The Influence of Boundary Layer Velocity Gradients and Bed Proximity on Vortex Shedding From Free Spanning Pipelines

Abstract

ABSTRACT The paper summarises the results of a laboratory study of the separate and combined effects of bed proximity and large velocity gradients on the frequency of vortex shedding from pipeline spans immersed in the thick boundary layers of tidal currents. Tests have been carried out using a rigid model pipeline cylinder in a water channel flow at subcritical Reynolds numbers. The measurements show that in the case of both sheared and uniform approach flows, with and without velocity gradients respectively, the Strouhal number defining the vortex shedding frequency progressively increases as the gap between the pipe base and the bed is reduced below two pipe diameters. The maximum increase in vortex shedding Strouhal number, recorded close to the bed in an approach flow with large velocity gradients, was of the order of 25 percent. A synthesis of these laboratory findings combined with the results of a linked full scale field investigation will be used to define threshold vibration criteria to be employed in assessing the stability of pipeline spans against vortex induced resonant oscillation. INTRODUCTION The laboratory investigation described in this paper, forms an early part of a project concerned with the behaviour of pipeline spans commissioned by the UK Department of Energy. A summary of the scope and objectives of the project is given in reference (11. The primary objective of the complete investigation, which also includes full scale tests, is to produce rationally based guidelines for assessing pipeline span conditions. Pipeline spans can occur due to local scour of bed sediment or where the pipeline is routed over irregular seabed. When bed currents flow across such lengths of freely suspended pipeline, separational vortices can form which are shed alternately from the top and bottom of the pipeline. This action generates fluctuating hydrodynamic forces which Can produce large amplitude resonant oscillation of the span in the cross-flow direction if the vortex shedding frequency is close to the span natural vibration frequency. Such oscillation is highly undesirable, particularly in the case of concrete coated pipelines where the coating may crack and possibly spall. In reaching a decision as to the possible need for remedial action, it is crucially important therefore to be able to predict threshold conditions for the onset of significant vortex produced oscillation for individual spans. Criteria for predicting incipient vortex induced vibration of flexible cylinders are often specified in terms of critical values of the 'reduced velocity' parameter, V = Ulf D, where U is the approach velocity f is the natural frequency, and D is the cylinder diameter. For example, in order to avoid cross-flow resonant oscillation, it is commonly recommended that V should be ? 3.5. Critical values of V are based largely on observations from tests carried out with cylinders in uniform approach flow remote from boundary influences.