Use and Conversion of an Existing Marine Drilling Riser for a System Integration Test for the Collection of Seafloor Polymetallic Nodules
Authors: Flamen S. , Taha M. , Barr A. M. 
Published: April 2024

The world's demand for metal is rising and new solutions must be explored to meet the needs associated with population growth, urbanization, decarbonized energy systems and circular economy goals. There is increasing recognition that seafloor minerals could help diversify the world's supply of responsibly sourced minerals and metals. As an example, polymetallic nodules found on the seafloor of the Clarion Clipperton Zone (CCZ) in the Pacific Ocean contain substantial quantities of nickel, cobalt, manganese and copper – all are minerals that have been highlighted as being important for the energy transition. A number of entities are undertaking mineral exploration in the CCZ, and some are developing technology for the collection of nodules on a commercial scale. One of the key components is a lift system to bring the nodules from the seafloor to a surface support vessel/platform. This study examines the feasibility of using a modified marine drilling riser for a Vertical Transport System (VTS) for deep-sea mining operations. In particular, it examines whether it would be feasible to use such a configuration during a System Integration Test (SIT), a full-scale mining trial which could be executed ahead of commercial mining operations. This paper examines several topics including: Rated capacity of existing marine drilling riser joints and handling equipment.Drill ship and drilling riser joint modifications.The arrangement of riser joints and pump modules, including the influence on riser dynamics.Effects of internal fluids on riser dynamics.Operability of the riser within the CCZ metocean environmentModified riser fatigue analysis.

Initially a sensitivity analysis was performed to determine which parameters have the most effect on the dynamic behavior of the riser. Afterwards, many iterations were simulated with different variables tested such as pump locations, riser joint types, buoyancy distributions, etc., in order to optimize the design of the riser. After validation of the in-house simulations, the second phase of the study analyzed the riser dynamics including the effects of internal fluids while the riser is pumping slurry. The operability and fatigue analysis were conducted using a 35-year hind-cast metocean data set, based on wave buoys, satellites and models.

The study resulted in a riser configuration that provided high operability (~99.9%) and sufficient fatigue life to conduct a test mining campaign in the Clarion Clipperton Zone. The proposed mining riser consists of existing riser joints and new-build joints hung-off on the spider-gimbal of the drill ship during operations (Hard hang-off). The existing riser joints need to undergo minor modifications to be able to reuse them for the mining riser. The riser modifications work with the vessel's existing handling equipment to deploy and retrieve the mining riser with minor changes.

The study concludes that a modified marine drilling riser offers an economical, technically and environmentally suitable solution for a deep-sea mining vertical transport system used during the SIT.