The Arctic is warming more rapidly than other latitudes, which can result in the release of additional greenhouse gasses, global sea level rise and increase in extreme weather events. Additionally, this causes the rapid decline of sea ice and an ice free Arctic might occur during the summer in the 2040s. The decreasing sea ice cover accelerates the warming of the Arctic, which is known as the albedo feedback system. Solar radiation management (SRM) can be a solution to diminish or possibly stop sea ice decline. Within SRM a proposed technology, known as Arctic Ice Management (AIM), is distributing water on top of existing sea ice to increase the ice thickness enough to survive the summer melt. This raises the question: What water volume should AIM distribute on top of existing sea ice to counteract the annual Arctic sea ice volume loss? Based on data obtained during the period 1979-2020, the September trends for ice extent, ice area and ice volume are -83 400 km2yr-1, -49 200 km2yr-1 and -322 km3yr-1 respectively. The ice volume is considered as target parameter, as it accounts for both absolute areal ice loss and overall decreasing ice thickness. There are two main ice drift patterns in the Arctic: The Beaufort Gyre in the Beaufort Sea and the Transpolar Drift, of which the latter exports ice through Fram Strait into the Greenland Sea. Literature shows the ice remains within the Arctic for about five years when located in the Beaufort Sea and one to two years when located in the Transpolar Drift. For both locations, the ice decay is determined using an analytical approach first. This approach shows resemblance for ice located in the Beaufort Sea, but generally overestimates the ice decay in the Transpolar Drift. For this reason, an empirical approach is developed to determine the survival ice thickness. This results in accurate trends for ice decay of -2.1 to -2.7 cm day-1 in the Beaufort Sea and -0.8 to -1.4 cm day-1 in the Transpolar Drift. Considering 91 melting days results in an average survival thickness of 2.18 and 1 m respectively. AIM can be used to increase the ice thickness beyond this survival thickness and an AIM model is developed to show ice growth including AIM. The model concludes the AIM thickness, initial ice thickness prior to flooding and freezing duration after AIM define the effective ice thickness increase. The model is validated with small scale experiments, which indicate a delay between the flooding phase and continued natural ice growth. This delay can be the effect of the duration required to restore the temperature profile in the ice after flooding as shown by COMSOL Multiphysics simulations. Considering the AIM model, it is discouraged to implement AIM on ice thicknesses below 0.6 m and suggested for ice thicknesses approaching 1 m or higher to optimize the effective increase. The required water volume to compensate the annual sea ice volume loss highly depends on the location, initial ice thickness and target ice thickness and varies between 707 to 1095 km3 in the Beaufort Sea and between 386 to 464 km3 in the Transpolar Drift for the methods discussed in this research. To pump up this water volume, the expected power requirements are 4.5 to 7.0 GW and 2.5 to 3.0 GW respectively.

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.

The world is transforming its energy production towards more sustainable sources of energy. In Europe, there is currently 25 GW of installed offshore wind power capacity. This is expected to grow with 29 GW over the next five years. Offshore wind farms can be expensive and challenging to build, design and maintain. Understanding the offshore environment will ensure that the to be produced offshore wind turbines are of sufficient quality while reducing costs. Monopiles are currently the most common sub-structure, but jacket sub-structures are becoming more relevant due to increasing water depth or changing soil conditions. Structures in icy waters, such as the Baltic Sea, may be subjected to ice induced vibrations while they encounter sea ice. These vibrations have to be considered in vertically-sided offshore structures' design and are the most critical load case when ice is concerned.

Multi-legged sub-structures, such as jackets, can have a problem that does not exist for monopiles, namely ice jamming, where ice fills the space between the legs of a multi-legged sub-structure. The legs and the jammed ice may then act as a single structural unit. Which leads to the main research question: how does an ice jam influence ice-induced vibrations of a multi-legged sub-structure?

First, a literature study of ice jams and multi-legged sub-structures was performed. This study concluded that different ice jamming situations are possible and have occurred with multi-legged structures, which not all have survived. The ratio between leg spacing and diameter plays a vital role in the ice action on multi-legged structures. Furthermore, the combination of ice-induced vibrations and ice jamming had not been studied yet.

