Dynamic Interaction Between Spans in a Multi Span Pipeline Subjected to Vortex Induced Vibrations

Pipelines are the most convenient and safe medium of oil and gas transportation from offshore fields. Such pipelines will often have free spans when passing through areas with uneven sea floor. These free spans may experience vibrations due to vortex shedding when subjected to ocean current, which may lead to unacceptable accumulation of fatigue damage. The bottom topography may result in multi-span pipelines with short contact area between adjacent spans. Such neighbor spans may interact dynamically, meaning that the response of one span can not be found unless its neighbors are taken into account. The purpose of the present paper is to illustrate possible consequences of interaction and present a consistent method for classification of interaction. Two aspects of interaction are studied, namely the possible energy transfer due to hydrodynamic forces from one span to its neighbor of different length, and how the stiffness properties and length of the contact area between spans influence the interaction. The first aspect is studied by using an empirical model for the analysis of vortex induced vibrations (VIV) of multi-span pipelines. This model applies a frequency domain solution procedure that must describe the interaction between the pipe and the seafloor by linear theory. It was observed from the results that the spans interact dynamically, and the distribution of lift forces along the length of pipe shows that energy is transferred from the span with smallest amplitude to the span with largest amplitude. The second aspect is studied by use of an analysis model where the contact between pipe and seafloor has a more realistic representation. This model is based on a non-linear finite element method, and the dynamic analysis is carried out in time domain. By using this model it is possible to characterize the mechanical coupling between the spans. A systematic study illustrates how seafloor stiffness and local profile of the seafloor will influence the interaction.