Dynamic response and fatigue behavior of a mining riser with variable wall thickness under irregular waves and currents

The structural integrity and dynamic performance of deep-sea mining risers are affected by waves, currents, and vessel motions. This study proposes a deep-sea mining riser consisting of four segments with wall thickness decreasing stepwise with ocean depth to enhance structural stability and reduce costs. A time-domain lumped-mass model is developed incorporating irregular waves, linear shear currents, and vessel motions. The dynamic responses of the segmented riser are compared with uniform-wall-thickness risers under varying wave heights and periods, followed by fatigue analysis. The results indicate that the segmented riser had considerably lower axial tension and higher mechanical performance than the uniform-wall-thickness risers did, indicating it is suitable for deep-sea mining. Fatigue evaluation indicated that cumulative damage was more considerable when waves were higher and the peak wave period was shorter. In addition, the dynamic response analysis revealed abrupt changes in bending moments at the connections between the third and fourth segments.