Session: 05-03-01 Deepsea Mining and Ocean Resources

Paper Number: 101679

101679 - 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 convey seawater to be discharged on the seafloor. This riser system shall have a subsea pump whose weight keeps the whole riser tensioned in the axial direction. Besides, the riser axial tension increases linearly upward due to the pipe and internal fluid weights.

One issue of this mining riser is that its top end has the most considerable axial tension. This axial tension 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, our aim is to reduce axial tension without buoys. At same time, we want to 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 those defined in the standards API Spec 5L and API Spec 5CT.

Next, we carried out numerical simulations in the time domain using commercial software for the mechanical analysis of riser systems. We assumed a 4,000 m riser system composed of two rigid pipes: an ore-lift pipe and a water-return pipe. These two pipes are attached to each other in the way they shall bend together. As external loads, we assumed a current profile and irregular waves typical for the Clarion-Clipperton Zone. A riser composed of all joints with the same diameter and thickness is considered as our reference. Then we repeated the simulation combining joints with different wall thicknesses. In this way, the thickest pipe was assembled on the riser’s top portion, and the joints with a narrower wall were assembled downward. Finally, the thinnest pipe was assembled in the bottom part.

In conclusion, our results show that the riser with an optimized structure had an axial tension reduction of 10% compared to the reference. Besides, the results of the structural failure criteria defined as Method 1 of API ST 2RD confirmed that both risers were tensioned below their yield strength. Thus, no structural failure should occur.

 

Presenting Author: Marcio Yamamoto National Maritime Research Institute

Presenting Author Biography: Marcio Yamamoto is a chief researcher in the National Maritime Research Institute, Japan.
His research is focused on subsea equipment for the exploitation of subsea minerals.

Authors:

Marcio Yamamoto National Maritime Research Institute
Joji Yamamoto National Maritime Research Institute
Sotaro Masanobu National Maritime Research Institute