Effects of Elastic Joints on 3-D Nonlinear Responses of a Deep-Ocean Pipe: Modeling and Boundary Conditions

Pipe vibration is often excited in the deep ocean by ship motions, wave forces and vortex shedding. Elastic joints along the pipe are modeled in an attempt to move the resonance frequencies away from the pipe system. The numerical examples focus on the investigation of single and multiple elastic joints along a long pipe and their effect on three-dimensional (3-D) nonlinear coupled pipe responses, including torsional coupling. The multi-substructure technique is introduced in order to get the governing equation of the entire pipe system. The pipe is subjected to a vertically varying current flow in establishing the static equilibrium configuration. Dynamic responses are excited by large-amplitude horizontal as well as vertical ship or pipe-top motion. Ocean-mining pipes 4,000 ft and 18,000 ft in length are used to investigate the effects of the joint stiffness and position on the pipe responses. The bending stiffness can affect the bending moments along the pipe and the associated maximum values, but has little influence on the bending deflection. However, the axial stiffness of the joint can greatly change the axial fundamental frequency, as well as static axial displacement, while it has little effect on the static internal axial force. The appropriate position of joints can have a greater influence on the static responses. The dynamic responses to the external excitation of a pipe with multiple elastic joints can be greatly reduced. The results are presented for both free and pinned bottom-end conditions of the pipe.