Time Domain VIV Prediction for Top Tensioned Risers

Abstract

Top Tensioned Risers (TTRs) have been widely used with floating production systems such as Spars and TLPs in deepwater field developments. A TTR system provides direct access to subsea wells from a floating platform for drilling, workover, and completion operations. It is often subjected to Vortex-Induced Vibration (VIV) caused by ambient ocean currents or vessel motions. This paper investigates time domain VIV prediction for TTRs used in a typical Spar floating production system. A typical TTR has strong nonlinear and time-varying dynamic characteristics. The existing gaps between the riser and keel guide and between riser top centralizers and the supporting conductor result in intermittent VIV behaviors of the riser. In addition, hydraulic tensioners are widely used to provide tension to a TTR. The tension from tensioners varies with the riser’s dynamic response especially in the vertical direction. The time domain approach, which has been benchmarked and published in about ten technical papers, is thus more appropriate to predict TTRs’ VIV performance than a frequency domain method. This paper first introduces a typical TTR structure and then presents the analysis methodology and features of the time domain VIV prediction program ABAVIV. An example TTR is used to illustrate intermittent VIV behaviors such as top tension, interaction load at the keel guide, and VIV response at the location of top centralizers. This paper further studies the sensitivity of the VIV response to different current profiles. It finally uses the time domain approach to analyze the VIV response of the riser with its boundary conditions fixed and compares the results with those from a frequency domain program. A conclusion is finally drawn about the use of time domain VIV prediction for Spar TTRs.