Marine risers are commonly exposed to sheared flows, and the Reynolds number is often high enough to ensure that the flow lies in the critical regime Despite this, the analysis of vortex induced vibration in marine risers is largely based on physical data obtained from model tests where the flow was subcritical and had a uniform profile.

This paper begins with a brief review of current knowledge pertaining to the effects of shear and critical flow on levels of vortex induced vibration Details are then presented of a new analytical model whit h can be used to predict amplitudes of vibration in sheared (and uniform) flows

In order to gain new information on the behaviour of marine risers in sheared and critical flows, and to validate the analytical prediction method, a large scale model test programme was carried out using a 6 m deep water flume

The presence of shear in the incident flow was found to reduce the peak levels of vibration, but the shear extended the range of velocity over which the vibration occurred These effects were, in general, predicted quite accurately by the analytical model

Critical flow was also found to reduce levels of vibration (by about 40%) and shifted the amplitude response curve to lower Reduced Velocities

Notation and symbol table (Available in full paper)

INTRODUCTION

The exploitation of offshore oil reserves involves the extensive use of slender cylindrical structures, such as drilling and production risers, which are often exposed to strong ocean currents Marine risers are compliant structures and their natural frequencies are, in many cases, similar to the natural frequency of vortex shedding caused by the local currents, they are, therefore, susceptible to vortex induced vibration

Marine risers are commonly exposed to sheared flows, and the Reynolds number is often high enough to ensure that the flow lies m the critical regime Despite this, th e analysis of vortex induced vibration in marine risers is largely based on physical data obtained from model tests where the flow was subcritical and had a uniform profile The main reason for this over-simplified approach is simply a lack of knowledge, the effect of critical flow on vibration levels is unknown and, although shear is known to reduce peak vibration levels, it is generally not possible to predict the magnitude of the reduction

The work reported in this paper was carried out to enable the effects of shear and critical flow to be quantified so that they can be included in the vortex induced vibration analysis of marine risers Fig 2–1 Circular cylinder in a linearly sheared flow profile (Available in full paper)

BACKGROUND

Consider a circular cylinder of length L and diameter D which is subjected to a linearly sheared flow profile, as illustrated in Fig 2 1 The degree of shear is characterise by the Shear Parameter, /3 as defined in the following equation (Available in full paper)

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