ABSTRACT

Wake Induced Oscillations (WIO) and amplification of Vortex Induced Vibration (VIV) are studied for two risers in tandem configuration. Criticality of riser interference is first investigated in term of non dimensional numbers for different types of risers. Theoretical, numerical and experimental approaches are then used simultaneously to define similitude rules and to investigate scale effects with respect to Reynolds number. Finally, experimental results demonstrating amplification of vortex induced vibration are given.

INTRODUCTION

Hydrodynamic riser interference is becoming a crucial issue as offshore technology moves into deep and ultra-deep water depth. The main concern about riser interference is to be able to predict the risk of collision between closely spaced risers, and eventually to design risers able to withstand possible impact events between each other. Besides turbulence buffeting, hydrodynamic interference between risers can be classified in two categories, namely Wake Induced Oscillation (WIO) and Vortex Induced Vibration (VIV).

Wake Induced Oscillation (WIO), also referred to as galloping, is a classical type of instability, see e.g. Blevins (1977, 1990, 2004), Axisa (2001) for a simplified description of the mechanism underlining the phenomena. Basically, spatial variations of the time averaged hydrodynamic coefficients (lift and drag) can create such instability at high reduced velocity. WIO is a low frequency motion which arises at the first natural period of the riser. Large amplitude, which may rise up to several tens of diameters, are predicted (see e.g. Wu, 2001). Vortex Induced Vibration are known to be self limited motion with a maximum of the order of one diameter for a single riser. However, in cylinders array, measurements have shown VIV amplification, up to two diameters. VIV frequencies may be high as they are closely correlated with current velocity through Strouhal relationship.

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