A parametric study of vortex-induced vibration of a long flexible marine riser

Ghimire, Anup (2008) A parametric study of vortex-induced vibration of a long flexible marine riser. Masters thesis, Memorial University of Newfoundland.

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Abstract

This thesis describes a parametric study of vortex induced vibration (VIV) on a marine riser, subjected to uniform and sheared flow. Raman-Nair and Baddour (2003), has developed a program that simulates the riser dynamics based on a time domain analysis. A parametric study was carried out, taking the code as a bench mark. The riser was simulated using lumped masses, connected by springs that model the riser's properties. This study proceeds by validating the code against the analytical proof and the experimental results. -- Force coefficients, drag and lift force primarily, were taken as the important parameters to study their effect upon the riser structural responses in terms of maximum bending moment, tension, tensile stress, and displacements. Other factors such as internal fluid flow and movement of riser top-end were also considered for the case where riser was subject to sheared flow. A certain configuration of riser was chosen with certain material properties. A steel catenary riser with an un-stretched length of 3000 meters was pinned at both ends and immersed in 2500 meters water depth with an outer and inner diameter of 0.5m and 0.4 m respectively. -- Design of Experiment (DOE) methodology was adopted for this parametric study. Full factorial and half fractional factorial designs were carried out for uniform and sheared flow respectively. Studies showed the drag force coefficients affected the in-line displacement and the lift force coefficient affected the cross-flow displacement. The maximum tension and maximum tensile stress on riser segments, all were affected by the internal fluid flow. The change in the position of the top end of riser reduced the bending moment and increased the tension and tensile stress. After identifying the significant parameters, magnitudes of the parameters were changed within the expected ranges, to determine if responses of the riser were varied by significant amount which would help in the riser analysis and design.

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