To limit global warming to 2 °C or below, the IPCC emphasizes the need for large-scale carbon dioxide removal (CDR) alongside emissions reductions. There is a growing recognition of ocean-based CDR techniques and expanded research is needed. This thesis investigated the long-term (>100 years) carbon sequestration potential of enhancing the biological carbon pump by ocean nitrogen fertilization, a research domain which so far has received little attention. The objective was to make an initial rough quantification of the (cumulative) CDR potential of one-off ocean nitrogen fertilization achieved within 5 years after the fertilization, to evaluate its potential as a global CDR technique and for commercial implementation.
An idealized one-dimensional vertical (1DV) model framework, coupling a marine ecosystem model with a vertical eddy diffusion physical model, was established to simulate ocean nitrogen fertilization, incorporating essential processes of ocean productivity and the biological carbon pump. Half of the ocean was excluded from the framework since these regions are incompatible with 1DV modelling. Additionally, regions where nitrogen is not the first limiting nutrient were excluded. Various fertilization simulations were performed in which the nitrogen limitation in the mixed layer of the remaining ocean areas was replenished by a one-off addition. Long-term carbon sequestration resulting from fertilization was assessed by comparing these simulations with baseline simulations of the biological carbon pump. An aggregated global outcome, forming the basis for feasibility assessment, was derived by averaging over simulations, while excluding hexagons with low sequestration efficiency as they contribute insignificantly to CDR.
In principle, the one-off nitrogen fertilization shows an effective carbon sequestration efficiency (2.95 kg CO2 per kg N). However, the high and challenging-to-reduce carbon footprint of current urea production, coupled with challenges in collecting manure and the priority of utilising it in agriculture rather than directing it for carbon removal, leads to a low overall global carbon sequestration potential. Nevertheless, it is demonstrated that implementing one-off nitrogen fertilization at a location with high sequestration efficiency using manure can be commercially viable. The cost per net tonne of CO2 sequestered is lower than the current market price of a carbon credit, considering the transportation is conducted with vessels that would have navigated the route regardless.
Based on the findings of this study, it is concluded that nitrogen fertilization has very limited potential to serve as a large-scale global carbon dioxide removal strategy, but can be effectively employed on company-level using surplus manure without agricultural purpose. The initial quantification, despite inherent limitations such as the use of a 1DV modelling approach, serves as a valuable addition to the scarcity of research in the domain of ocean nitrogen fertilization. Further more detailed research into strategic nitrogen fertilization for company-level CDR is justified, with this study providing valuable guidance on optimal research locations.

In recent years, several logistic optimisation models have emerged as powerful tools to assess and optimise the planning, costs, and workability of marine operations. Nevertheless, these models often rely on two underlying assumptions: (1) the significant wave height and peak wave period adequately describe (directional) wave fields, and (2) these two parameters sufficiently describe the conditions causing weather downtime. However, little is known about how these underlying assumptions affect the reliability of logistic optimisation tools. This study, therefore, presents an alternative model that integrates response motions of vessels and turbine structures into a logistic optimisation tool to address and expose the implications that follow from using these assumptions. A case-study approach, on two recently realised offshore wind farm projects, showed that integrating response motions results in, up to 10%, less favourable workability conditions. Further analysis on the data showed that it is crucial to include the two-dimensional wave energy distribution to expose more complex sea states that induce weather downtime. Moreover, a failure analysis approach found that the conditions inducing downtime events are more accurately described by response motions instead of sea state parameters. Therefore, the findings of this study suggest that integrating response motions into logistic optimisation models improves the reliability of the model estimates. Besides, this study suggest that the industry’s approach potentially overestimates the true workability and, therefore, imposes unnecessary operational risks. Hence, the results of this study demonstrate the importance of integrating hydrodynamic engineering knowledge into the assessment and optimisation of project planning, costs, and workability.

The amount of suspended sediment spill released during dredging activities in the Fehmarnbelt strait is monitored along the excavated trench to prevent negative effects on local ecology. Stratification due to the interaction of saline water from the North Sea and fresh water from the Baltic Sea forces bi-directional flow of the layer above and below the density gradient, causing bi-directional spread of the dredging plume perpendicular to the trench as well. During such an event, monitoring on both sides of the trench is required. By mapping out the spatial and temporal behaviour of stratification, a prediction can be given on where and when measuring on both sides of the trench will be needed to include the baroclinic effect. Figures created with monitoring data show that the density profile is influenced by the salinity, rather than the temperature. Therefore, stratification predominantly depends on the in and outflow of water with respect to the Baltic Sea. The figures also show a larger density gradient during out than inflow. It can also be observed that where the water depth is restricted to 10 meters, wind and bottom friction mix the entire water column. Therefore, stratification occurs predominantly during outflow in sections deeper than 10 meters, indicating the need for monitoring the bi-directional plume spread during such circumstances. Whether stratification occurs during inflow in sections deeper than 10 meters likely depends on the duration and strength
of the wind forcing and the initial strength of the density gradient prior to the inflow event. Further analysis should be done to confirm this. Signs of Ekman transport, return flows and the deflection of the currents towards deeper water can also be observed in the measurement figures. Since these processes affect the plume spread direction, additional research can be done on the behaviour of the current direction in the Fehmarnbelt.

This study aims to get an insight into the particle trajectories that microplastics follow, after having been released in the North Sea.

For the computations daily-mean values of the surface currents are used, retrieved from the Mercator global ocean model. 2D particles trajectories are simulated for a year, with a 3rd party Python toolbox for Lagrangian simulation of particles: OceanParcels. Particles released from any location in the North Sea eventually get trapped in the Norwegian Coastal Current (NCC). From here they are being further advected to the North, at different moments in time for the particles released at different locations. The coastal processes in the NCC are mainly linked to wind and stratification, hence variations in ow patterns near the coast are linked to the seasons. When these ow pattern include large scale eddies, the particles follow a meandering and erratic path. Floating plastic particles released in the North Sea will flow northwards along the coast of Norway. Eventually those particles will end up in the Arctic region or get
trapped in the Norwegian fjords, independently of the location of release. However, the time scale of the northward advection depends both on where the particle has been released and the environmental conditions.