Vortex Excited Structura Osilation at ions of a Circular Cylinder in Steady Currents
Authors: King Roger 
Published: October 2008

This paper is applicable to offshore structures in locations where significant steady currents are encountered (e. g. the N. Sea). It emphasizes the importance of considering excitation of fundamental and higher normal modes of structural oscillations in the flow direction, as these can be excited in velocities one quarter those required for-cross-flow excitation.

The use of free surface hydro elastic models is justified by quantitative correlation with full scale test results.

1. INTRODUCTION

Many of the offshore structures now being designed and built include a variety of circular cylinders; braced members, jacket legs and delivery tubes. The flow of water past these constructions can cause oscillations of individual members or of the complete structure, which could, at the least, make working conditions unpleasant for the personnel on site, and might cause fatigue failure or structural collapse. Designers of offshore drilling platforms, mooring dolphins or oil jetties must therefore match the static and dynamic structural integrity with economic viability.

Unfortunately, design data are not always comprehensive and may conflict with data from other sources. In such situations, testing scaled models offers an attractive alternative to the tedium of possibly inconclusive calculations, by demonstrating effects which are difficult or impossible to anticipate.

Flow-induced oscillations in the cross-flow direction of circular structures in air are well documented but the excitation of flow-direction (in-line) oscillations of marine piles during construction of an oil terminal was totally unexpected, particularly as they were excited by velocities of one quarter those anticipated for the excitation of cross-flow oscillations. To investigate the causes of these observed in-line oscillations, a full-scale test rig consisting of three separate piles, was built and tested on site, where steady currents of up to 8 ft. per second were recorded. An independent programme of basic research was instituted by BHRA and the general results of tisane reviewed in Section 3 of this paper. More specifically, hydro elastic models (built to scales of 1/27 and 1/30) of two of the full scale test piles yielded accurate qualitative and quantitative representations of full scale behavior, as detailed in Section 5. It is acknowledged that these are relatively simple models, the use of which is justified in one particular situation, but the basic technique undoubtedly can be extended to models of more complex structures in steady flow.

The importance of considering the effects of steady currents was emphasised recently by estimates that they could cause increases of up to 30% in the stress levels of some of the larger North Sea oil rigs.

2. DESCRIPTION OF FLOW ABOUT STATIONARY AND OSCILLATINO- CYLINDERS

2.1. Flow about a stationary cylinder

When water flows past a stationary cylinder, a periodic wake is formed by vortices shed from alternate sides of the cylinder. The frequency (fv) at which pairs of vortices are shed is related to the cylinder diameter (d) and flow velocity (Y) through the non-dimensional Strouhal number S, where

(Equation Available In Full Paper)