Prediction of Combined Inline and Crossflow Vortex-Induced Vibrations Response of Deepwater Risers

Abstract

Deepwater risers are susceptible to vortex-induced vibrations (VIV) when subjected to currents. When responding at high modes, fatigue damage in the inline (IL) direction may become equally important as the crossflow (CF) components. Accurate calculation of both IL and CF responses is therefore needed. Empirical VIV prediction programs, such as VIVANA “Passano et al. (2016, “VIVANA—Theory Manual Version 4.8,” Trondheim, Norway),” SHEAR7 “(Vandiver, J. K., and Li, L., 2007, “Shear7 v4.5 Program Theoretical Manual,” Department of Ocean Engineering, Massachusetts Institute of Technology, Cambridge, MA),” and VIVA “Triantafyllou et al. (1999, “Pragmatic Riser VIV Analysis,” Offshore Technology Conference, Houston, TX, May 3–6, Paper No. OTC-10931-MS.)” are the most common tools used by the offshore industry. Progress has been seen in the prediction of CF responses. Efforts have also been made to include an IL load model in VIVANA. A set of excitation coefficient parameters were obtained from rigid cylinder test and adjusted using measured responses of one of the flexible cylinder VIV tests. This set of excitation coefficient parameters is still considered preliminary and further validation is required. Without an accurate IL response prediction, a conservative approach in VIV analysis has to be followed, i.e., all current profiles have to be assumed to be unidirectional or acting in the same direction. The purpose of this paper is to provide a reliable combined IL and CF load model for the empirical VIV prediction programs. VIV prediction using the existing combined IL and CF load model in VIVANA is validated against selected flexible cylinder test data. A case study of a deepwater top tension riser (TTR) has been carried out. The results indicate that VIV fatigue damage using two-dimensional directional current profiles is less conservative compared to the traditional way of using unidirectional current profiles.