The rising demand for minerals and metals and the depletion of land-based resources has created a renewed interest in the extraction of resources from the deep ocean, i.g., deep sea mining. This research focuses on the mining of manganese nodules, these are potato-shaped objects, 5-10cm in size, composed of metals half buried in the seafloor. A Seafloor Mining Tool (SMT) is used to mine the nodules, however, during mining besides the nodules, also the sediment around the nodules is collected. This sediment is separated in the SMT and discharged through a horizontal diffuser located at the rear of the SMT, thereby creating a sediment plume. The sediment plume needs to be reduced for environmental, economic and legal reasons. The sediment plume causes changes in the sediment characteristics, clogging of pores of suspension feeding organisms and burying of the benthic fauna. The plume will spread in all directions, thereby burying still to be collected nodules and reducing the pick-up efficiency, making the process inefficient from an economical perspective. Legal issues will arise when the discharged sediment plume spreads to other exploration areas owned by other parties or to environmentally protected areas. The aim of this research is to investigate the influence of the choice of diffuser height on the reduction of the sediment plume. To achieve this goal a Matlab model was made and small-scale experiments in the laboratory of Dredging Engineering were done. The Matlab model is based on the JETLAG model, described in 'Jets and Plumes' by Lee and Chu (2003), and is a Lagrangian model predicting the centerline trajectory of the plume. The small-scale experiments provide insight in the behaviour of the horizontal discharge plume at different diffuser heights and generate measurements to validate the model. A diffuser designed specifically for these experiments is produced and the height and flow velocity are differed. The experiments consist of visualization experiments, capturing top view and side view images of the buoyant jet, and experiments measuring the velocity and bottom concentration at different locations. The velocity profiles, sediment concentration, deposition pattern and impingement range are compared and analysed. The combined results from the model and the experiments show that the start of the impingement range increases with increasing diffuser height. Furthermore, a lower diffuser height increases the amount of bottom deposition near the diffuser outlet. The measured velocity profiles, near bottom concentration and impingement range are in agreement with this observation. The measured near bottom velocities at 15D from the diffuser outlet are approximately 0.1 and 0.2m/s, depending on the discharge velocity, both have the potential of resuspending settled sediment. This research suggests that the spreading of the plume can be reduced by the choice of diffuser height above the seafloor. Based on the diffuser heights tested in this research a diffuser height of 100mm is recommended. Since a higher diffuser decreases the amount of particles settling close to the source and a lower diffuser increases the potential for erosion. This research has shown that up to a distance of 15D from the diffuser, the near bottom velocities are large enough to cause erosion. Research into the near bottom velocity decay and magnitude further away from the source, inside the turbidity current, is necessary as well to understand the full spreading behaviour and the potential for erosion. Furthermore it is recommended to do research into spreading behaviour of finer particles, especially the behaviour of the actual deep sea sediment in combination with salt water causing flocculation. Moreover, research into the influence of the forward motion of the SMT is recommended since the SMT will create a wake that will influence the spreading behaviour of the plume. Research into the potential creation of a recirculation region caused by the limited entrainment at the rear of the SMT is recommended as well, this can be done by aligning the diffuser outlet with a screen.

Ocean Thermal Energy Conversion (OTEC) has been heavily researched and developed over the last decade. However, no commercial scale for OTEC has been reached yet. OTEC is a clean and renewable source of energy which utilizes the seawater temperature difference between the upper and deep ocean layers to generate electricity. The heat exchangers and the cold water pipe design are considered to be the most challenging part. The cold water pipe is often stated to be the largest risk. The installation of the cold water pipe, the high investment costs of the cold water pipe and the large forces acting upon the cold water pipe are crucial factors for the future success of OTEC. In this thesis, a new kind of cold water pipe concept is introduced. The FLEX-hose concept consists of a thin sheet of nylon, supported by concrete rings and polyester cables. The total length of the foldable concept design is 1000 m. The nylon sheet is attached to the concrete rings and the cables are attached to the concrete rings. The cables run through the concrete rings and can be used during installation to lower the pipe. Deployment of the foldable pipe is expected to be relatively easy thereby lowering the installation costs. The Multi Criteria Analysis performed in this thesis show the FLEX-hose foldable concept to be attractive compared to a foldable spiral concept, a flexible composite concept, a rigid composite pipe and a HDPE pipe. However, the inclination angle occurring while pumping may not become significant, meaning above 7.5 degrees, since this will lead to lower energy efficiency. In addition, no current design is available on the market, meaning heavy research and development is needed for the foldable concept to succeed. Possible challenges for the FLEX-hose concept that have been established through this research include instability occurring due to buckling and a pressure drop becoming larger than 0.6 bar, thereby significantly reducing the net power of the OTEC plant. The maximum allowed angle of inclination of the sheet to stay below 0.6 bar is calculated to be 7.5 degrees at a flow speed of 2 m/s. During pumping a discharge of 20.000 kg/s is pumped through a thin walled pipe with a diameter of 3.5 m, which leads to a radial displacement of the nylon sheet. Collapse and too significant deformations of the thin walled sheet are prevented by applying 10 MN of pre-tension in the thin walled sheet. However, in order for the nylon sheet to stay in the linear elastic regime, the calculated wall thickness is 0.04 m, thereby making foldability questionable. A total of 200 concrete rings is present in the design, one every 5 m over the total length of 1000 m of the pipe. In addition, a large clump weight of 12 MN is needed at the bottom of the pipe. The total system weight of the foldable pipe with concrete rings is calculated to be 1840 mT, which could be installed by an offshore installation vessel. Finally, creep has also been established to be a challenge due to the life-time of 30 years for the cold water pipe. A synthetic plastic material such as nylon experiences relaxation of the material when stressed for a longer period of time, leading to large deformations.

Deep-sea mining is becoming more popular. The Atlantis II Deep is a deep sea mining field of metalliferous muds in the Red Sea. This field could be exploited by transporting the sediments through a pipeline laid from shore onto the sea bed to the field. From determined concentrations with their shear stress-shear rate relation, parameters for the power law and Bingham plastic model were determined, relayed to test data, so that in the end the Bingham Plastic model was used to calculate pwoer consumption for a variety of cases.