Due to urbanization, improved living standards and electrification, approximately five times more raw minerals are necessary in 2050 compared to 2018. In deep oceans, the seafloor contains these minerals in the form of polymetallic nodules. Nodules are about the size of golf balls that grow throughout the ocean at depths between 3500 m and 6000 m. They contain a wide variety of metals, such as manganese, copper, nickel, cobalt. Nowadays, for large-scale applications, hydraulic lifting is almost exclusively considered for vertical transportation through the water column. However, there is little research available about using other techniques instead. To tackle this knowledge gap, this thesis studies the feasibility of transporting the nodules using a concept of mechanical lifting. The concept used in this thesis consists of two alternating containers that are lowered and hoisted by lifting and guidance wires. Due to the conditions, such as the large depth, the environmental characteristics and the positioning and heading of the vehicles, there are technical uncertainties regarding mechanical lifting. Risks include the yaw rotation of the container, which might result in rope entanglement and wearing of the ropes. This thesis presents a study into the yawing stability of the concept of mechanical lifting for the vertical transportation of polymetallic nodules, which is a crucial factor to operate reliably.

The research question is answered by performing an experimental test and a CFD analysis. The experimental tests include the dynamics of the system while testing various configurations and is validated by an analytical integration in time and a CFD simulation at model scale. The CFD analysis takes away the uncertainties and unknowns: the drag force, the yawing moment and the fluctuation magnitudes and frequencies. The CFD analysis is performed using the open-source software OpenFOAM and simulates multiple configurations. The results of the simulations are compared to the restoring moment by the guidance wires, by transforming the excitation moments into static and dynamic responses of the system. The CFD model is validated by testing the model with a 2D cylinder and 3D sphere, and by performing a mesh convergence study. The CFD simulations are validated by literature. With the obtained drag forces, the energy consumption is calculated.

From the results, it can be concluded that the system can stably be transported at 2 m/s, as the static and dynamic responses are well within the safety limits. The largest response occurs in the middle of the water column, as the rotational stiffness is the smallest at that location. The dynamic response is smaller compared to the static response, as the high frequent fluctuations (f > 0.075 Hz) are damped. Rope entanglement will not occur during normal operation at 2 m/s. However, critical situations due to incidental events can arise, including a winch failure, friction or a sudden high current. This has not been evaluated in this research and therefore stability cannot be guaranteed. As lowering at 3 m/s with an inclined system and including the current results in a static maximum yawing rotation larger than the safety limit, the stability cannot be guaranteed for operating at 3 m/s.