An analysis of a deep-sea mining nodule mining system

Abstract

Deep sea minerals can offer an additional resource to meet the increasing mineral demands, instigated by population growth and technological advancements. Deep sea minerals exist in different forms at the bottom of the ocean. In this research, polymetallic or manganese nodules are the kind that are of interest. The nodules are 1 to 12 cm large and contain a variety of minerals like copper, nickel and cobalt, but owe their name to its main component manganese. The region with the highest approximated resource of polymetallic nodules is the Clarion-Clipperton Zone (CCZ), situated in the Pacific Ocean between Hawaii and Mexico. The CCZ has water depth reaching 6000 meters, which is a significantly larger working depth than state-of-the-art deep sea projects within the offshore industry. These depths are accompanied by challenging environmental conditions exerted on the deep sea mining system. A deep sea mining system typically consists out of three components: 1) Production Support Vessel (PSV), 2) Vertical Transport System (VTS) and 3) Seafloor Production Tool (SPT). The SPT harvests the nodules from the seabed, the VTS transports the mined nodules through the water column to the surface where they are transferred to the PSV. The focus in this thesis will be on the Vertical Transport System. Where most deep-sea mining developments are considering hydraulic vertical transport with a riser, Boskalis introduces a concept that utilizes mechanical lifting for the vertical transport. This allows for energy efficient transport, relative simplicity of concept and a maximization of the amount of power units above water. The objective of this thesis is captured in the following research question: What is the behaviour of the combined mining system (ropes, skip and SPT) during vertical transportation by means of mechanical lifting? To answer this question, a wide overview is given of the deep-sea minerals that exist on the seafloor and the existing technologies to harvest them. Whilst the system is in principle relatively simple (just two containers (skips) that are alternatingly filled, hoisted to the surface, emptied and lowered again) many challenges arise. To identify these challenges, the system and the production cycle are discussed in detail. Literature research has been done to ensure realistic modelling of characteristics like structural damping of the rope, drag forces and added mass. The safe working load of steel wires is mostly consumed by its self-weight at a length of 4000 meters, making them unsuitable for deep-sea mining. Instead, the less common but naturally buoyant synthetic fibre rope is envisioned. Many of the challenges in this deep-sea mining system originate from the environment as the system is subject to wave action and currents. Therefore, the current profile and wave spectrum typical for the Clarion-Clipperton Zone are obtained to serve as input for further investigation in the hydrodynamic analysis software Orcaflex. The system will be deployed over the entire 6000 m water column, causing the current but also the forward velocity of the system to possibly lead to high drag forces. A reduction of the forward velocity by introducing a new harvesting method is implemented, resulting in a large reduction of the drag forces and offset. The system consists of at least eight ropes, with two moving skips. Combined with the current and vessel motion, rope entanglement is a risk. A solution to prevent the rope entanglement is presented in this thesis. Possible occurrence of vortex-induced-vibrations (VIV) is identified and future research is recommended. The offset analysis shows that a large offset (500m) between the PSV and SPT results in relatively low horizontal forces on the harvester. The system is connected to the PSV, which is subjected to the Pierson-Moskowitz wave spectrum environment it is situated in, resulting in vessel motions. These motions will govern the dynamic behaviour of the system. The skips with attached fibre ropes have different eigenfrequencies on different water depths, as a longer rope will make for a softer system. This causes both skips, full and empty, to resonate in some regions. Consequently, the dynamic tension in the ropes is higher than the static tension, although it does not come forward as problematic. However, undesired slack rope conditions can occur when lowering the empty skip. To conclude, an analysis of the deep-sea mining system has been done in which the eventual design has been modelled to the best extent currently possible. This research underlines the technical feasibility of this deep-sea mining concept. This research also evaluates the questions that have not been answered yet and recommends a variety of interesting topics for future research.