The effect of internal flow on marine riser dynamics

A mathematical model for the dynamic analysis of the riser system with the inclusion of internal flow and nonlinear effects due to large structural displacements is developed to investigate the effect of internal flow on marine riser dynamics. The riser system accounts for the nonlinear boundary conditions and includes a steady flow inside the pipe which is modeled as an extensible or inextensible, tubular beam subject to nonlinear three-dimensional hydrodynamic loads such as current or wave excitation. Combining Iwan-Blevin's model into the approximated form of the nonlinear model derived in this study, current-vortex model is derived to examine the effect of internal flow on vortex-induced vibration due to inline current.;A semi-analytical method using series expansion is employed to derive Eigenvalue problem to facilitate the evaluation of the system frequencies, and its validity is given through the comparison of the solutions with the conventional method using system matrices. Galerkin's finite element approximation and time-incremental operator are implemented to derive the matrix equation of equilibrium for the finite element system and the extensibility or inextensibility condition is used to reduce the degrees of freedom of the system and the required computational time in the case of a nonlinear model. The algorithm is implemented to develop computer programs used in several numerical applications.;The investigations of the effect of internal flow on system frequency, vortex-induced vibration, and vibration due to current or wave loading are performed according to the change of various parameters such as top tension, internal flow velocity, current velocity, wave period, and so on. It is found that the effect of internal flow can be controlled by the increase of top tension. However, careful consideration has to be given in the design point, particularly for the long riser under the harmonic loading such as vortex-shedding or waves. And also, the consideration of nonlinear effects due to large structural displacements increases the effect of internal flow on riser dynamics.