An insight into the interaction between the in-plane and out-of-plane responses of a catenary flexible riser

Nonlinear dynamic response of catenary flexible risers excited by ocean currents is the main reason for fatigue failures. In this work, a series of experiments are conducted to improve the understanding of the intrinsic relationship between the in-plane and out-of-plane responses of a catenary flexible riser subjected to a sheared current in the depth-averaged Reynold number range of 150-1880. It is found that the interaction between the in-plane and out-of-plane responses is sensitive to the reduced velocity. Four patterns are identified, including the weak coupling, the partial coupling, the strong coupling, and the strong coupling+. The interaction is negligible at low reduced velocities. In contrast, when the reduced velocity is sufficiently large, the in-plane dominant frequency coincides with the out-of-plane one, signifying a strong coupling. As the reduced velocity increases, only a part of the riser possesses the same dominant frequency in the in-plane and out-of-plane directions, termed partial coupling. At higher reduced velocities, the coupled pipe section is shortened and the interaction is finally attenuated.