This work presents the experimental results focused on the dynamic characteristics of in-line response of a long flexible riser. The riser model is 28.04 m in length and 0.016 m in diameter, giving an aspect ratio (length to diameter) of around 1750. The mass ratio (structural mass over displaced fluid mass per unit) of the riser model approximates 1.0. By means of towing the riser model in a wave basin, the uniform current was generated. The time series of strains of the riser model under various current speeds (ranging from 0.15 m/s to 0.60 m/s) were monitored using the Fiber Bragg Grating (FBG) sensors. The mode decomposition method was employed to analyze the measured strain signals. The dominant frequency, displacement traces, standard deviation of displacements and fatigue life associated with the in-line vibration were obtained. The experimental results indicated that the higher order response mode from the in-line vibration together with the large deflection may reduce the fatigue life of risers as severely as that from the cross-flow vibration, which should be paid much more attentions to the practical engineering applications.
Many flexible slender structures in offshore engineering, such as deepwater risers, TLP tendons and hanging cables are subjected to marine currents. As the fluid flow passes these cylindrical structures, the well-known vortex shedding is observed, resulting in the fluctuating forces on the structures and finally inducing the vibrations of structures in both cross-flow and in-line directions, The vast research efforts have been focused on understanding and predicting the dynamic response of vortex-induced vibration by means of experimental and numerical approaches. Contrasted to the extensive investigations on the cross-flow response of the long flexible risers, the in-line dynamic response has received little attentions so far due to its rather smaller oscillating amplitude (Søreide, 2002).