Secondly, a model is made based on a phenomenological ice crushing model using COMSOL Multiphysics and MATLAB to simulate the structural response. The sub-structure is based on the jacket design for the NREL 5-MW reference turbine. Different situations from the literature study are used to make several design scenarios for which the structural response is calculated. In total, there are five situations: a base case, an angled base case, an internal jam, a frontal jam and an angled frontal ice jam. The base case does not have an ice jam, and the angled frontal jam has an increased thickness of the jam to twice the incoming ice. For the other jams, the thickness is equal to the incoming ice.

The different scenarios are simulated for a range of ice drift velocities to capture the different ice-induced vibration regimes and see how the structural response changes due to the presence of an ice jam. First, a baseline was established of the jacket's structural response for the base case. Afterwards, the three different ice jams were simulated. Results show that the base case is excited in all three ice-induced vibration regimes. At lower ice drift speeds, intermittent crushing is observed. Then at around 0.05 ms^-1, it transitions into the frequency lock-in regime. Here the structure is excited at its second natural frequency. For higher ice drift velocities (>0.2 ms^-1), continuous brittle crushing is seen. For the angled base case, the transition between intermittent crushing and frequency lock-in happens at around 0.1 ms^-1, and it stays longer in the frequency lock-in regime. The internal stresses around the contact area between ice and leg for the internal and frontal jam did significantly exceed the ice strength. Thus these jams would have failed on crushing at the contact area. The stresses inside the angled frontal jam exceed the ice strength but by a small margin. With all the assumptions made taken into account, the jam might hold. The structural response shows an increase in period for intermittent crushing and a lower amplitude in structural displacement than the base case.

The main conclusion is that an ice jam that would significantly impact the ice-induced vibrations cannot be sustained. The internal stresses exceed the ice strength which would cause the jam to fail. The ice jam that can be sustained acts as additional stiffness for the system and decreases the structure's displacement amplitude for the intermittent crushing regime. In the frequency lock-in regime the structure's displacement frequency increases a bit. But the amplitude is similar in all scenarios because the maximum velocity of the structure will roughly be the same as the incoming ice floe because that is what excites the structure, and this doesn't change.

Offshore wind farms are being developed at locations with moderate ice conditions such as the Baltic Sea, where drifting sea ice upon interacting with offshore structures could lead to the development of a phenomenon known as Ice-induced vibrations (IIV). These vibrations are especially severe when the turbine is idling. Current mitigation measures consist of an expensive solution of ice cones, which are only favourable when ice occurs seasonally. The main objective of this study is to investigate numerically a novel approach to mitigate the ice-induced vibrations of offshore wind turbines by means of control idling.
Three regimes of IIV are generally distinguished, viz. intermittent crushing (ICR), frequency lock-in (FLI), and continuous brittle crushing (CBR). Among these regimes, the ICR and FLI can cause significant vibrations in the offshore structure. Preceding the ice action, the rotor aerodynamics during the parked condition shows that for the wind speeds below the cut-in wind speed of the rotor, the turbine operates in the unsteady aerodynamics termed as dynamic inflow. The comparative analysis is made between the two cases: one with the ice action only, and, the other with the combined effect of ice and wind, where the rotational rotor speeds chosen are 3.0rpm, 6.9rpm and 12.1rpm. In the ice-action case, it is found that the structural response frequency during the ICR and FLI is around the first and the second natural frequency of the structure, respectively. In the case of ice and wind, it is found that the unsteady BEM method has certain limitations, especially in the ICR regime. Also, the aerodynamic damping has no notable effect on the range of IIV regimes for the rotor speed of 6.9rpm and 12.1rpm. However, it does have a significant effect for the rotor speed of 3.0rpm. The quantitative comparison of fatigue damage between the two cases showcases that for the majority of ice-sheet velocities during ICR and FLI, the damage is found to be greater in the ice and wind case. Based on the results, it is concluded that the rotor aerodynamics does help in damping the vibrations in the ICR regime, but in the FLI regime, it has no significant impact when specific ice-drift speeds are considered. It can also be confirmed that by the careful selection of the rotational rotor speed, the range of IIV regime can be influenced. However, to draw the general conclusion, the analysis needs to be conducted for varied ranges of rotor speeds. Also, the present framework of the aerodynamic model needs to be improved to capture the vortex-ring flow state to predict the rotor aerodynamics accurately for all the ice-sheet velocities.