A Lumped Vertical Motion Model for Pipeline Flotation during Trench Backfilling

The occurrence of flotation phenomena during trench backfilling remains a well-recognized threat to the successful installation of offshore pipelines, with direct implications for both safety and cost. This study developed and assessed a simplified model based on Kynch’s batch sedimentation theory to predict the buoyant response of pipelines during sand–water backfilling. The formulation couples the temporal evolution of solid concentration with the dynamic equilibrium of the pipe, accounting for both buoyancy and Bingham-type drag forces. The model was assessed through comparison with a set of small-scale laboratory test results, and good agreement was found in terms of concentration evolution, uplift onset time, and pipe displacement evolution. The results of numerical parametric studies provide insight into the influence of pipe specific gravity, revealing its impact on the evolving balance between driving and resisting forces—from the onset of flotation to eventual resettlement. The results confirm that, despite the simplifying assumptions, the formulation captures the fundamental physics of pipe flotation and resettlement. The model offers a sound theoretical framework that can support future design-oriented studies aimed at identifying safe combinations of pipe density, trench geometry, and backfilling conditions in offshore installations.