Large amplitude vibrations of a deepwater riser conveying oscillatory internal fluid flow

Abstract

A marine riser operated in deepwater could encounter harsh environmental conditions and exhibit complex vibration phenomena which require thorough investigation, especially using nonlinear geometric diagnosis. The aim of this paper was to propose a theoretical formulation for undertaking large amplitude vibration analysis of a deepwater marine riser conveying oscillatory internal fluid flow. The work-energy principle based on virtual displacement was utilized to formulate the riser mathematical model, accounting for internal strain energy and external work done. The geometric nonlinearity of the riser regarding extensible elastica theory was considered, which accounted for axial stretching and the large deformed curvature of the riser. The finite element method was utilized to form the equation of the motion system for solving numerical solutions. Based on Taylor's series expansion, the stiffness matrices with second-order nonlinear dynamic displacements of the riser were regarded in the equation of motion. The hydrodynamic ocean force modeled in terms of the squared riser velocity also yielded the nonlinear hydrodynamic damping matrix. The inertial force initiated by the pulsatile flow of transported fluid was taken into account as a harmonic oscillation representing the pump operation. Performing the Newmark time integration incorporated direct iteration on the equation of motion provided the nonlinear vibration response. A thorough parametric study was accomplished which investigated the influences of geometric nonlinearity, the forcing frequencies of a hydrodynamic wave, and an internal pulsatile flow on the large amplitude dynamic response characteristics of a deepwater riser.