Fig. 01, a sketch of offshore platform (red line is the drilling riser)

Flexible Structure Vortex-Induced Vibrations (VIVs)

Long slender structures within the steady oncoming flow are subject to vibrations caused by vortical structures forming due to a distributed flow instability in their wake. The problem has considerable theoretical interest as it constitutes a fundamental nonlinear flow-structure interaction system, while it is very important for the design of offshore industry systems, such as risers, cables, and hawsers, which are subject to large drag loads and potentially catastrophic fatigue damage as a consequence of the vortex-induced vibrations.

The key questions I addressed in my research are:

  • The role of the added mass distribution along the flexible structure

  • Validity of the strip assumption: rigid V.S. flexible structures

  • Characters of the multiple-frequency VIVs

  • VIVs of riser with various complex configurations

01. Hydrodynamic Properties of Single-Frequency VIV (Paper 01, Paper 02, Paper 03, Paper 04)

Through laboratory-scale experiment, show in Fig. 02, we observed the uniform cylinder in the uniform flow will behaves as follows, 

  • the model responds in two types of modal groups, with amplitude increasing in the same group and jumping between two.

  • added mass distribution varies significantly along the model, and have a good comparison of the fluid force distribution with the rigid model prediction. 

  • vortex tube shed in cells corresponding to the in-line mode number, and the vortex pattern is strongly correlated to the sign of the Cmy.

Fig. 02, model sketch and experimental setup 

Structural response:

Fig. 03, 3d visualization of the CF displacement response 

Fig. 04, maximum CF displacement and frequency response over Ur.

Fluid force distribution along the flexible model:

Fig. 05, a comparison between the fluid force distribution along the flexible structure (solid line) and rigid model prediction (dashed line): first row, Clv; second row, Cmy

Wake structure behind a flexible cylinder under VIVs:

Fig. 06, 3d wake structure behind a uniform cylinder in the uniform flow at Re = 900, Ur = 17.22

Mov. 01, structural and wake response of a uniform cylinder in the uniform flow at Re = 900, Ur = 17.22

02. Multiple-Frequency VIVs (Paper 05)

We use cylinders of non-uniform diameters to create multiple-frequency VIVs to understanding the real riser response in the non-uniform current profile in the ocean. And from the result of the stepped flexible cylinder, we observed, 

  • The non-uniform cylinder successfully create stably coexisting multiple-frequency vibrations. 

  • When separated by the frequency component, the maximum amplitude response resembles that of the uniform cylinder in the uniform inflow. 

  • The hydrodynamic coefficient (Clv) of the stepped cylinder can be predicted by the rigid model, but it requires additional input.   

Fig. 07, a sketch of the flexible stepped cylinder model

Structural response:

Fig. 08, frequency response shows the coexistence of the two frequencies at the same time

Fig. 09, separated by the frequency component, the structural response of the stepped flexible cylinder is found to be similar to that of the uniform cylinder in the uniform flow

Fluid force distribution along the flexible model:

Fig. 10, the comparison between the fluid force distribution along the flexible cylinder and the rigid model prediction from two type of forced vibration experiments show the validity and the limitation of the strip theory. 

03. Optical Measurement & Inverse Force Reconstruction (Talk)

For flexible structure FSI problems, the traditional measurement techniques apply accelerometers and strain gauges inside the model, and such methods suffer several drawbacks, including drafting issue, invasiveness, low robustness, long preparation time and sparse spatial resolution. I developed a robust underwater optical measurement system, using multiple high speed camera. shown in Fig. 11.

Fig. 11, a comparison between traditional [1] and current measurement system for flexible cylinder VIVs. 

Mov. 02, a demonstration of the underwater optical measurement system.

Fig. 12, a sketch of a tensioned beam under external forces.

The newly developed optical measurement system allows an evaluation of the fluid force distribution along the flexible structure via inverse force reconstruction method, as it provides a both temporally and spatially dense measurement, shown in Fig. 12

This system has been used to study the flexible cylinder VIV of various configurations, such as buoyancy module riser response (paper), piggyback pipeline response (paper), etc. 

[1] Chaplin, J. R., P. W. Bearman, FJ Huera Huarte, and R. J. Pattenden. "Laboratory measurements of vortex-induced vibrations of a vertical tension riser in a stepped current." Journal of Fluids and Structures 21, no. 1 (2005): 3-24.

 

© 2019 by Dixia Fan, a "lazy" fluid mechanist