It is becoming increasingly clear that the effects of climate change should be decreased or even mitigated. Green alternative sources of energy are being explored, and wind energy emerges as an important option that can be exploited on a large scale. Wind turbines are placed more and more often offshore due to larger and more stable wind resources. The most common foundations for these turbines are monopiles. The future outlook for these turbines and their foundations is that they will become bigger. An important design characteristic of monopiles is the natural frequency of the pile. In order to keep dynamic effects to a minimum, excitation frequencies should not coincide with the natural frequency. The primary source of excitation of monopiles are bending moments. Therefore, this thesis will take a closer look at the dynamic bending capability of monopiles. There is a lack of knowledge on dynamic effects of bending moments on steel cylindrical shells (monopiles). While the dynamic axial buckling capacity of cylindrical shells has been researched extensively, there is little known on the dynamic bending buckling capacity of such shells. This thesis investigates the influence of loading rate on the dynamic buckling capacity of monopiles subjected to bending moments. For this, a finite element model is constructed which is validated by analytical models. Later on, the finite element model is used to conduct a parametric study to investigate the effect of loading rate on buckling capacity. The scope of this study excludes factors such as soil dynamics, fluid structure interactions, residual stresses, and lateral forces. The focus is solely on the cylindrical shell of monopiles, excluding secondary steel from consideration. Initial geometric imperfections are considered as local perturbations necessary to initiate buckling, while other factors that may affect lateral forces or overall structural capacity are excluded. This study will show that different parameters play part in the dynamic bending buckling behavior of cylindrical shells. The natural frequency of the cylinder, as well as the non-dimensional length together with the yield stress of the material play an important role in the dynamic buckling capacity. This research concludes that cylindrical shells with higher natural periods are more influenced by dynamic bending moments than cylinders with shorter periods. Next, shorter, stocky cylinders exhibit higher dynamic buckling capacities than slender cylinders. Also, imperfections are found to decrease the buckling capacity of cylindrical shells, but this effect diminishes for increasing loading rates. Keywords: Offshore wind energy; Cylindrical shells; Dynamic buckling; Loading rate; Imperfections; FEM;

The aim of this thesis is to determine the influence of the Y/T, yield to tensile strength, ratio on the stress fields of high strength steels. This is relevant to the offshore industry as regulations restrict the use of steels with a high yield to tensile strength ratio, which is a characteristic of high­-strength and ultra high­-strength steels. Identifying the influence of these parameters on behavior could lead to a scientific underpinning and review of the rules. The focus of this thesis is on stress concentrations at circular cutouts under uniform tension. The local stress and strain was found, which could be used to assess the risk of strain localization and fracture. Analytical models of Stowell, Neuber and Irwin are used to establish new relationships between the Y/T ratio and the stress field. The analytical methods of Stowell and Neuber were able to account for strain hardening, while the Irwin analytical relationship only provides a elastic-­perfectly plastic solution. Besides the stress field, an analytical formulation for the local strain based on Stowell’s theory is also established. Numerical simulations were carried out for S690, S960, and S1100 materials. The numerical material models are later used in the research to validate analytical approaches. Also, an infinite plate approach is applied to the specimens by scaling the ligaments of the specimens by a factor of 6. A parametric study has been carried out to gain insights into the influence of the governing parameters in relation to local strain. The parameters studied are the Y/T, yield strength, strain hardening exponent, and strength coefficient. Each parameter is varied separately. The main conclusions are that a high Y/T ratio causes a slight decrease in plastic stress concentration factor, resulting in a slight increase in local strain. The strain hardening exponent shows the same trend, but the changes are even less significant. The yield strength and strength coefficient have no effect on the stress concentration factor or local strain. None of the parameters show any influence on the extent of the plastic zone of the material.

The forming of double curvature shipbuilding steel plates is performed by experienced craftsmen at IHC Metalix. Their experience will get lost when they retire; therefore, it can be valuable for future metal workers to capture the forming process of a double curved plate theoretically. An analytical description and understanding of the forming process will help craftsmen. In the future, it could even lead to the development of automatic forming machines. This research investigates plastic deformation initiated by a rolling and a three point bending machine forming a saddle shaped plate. A 2Doptimization for the three point bending process is performed using beam theory. A single bend is validated by a 2D Finite Element Analysis (FEA). The rolling process is analytically described by equations obtained from literature. These equations are used to find a relation between the material stretching/membrane strains and the rolling forces. Furthermore, an elastic perfectly plastic stress strain curve is used. Material hardening and residual stresses due to repeated bending or rolling operations are not taken into account. The bending optimization resulted in the least number of bending operations needed over a cross section, assuming the cross section behaves like a beam. The rolling equations resulted in required rolling forces for a desired membrane strain. The craftsmen know by experience that bending operations should be performed first to initiate a first curvature. Thereafter, the plate is rolled to initiate the second curvature and form the saddle shape. In this thesis, the first steps are taken to capture and predict the manual forming process of double curvature steel plates. Future work on this topic should take the 3D-effects of the three point bending into account which lead to more accurate analytical descriptions. Furthermore, other assumptions could be analyzed to get better understanding of their contribution.