ABSTRACT

A highly nonlinear response and a self-regulated process are usually associated with the dynamic response of a flexible riser. In addition, the formation of vortices shedding from the riser related to Vortex-Induced Vibration (VIV) may adversely affect its structural integrity. In this paper a response prediction model for oscillating flexible risers is presented. It is based on the Finite Element Method (FEM) and a harmonic model for the prediction of the cross-flow forces. An increased mean drag coefficient model and amplitude-dependent lift coefficients are also considered. The simulation results are compared with experimental data obtained from a 35-m riser model. Good agreement in amplitude response is observed in these comparisons.

INTRODUCTION

One of the main challenges of deep petroleum production is the response prediction of the riser system employed to transport oil from the seabed to floating offshore structures. A complex dynamic response and considerable economical impact due to large structural degradation, mainly caused by vortex-induced vibrations (VIV), are commonly associated with deep-water risers. In addition, the resonant-type VIV, when the vortex shedding frequency approaches, or is coincident with, a natural frequency of a riser, may cause considerable cross-flow oscillations of the riser. The VIV analysis of a deep-water riser is still challenging due to the fact the riser can be excited along its length in different modes and at different frequencies, leading to a modal response dominated by mode interference, multi-mode response, mode switching and frequency dependence of the added mass. Basically2 approaches are widely used to predict the dynamic response of a flexible riser; the main difference between these is related to the procedure employed to compute the hydrodynamic forces. The first approach usually incorporates a computational fluid dynamics-based procedure to solve the Navier-Stokes equations in order to obtain the hydrodynamic forces in 2-dimensional planes and then input them into an FE model of a riser.

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