Vortex Induced Vibrations of Deep Water Risers: Sensitivity to Current Profile, Shear and Directionality

Abstract

Slender offshore structures such as risers experience vortex induced vibrations (VIV) when they are exposed to currents and accumulate significant fatigue damage through that process. VIV will depend on several structural properties of the riser and on the current profile that the structure is exposed to. In deep water regions, risers will be subject to intricate circulation systems that impose currents profiles which may vary in intensity, shear and direction throughout the water column. The increased complexity of currents will make the prediction of VIV more difficult and represents a clear challenge to the Oil and Gas Industry. The objective of this study is to investigate how selected properties of a current profile affect the development and excitation of VIV for a deep water tensioned riser. We employ a semi-empirical frequency-domain program to perform a series of numerical sensitivity analyses where the riser model is subject to current profiles that vary in complexity and include uniform profiles, linearly-sheared profiles and more realistic profiles that represent offshore boundary current regimes from SE Brazil. We address the sensitivity of the VIV response to current intensity, shear and directionality. Our results demonstrate that those properties of the current profile have significant influence on the range of VIV modes that are excited and on the VIV response. Overall, uniform profiles produced the largest responses and the linearly-sheared profiles demonstrated the large range of VIV modes that can be excited. The realistic profiles also excited a broad range of VIV modes and variations between the profiles produced changes in the VIV response. This study highlights the need to further understand how complex current profiles in the offshore region affect VIV development in comparison to simpler profiles that are recurrent in model test conditions.