Study on the Mechanism of Uplift Instability of Pipelines on Liquefiable Seabed Under Wave Action

The changes in the flow field around a liquefiable seabed have significant implications for the dynamic stability of subsea pipelines. The motion of seawater (waves) and the pore fluid within the seabed is described using the RANS (Reynolds-Averaged Navier-Stokes) equations and Biot's dynamic consolidation equation, respectively. Based on the extended Masing rule, a visco-elastoplastic constitutive model capable of simulating cyclic softening and large deformation due to liquefaction of the seabed was constructed. By implementing real-time transfer of hydrodynamic pressure at the seawater-seabed interface, an integrated numerical model for wave-seabed-pipeline interaction was established. This model simulated Sumer et al.'s wave flume experiments to verify its reliability. Using a typical borehole from the Jinqimen area as site conditions, numerical analysis was conducted on the dynamic response of pipelines under long-term wave action. The results show that: (1) The presence of pipelines alters the spatial distribution of liquefied zones in the seabed. Liquefaction initiates at the seabed surface and beneath the pipeline, with subsequent liquefied areas evolving upwards along the outer walls of the pipeline; (2) The rate of reduction in shear stress is more pronounced in the near-field compared to the far-field, leading to significant strain concentration around the pipeline. Additionally, the cyclic shear stress ratio (CSSR) near the pipeline is higher than in the far-field, resulting in faster liquefaction rates in the near-field; (3) Intense seabed-pipeline interactions exacerbate the depth and extent of liquefaction in the near-field seabed, increasing buoyancy forces on the pipeline while reducing frictional resistance. Plastic flow of soil within the affected zone causes soil accumulation and compression under the pipeline, collectively leading to pipeline uplift and lateral displacement.