Abstract: In order to solve the vibration problem of a Pipe-in-pipe (PIP) system under the action of seawater, the dynamic response of the vortex induced forced vibration of the PIP system is studied. Then the numerical analysis is carried out to study influences of the outer pipe diameter, axial load, and external excitation frequency on displacement responses of the PIP system under the action of vortex. Based on Euler-Bernoulli beams theory, Lamb-Oseen vortex model is used to establish the dynamic model the PIP system, and Green function method is used to obtain the steady-state response of the forced vibration of the PIP system. Results show that as the diameter of the outer pipe increases, the vortex excitation force increases, and the position of the maximum amplitude becomes farther from the outer pipe; the axial tension has a greater impact on the outer pipe, but has a smaller effect on the inner pipe; when the dimensionless external excitation frequency sets to 0.4, the displacement of the outer pipe exceeds the limit of allowable deformation, and periodic collisions between the inner and outer pipe can easily happen, which cause damage to the PIP system.

Key words: pipe-in-pipe, vortex induced forced vibration, steady-state response, Green's function method, Euler-Bernoulli double-beams