certain fatigue crack growth step, the location of this crack can be determined by using the differences in time of arrival of the signals at these sensor locations. This localization is performed by the method of multilateration, which is the same method that is used to calculate the epicentre of earthquakes. TU Delft and the 4D-Fatigue joint industry project will use acoustic emission to predict fatigue crack depth, in order to determine the crack growth rate in a multi-axial fatigue loaded test specimen. The location of the fatigue crack will be known, so sensors can be applied very close to it. This use of acoustic emission is unconventional due to the fact that most appliances focus on a much larger scale, since this is one of the advantages of this method. The accuracy goal which is set during this project is also far exceeding traditional use, aiming at a crack depth prediction with an error less than 1mm. Little literature on acoustic emission focuses on this close range, inducing the need for additional research. The goal of this thesis is to perform finite element simulations, in order to determine the behaviour of acoustic emission waves in close proximity of the crack. This knowledge should be used to determine what accuracy values are achievable, and whether there are ways to improve on this accuracy. The first step in order to achieve this goal was the simulation of a Hsu-Nielsen source, more commonly referred to as a pencil lead break. This is a very simple to perform acoustic emission source, and will serve to establish appropriate finite element simulation parameters. This simple first step is also convenient to establish the effect the acoustic emission monitoring equipment has on the obtained signals. The multiple filter steps which are performed by the hardware, and the frequency response of the sensor have a great influence on the signal. Pencil lead breaks were performed on a steel plate of 5mm thickness, at several distances of a sensor. Signals produced by the simulations were verified by the experimental signals in a range of 5mm-70mm, verifying the used simulation methods. The next step was to simulate actual fatigue growth in the same steel plate of 5mm thickness. Experimental signals were attempted to be acquired during fatigue tests of the plate. The fatigue machine that was used however, turned out to produce high amplitude noise, making it not possible to obtain the desired signals. Since verification of the simulated signals would not be possible, several source models were investigated in order to simulate fatigue crack growth acoustic emission, comparing the localization results with each other. Localization using conventional methods were able to achieve an accuracy of 0.6mm. Using a new method of wave speed determination however, which makes use of the simulation results, the localization accuracy was improved to 0.4mm. The final step was to perform a simulation of a specimen which is used by the 4D-Fatigue JIP. This tubular specimen contains flanges, which reflect inbound waves back to the sensors, potentially influencing the localization accuracy. The simulations however, concluded that these reflections arrive after the initial wave has passed. The effect on localization was found to be negligible. Localization using conventional methods was not found result in reliable data. This decrease in accuracy compared to the accuracy found in the 5mm plate, is attributed to the larger differences in travel distance the signals have, due to the 10mm thick body of the tubular specimen. Localization using the new speed determination method, resulted in a localization accuracy of approximately 1mm. Although localization accuracy was found to be sufficient to satisfy the initial goal of 1mm, it should be noted that the localization was performed using an estimated noise amplitude value. Also, during these numerical simulations the sensor is exactly known, which is not the case during experiments, so additional location error should be taken into account. The work in this thesis does however describe multiple phenomena which influence the localization accuracy, and forms basis for further research to improve on it.

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.

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

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

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.

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.

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.

Ship and offshore structures are prone to fatigue damage as they are cyclically loaded by waves. Therefore, regular inspection is needed in order to confirm adequate structural integrity throughout the entire service life of the structure. Detected fatigue cracks that are too long for safe operation need to be repaired. Detected cracks of acceptable length need to be at least inspected more frequently. These inspections are costly, time consuming, and hazardous, so additional inspections on top of the periodical class approval surveys are to be avoided if possible.@en