STUDY ON THE AXIAL TENSION REDUCTION OF A DUAL-BORE VERTICAL RISER SYSTEM FOR DEEP SEA MINING

Exploiting polymetallic nodules in the Clarion-Clipperton Zone shall require a vertical riser with two separate lines. First, the ore-lift line shall convey the slurry composed of seawater and nodules from the seafloor up to the floating platform. The second line is the water- return line that shall bring this seawater to be discharged on the seafloor. One issue of this mining riser is that its top end has the highest axial tension, which may overcome the material's yield strength resulting in structural failure. For instance, the drilling riser used in the Petroleum Industry must have buoys attached along its length to address the same issue. The problem with the attached buoy is the hydrodynamic loads increase linearly with the buoy diameter. Thus, we aim to reduce axial tension without attached buoys and keep the internal diameter as constant as possible to avoid flow assurance issues. Our approach is to assemble the mining riser using riser joints with the same external diameter but different wall thicknesses. The pipe's outer diameter and wall thicknesses assumed in our analyses are defined in the standards API Spec 5CT. Next, we proposed four different optimized configurations for the mining riser. Then, we carried out numerical simulations in the time domain, assuming a 4,000 m riser system. As external loads, we adopted a current profile and irregular waves typical for the Clarion-Clipperton Zone. Our reference is a riser composed of all joints with the same diameter and thickness. Our results show that the riser with the best-optimized structure had an axial tension reduction of 15% compared to the reference. However, this riser has the highest risk of structural failure criteria defined as Method 1 of API ST 2RD in case of worse external loads.