Study of vortex-induced vibrations in a riser under low Keulegan-Carpenter numbers

Abstract

In this study, a marine riser is subjected to excitations at its top with a certain period and amplitude in order to investigate the resultant vortexinduced vibration (VIV) effects. The strain in the inline and cross-flow directions of the riser is measured using a fiber Bragg grating (FBG) sensor that detects the dynamic behavior of the riser by capturing the wavelength variation between the emission and the absorption of a signal. The dynamic behavior of the riser varies along its length in the x-y plane because of complex VIVs under high Keulegan-Carpenter (KC) numbers, even though a response peak is observed at the same frequency. For the same KC number, the inline response is inversely proportional to the excitation frequency. On the other hand, the crossflow response is proportional to the excitation frequency. It is found that the component of the excitation frequency region in the cross-flow response is dominant at low KC numbers. As the KC number increases, the component of the twofold excitation frequency becomes dominant in the cross-flow response. The structural propagation wave induces the same response peak regions at all sensors along the riser length. Copyright ？ 2012 by the International Society of Offshore and Polar Engineers (ISOPE).