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 mining riser comprising four segments of differing wall thickness. The wall thickness decreases segmentally with increasing ocean depth. The focus of this design is to improve the structural stability of the riser while reducing its manufacturing, transportation, and maintenance costs. A time-domain model based on the lumped mass method was established by considering the combined excitations of irregular waves, linear shear currents, and vessel motions. Subsequently, this model was employed to compare the dynamic responses of a segmented mining riser and risers with various uniform wall thicknesses. A parametric analysis was conducted to investigate the risers’ dynamic response under nominal wave heights and periods. Subsequently, a fatigue analysis was conducted. 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.