Effects of Axial Dampers And Elastic Joints On the 3-D Dynamic Responses of a Deep-Ocean Pipe With Torsional Coupling

Reduction in the axial stress near the axial resonance was previously achieved by Chung and Whitney (1981) with the axial damping of a buffer: equipment with a mass attached to the bottom end of a long vertical pipe. Recently, another method to reduce dynamic axial stress of a long vertical pipe was investigated by Aso et aI. (1991, 1994), applying axial vibration absorbers along the pipe in solving it as pure axial motion in the frequency domain, and showed that the absorbers can effectively reduce the axial stress near its resonance. The 3-D nonlinear pipe code, 3DNLPIPE, is updated with the modeling of axial vibration dampers attached on a very long vertical pipe, and the numerical investigations include the 3-D, as well as torsional, responses with and without elastic joints at selected locations along the vertical pipe. During the ocean mining operations in the deep ocean, the vertical pipe undergoes a large deflection (Chung, Cheng and Zheng, 1995), with the bending (x- and y-), axial (z-) and torsional (Ѳz-) displacements being coupled. For this" case, the present analysis of the 3-D nonlinear coupled responses shows that proper arrangements of the axial vibration dampers at proper positions along the pipe without the elastic joints can reduce the axial stress. The axial dampers can slightly decrease the mean bending deflection, while the elastic joints slightly increase it. The elastic (flexible) joints installed along the pipe can be arranged to be effective in reducing the axial stress and bending moments. Furthermore, a best combination of the multiple elastic joints and tbe axial dampers along the pipe can be used to reduce static, as well as mean dynamic, deflections, the axial stress, bending moments and torsional deformation. The elastic joints can make the biaxial and torsional vibrations reach steady state, while the axial dampers do not.