Deep-sea nodule mining

Abstract

With a rapidly growing population and subsequent technological developments, an increase in demand of rare-earth metals evolves. Land-based deposits seem to be insufficient and shortage impends within the coming decades. Mineral resources from the deep ocean are considered as a possible complement. These minerals occur in various forms. One of these forms is lying on the subsea sediment of the deep ocean; polymetallic nodules. The nodules can be best described as potato shaped objects containing amounts of rare earth minerals like copper, nickel and cobalt. The economic feasibility depends on nodule abundance per square meter. An exploitation region with high prospectiveness in terms of economic feasibility is the Clarion-Clipperton Zone, a region in the centre of the Pacific Ocean. As deep-sea mining implies, the mining site is located at a water depth of approximately 4000m. Characteristics such as these automatically result into challenging conditions. Exploitation of the nodules includes subsea harvesting and vertical transportation towards the sea surface. Activities which are within the scope of Boskalis, appealing their know-how and experience in dredging and marine operations. Currently, most developments are focused on vertical transport by means of hydraulic lifting through a riser. In order to avoid challenges such as flow assurance, riser handling and efficiency, Boskalis proposes an alternative solution; mechanical lifting.

Mechanical lifting can be considered as a newcomer in the relative early field of deep-sea mining. However, experience can be gathered from successive deep-sea lowering operations in the oil and gas industry. The objective of this thesis is to investigate the static configuration and dynamic response of the proposed mining system during operation, identify potential showstoppers and in the end asses whether deep-sea mining by mechanical lifting is technical feasible. When simplified, it is merely a matter of collection of nodules in transportation skips and hoisting of these skips towards the sea surface. Nevertheless, within this simple procedure various challenges arise. Therefore a start is made with a proper description of the proposed production method and identification of potential hazards and showstoppers. In complementing the initial research, the project site has been studied in terms of sea floor characteristics and metocean data. These obtained results served as input in further analysis. For example a sheared current is present, influencing the geometry of the hoisting wires. The sheared current phenomenom has been studied in which a quasi-static analysis is performed. The analysis has been carried out by discretization of the hoisting wires in a number of elements with a payload attached at the tip acting as a sinker. Here it became clear that for the applied hoisting setup, the influence of the local current is minimal and the influence of a forward vessel motion significant.

When acting in an offshore environment, the vessel motions might affect the deep-sea mining operation as a result of the connection with the nodule collector. Preliminary to a dynamic analysis, typical vessel motions are determined. This is done by selection of an appropriate mining vessel and implementation of a characteristic sea state using a Pierson-Moskowitz spectrum. Vessel motions are calculated with the software package Seaway, resulting in an output in the frequency domain. The suspended hoisting wires are still numerically modelled by a discretization method. Axial analysis for the applied hoisting setup shows significant magnification of the response of the attached payload at full depth (4000m). This hurdle should be overcome by applying heave compensation, which has become a commonplace in this type of marine operations. Transversal motions for the applied setup are expected to be minimal as a result of the submerged state.

Conclusively, within this research first insight in technical feasibility is gained, but as a matter of fact yield further valuable questions along the lines of deep-sea technology. Therefore, we can conclude that deep-sea mining by means of mechanical lifting is feasible, but still offers numerous interesting topics in future research exceeding the current standards.