Testing to Reveal Vertical Soil Stiffness and Damping at Pipeline Free-Span ‘Shoulders’

Pipeline (or cable) engineers routinely have to manage freespans - i.e. sections of the pipeline which are or which may become unsupported by the seabed - during their operational lifetime. Free-spans can make the pipeline susceptible to Vortex- Induced-Vibration (VIV), with associated potential of fatigue failure, leading to the requirement for regular pipeline inspections and expensive free-span mitigation for critical spans. VIV can lead to significant dynamic movement of the pipelines in the free-span and requires consideration of the metocean inputs, the local hydrodynamics, seabed changes due to scour, the structural response of the pipeline and the seabed support conditions ('pipe-soil interaction'). Although recommended practice guidelines exist for pipesoil- interaction of submarine pipelines, there is a knowledge gap in the realistic selection of non-linear vertical soil springs and dashpots at the free-span shoulders (i.e. sections of continuous pipeline support on either side of a free-span), which may lead to increased conservatism and potentially unnecessary remediation. This paper reports an experimental investigation into the variation of soil stiffness and damping ratio for a pipeline section on the seabed that can be idealised as a segment (representing part of the free-span shoulder) when subject to different cyclic vertical loading conditions. Results from the tests are compared with state-of- practice guidelines, and show that both the load history and amplitude of the cyclic loading have a significant impact on the soil response. The reported results will contribute to better understanding of pipe-soil-interaction for free-span shoulders and improved selection of non-linear soil springs and dampers.