On the Coupling Between In-Line and Cross-Flow VIV Response

As offshore installations are moving into deeper water, engineers have to face new challenges in design of structures. Risers and free-span pipelines, subjected to heavy wave loads and large current velocities, are important components of these installations. Vortex induced vibrations (VIV) is a well known subject for most offshore engineers. VIV can cause large stresses and fatigue damage of slender marine structures. Hence, large safety factors are applied to the fatigue limit state design criterion (FLS), due to uncertainties regarding VIV. The present paper describes the preliminary investigation into the coupling between in-line and cross-flow VIV response. Most experimental data so far has been concentrated on predicting the cross-flow response. However, in-line displacements can make a valuable contribution. In fact, it has been proved that in-line responses may decrease the cross-flow response significantly when allowing the pipe to oscillate in both directions. The paper is based on a master of science thesis at the Norwegian University of Science and Technology (NTNU).