ABSTRACT:

This paper presents a new theoretical model capable of predicting the vortex-induced vibration response of a steel catenary riser subject to a steady uniform current. The equations governing riser in-plane/out-ofplane (cross-flow/in-line) motion are based on a pinned beam-cable model accounting for overall effects of bending, extensibility, sag, inclination and structural nonlinearities. The empirically hydrodynamic model is based on nonlinear wake oscillators describing the fluctuating lift/drag forces. Depending on the potentially vortex-induced modes and system parameters, a reduced-order fluid-structure interaction model is derived which entails a significantly reduced computational time effort. Parametric results reveal maximum response amplitudes of risers, along with the occurrence of uni-modal lock-in phenomenon.

INTRODUCTION

Steel catenary riser (SCR) has become a primary candidate for future ultra deepwater oil/gas industry because it offers the most promising technological and commercial solution. One of the key issues in the analysis and design of SCRs is to estimate and control the fatigue damage due to vortex-induced vibration (VIV). Nevertheless, current industrial knowledge of VIV prediction is still based on an empirical science and on a simplified linearized model of straight (e.g., toptensioned drilling/production) risers and pipelines. Therefore, many uncertainties arise when designing the SCRs which are actually flexible inclined cylinders, having initial sags and varying curvatures. As a matter of fact, SCRs are substantially different from top-tensioned risers (TTRs), in view of the current flow direction relative to the pipe axis, which is arbitrary and different from 90o when the flow aligns with the SCR plane of curvature. Moreover, a slender long beam-cable system has multiple natural frequencies which potentially give rise to different in-plane/out-of-plane multi modes in cross-flow/in-line VIV. Nowadays, numerous frequency and time domain tools for predicting nonlinear dynamic responses of straight vertical risers experiencing VIV are available in industry.

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