Pipeline laid on irregular seabed terrain may have free spans. Due to current, such spans may experiences vortex induced vibrations (VIV), which may lead to fatigue failure. The dynamic properties of free spanning pipelines cause a very complex response pattern and adjacent spans may also have some kind of dynamic interaction. A first set of experiments with flexible pipe model, see Soni & Larsen (2006), showed that the maximum response amplitudes for two interacting spans are higher than for equivalent single span cases. The interaction between IL and CF response will probably have some influence on the response level, in addition to the interaction between adjacent spans. A second set of experiments has been conducted with a motion controlled rigid cylinder in order to find the hydrodynamic coefficients for different flow conditions and also to observe how combination of IL and CF motions will influence the hydrodynamic forces. The cylinder was forced to follow an oscillatory pattern found from the first set of experiments with flexible pipe model. The Reynolds’s number and the dimensionless frequency were kept the same for both types of tests in order to ensure that the flow conditions are identical. The vortex shedding process for the motion controlled rigid cylinder has been mapped using Particle Imaging Velocimetry (PIV) under varying oscillation conditions. Improved understanding of correlation along a flexible beam and the interaction between cylinder motions and vortex shedding is hence obtained. The variation of lift coefficient along the pipe length supports the theory given by the authors; see Soni & Larsen (2005), for energy transfer between the spans. Thus, the spans interact dynamically.

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