Analysis of vibration characteristics of deep-sea mining risers under emergency disconnection state

The extreme marine environment poses a significant threat to deep-sea mining operations. For the sake of security, the jack-up diving platform and riser system are still deployed in the sea, and the emergency evacuation of personnel and mining vessels is one of the common coping strategies. To address the issue of mineral particles naturally settling and stratifying under the influence of gravity in the riser during emergency state, which leads to the nonuniform distribution of the tension of the riser, a simplified mathematical model for the transverse vibration of the segmented riser is established and solved by the new method based on perturbation. The analytical expression of the natural frequencies and the corresponding mode functions of the transverse vibration of the riser under the emergency state is theoretically given. The change in the vibration characteristics of the risers in emergency and operation states is compared and analyzed. The influence of the concentration of mineral particles in the riser in operation state and the concentration of mineral particles in the riser under emergency disconnection state and the density of mined mineral particles on the vibration characteristics of the riser is analyzed. The results indicate that the natural frequencies of segmented risers increase under emergency conditions, with variations within 10% of the operational frequencies. The first-order frequency exhibits the most significant change. The upper segment riser vibrates more violently than the bottom segment riser, and every ventral point in each order of mode moves to the negative x-axis direction compared with the operation state. When the concentration of mineral particles in the riser under operation state increases, the natural frequency of each order decreases. When the concentration of mineral particles in the bottom mineral segment riser of the emergency state increases, the first order and the second order natural frequencies will increase, the third order and the sixth order natural frequencies will first decrease and then increase, and the fourth order and the fifth order natural frequencies will first increase and then decrease. When the mineral density increases, the natural frequency of each order will decrease. Each ventral point in each mode moves regularly with the change of mineral particle density and concentration.