Pipe-Soil Interaction for VIV of Free Spans in Carbonate Silty Sand

Abstract Pipeline free-spans are sections of the pipeline that are not supported by the seabed, and can occur either due to natural processes such as seabed undulations and scour, or exist as engineered sections including pipeline crossings or buckle initiators. Free-spans can leave the pipeline susceptible to inline (horizontal) or cross-flow (vertical) Vortex-Induced-Vibrations (VIV) that could potentially result in fatigue failure. For spans deemed to have a high probability of failure, i.e. critical spans, or for spans evolving over time due to seabed mobility, expensive mitigation can be required. To assess the remaining fatigue life of a free-span, the system response must be quantified, including interactions with the seabed at the span shoulders, which are usually defined in terms of dynamic soil spring stiffness and soil damping values. The paper presents findings from an experimental study to determine soil stiffness and damping during simulated VIV motions for pipeline ‘elements’ (representing parts of the free-span ‘shoulder’) at varied pipeline vertical pressures and installation modelling scenarios. Testing was performed in a carbonate silty sand sample. The paper focuses on in-line VIV, although selected results for cross-flow VIV are also presented. The results indicate that the initial embedment, amplitude of the cyclic motion and pipeline self-weight significantly affect the soil response. These findings will improve understanding of Pipe-Soil-Interaction (PSI) for free-span shoulder sections, resulting in increased confidence in the selection of non-linear soil springs and dashpots, and contributing to more informed assessment of free-span behavior.