Laboratory measurements of the effects of interlaminar slip on vortex-induced vibrations of the unbonded umbilical

Abstract

Laboratory tests were performed to investigate the influence of interlaminar slip on the vortex-induced vibrations (VIV) of the unbonded umbilicals. Two types of umbilical models with the same size were fabricated by using an acrylic tube enclosing seven-core spiral copper cable, in which the unbonded umbilical contains relative slip between its tube and cable, and the interior structure of the bonded one was fixed as a contrast. Both umbilical models under a same top tension were excited respectively in a flume with current velocity varying from 0.1 m/s to 0.7 m/s. Fiber Bragg Grating (FBG) sensors were employed to measure the VIV response in both cross-flow (CF) and in-line (IL) directions. Experimental results confirm that the unbonded umbilical is more susceptive to the reduced velocity in terms of displacement, frequency and trajectory, which is attributed to the nonlinear variation in bending stiffness resulting from the relative slip. Furthermore, the onset of lock-in region for the unbonded model occurs at a lower velocity than that for the bonded model, but with a similar width of lock-in region. Although VIV displacement in the mode transition is relatively small, the interlaminar slip still works due to the influence of its high frequency.