An experimental study on vortex-induced motion responses of a moored semi-submersible with and without riser

Abstract

This article presents an experimental study on the “vortex-induced motion” responses of a moored “semi-submersible without catenary riser” and “semi-submersible with catenary riser.” An eight-point catenary mooring system is adopted for the study in which eight rollers are installed on the pontoons for guiding the mooring lines and the semi-submersible design is of two rectangular pontoon and two circular columns on each of the pontoons. To replicate the real-world scene, the experiments are conducted in wave-cum-current flume with five directions of current (i.e. 0°, 30°, 45°, 60°, and 90°) to study the vortex-induced motion under the surge and sway degrees of freedom at high dimensionless reduced velocities (i.e. 6<U_rs<50.86). Along with the surge and sway degrees of motion, the roll and pitch degrees of motion are investigated too with their paths for semi-submersible (i.e. amplitude traces). A “stainless steel flexible braided hose” with an aspect ratio of 157 till the touchdown point is used as a catenary riser. Results are presented with the definitions of degrees of freedom with respect to the axes defined at the center of gravity of semi-submersible and show that above the reduced velocity of 25 for the sway and surge degrees of freedom, the vortex-induced motion responses show a complex variation without any monotonicity. Furthermore, it is observed that (1) in traces there are no eight-shaped trajectories; (2) for both the semi-submersible without catenary riser and semi-submersible with catenary riser, the higher responses occur along the diagonals; (3) there are couplings of the pitch with surge, and roll with sway dofs; (4) the catenary riser reduces the vortex-induced motion responses for most of the current directions; and (5) galloping instabilities may be occurring and higher amplitudes may be related to them. Finally, it is shown that the vortex-induced motion response of semi-submersible reduces the vortex-induced vibration response of riser and that the motions in pitch and roll are likely to become critical and they demand a detailed analysis to be addressed in future to improve the operational range of semi-submersible and catenary riser in deep water depths under harsh marine environment of high current velocities.

Keywords

Vortex-induced motion semi-submersible stainless steel flexible braided hose catenary riser current velocities

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