The Seafloor Nodule Collector: Best Available Techniques and Future Developments

Abstract

Abstract On top of the abyssal plains of the Clarion-Clipperton Zone (CCZ) in the Pacific Ocean lie trillions of polymetallic nodules that contain many of the metals needed for transitioning to a low carbon future. In 2021, the pre-protype seafloor nodule collector ‘Patania II’ performed successful in-situ trials at 4,500 m (>14,750 ft) water depth. The trials entailed collecting nodules using a hydraulic collector head and were complemented by an extensive environmental monitoring program. The gathered results will feed directly into a Seafloor Nodule Collector (SNC) and collector head design iteration study aimed at further improving performance. This paper presents the initial results of the Seafloor Nodule Collector improvement study using Computational Fluid Dynamics (CFD). The scope of the study is twofold, firstly the collector head design is improved and secondly, the wake behind the SNC is minimized. The combination of field experience and data with high-fidelity CFD simulation technology will result in a reliable impact prediction of the design improvements to be done. This paper will provide a useful methodology and reference for further design improvements for seafloor nodule collector vehicles, not only to enhance the hydraulic performance but also to reduce the environmental impact of deep-sea mining operations. Key performance indicators are being identified that contribute to sediment plume dispersion. First, with respect to the collector head, the collector performance is influenced by its geometry, the jet- and surrounding water inflow and forward speed. The combination of these factors will create a pressure profile on the seafloor, which allows nodule pick-up and hydraulic transport. The trials showed that the balance between performance, robustness and unwanted water/sediment intake is impossible to determine in lab conditions. The in-situ gathered performance data, like pick-up efficiencies related to jet velocities, was used to validate CFD simulations and establish a reliable collector head numerical model. Based on measurement data and CFD results, a seafloor pressure profile base-case has been defined. A step-by-step approach was followed by simulating several modifications to the collector head geometry, by varying the jet flow and by applying recirculation of the discharge water to the suction head. This was carried out while consistently maintaining the base-case pressure profile as a reference. The modified numerical model shows that it is feasible to re-circulate and re-use a significant amount of sediment-laden jet water, while maintaining the pick-up performance and reliability. The set-up allows controllability on the amount of re-circulation. The obtained re-circulation results in a significant increase of sediment density, in combination with a decrease of diffuser outlet flow. This will improve the diffuser discharge characteristics and further reduce the sediment plume and associated environmental impact. Secondly, a CFD study for wake modelling has been performed on Patania II as a base case. An optimization study was performed, which involved improving the SNC geometry and diffuser location with the aim to reduce the turbulence behind the vehicle and consequently reduce local sediment dispersion and related environmental impact.