Lock-in prediction for vortex-induced vibrations of a long hanged and weighted riser in internal fluid flow and external currents

Abstract

The occurrence of lock-in, a local synchronization between the vortex shedding and cross-flow (CF) structural vibration frequencies, is investigated for a long flexible riser, of which the top is hanged and bottom is weighted and moveable in both the in-line (IL) and CF directions, conveying internal fluid flow (IF) and immersed in uniform, and linear shear currents (flows). Our established models are used to conduct a numerical simulation, in which unsteady hydrodynamic forces associated with wake dynamics are described using two van der Pol wake oscillators in both the IL and CF directions that are coupled with the riser. The riser is discretized into a string of spatial slender beam elements, and the wake oscillators are distributed on every immersed element nodes. MATLAB (R2014a) is used to establish a computational program, which is verified using existing experimental data. The lock-in frequency and location are captured by comparing the CF oscillation frequency of the wake model with the CF vibration frequency of the riser at every node of the finite element system when both coincide with each other at the frequency as well as location. The influences of the oncoming flow velocity (OFV), IF velocity (IFV) and bottom hanging weight (BHW) on the lock-in frequency, as well as the lock-in distribution along the riser axis are discussed. In UF, a high IFV (4.5 m/s) makes lock-in occurrence in riser’s “whole oncoming velocity region” in a single-frequency mode but a zero or low IFV (1.5 m/s) makes it null. In linear shear flow, a zero or low IFV (1.5 m/s) makes the lock-in occurrence in riser’s “low and middle oncoming velocity region” in a single-frequency mode and a high IFV (4.5 m/s) makes it in the “whole oncoming velocity region” in the multi-frequency modes, the lock-in frequency mode (but not the lock-in amount) is regardless of OFV and BHW in the context of this paper.