Although the concept of a submerged floating tunnel (SFT) originates from the early 1900s, the cross-section is always assumed to be circular or rectangular. In this research, it is investigated whether this assumption is valid. In order to find the optimal cross-section, the optimization process is split into two targets. The first targets aims for an optimization of material use considering the large hydrostatic water pressure. The second target guarantees a tensile force in the tethers that support the SFT, while keeping the buoyancy weight ratio (BWR) as close to 1.0 as possible. This is beneficial for both the tunnel tube and the tether system. The optimal cross-section considering the optimization of material use is an 'egg-shape' or an oval which deviates slightly from a circle. This is modelled by the form-finding process as executed in the Grasshopper software. This is due to the relative pressure difference between SFT-top and SFT-bottom. The ovalization decreases for increasing depth. The second target results in an ellipsoidal shape, with a flat bottom and convexly shaped top (in case the SFT is anchored to the seabed). This shape reduces the drag and turbulence on the SFT, while a lift force is generated contributing to the tensile force in the tethers. An analogy with an aeroplane wing is made. The magnitude of the lift and drag force is evaluated by solving for the Panel Method in Python. A significant reduction in drag and turbulence becomes visible. Moreover, some significant lift forces are generated by the encountering flow. The wave load must be compensated by the BWR to assure a tensile force in the tethers. All together, two types of solutions are presented. The ideal solution would be an inner (concrete) tube which absorbs the water pressure with a steel exoskeleton to reduce the drag and generate a lift force. The exoskeleton has a flat bottom and a convexly shaped top (when anchored to the seabed) A compromise between the targets is also possible and presented within the research. This compromise also has a flatter bottom and a more convexly shaped top, but still has some appearances of a circle.

A submerged floating tunnel (SFT) can be a promising solution for crossing a deep or wide waterway. This tunnel concept will consist of an immersed tube, either attached with anchor cables to the seabed or attached to pontoons floating on the water surface. The reliability of the tether-stabilized SFT is assessed in this research. A suitable target reliability is determined in order to design a full probabilistic SFT. Subsequently, a calibration of partial factors from Eurocode is performed. The robustness of the structure is also analyzed and improvements are suggested. Important failure mechanisms are defined as yielding and slackening of the tethers, longitudinal failure and transverse shear failure of the tube. A first-order reliability method (FORM) and a Monte Carlo simulation (MC) are performed for the limit state functions of these mechanisms. Design parameters are determined so that a target reliability index of 3.8 is met, because of consistency with Eurocode. Consequently, the design points from FORM are used to calculate partial factors for different loading types. The calculated factors and the general partial factors from Eurocode are compared. Slackening of the tethers proved to be the governing failure mechanism in this analysis. The resistance against slackening depends on the force equilibrium, whereas the resistance of the other mechanisms depends on structural strength. The influence factors from the FORM analysis indicated that permanent loading parameters were dominant, i.e.\ concrete density, water density and tube diameter. It was found that for the strength (STR) mechanisms, the factors from Eurocode result in an overly safe design of the SFT. The calculated partial factors for unfavorable permanent load and variable load are significantly lower than the corresponding general factors from Eurocode. For the equilibrium (EQU) case, Eurocode is not safe to be applied. The general partial factor for the unfavorable permanent loading is insufficient. The robustness of the structure is assessed by considering important scenarios. Excessive leakage has large consequences and will result in global structural failure. However, it has a low probability of occurrence. Mitigating measures are available to prevent failure due to leakage. Furthermore, an SFT will be constructed at a specific location. Wave conditions and geolocation need to be taken into account to reach an optimal design. At a depth of 30 meters, the impact of waves becomes insignificant. Lastly, failure of a single tether should not result in failure of adjacent tethers (i.e. progressive failure). A redundant system can be created by installing more or higher quality tethers. Consequently, when all four tethers of one element fail at the same time, this does not result in longitudinal failure. Overall, it was demonstrated that the reliability requirements of the SFT can be met in the design. Moreover, the design can be optimized by a full probabilistic calibration of partial factors.

The main focus of this thesis is the influence of different real boundary conditions on the failure mechanism of local buckling of steel tubular piles. The research was started with the theoretical method of describing boundary condition by specifying the degrees of freedom per boundary condition. The method makes use of the radial, meridional and rotational degrees of freedom. This method is also used in the current design code for steel shell structures, the NEN-EN 1993-1-6 \cite{NEN-EN_1993-1-6}. By varying the amount and types of constrained degrees of freedom, eight possible boundary conditions can be generated. Looking to the practical applications, a ninth variant with a non-homogeneous boundary completes a list of nine. From the nine boundary conditions, eight conditions are selected to be included in a FEM model study. The current design code describes only five of the nine conditions, what allows for a comparison of the known and unknown conditions with the prescribed rules. Two types of parameters that influence the local buckling failure mechanism are material and geometrical imperfections. In the study to geometrical imperfections, initial ovalisation, accidental eccentricity and dimples are considered. The effect on the design yield stress of the different imperfections is determined following the calculation methods described by the current design code. The calculated reduction of the design yield stress by specific boundary condition and an increasing imperfection is compared to results of ABAQUS FEM model tests. Residual stresses are investigated on the side of material imperfections. The used residual stresses are validated with a simple FEM ring model test that shows deformations due to the introduced residual stresses. The observed displacements could be compared with measured data of real experiments. A range of 36 different tubes is selected to investigate the influences of the eight different boundary conditions on the buckling failure behaviour. This test range covers a wide range of tubes with cross-section class 3 and 4 known for their sensitivity for local buckling. A base model is generated for all the 288 model tests, 36 tubes with all 8 boundary conditions per tube. The results from all tests are displayed in normalized moment-curvature diagrams. Because of a lack of test data of real experiments with boundary conditions involved, a validation with an analytical method is used that describes a linear and bi-linear part of the moment-curvature relation. Finally the results of the FEM model tests are analyzed per parameter but also the overall behaviour is discussed. From the model results could be concluded that residual stresses hardly influence the moment capacity but causes an increase in the curvature of most boundary condition. Geometrical imperfections cause a decreasing moment capacity and an increasing critical curvature whit an increasing imperfection. The trend of the influence of dimple imperfections is decreasing slightly faster compared to the trend observed from the code calculations. A safety of approximately 20\% is observed between the failure stresses of the test results and the design stresses from the code. With an increasing diameter over thickness (D/t) ratio or yield stress, the moment capacity decreases and the critical curvature increases. The relative influence of the D/t ratio within the tested range is larger compared to the yield stress. To investigate the influence of the different boundary conditions, the effect of all parameters is investigated per boundary condition. Following the test results, all boundary conditions except BC4F show the same influence on the maximum moment and corresponding curvature. Boundary condition BC4F shows a moment resistance reduction of 14\% and a critical curvature reduction of 26\% compared to the other conditions. Small observed differences in the post buckling behaviour between the other different boundary conditions are reported to complete this study.