Vortex Induced VIBRATION Model Test for Step Riser configuration

Abstract

Abstract Deepwater flexible riser development is often limited by strength and fatigue shortages at the hang off region due to the huge tension induced by the pipe's weight in the deepwater column. A step riser configuration has been developed to reduce these limitations in deep and ultra-deepwater applications. The step riser configuration is comprised of a series of buoyancy cans or modules distributed around the endfitting area to provide a relative large uplift force to compensate for the weight of the flexible riser. Numerical simulations indicate that this step riser configuration is very effective in reducing the tensions at hangoff, thus improving the strength and reducing the fatigue of the flexible riser. The vortex induced vibration (VIV) in the step riser configuration needs to be closely monitored since the deepwater depth and relative large OD of the buoyancy cans or modules will result in greater current loads exerted on the pipe, increasing the drag area. A 1:40 ratio model test has been performed to investigate the excitation of VIV to the step riser configuration. The test section was composed of flexible riser, buoyancy cans, endfitting and its vicinity area. Strain gauges and optical fibers were used to measure the deformation of the flexible pipe and joint mechanism of the step riser configuration. Real time strain signals were collected and post-processed into the frequency domain. System lock-in phenomena was observed during the model test. The intensity of VIV excitation in the model test for normal current ranges in deep waters will be described. Based on the model test, the VIV vibration amplitude in both flow and perpendicular flow directions will be summarized and discussed. The VIV frequency and energy spectrum will also be presented and discussed in this paper.