This paper presents some results of experimental tests performed to investigate the coupling between in-line and cross-flow vortex induced vibrations (VIV). Traditionally, engineers have concentrated on crossflow vibrations, assuming that in-line oscillations are less important. In-line amplitudes are generally lower, but the high-frequency nature of these oscillations can result in significant fatigue damage. A simple pendulum test set-up was applied in order to study how in-line response influenced the cross-flow one and vice versa. The test programme covered a wide range of reduced velocities as well as four different frequency ratios between in-line and cross-flow natural frequencies. The paper is based on a master of engineering thesis at the Norwegian University of Science and Technology (NTNU).
One of the major challenges for many offshore projects is uneven seabed. As a result, a significant amount is spent on pipeline free span rectification work. Both Snøhvit (Statoil operated) and Ormen Lange (Norsk Hydro operated) field developments comprises small and large diameter pipelines with free span challenges. Increased understanding of the behaviour of free spans will contribute to increased reliability of the pipelines during their operational life. Improved guidelines are therefore of interest to the industry. As fluid particles move close to the surface of a cylinder, vortices are shed from the structure at the separation point. Typically, the shedding process gives a vortex pattern in which the in-line excitation frequency is twice that in the cross-flow direction. A riser or a pipeline may be forced into resonant vibrations, as the natural frequency may coincide with the shedding frequency. Then the motion of the cylinder seems to control the vortex shedding in such a way that the shedding frequency locks on to the resonant frequency of the structure.