VIV in Free Spanning Pipelines

Abstract

Two important aspects of vortex induced vibrations (VIV) in free spanning pipelines are so far not satisfactorily understood; the complex interaction between in-line (IL) and cross- ow (CF) response, and the signicance of higher order frequencies. The focus of this work is the CF-IL interaction, with emphasis on the transition zone from IL to CF dominated response. It also discusses the in uence of the higher order frequencies.

The basis for this study is a set of exible beam experiments conducted as a part of the Ormen Lange (OL) pipeline study. Trajectories from selected cross sections of these tests were used as forced motions for a rigid cylinder, keeping the same A/D, nondimensional frequency and Reynolds number. Assuming strip theory to be valid, the local ow around the cross section of the OL exible beam was then reconstructed.

The test methodology and robustness of the results were evaluated and found to be satisfactory. The energy balance along the exible beam found from the rigid cylinder tests was identied, and discussed in relation to the use of strip theory when predicting VIV in free spanning pipelines.

Hydrodynamic coecients from the forced motion experiments were calculated, and their time-wise stability was evaluated. The results show that added mass changes both along the beam and for varying velocities, and thereby creates resonance for both IL and CF response. Furthermore, span length, higher harmonic forces and motion components, and vortex shedding characteristics are among the factors that are found to be important for the composition of the vibrations generated by vortex shedding on a free spanning pipeline.

This thesis shows that the interaction between in-line and cross- ow response, as well as the eect of higher harmonics, is important when predicting the response from vortex shedding in free spanning pipelines.