INVESTIGATION OF VIV FATIGUE PREDICTION FOR A TOP TENSIONED RISER

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 wellheads from a floating platform for drilling, workover, and completion operations. It is often subjected to Vortex-Induced Vibrations (VIVs) caused by ambient ocean currents. This paper investigates the VIV fatigue prediction for a TTR used with a Spar in deep water. The riser VIV predictions are mainly based on the frequency domain program SHEAR7 that is widely used in the offshore industry. The nonlinear finite element program ABAQUS is used to model the riser for the generation of modal data, including natural frequencies, mode shapes and mode curvatures. The riser is modeled using generic beam elements with equivalent section properties. The model considers the lateral supports from the keel guide and hard tank. The riser tensioner is modeled with non-linear springs. The riser VIV is predicted using different SHEAR7 versions. This paper investigates the sensitivity of the results to key parameters such as the type of lift coefficients, mode bandwidth, mode cutoff, and the Strouhal number. In addition, the time domain VIV prediction program ABAVIV is used to compute the VIV response for a few governing current profiles. The results by SHEAR7 are compared with those by ABAVIV.