Research on the Dynamic Behavior of 6000M-Class Rigid Risers in Hang-Off Configuration Under Regular Wave in Deep Sea Mining Operations

Abstract This paper investigated the stability and dynamic response of the free end of rigid risers used in deep-sea mining operations, focusing on extracting polymetallic nodules from 6000 m around Minamitorisima Island, Japan. To simulate the dynamic behavior, we first hypothesized the wave height and wave period interval based on the Airy theory and incorporated the current by power law method. Then we established a model composed of Vessel, Riser, and Buffer in the FDM software to evaluate the hang-off configuration’s behavior of the riser by time domain simulation, involving bending moment, effective tension, lateral lateral displacement and steady state, under regular wave and current condition. In addition, the factors influencing the riser bottom’s dynamics were estimated quantitatively, like the buffer mass (connect to the bottom). The results revealed that the buffer mass significantly impacted the bottom’s dynamic behavior, reducing the maximum bending moment and lateral displacement while increasing the holistic effective tension. Besides that, in most cases with different buffer masses, after approximately 2500 seconds from the start of the simulation, the dynamic changes of the riser bottom began to exhibit periodicity and fluctuate within a relatively fixed numerical range. The findings suggest that the optimal buffer mass of 25% to 30% riser mass could offer enhanced stability and reliability for deep-sea mining operations.