Experimental investigation on the coupling between the platform motion and the catenary flexible riser response

Abstract

The vortex-induced vibration of a catenary flexible riser hinged underneath a two-degree-of-freedom horizontally oscillating platform is experimentally investigated in this work. The catenary riser with an aspect ratio of 125 was arranged in the water tank with its concave facing the oncoming flow. The dynamic behavior of the riser was measured in the depth-averaged Reynolds number range of 150–1880 using the non-intrusive optical technique with high-speed cameras. Meanwhile, the sway and surge of the platform were monitored with two laser displacement sensors. The experimental results indicate that the out-of-plane response of the riser is less affected by the sway of the platform in spite of the top-linked motion. There is no distinct alteration in both the response amplitude and frequency in comparison with a top fixed riser. In contrast, the surge of the platform has a great influence on the in-plane response of the riser. Four types of relative motion between the platform and flexible riser are identified. The relative motion and mode competition are the two main reasons for that. In addition, strong coupling and weak coupling between the platform motion and riser vibration are identified. A coupling length is proposed to quantify the interaction between the platform and the riser, which is closely related to the spatial mode competition. As the reduced velocity grows, the enhanced oscillation of the platform contributes to the elongated coupling length of the riser. However, the influence is gradually attenuated at higher depth-averaged reduced velocities, especially in the out-of-plane response. The coupling also exists between the in-plane and out-of-plane responses of the flexible riser. The platform motion has a suppressing effect on this coupling, especially at high-depth-averaged reduced velocities.