Improvements in the Optimization of a 4,000 M Class Riser System for Deep Sea Mining

Abstract This article explores optimizing buoy quantity and placement along a riser system for deep-sea mining. Initially, an algorithm was developed to place buoys along a hang-off riser which the main tube has a constant wall thickness, setting the von Mises stress to one-third of the grade X65 pipe’s yield strength. After this first algorithm, the resulting riser is referred to as Configuration 1. However, Configuration 1 resulted in resonance due to the natural period of the first axial mode being close to the wave’s peak period. To address this, a second optimization algorithm was developed, incorporating main tubes with varying wall thicknesses. Thinner tubes were used at the bottom, progressing to thicker tubes upward as needed. Buoys were strategically placed to maintain the same stress level mentioned above. After this second algorithm, the resulting riser is referred to as Configuration 2, which successfully mitigated axial resonance by shifting the natural frequency away from the wave’s peak period. Time domain simulations and eigenvalue analysis confirmed that Configuration 1 experienced significant axial resonance, while Configuration 2 reduced the resonance effect, aligning natural periods away from the wave’s peak period, which shall minimize cumulative fatigue damage.