Development of a Coupled Hydro-Mechanical Model for Predicting the Structural Response of Buried Pipelines to Geotechnical Instability in the Swampy Terrain of the Niger Delta

Buried oil and gas transmission pipelines in the swampy, water-saturated terrains of the Niger Delta are subjected to complex and poorly understood hydro-mechanical loading regimes resulting from seasonal groundwater fluctuation, lateral soil movement from creep and differential consolidation, and buoyancy instability in waterlogged backfill, all of which contribute to pipeline bending, ovality, and joint distress but have not been represented in a unified coupled model suitable for engineering integrity assessment. This research develops a fully coupled hydro-mechanical finite element model for the structural response of buried flexible pipelines to geotechnical instability mechanisms characteristic of the Niger Delta swamp environment. The model couples a porous media consolidation formulation for saturated soil behaviour with a pipeline structural model incorporating nonlinear soil-pipe interaction springs calibrated against physical pull-out and lateral loading tests on instrumented pipeline specimens embedded in reconstituted Niger Delta soil samples. The hydro-mechanical coupling captures the dynamic pore pressure redistribution effects on effective stress and lateral resistance during flood and dry season cycles, which are shown theoretically to produce load reversal sequences particularly damaging to welded joint fatigue. Validation against pipeline strain gauge measurements from an instrumented segment of an existing Niger Delta crude oil trunk line is conducted, demonstrating model prediction accuracy within 15 percent for peak bending strain. The model provides a theoretical and computational tool enabling fitness-for-service assessment, rerouting optimisation, and cathodic protection interference analysis for pipeline infrastructure in the geotechnically challenging Niger Delta swamp corridor. Keywords: buried pipeline, hydro-mechanical coupling, geotechnical instability, Niger Delta swamp, finite element model.