For safe operations in deep ocean, changes in the vibration characteristics of a long pipe or riser are desirable in order to make the vibration of a pipe system controllable, thus increasing its fatigue life. The pipe vibration is often excited by the ship motions, wave forces, and vortex-shedding. Elastic joints along the pipe are presented in an attempt to move the resource frequencies away from the pipe system. The numerical examples focus on the investigation of single and multiple flexible joints along a long pipe and their effect on three-dimensional nonlinear coupled pipe responses, including torsional coupling. The multisubstructure technique is introduced in order to get the governing equation of the entire pipe system. The pipe is subjected to a vertically-varying current Row 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 of 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 bas little influence on the bending direction. 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 "citation of a pipe with multiple flexible joints can greatly be reduced.
Changes in or control of the axial or bending resonance frequencies are often desired in design and ocean operations. One of the methods applied here is to change the fundamental axial frequency and the static equilibrium state of a pipe.