Dynamic Analysis to Identify Mathieu Instability on Hanging Riser for Exploitation of Subsea Minerals

Abstract

Abstract We investigated the occurrence of Mathieu Instability on a hanging riser under dynamic loads from irregular waves and platform motion. Mathieu Instability shall occur in risers by combining dynamic axial and radial loads with reduced axial tension, resulting in high amplitudes of bending moment and axial tension. Our model is a 20" drilling riser in the hang-off configuration, including the 6-5/8" Choke/Kill lines. The simulation was repeated for 1,500 m and 4,000 m of water depths, typical depths for polymetallic crusts and nodules, respectively. Besides, we removed the subsea slurry pump to increase the risk of the Mathieu Instability occurrence. However, the Mathieu Instability did not occur in all cases because the numerical results of the axial tension and bending moment amplitudes were reduced. We hypothesize that the drilling riser has enough bending stiffness to mitigate the Mathieu Instability. Besides, the top portion of the 4,000 m riser was tensioned beyond the material's yield strength due to effective tension. Therefore, we expect a structural failure in this case.