Effect of elastic joints on eigenfrequency of a vertical deep-ocean pipe

Book · Fri Dec 01 04:00:00 EST 1995

OSTI ID:131705

Chung, J S ^[1]; Cheng, B R ^[2]

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1. Colorado School of Mines, Golden, CO (United States). Dept. of Engineering
2. Tsinghua Univ., Beijing (China). Dept. of Engineering Mechanics

The dynamic axial force is one of the most important design parameters for a long vertical pipe in the ocean. In design and ocean operations, changes in the vibration characteristics of a long pipe or riser are desirable in order to make the vibration of a pipe system controllable and its fatigue life longer. The pipe vibration is often excited by the horizontal as well as vertical ship or pipe-top motions and the vortex-shedding. In the present investigation a FEM modeling of elastic joints along the pipe and a substructure technique are presented in an attempt to move the resonance frequencies away from those of the pipe system, using elastic joints. For a very long pipe (e.g., 18,000 ft), a modal synthesis is adopted to reduce the large number of degrees of freedom required to get the accuracy of the axial force variation in the entire pipe. Numerical examples are presented for two pipe configurations: (a) top-fixed and bottom-free and (b) top-pinned and bottom-free. The results include effects of the elastic joints on the fundamental axial and bending frequencies. As the pipe gets shorter (e.g., 400 ft), the effect becomes very substantial. The elasticity of the joint affects the fundamental axial eigenfrequencies much more than the fundamental bending frequencies. Regardless of the pipe length, its axial eigenfrequencies can be changed by proper arrangements of the joints along the pipe within a certain range of stiffness. The less the stiffness of the joint, the more pronounced are the effects. The present elastic joints can be used to control the coupled dynamic response characteristics of a deep-ocean pipe.

• OSTI ID: 131705
• Report Number(s): CONF-950740--; ISBN 0-7918-1329-0
• Country of Publication: United States
• Language: English

Related Subjects

02 PETROLEUM
42 ENGINEERING
ELASTICITY
FINITE ELEMENT METHOD
FLUID-STRUCTURE INTERACTIONS
JOINTS
MARINE RISERS
MECHANICAL VIBRATIONS
OFFSHORE OPERATIONS
SERVICE LIFE
THEORETICAL DATA