Structures built in lakes and rivers or at sea are under huge forces induced by ice. These cause friction at the ice-structure interface that may pose a threat to the structural integrity of the construction. For the detailed design and realisation of structures in such regions an understanding is required of this friction process. The friction process at the ice-structure interface is driven by a phenomenon called the stick-slip phenomenon, where during stick mode the structure and the ice move simultaneously with the same velocity, and during slip mode the ice and structure interface slide over one another. This phenomenon has been observed and researched extensively, yet the corresponding static and kinetic friction coefficients reported show a wide range and the relation between the involved parameters and the friction coefficients is inconclusive. The most common theory used in describing general friction processes is Coulomb's Law of Friction. This thesis aims to determine whether and when ice-structure interaction can or cannot be described by Coulomb's Law of Friction in particular cases.

For this graduation project, first an analysis of several test set-ups was carried out to determine which set-up was best suited for experiments with stick-slip behaviour. The chosen test set-up consists of a one-dimensional rotating conveyor belt made from sandpaper on which an ice sample would be placed. This sample would be held in place by four springs attached to the back and front of the sample. This would allow the ice sample to move over the rotating belt, but remain relatively in its place. This would enable to the ice to displace in one direction, the direction of the rotating conveyor belt. This test set-up would be compact and it would be easy to switch the belt or ice sample if they had deteriorated too much. However, the use of a 1D conveyor belt prohibits construction materials such as steel or concrete to be used. Using sandpaper with similar roughness as concrete or steel would serve as a substitute to overcome this problem.

Ultimately, it proved to be impossible to realise the set-up and the focus of this thesis switched towards modelling the two-dimensional stick-slip behaviour based on data from previous research. For this a three degree of freedom (3DOF) model was created. As the forces exerted by the springs on the mass act under an angle that is depending on the position of the mass four different approaches were tested to see how this angle could best be incorporated. Furthermore, the friction force acting on the ice-structure interface was modelled using the friction coefficients found by previous research, and using two different approaches to find out whether the friction force should be modelled using the relative sliding velocity at the interface or not. It proved that the angle between the springs’ original position and it’s instantaneous position should be updated every step during slip mode, and that the friction force should be modelled using the relative sliding velocity. Both are cause for higher computational time. When validating this model against the experimental data it was found that friction coefficients were overestimated substantially for the 3DOF model. Using this the 3DOF model was updated and it was found that it now matches with the experimental data.

After analysing the output of the model, it was found that the kinetic friction coefficients are now in the range of 0.05-0.11 for the used parameters (mass 0.67 kg – 2.03 kg, slab velocity 0.15 m/s – 0.82 m/s, stiffness 20.17 N/m – 68.22 N/m). Also, the friction coefficients show a linear dependency with mass, and an slight inverse linear relationship with the slab velocity. Furthermore, with a low stiffness varying friction coefficients were found when varying the slab velocity, but when increasing the stiffness the friction coefficients converge and this variation decreases. Although further improvements can be made to create a better model, in general it is concluded that under the made assumptions, the 3DOF model describes the two-dimensional behaviour well, and shows how research into friction can be carried out while still using construction materials in an experimental set-up.

A single point mooring (SPM) system is one of the possible mooring systems in the field of Arctic floating production. It consists of a moored buoy to provide its anchoring unit with the capacity of mooring and ice sheltering. In the preliminary design phase, the performance of the mooring buoy must be assessed. This can be done by numerical modeling. This thesis provides a model for the two-dimensional numerical simulation of a moored buoy under ice loading, and it optimizes the buoy design based on the results of the numerical simulation.

The model is constructed based on the discrete element method (DEM), where the elements consist of the elements describing the level ice, and one element describing the buoy. The numerical model is capable of simulating several ice-structure interaction mechanisms, including the ice compressive failure, the ice bending failure, and friction.

The model consists of three parts. A compliant contact formulation describes the behavior of contacts between elements. Contact forces are solved implicitly using a Lagrange multiplier formulation. Lastly, the mooring system is described using the catenary equation.

The model is validated against a number of analytical solutions, demonstrating that the model is capable of simulating the buoy-ice interaction. An example of the buoy design optimization process is demonstrated. The optimization aims to obtain a concept design of a satisfactorily performing moored buoy with minimized dimensions under a specific ice condition. Design criteria were determined for the maximum pitch angle and the maximum horizontal displacement of the buoy. Given the desired design criteria, one can obtain the most optimized buoy design after several phases of selection.

Offshore wind energy has very recently begun expanding into subarctic regions with seasonally ice-infested waters like the Baltic Sea. For offshore wind turbines with conventional support structures such as monopiles and jackets that are typically flexible and vertically sided, the phenomenon known as ice-induced vibrations can develop. In response to the growing interest in offshore wind in the Baltic Sea and in further validating a state-of-the-art numerical model for ice-structure interaction, the Ice-induced Vibrations of Offshore Structures
(IVOS) project has been coordinated with the Hamburg Ship Model Basin (HSVA) and many academic and industry partners—including TU Delft and NTNU—in order to enhance understanding of the topic via a comprehensive laboratory testing campaign.
In this thesis, specific data are selected fromthe extensive IVOS Phase 2 tests such that a comparative analysis is performed between two different model-scale structures, one with circular cross-section and another with rectangular cross-section, that were subjected to ice-induced vibrations in the frequency lock-in regime. Preceding the comparative analysis, the data from the IVOS Phase 2 tests are organized into a database and post-processing tools are developed to facilitate further research by supplying analysis-ready data sets. Included in the post-processing tools is an attempt to determine ice-induced global loads from uncalibrated tactile sensor data, which offers insight into the relationship between the relative pressure distributions along the ice-structure interface and the ice-induced global forces on the model structure.
The comparative analysis garnered qualitative information about the frequency lock-in regime and buckling failure of ice. It is observed that frequency lock-in vibrations usually persisted regardless of buckling events. For the frequency lock-in regime, the ice-induced global loads on the circular cross-section structure are generally lower than those for the rectangular cross-section structure. It is unclear whether the difference in global loads between the structures is caused by the difference in cross-sectional shape, other structural properties, ice properties, or combinations thereof. The analysis of the energy of the system is intriguing but does not offer lucid conclusions. However, the quasi-figure-eight pattern from the x-direction and derived y-direction structural displacements is an enlightening discovery that may explain inconsistencies in the energy of the system. Based on the general configuration of the test apparatus from the IVOS Phase 2 tests and the results from the comparative analysis, it can be concluded that ice-induced vibrations of the model-scale structures in the frequency lock-in regime should be regarded as a two-dimensional problem.