Dynamic Response of a Deepsea Mining Riser

Li, Ting Ting (2H Offshore) | Lim, Frank (2H Offshore) | Sha, Yong (2H Offshore) | Zhang, Hui (2H Offshore)

OnePetro 

ABSTRACT

Seabed mining in deep oceans has taken on new interests around the world after a hiatus since the initial efforts in the 1970’s. Exploration campaigns using the latest underwater survey and mapping technologies have shown certain ocean floors rich in polymetallic mineral deposits in the forms of SMnN (Seabed Manganese Nodules). The few mining systems developed thus far typically involve a VTS (Vertical Transport System) which has a free-hanging rigid riser with pump(s), and sometimes a buffer, located along it. The free-hanging riser presents a special design challenge because of its long unsupported span, heavy weight, and the lack of tension to control its curvature/bending stress. This paper investigates the dynamic response of such a mining riser during operation and installation in 5000m water depth. The analyses conducted include wave induced riser motions and current induced VIVs (Vortex Induced Vibrations) in an exercise to identify design issues and determine operational limitations. Particular attention is given to the scenario when the riser is deployed with the seabed mining crawler hanging in tandem. It is found that an additional clump weight may have to be added to the riser bottom to improve its lateral stability, VIVs dominates the riser fatigue behavior, and the riser axial response dictates the installation weather windows.

INTRODUCTION

With the increasing global demand for strategic metals, deepsea mining has taken on new interests around the world after a hiatus since the initial efforts in the 1970’s. Polymetallic mineral deposits in the forms of SMnN (Seabed Manganese Nodules) have been found in certain ocean floors in water depth of 4000m to 6000m by using the latest underwater survey and mapping technologies. The few mining systems developed thus far typically consists of a mining supporting vessel, VTS (Vertical Transport System) which has a free-hanging rigid riser with pump(s) and sometimes a buffer located along it, flexible jumper and seafloor mining machine.